Dissertations / Theses: 'Seismic ambiant noise'

1

Froment, Bérénice. "Utilisation du bruit sismique ambiant dans le suivi temporel de structures géologiques." Thesis, Grenoble, 2011. http://www.theses.fr/2011GRENU035/document.

Full text

Abstract:

La technique des corrélations de bruit ambiant est aujourd'hui largement utilisée en sismologie. Elle présente l'avantage essentiel de fournir des données qui ne dépendent pas de l'occurrence des séismes. Cette technique a été utilisée pour imager avec une bonne résolution la croûte dans plusieurs régions du monde. Depuis plus récemment, les corrélations de bruit sont également utilisées pour détecter des variations des propriétés élastiques de la croûte terrestre, et suivre l'évolution temporelle de structures géologiques potentiellement dangereuses. Dans ce contexte, mon travail de thèse a consisté en deux parties principales : 1/ Une étude méthodologique pour améliorer la qualité des mesures sur les corrélations de bruit. La distribution des sources de bruit empêche généralement la reconstruction parfaite de la fonction de Green dans les corrélations de bruit, pouvant introduire des biais dans les mesures basées sur le bruit sismique ambiant. Nous présentons dans ce manuscrit deux approches différentes pour essayer d'éliminer l'influence des sources de bruit dans nos mesures. 2/ Une étude de suivi temporel dans la province du Sichuan (Chine), autour du séisme du Wenchuan (12 mai 2008, Mw = 7.9). Cette application offre des conditions exceptionnelles à une application de suivi temporel, avec l'étude d'un très puissant séisme continental et un très grand nombre de données. Nous nous sommes en particulier intéressés au comportement de la croûte à différentes profondeurs, en réponse au séisme. Nous présentons également une méthode d'inversion pour localiser les variations dans le milieu
The ambient noise correlation technique is now widely used in seismology. It is especially useful since data provided do not depend on the occurrence of earthquakes. This technique has been used to construct high-resolution tomographic images of the crust in various regions of the world. More recently, ambient noise correlations have been used to detect changes in elastic properties of the Earth's crust, and monitor the temporal evolution of potentially dangerous geological structures. In this scientific context, my research work has consisted of two main parts : 1/A methodological study to improve the quality of noise-based measurements. Usually, the distribution of noise sources prevents the exact reconstruction of the Green's function in noise correlations, that may introduce bias in noise-based measurements. We propose here two different approaches that can help in suppressing noise source influence in measurements. 2/A monitoring case study in Sichuan (China), associated with the Wenchuan earthquake (12 May 2008, Mw = 7.9). This case study provides many opportunities, with the study of a great continental earthquake and a large amount of data available. We especially investigated the behavior of the crust at different depths, in response to the Wenchuan earthquake.We also present an inversion method in order to locate variations in the medium

APA, Harvard, Vancouver, ISO, and other styles

2

Boué, Pierre. "Traitement d'antenne et corrélation du bruit sismique ambiant : applications multi-échelles." Phd thesis, Université de Grenoble, 2013. http://tel.archives-ouvertes.fr/tel-00988090.

Full text

Abstract:

L'utilisation d'un grand nombre de capteurs sismiques est de plus en plus courant pour imager l'intérieur de notre planète depuis sa surface pour la prospection sismique, jusqu'à sa structure profonde avec la sismologie continentale et globale. L'application d'un traitement d'antenne aux enregistrements issus de réseaux de capteurs permet l'extraction de nouvelles observables et une meilleure compréhension de la propagation des ondes dans les milieux complexes. Parmi ces méthodes, on s'intéresse particulièrement aux traitements simultanés en émission-réception de type double formation de voies (DFV). A l'échelle de la prospection sismique, la DFV est utilisée pour extraire des ondes de volume pouvant être masquées par des ondes de surface plus énergétiques. A l'échelle continentale, les réseaux de sources étant plus rares, on propose d'appliquer la méthode DFV à des signaux reconstruits par corrélation du bruit sismique ambiant. De la même manière que pour un couple de stations, la corrélation d'enregistrements continus permet d'évaluer la fonction de Green entre deux antennes réceptrices. Cette méthode est appliquée à des données du réseau Transportable Array (USArray) afin de mesurer et cartographier la vitesse de phase des ondes de surface au centre des USA. Enfin à l'échelle globale, une combinaison de plusieurs grands réseaux sismologiques est utilisée pour démontrer que la corrélation d'enregistrements continus, dans la gamme de périodes, 5-100s permet la reconstruction des ondes de volume à des distances télésismiques. Une analyse de la contribution respective du bruit ambiant, d'origine océanique, et des séismes est réalisée. On montre que les arrivées tardives des forts séismes, réverbérées à l'intérieur du globe, contribuent de manière importante à la reconstruction des phases profondes. Les ondes de volume reconstruites à partir du bruit ambiant constituent une nouvelle source d'information, complémentaire aux données issues des séismes, et pouvant être utilisée pour imager notre planète.

APA, Harvard, Vancouver, ISO, and other styles

3

Lehujeur, Maximilien. "Étude d'un réservoir géothermique profond par corrélation de bruit sismique ambiant." Thesis, Strasbourg, 2015. http://www.theses.fr/2015STRAH013/document.

Full text

Abstract:

Cette thèse porte sur l’application de la technique de corrélation de bruit sismologique ambiant pour l'imagerie et le suivi des réservoirs géothermiques de Rittershoffen (ECOGI) et de Soultz-sous-Forêts (GEIE-EMC). La forte variabilité spatio-temporelle des sources du bruit de fond sismologique dans la gamme de période 0.2-7s limite la reconstruction des fonctions de Green. Cela induit des erreurs dans la construction des modèles de vitesse. Deux approches sont proposées pour s’affranchir des effets de la non-uniformité spatiale du bruit. Par ailleurs, la variabilité temporelle des sources de bruit est un facteur limitant pour le suivi du réservoir. On estime que les perturbations de vitesse doivent être de l’ordre de 0.1% à 1% pour pouvoir être détectées par les réseaux disponibles. Ce seuil n’a pas été franchi lors de la construction du site Rittershoffen mais une modification probable des propriétés diffractantes du milieu a été observée à la suite d’une stimulation
This work focuses on the application of the ambient seismic noise correlation technique for the imaging and monitoring of deep geothermal reservoirs near Rittershoffen (ECOGI) and Soultz-sous-Forêts (GEIE-EMC). The strong spatial and temporal variability of the noise sources in the period range 0.2-7s limits the reconstruction of the Green’s functions. This results in significant errors in the velocity models. Two approaches are proposed to overcome the spatial non-uniformity of the noise and to improve the quality of the velocity models. Besides that, the temporal variability of the noise sources is a limiting factor for monitoring purposes. We estimate that the speed variations should be larger than 0.1% to 1% to be detected by the available networks. This threshold was not reached at Rittershoffen during the drillings or the stimulations. However, a probable change of the diffracting properties of the medium was observed following a hydraulic stimulation

APA, Harvard, Vancouver, ISO, and other styles

4

El, khoury Christine. "Seismic ambient noise amplitude anomalies induced by an anticline structure : application to the context of gas reservoirs." Electronic Thesis or Diss., Université Paris sciences et lettres, 2022. http://www.theses.fr/2022UPSLM100.

Full text

Abstract:

Des anomalies spectrales dans le bruit sismique ambiant, définies comme de fortes amplifications spectrales de la composante verticale pour des fréquences de 1 à 5 Hz, sont couramment observées au droit de réservoirs de gaz. Si elles sont bien comprises, ces anomalies pourraient avoir un potentiel pour l'exploration ou la surveillance des réservoirs de fluides. La nature de ces anomalies s'explique principalement par la structure géologique plutôt que par le réservoir lui-même, pour une modélisation purement élastique. L'analyse sur des jeux de données réels de bruit ambiant montre que les ondes de surface, principalement le mode fondamental, sont les principaux contributeurs à l'anomalie. Ce travail explore comment les ondes de Rayleigh interagissent avec une structure anticlinale, un piège géologique typique pour un réservoir d'hydrocarbures. L'analyse de la signature spectrale d'une structure anticlinale est effectuée pour différentes configurations de source, pour finalement fournir une relation reliant les paramètres géométriques et élastiques de l'anticlinal à l'anomalie induite pour le cas du mode fondamental des ondes de Rayleigh
Spectral anomalies in the seismic ambient noise, defined as strong spectral amplification of the vertical component at frequencies of 1 to 5 Hertz, are commonly observed above gas reservoirs. If properly understood, these anomalies could have a potential for fluid reservoir exploration or monitoring. The nature of these anomalies was mainly explained by the geological structure more than the fluid reservoir itself, for purely elastic waves. Analysis on real datasets of ambient noise measurements shows that surface waves, mainly fundamental mode, are the main contributors to the anomaly. This work further explores how Rayleigh waves interact with an anticline structure, a typical geological trap for a hydrocarbon reservoir. Analysis of the spectral signature of an anticline structure is performed for different source configurations, to finally provide a relationship linking the anticline geometrical and elastic parameters to the induced anomaly for the case of the fundamental mode Rayleigh waves

APA, Harvard, Vancouver, ISO, and other styles

5

Kazantsev, Alexandre. "Perturbations d'amplitude du bruit ambiant au droit des hétérogéneités : étude de faisabilité pour l'exploration et la surveillance de réservoirs multi-fluide." Thesis, Paris Sciences et Lettres (ComUE), 2018. http://www.theses.fr/2018PSLEM075/document.

Full text

Abstract:

L'objet de cette thèse est l'étude des possibles mécanismes élastiques expliquant l'amplification du bruit ambiant au droit de certains réservoirs multi-phasiques. Trois jeux de données sont traités. La signature spectrale observée d'un réservoir de vapeur géothermique est différente de celle d'un stockage de gaz. Dans une approche empirique, un algorithme de classification permet d'extraire et de cartographier les anomalies que l'on présume liées au réservoir. Un travail de modélisation est effectué pour tenter d'expliquer les anomalies mesurées. Dans les données réelles, une forte présence de modes supérieurs d'ondes de Rayleigh est détectée. On modélise numériquement en 2D la propagation de ces modes à travers un réservoir placé au sein d'une structure géologique réaliste. La réponse simulée du réservoir se révèle trop faible par rapport aux observations de terrain. Néanmoins, on parvient à inverser les faibles perturbations d'amplitude synthétiques pour la position du réservoir, dans des modèles de référence simples. Cette méthode pourrait être utilisable pour l'imagerie à partir de faibles variations d'amplitudes dans le cadre du monitoring. Pour ce qui est de fortes anomalies observées sur le terrain , il est à noter que les effets visco-élastiques, les effets 3D, et les effets liés à un éventuel champ incident diffus n'ont pas été pris en compte dans la modélisation. Ainsi ce travail n'exclut pas la possibilité de telles anomalies liées à la présence d'un réservoir
This PhD work investigates the possible elastic mechanisms behind the ambient noise amplification above multi-phase fluid reservoirs. Three datasets are analysed above different reservoirs. The observed spectral signature is different in the gas storage and geothermal contexts. A non-supervised algorithm for amplitude spectrum classification is developed, allowing to extract and map the relevant attributes of a multi-phase fluid presence. As a first modelling step, a wavefield characterisation methodology is applied to determine the composition of the ambient noise. It reveals the presence of strong Rayleigh overtones. Numerical 2D elastic modelling is used to simulate the propagation of overtones across a reservoir within a realistic geological structure. The modelled reservoir response is too small compared to the real data. However, the small amplitude perturbations arising in the numerical simulations are successfully inverted for the position of the reservoir, in simple background models. The developed method could in theory be used for imaging small time-lapse amplitude variations (monitoring), despite the obstacles remaining to be overcome before a real-data application. Neither visco-elastic nor 3D effects are adressed. Thus this work does not exclude the possibility of strong reservoir-specific spectral anomalies

APA, Harvard, Vancouver, ISO, and other styles

6

Kazantsev, Alexandre. "Perturbations d'amplitude du bruit ambiant au droit des hétérogéneités : étude de faisabilité pour l'exploration et la surveillance de réservoirs multi-fluide." Electronic Thesis or Diss., Paris Sciences et Lettres (ComUE), 2018. http://www.theses.fr/2018PSLEM075.

Full text

Abstract:

L'objet de cette thèse est l'étude des possibles mécanismes élastiques expliquant l'amplification du bruit ambiant au droit de certains réservoirs multi-phasiques. Trois jeux de données sont traités. La signature spectrale observée d'un réservoir de vapeur géothermique est différente de celle d'un stockage de gaz. Dans une approche empirique, un algorithme de classification permet d'extraire et de cartographier les anomalies que l'on présume liées au réservoir. Un travail de modélisation est effectué pour tenter d'expliquer les anomalies mesurées. Dans les données réelles, une forte présence de modes supérieurs d'ondes de Rayleigh est détectée. On modélise numériquement en 2D la propagation de ces modes à travers un réservoir placé au sein d'une structure géologique réaliste. La réponse simulée du réservoir se révèle trop faible par rapport aux observations de terrain. Néanmoins, on parvient à inverser les faibles perturbations d'amplitude synthétiques pour la position du réservoir, dans des modèles de référence simples. Cette méthode pourrait être utilisable pour l'imagerie à partir de faibles variations d'amplitudes dans le cadre du monitoring. Pour ce qui est de fortes anomalies observées sur le terrain , il est à noter que les effets visco-élastiques, les effets 3D, et les effets liés à un éventuel champ incident diffus n'ont pas été pris en compte dans la modélisation. Ainsi ce travail n'exclut pas la possibilité de telles anomalies liées à la présence d'un réservoir
This PhD work investigates the possible elastic mechanisms behind the ambient noise amplification above multi-phase fluid reservoirs. Three datasets are analysed above different reservoirs. The observed spectral signature is different in the gas storage and geothermal contexts. A non-supervised algorithm for amplitude spectrum classification is developed, allowing to extract and map the relevant attributes of a multi-phase fluid presence. As a first modelling step, a wavefield characterisation methodology is applied to determine the composition of the ambient noise. It reveals the presence of strong Rayleigh overtones. Numerical 2D elastic modelling is used to simulate the propagation of overtones across a reservoir within a realistic geological structure. The modelled reservoir response is too small compared to the real data. However, the small amplitude perturbations arising in the numerical simulations are successfully inverted for the position of the reservoir, in simple background models. The developed method could in theory be used for imaging small time-lapse amplitude variations (monitoring), despite the obstacles remaining to be overcome before a real-data application. Neither visco-elastic nor 3D effects are adressed. Thus this work does not exclude the possibility of strong reservoir-specific spectral anomalies

APA, Harvard, Vancouver, ISO, and other styles

7

Alder, Chloé. "Etude de la déformation de la croûte et du manteau terrestres à partir de l'anisotropie sismique des modèles tomographiques." Thesis, Lyon, 2020. http://www.theses.fr/2020LYSEN021.

Full text

Abstract:

L’anisotropie est souvent utilisée en sismologie comme proxy de la déformation des roches. Elle se développe lorsque des minéraux acquièrent une orientation préférentielle. C’est le cas de l’olivine des péridotites du manteau qui s’aligne selon la direction des flux de matière. Néanmoins, l’anisotropie observée en tomographie peut aussi provenir d’hétérogénéités chimiques non résolues par les ondes sismiques longues périodes : elle s’apparente alors à̀ de l’anisotropie artificielle. Les théories et développements méthodologiques actuels ne permettent pas de distinguer l’anisotropie réelle de l’anisotropie artificielle dans les signaux sismiques. Dans cette thèse nous avons utilisé des modèles simples de manteau terrestre pour étudier théoriquement et numériquement le lien entre le niveau d’hétérogénéités non résolues et l’anisotropie radiale artificielle. Nous en avons conclu que l’anisotropie est proportionnelle au carré des hétérogénéités et que 10% de contraste de vitesse sismique peut engendrer plus de 3% d’anisotropie, un niveau non négligeable au regard des observations tomographiques. Une tomographie 3-D anisotrope de l’Europe, centrée sur les Alpes et les Apennins, a également enté réalisée. Les données d’ondes de surface utilisées sont issues de corrélations de bruit sismique et permettent d’imager la croûte et le manteau supérieur. La structure isotrope de notre modelé illumine particulièrement bien la plaque adriatique en subduction sous les Apennins ainsi qu’une rupture de ce panneau située au sud de la chaîne. Par ailleurs, la méthode d’inversion en profondeur utilisée prend en compte le biais entre anisotropie et hétérogénéités. Nous présentons ainsi le premier modelé 3-D d’anisotropie radiale en Europe réalisé pour des profondeurs aussi faibles. Nos résultats suggèrent que la croûte inférieure est marquée par une structuration horizontale dans les Apennins, probablement en lien avec la déformation extensive actuelle observée dans la région
Seismic anisotropy is often used as a proxy for rock deformation. It arises from the preferred orientation of anisotropic minerals. For instance, olivine in mantellic peridotites align according to mantle flow. However, anisotropy in tomographic models can be the result of small scale heterogeneities unresolved by long period seismic waves. It is thus considered as artificial. Theories and methodological developments do not allow to distinguish the relative contributions of real and artificial anisotropy in seismic signals. In this thesis, we used simple models of the Earth’s mantle to analytically and numerically study the link between unresolved heterogeneities and the level of artificial radial anisotropy. We concluded that anisotropy is proportional to the square of heterogeneities and that 10% of velocity contrast can be responsible for 3% of anisotropy, which is non negligible compared to the observed anisotropy in tomography. A 3-D anisotropic model of Europe, focusing on the Alps and Apennines, was constructed from surface waves data. The dispersion measurements were made from noise correlation and allow to image the crust and uppermost mantle. The isotropic structure of the model shows particularly well the Adriatic plate subducting under the Apennines, as well as a slab break-off in the Southern part of the chain. The method used for the depth inversion takes into account the trade-off between layering and anisotropy. Our model is therefore the first 3-D model of radial anisotropy built at shallow depths in Europe. Our results suggest that the lower crust has a horizontal organization in the Apennines, probably related to the extensive regime observed in the area

APA, Harvard, Vancouver, ISO, and other styles

8

Le, Breton Mathieu. "Suivi temporel d'un glissement de terrain à l'aide d'étiquettes RFID passives, couplé à l'observation de pluviométrie et de bruit sismique ambiant." Thesis, Université Grenoble Alpes (ComUE), 2019. http://www.theses.fr/2019GREAU013.

Full text

Abstract:

La surveillance d’un glissement de terrain vise à anticiper sa rupture pour réduire le risque d’accident. Elle s'opère généralement en mesurant les déplacements du sol. Ce travail propose une nouvelle technique de mesure de déplacement de glissements, flexible et à bas coût, basée sur l’utilisation d’étiquettes d’identification radiofréquence (RFID). La méthode de localisation d’étiquettes par différence de phase à 866 MHz est explorée en conditions extérieures et sur de longues durées. Cette étude a montré une détérioration de la mesure causée par les variations de température, d’humidité, de neige et de végétation. Après application de corrections, la précision de mesure a été améliorée, passant de ±20 cm à ±1 cm en conditions extérieures courantes. Cette technique fonctionne également en conditions neigeuses et en présence d’herbes hautes, mais avec une incertitude de mesure plus élevée (±8 et 4 cm respectivement). Ces erreurs de mesure sont provoquées par des effets de propagation, d’interférence multitrajets, et de per-turbations à proximité des antennes. Un système de mesure en continu a été déployé sur le glissement de terrain de Pont-Bourquin, en Suisse, pendant cinq mois. Ce dispositif a validé l’efficacité de la technique en conditions réelles. De plus, la mesure résiste bien aux intempéries et le dispositif demande peu de maintenance, en comparaison avec les techniques conven-tionnelles (extensomètre, GPS, station totale).Deux méthodes de mesure complémentaires aux déplacements ont ensuite été étudiées. La méthode de corrélation de vibrations ambiantes est prometteuse, mais n’a pas encore été utilisée en surveillance opérationnelle. Une étude bibliographique souligne plusieurs verrous à lever, tels que la correction des variations saisonnières et journalières, l’augmentation de la résolution temporelle, et le choix des paramètres de traitement adaptés au site surveillé. La méthode qui consiste à inverser une fonction de transfert entre des données de pluie et de déplacements est ensuite étudiée. Une inversion haute résolution de cette fonction est proposée. Elle permet d’identifier des comportements hydrologiques complexes (ex : infiltration à deux vitesses sur le site de Pont-Bourquin) et de mesurer leur évolution. Les avancées de cette thèse vont permettre d’améliorer la surveillance opérationnelle tout en réduisant son coût, répondant aux besoins des collectivités territoriales
Landslide early-warning systems are based primarily on monitoring the displacement of the landslide. This work develops a new technique for monitoring these displacements, using radio-frequency identification (RFID) passive tags and phase-based location technique. This technique is deployed for the first time outdoors and for several months. Outdoor conditions revealed strong environmental influences due to temperature variations, moisture, snow and vegetation. These can cause a ±20 cm measurement uncertainty over a year, which is too large for landslide monitoring applications. The correction of these effects allows reaching the accuracy of ±1 cm under normal conditions, ±8 cm with snow and ±4 cm with dense high grass. The remaining effects due to snow and grass are explained by the influence of this material on the direct propagation, on the multipath interferences and on the antennas. This measurement system has been deployed on the Pont-Bourquin landslide for five months. The results validate the technique for landslide monitoring applications. The technique also shows the operational benefits of robustness to bad weather, easy maintenance and low-cost material, compared to conventional techniques (extensometer, GPS, total station).This thesis then studies two complementary monitoring methods that had recently been shown to provide precursors to landslide rupture. First, ambient seismic noise interferometry is used to detect a drop of shear-wave velocity prior to a rupture. The seismic method was studied in the literature to identify what must be developed to use this technique in an operational early-warning system. It requires getting rid of daily and seasonal environmental influences, choosing the processing parameters appropriate to the monitored landslide, and improving the temporal resolution below one day while keeping a stable enough signal. The other method consists of inverting an impulse response between rainfall and displacement rate, with a high resolution. It can shed light on complex infiltration processes (e.g. infiltration with two different delays at Pont-Bourquin) and detect their abnormal evolution across time. These developments should improve landslide operational monitoring with a low budget

APA, Harvard, Vancouver, ISO, and other styles

9

Perron, Vincent. "Apport des enregistrements de séismes et de bruit de fond pour l'évaluation site-spécifique de l'aléa sismique en zone de sismicité faible à modérée." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAU020.

Full text

Abstract:

Les effets de site peuvent augmenter fortement la durée et l’amplitude des sollicitations sismiques imposées aux structures. Les effets de site 2D-3D induisent des amplifications sur une large bande de fréquence qui ne peuvent pas être simulées numériquement jusqu’à haute fréquence (>2-4 Hz) du fait de la limite de résolution des connaissances géologiques, géophysiques et/ou géotechniques du sous-sol. Les évaluations empiriques des effets de site sont donc indispensables pour pouvoir observer ce phénomène complexe de façon fiable jusqu’à haute fréquence. De telles évaluations nécessitent l'enregistrement de bonne qualité de nombreux séismes ce qui rend leur obtention rapide difficile dans les régions faiblement actives.Ce travail présente une analyse comparative de ces évaluations empiriques sur deux sites très différents, l’un en contexte sismique modéré (Provence, France) et l’autre en contexte très actif (Céphalonie, Grèce). Sur le site provençal, 500 séismes ont pu être enregistrés en l’espace de 2½ ans grâce à l’utilisation de vélocimètres. Une mesure du paramètre d’atténuation de site k_0 a ainsi pu être réalisée via l’analyse des spectres en accélération (k_(0_AS)) et en déplacement (k_(0_DS)). La mesure de k_0 n’est relativement fiable que sur les sites au rocher du fait des amplifications de site au sédiment. La méthode des rapports spectraux sismique (SSR) permet l’évaluation des effets de site relatif à partir des nombreux enregistrements de mouvement faible. Les résultats montrent une forte variabilité épistémique attribuable à l’éclairage induit par la position de la source sismique vis-à-vis du bassin. Ainsi, une estimation fiable de la réponse des sites est possible à partir de quelques évènements seulement dans le cas 1D, mais nécessite un nombre beaucoup plus important de séismes répartis de façon homogène autour du site dans les cas 2D-3D. Les résultats SSR moyens sont ensuite comparés à ceux issus des méthodes utilisant le bruit ambiant. Conformément aux attentes, la méthode des rapports spectraux H/V (HVSR) ne permet que de caractériser la fréquence fondamentale pour certains sites. À l’inverse, la méthode SSR appliquée au bruit ambiant (SSRn) montre des résultats très comparables à ceux de la méthode SSR jusqu’à haute fréquence à la condition que le site de référence soit pris dans le bassin sédimentaire. Les approches par corrélation de bruit (cohérence et ANIRF) révèlent qu’une estimation de la fonction de transfert relative à un site de référence au rocher est possible au moins jusqu’à basse fréquence (<4 Hz). L’utilisation du bruit ambiant permet ainsi d’envisager l’évaluation rapide de la réponse des sites et de sa variabilité spatiale (microzonage), même lorsque la sismicité est faible. Partout, les méthodes empiriques d’évaluation des effets de site apportent un complément essentiel aux approches numériques qui reste indispensables.Ces résultats permettent de proposer une méthodologie d’évaluation de l’aléa sismique site-spécifique qui se décompose en trois étapes principales : (i) ajustement des équations de prédictions de mouvement du sol (GMPEs) sur le site de référence au rocher au moyen, entre autres, de k_(0_DS) ; (ii) évaluation empirique fiable de la réponse d’au moins un site dans le bassin relativement au site de référence (pour lequel les GMPEs ont été ajustées) par la méthode SSR; (iii) réalisation de cette évaluation à partir de l’ANIRF dans les régions les moins actives sismiquement et/ou extension à toute la zone d’étude à partir du SSRn. Cette méthodologie permet d’envisager une bien meilleure prise en compte des effets de site (en particulier 2D-3D) et une réduction importante des incertitudes dans les évaluations de l’aléa sismique spécifique à un site. Elle nécessite simplement l’enregistrement simultané de séismes sur au moins deux sites équipé de vélocimètres sensibles, et la réalisation de campagnes de mesure temporaire du bruit ambiant, si nécessaire
Site effects can greatly increase both the duration and the amplitude of the seismic solicitation imposed on structures. 2D-3D site effects induce broadband amplifications that cannot be simulated up to high frequency (>2-4 Hz) due to the limited resolution of the geological, geophysical and/or geotechnical information. Empirical site effect assessment is therefore essential for reliable observations of this complex phenomenon up to high frequency. However, such assessments often require good quality records from many earthquakes that cannot be rapidly obtained in low seismicity areas.This work presents a comparative analysis of these empirical evaluations on two very different sites, the first in a moderate seismicity context (Provence, France) and the second in a very active context (Kefalonia, Greece). For the Provençal site, nearly 500 earthquakes were recorded in only 2½ years thanks to the use of velocimeters. The site attenuation parameter κ_0 could thus be measured both on the acceleration (κ_(0_AS)) and displacement (κ_(0_DS)) spectra. Our results show that the measurement of κ_0 is relatively reliable on rock sites only, mainly due to the too great disturbance by the amplification for sedimentary sites, even for those that are quite stiff. The standard spectral ratio (SSR) approach provides the relative site effects from the numerous weak motion recordings available at sites located in Provence and Kefalonia. The results show a strong epistemic variability due to the lighting induced by the position of the seismic source with respect to the basin. Thus, while a reliable site response estimation is possible from only a few events for 1D geometries, it requires much more earthquakes evenly distributed around the site when the geometry is 2D-3D. The mean SSR results are then compared with those obtained from methods using the ambient noise. As expected, the H/V spectral ratio approach (HVSR) provides only the fundamental frequency for some sites. Conversely, SSR applied to ambient noise (SSRn) shows very similar results to the SSR method up to high frequency, provided that the reference site is taken in the sedimentary basin. Approaches using the noise correlation (coherence and ANIRF) reveal that the transfer function relative to a rock reference site can be estimated at least up to low frequency (<4 Hz). Methods using ambient noise are promising for a rapid evaluation of the site response and its spatial variability (microzoning), even when seismicity is low. Empirical site effects methods are therefore applicable everywhere. They provide an essential complement to numerical approaches, which remain inevitable when the coverage of available earthquakes is not homogeneous or when soils are likely to present non-linear behaviors.These results led us to propose a methodology for the evaluation of the site-specific seismic hazard, which consists of three main stages: (i) adjustment to the rock reference site of the ground motion prediction equations (GMPEs) using, among others, κ_(0_DS); (ii) Reliable evaluation of the SSR transfer function between at least one site in the basin and the reference site (for which the GMPEs were adjusted); (iii) carrying out this assessment from the ANIRF in the less seismically active regions and/or extension to the entire study area with the SSRn. This methodology allows a much better consideration of the site effects (especially 2D-3D) and a significant reduction of the uncertainties in the evaluations of the site-specific seismic hazard. It only required, simultaneous recording of earthquakes on at least two sites equipped with sensible velocimeters, and to carrying out temporary campaign of ambient noise measurements, if necessary

APA, Harvard, Vancouver, ISO, and other styles

10

Fores, Benjamin. "Gravimétrie et surveillance sismique pour la modélisation hydrologique en milieu karstique : application au bassin du Durzon (Larzac, France)." Thesis, Montpellier, 2016. http://www.theses.fr/2016MONTT128/document.

Full text

Abstract:

Les aquifères karstiques représentent des ressources en eau essentielles dans de nombreuses régions du monde comme le bassin Méditerranéen. Cependant, de par les processus complexes de karstification, ces aquifères sont hétérogènes à de nombreuses échelles et vulnérables. Dans cette thèse, nous étudions le potentiel de la gravimétrie et du bruit sismique ambiant pour la modélisation hydrologique en milieu karstique.Le site dolomitique de l’observatoire « GEK », sur le bassin du Durzon dans le Larzac, est le site d’étude privilégié de ces travaux. Dans l’observatoire, un gravimètre supraconducteur dédié à l’hydrologie mesure depuis 2011 les variations de gravité en continu et à une très haute précision, pour la première fois sur un karst. Des modèles hydrologiques conceptuels ont été réalisés à partir de cette surveillance gravimétrique et ont permis de poser les bases de modèles physiques d’écoulements 1-D. En effet la gravimétrie, intégratrice, permet 1) de considérer l’épikarst localement hétérogène comme un milieu tabulaire équivalent et 2) de définir les types de transfert à l’œuvre sur le site. En particulier, l’absence de transfert rapide dans l’épikarst a été quantifiée avec précision pour la première fois à l’échelle du terrain (~100m). A l’aide de données météorologiques locales, un bilan de masse précis a permis de définir le flux en limite inférieure du modèle à 1 mm.jour-1. Ce flux s’est montré représentatif du débit de basses-eaux de la source drainant l’ensemble du bassin. Ce résultat suppose une homogénéité de l’épikarst dolomitique quasiment à l’échelle du bassin. Les paramètres des modèles physiques ont ensuite pu être calibrés à l’aide d’un an d’intercorrélation du bruit sismique ambiant entre deux stations. Les variations de vitesses de phase obtenues entre 6 et 8Hz nous ont servi de « chronomètre » pour suivre l’infiltration entre 30 et 60m de profondeur. La surveillance passive des variations de vitesses sismiques par intercorrélation du bruit sismique ambiant montre ainsi un fort potentiel pour l’étude des zones critiques profondes et complexes à l’échelle du terrain et peut combler la lacune instrumentale qui existe actuellement en hydrologie.Des campagnes répétées de mesures avec un gravimètre portable à ressort ont également mis en évidence le fonctionnement différent de deux épikarsts et leur variabilité à l’échelle de la centaine de mètres. Des mesures mensuelles autour de l’observatoire ont mis en évidence l’homogénéité spatiale de cet épikarst dolomitique : toutes les stations ont les mêmes variations temporelles de stock d’eau. Au contraire, des mesures saisonnières en surface et en profondeur le long de la galerie souterraine calcaire de l’abîme de Saint-Ferréol ont montré une variabilité spatiale forte du stockage ainsi que du transfert rapide. La lithologie de l’épikarst est donc suspectée de jouer un rôle dans sa capacité de stockage. Lors de ces campagnes, la faiblesse du signal recherché a nécessité une méthodologie précautionneuse et un effet de température sur les mesures des gravimètres relatifs à ressort a été observé sur le terrain et quantifié en laboratoire
Karstic aquifers represent the most important fresh water reservoirs in many regions of the world like the Mediterranean Basin. However, because of complex processes of karstification, those aquifers are highly heterogeneous at all spatial scales and vulnerable to contamination. In this dissertation, we studied the potential of gravimetry and ambient seismic noise for hydrological modeling in karstic areas.The dolomitic area surrounding the “GEK” observatory in the Durzon catchment on the Larzac plateau, in France, is the preferred site for these studies. Inside the observatory, a superconducting gravimeter dedicated to hydrology has continuously measured gravity changes since 2011 with high precision, undertaken for the first time on a karst. From this gravity monitoring, we made conceptual hydrological models which laid the foundation of 1-D flow physical models. Indeed, gravimetry is an integrative hydro-geophysical method which allows 1) to consider the epikarst, locally heterogeneous as an equivalent tabular medium and 2) to define the types of transfer (fast & slow) occurring at the site. Especially, the lack of fast transfer through the GEK epikarst was precisely quantified for the first time at the field scale (~100m). Gravity-driven water mass balance with local meteorological data (evapotranspiration from a flux tower and precipitation) allowed setting the bottom outlet of the model to 1 mm.day-1. This flow has proved to be representative of the low-flow discharge at the only spring which represents all groundwater outflows from the catchment. This result supposes the homogeneity of the dolomitic epikarst almost at the basin scale. Model parameters were next calibrated using one year of ambient seismic noise monitored at two stations. Phase velocity changes obtained by cross-correlating the noise between 6 and 8 Hz were used as a ”timer” to follow the water infiltration between a depth of 30 and 60 meters. Thus, monitoring seismic velocity changes using ambient seismic noise demonstrates great potential for the study of deep and complex critical zones and could fill the instrumental gap currently existing in hydrology.Time-lapse gravity measurements with a spring-based portable gravimeter have also demonstrated the different behavior of two epikarsts and their variability at the scale of a few hundred meters. Monthly measurements around the observatory revealed the spatial homogeneity of this dolomitic epikarst: all the stations showed the same water storage changes. On the contrary, seasonal surface to depth gravity measurements along the underground passage of the Saint-Ferréol sinkhole, in limestone, have shown fast transfer and strong spatial variability of water storage. Lithology is then expected to play a part in the epikarst capacity to retain water. The precision needed to measure the weak hydrological induced signals during those surveys required robust methodology and an ambient temperature effect on measurements with spring-based gravimeter was observed in the field and quantified in the laboratory

APA, Harvard, Vancouver, ISO, and other styles

11

Mohamadian, Sarvandani Mohamadhasan. "Seismic tomography of an amagmatic ultra-slow spreading ridge." Electronic Thesis or Diss., Sorbonne université, 2022. http://www.theses.fr/2022SORUS467.

Full text

Abstract:

Les dorsales ultra-lentes quasi-amagmatiques constituent une nouvelle catégorie de dorsales océaniques caractérisées par une accrétion crustale, exposant sur le fond marin des quantités considérables de péridotites provenant du manteau. L’étude de la contribution des processus tectoniques, magmatiques et d’autres processus impliqués est nécessaire pour obtenir un modèle conceptuel complet des dorsales océaniques à accrétion ultra-lente. L’imagerie des structures de la croûte et du manteau supérieur peut nous aider à comprendre les activités géologiques passées et actuelles sur les dorsales à accrétion océanique ultra-lente. L’objectif du projet est de comprendre la croûte océanique formée dans une dorsale à accrétion ultra-lente appelée ride sud-ouest indienne, à faible apport de magma. Notre projet de recherche est basé sur le traitement et la modélisation de données sismiques actives et passives dans la partie la plus orientale de la dorsale Sud-Ouest Indienne. L’acquisition des données géophysiques a eu lieu en 2014 lors de la campagne SISMOSMOOTH, à bord du N/O Marion-Dufresne. Nous avons analysé les enregistrements des composantes verticales de 43 sismomètres fond de mer (OBS) dans notre approche sismique passive et les composantes hydrophones de 16 sismomètres fond de mer pour l’approche sismique active. L’interférométrie de bruit ambiant et l’inversion de forme d’onde complète (FWI) des données de réfraction ont été utilisées pour imager les structures internes de la croûte et de la lithosphère. Grâce à la modélisation de l’interférométrie de bruit ambiant, on trouve une épaisseur moyenne de croûte de 7 km avec une couche peu profonde de faibles vitesses de cisaillement. De plus, nous en déduisons que les 2 km supérieurs sont très poreux et peuvent être fortement serpentinisés. La vitesse moyenne des ondes de cisaillement entre la base de la croûte et la profondeur maximale de notre modèle (15 km) est inférieure à la valeur de référence globale de 4.5 km/s et peut s’expliquer par le jeune âge des fonds marins de notre zone. Notre modèle bi-dimensionnel de vitesse des ondes P obtenu à partir de notre analyse FWI suggère des variations considérables de composition dans la partie supérieure le long du profil parallèle à l’axe. Notre étude propose un domaine de transition entre un domaine à prédominance volcanique et un non magmatique, entre ∼65 à 95 km de distance sur le profil. Des injections magmatiques dans des dikes sont proposées dans le domaine oriental non volcanique. Une augmentation vers l’ouest de l’apport de matériel magmatique est confirmée pour le mode d’accrétion océanique. Le modèle de vitesse des ondes P associé aux variations de serpentinisation suggère que le Moho est une transition graduelle d’une péridotite hydratéevers une péridotite non altérée
Ultra-slow spreading ridges are a new category of spreading ridges characterized by quasi-amagmatic crustal accretion, exposing considerable amounts of mantle derived peridotites on the seafloor. Investigating the contributions of tectonic, magmatic, and other involved processes is necessary to gain a comprehensive conceptual model of ultra-slow spreading ridges. Imaging the crustal and upper mantle structures can help us to understand the past and current geological activities in the ultra-slow spreading ridges. The aim of the project is to understand the oceanic crust formed in an ultra-slow spreading ridge called the Southwest Indian Ridge with a low melt supply. Our research project is based on the processing and modeling of the active and passive seismic data in the easternmost part of Southwest Indian Ridge. The data acquisition took place in 2014 during the SISMOSMOOTH cruise. We analyzed vertical component recordings from 43 ocean-bottom seismometers in our passive seismic approach and the hydrophone components of 16 ocean-bottom seismometers in the active seismic approach. Ambient-noise interferometry and full-waveform inversion (FWI) of refraction data were used to image the internal structures of the lithosphere. In the modeling of ambient-noise interferometry, we find an average crustal thickness of 7 km with a shallow layer of low shear velocities. Moreover, we infer that the uppermost 2 km are highly porous and may be strongly serpentinized. The average shear wave velocity between the base of the crust and the maximum depth of our model (15 km) was less than the global reference value of 4.5 km/s and was explained by the younger age of the seafloor in our area. Our two-dimensional P-wave velocity model obtained from FWI suggests considerable variations in the upper lithospheric compositions along the axis-parallel profile. A transition is expected at a distance of ∼65-95 km along the profile from the predominantly volcanic domain in the western zone to variable serpentinized peridotite in the eastern zone. Dike injections are predicted in this area. A westward increase in melt supply is proposed in the seafloor accretion mode. The serpentinization and P-wave velocity model suggests that the Moho is a gradual transition from hydrated to unaltered peridotite

APA, Harvard, Vancouver, ISO, and other styles

12

Arogundade, Simisola M. "Numerical modeling of ambient noise seismic interferometry." Thesis, Michigan Technological University, 2016. http://pqdtopen.proquest.com/#viewpdf?dispub=10125274.

Full text

Abstract:

CO₂ sequestration involves storing CO₂ in a deep geological formation and may help to mitigate the increasing emission of carbon. To monitor the migration of injected fluid in the reservoir, seismic observations may be used to observe changes in reflection character. Conventional methods to image the subsurface, using active seismic measurements, with man-made sources, have been applied at a few test sites, and the use of passive measurements, with natural sources, has been considered as a probable cost-efficient method to monitor CO₂ migration and leakage. This numerical modeling study examines the use of seismic interferometry to retrieve weak seismic reflections from background noise, a form of passive monitoring.

The factors that influence the quality of the retrieved reflections from interferometry include geophone interval, geophone depth, and effect of shallow noise sources, assuming we seek reflections from deep noise sources, representing either teleseismic events or local events as expected in a field of active injection. Using model data, geophone interval had no significant effect on the reflection quality, but buried geophones produce ghost reflections, suggesting that shallow geophones might be optimal. Shallow noise sources produce a destructive effect on the reflections from deeper noise sources and damage the resulting image.

APA, Harvard, Vancouver, ISO, and other styles

13

Olivier, Gerrit. "Seismic imaging and monitoring in mines with ambient seismic noise correlations." Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GREAU018/document.

Full text

Abstract:

Cette thèse s'intéresse au développement des méthodes d'auscultation sismique passive pour l'imagerie et la surveillance des mines profondes. Les résultats marquants sont 1/ la possibilité d'imager en profondeur les structures géologiques d'intérêt et 2/ la possibilité de suivre dans le temps les propriétés mécaniques des roches qui subissent les sollicitation associés à l'exploitation minière. Ce travail ouvre des perspectives quant à l'amélioration de la sécurité dans les mines profondes
This work focus on using passive noise-based seismic methods to image and monitor the rock mass in underground mines. The main results show that it is possible to gain benefit from the diffuse ambient seismic field in mines to 1/ image the rock mass and 2/ monitor its mechanical property changes over time. This work opens a way to improve safety in deep underground mines

APA, Harvard, Vancouver, ISO, and other styles

14

Legovini, Paride <1983&gt. "Ambient seismic noise tomography of the Antarctic continent." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2019. http://amsdottorato.unibo.it/9023/1/tesi-legovini.pdf.

Full text

Abstract:

The lithosphere of Antarctica reacts to both the stress variations due to the dynamics of the mantle and the variation of the glacial load due to changes in climate. These two factors act on the dynamics of the glacial mass, on the evolution of the continent's topography and on the plate seismicity. This means that the knowledge of spatial variations in lithospheric thickness is needed to comprehend of the processes that involve interactions between climate and the geosphere. The main aim of this work is to produce a tomographic model of the continent. The classic data source for tomographic studies is a set of seismic signals produced by earthquakes. In this work I use a different technique: information is extracted by correlating the ambient seismic noise recorded in different locations. Instead of a the classic cross-correlation, this work uses the phase cross correlation technique, which appears to be more robust in our use case. Signal phases are used even to improve the correlogram stacks, by weighting the stack samples according to the correlograms' coherence. In Antarctica, the 30s Rayleigh wave group velocity map shows lower velocities in East Antarctica, and higher velocities in West Antarctica, coherently with expectations related to a thicker cratonic crust in the East, and thinner extending crust in the West. Deeper sensitivity of longer periods make the color pattern switch for the 80s map, sensing the upper mantle and consequently higher velocity at the cold roots of cratonic East Antarctica and lower velocity in the tectonically active West. I also contributed to the maintenance and development of the seismic observational infrastructure in Antarctica. This thesis recaps the activities I carried out during my participation to the 31st campaign of the Italian National Antarctic Research Program, to which I participated during the PhD course.

APA, Harvard, Vancouver, ISO, and other styles

15

Allmark, Claire Lindsay. "Analysing the Earth's near surface using ambient seismic noise." Thesis, University of Edinburgh, 2018. http://hdl.handle.net/1842/29639.

Full text

Abstract:

Near surface measurements of seismic velocity and Q are useful in a number of situations, for example for when carrying out re-datuming and migration for depth images, or when analysing ground conditions for building. This thesis concentrates on the estimation of surface wave group and phase velocity as well as Q structure through the use of cross correlations of ambient noise recordings. Linearised tomography estimates are made for the British Isles, the Permain Basin of Texas and New Mexico, the Ekofisk Life of Field Seismic (LoFS) array and the Aquistore CO2 storage site. The results correspond well with the known geological structure and/or structure observed in velocity maps by other researchers. For the Ekofisk array a non-linearised non-linear method was also applied and the results estimated by these two methods for the Ekofisk LoFS array are compared. By non-linearised non-linear it is meant that the inversion method is fully non-linear and no linearisation has taken place in the method, this term will be used throughout this thesis for all methods which fall into this category. The tomography results from the two methods had similarities in their general structure but differences in the finer details, and so suggest that the substantial increase in time required for the non-linearised non-linear method is not justified. Linearised tomographic inversion of the Aquistore array was used to determine the potential of using ambient noise tomography for monitoring of CO2 storage sites. It was found that the repeatability of the tomography at the Aquistore site was not good enough to allow ambient noise tomography to be used for monitoring; however, it may be possible to apply the method at other sites. A Q and phase velocity inversion of the Ekofisk array is also presented, with results mostly showing excellent correlation with known geological features. It is shown that the higher frequencies are more sensitive to the effects of sea floor subsidence at the site, while lower frequencies are more sensitive to the effects of faulting. A final near surface method called ambient noise gradiometry was applied, this concentrates on estimating locations of sources of seismic energy within receiver arrays. Ambient noise gradiometry is applied to synthetic and real data for this purpose. It was found that using ambient noise gradiometry allows internal sources of energy to be identified but they produce a bias in the phase velocity tomography result. Two methods of reducing this bias are presented, both of which also provide an estimate of the source term for different sections of time of the recording.

APA, Harvard, Vancouver, ISO, and other styles

16

Neale, Jennifer F. Ward. "An investigation into ocean wave sources of ambient seismic noise." Thesis, University of Southampton, 2017. https://eprints.soton.ac.uk/412555/.

Full text

Abstract:

Periodic pressure fluctuations beneath ocean waves deform the seabed beneath them and can generate seismic waves that propagate around the globe. These pressure fluctuations are recorded on seafloor pressure gauges and the associated ground displacements on seismometers, where they contribute to ambient seismic noise. The signals offer an opportunity to study or monitor ocean waves that are traditionally difficult to measure because of their low wave heights (deep water infragravity waves) or their remote locations (deep water swell waves). However, the link between ocean waves and the oscillations of the pressure and seismic wavefields has remained unclear. The aim of this thesis was to increase our understanding of ocean sources of ambient seismic noise, including their location, how well they can be located, and their relationship with ocean wave parameters. Using cross-correlated pairs of seafloor pressure records, infragravity waves offshore California/Oregon were found to originate mostly from local coastlines during northern-hemisphere winter and from the south during summer. A first attempt to estimate the coastal reflection coefficient of remote arrivals was made and found to be 0.49-0.74, which has implications for infragravity energy in the deep ocean and may be important for models in which infragravity waves are propagated across ocean basins. P-wave sources in the North Pacific during winter were located using seismometers in California and found to be concentrated around 40-50◦N 160-180◦E. Observed source locations were within 10◦ of the modelled source locations. Significant wave height in the deep ocean was estimated from P-waves and correlated with modelled wave height with a correlation coefficient of 0.63. Previous work only attempted to estimate coastal wave heights. Combining additional records from Japan and Europe improved source location, including imaging of multiple sources. Accuracy in source location and amplitude estimation are essential if microseisms are to be used to monitor wave activity in the deep ocean.

APA, Harvard, Vancouver, ISO, and other styles

17

Fang, Lihua. "Rayleigh wave tomography in North-China from ambient seismic noise." Doctoral thesis, Università degli studi di Trieste, 2010. http://hdl.handle.net/10077/3623.

Full text

Abstract:

2008/2009
The theory and methodology of ambient noise tomography has been studied and applied to North-China successfully. Continuous vertical-component seismograms, spanning the period from January 1, 2007 to February 28, 2008 recorded by 190 broadband stations and 10 very broadband stations, have been used. The cross correlation technique has been applied to ambient noise data recorded by North-China Seismic Array for each station pairs of the array. Rayleigh wave group velocity dispersion curves are measured at periods between 4 s and 40 s by multiple filter technique. We obtain 5630 high quality dispersion curves. Surface wave tomography is conducted to generate group velocity maps with a grid spacing of 0.25º×0.25º. These maps display higher resolution and extend to shorter periods than previous surface wave tomography maps. Then genetic algorithm is used to invert pure path dispersion curves. The 3-D shear wave velocity structure from 0 to 50 km depth is readily constructed. To the authors' knowledge, the resolution presented here is, so far, the highest one in China mainland. The original results of this thesis are: 1, The SNR of Green Function is proportional to the square root of observation time and can be enhanced by using the symmetric component. The inhomogeneous distribution of seismic noise gives rise to the asymmetry of Green Function. Using more than one year's data, one can get more symmetric and higher SNR Green Function. 2, The characteristics of ambient seismic noise are different for different period bands. In 4-10 s, a coherent phase with large amplitude near zero lag time is observed. In 10-20 s, the sources of ambient seismic noise have a very clear seasonal variability. The azimuthal distributions of noise share a great similarity with the map of average ocean wave height map obtained by TOPEX-Poseidon. In 20-50s range, Rayleigh wave Green Functions are almost symmetrical and show less seasonal variation in both signal strength and directivity, which indicates that the distribution of noise is - ii - almost homogeneous. In 4-20s range, the amplitudes of positive and negative components of Green Functions are obviously asymmetrical, but the arrival times are almost identical, indicating that the distribution of noise has much influence on the amplitude of Green Function, but less influence on arrival time. 3, Tomographic maps at short periods reveal an evident lateral heterogeneity in the curst of North-China, quite well in agreement with known geological and tectonic features. The North China Basin is imaged as a broad low velocity area, while the Taihangshan and Yanshan uplifts and Ordos block are imaged as high velocity zones, and the Quaternary intermountain basins show up as small low-velocity anomalies. 4, The 3-D S-wave crustal velocity model in North China shows a distinct low velocity belt with NW trend at 10 km of depth near Zhangjiakou-Bohai seismic zone. This low velocity belt and the southern margin of Yanshan high velocity anomaly draw the outline of Zhangjiakou-Bohai seismic zone and its northern border line. There is a well-defined low velocity zone in middle-to-lower crust (15-25 km) in the Beijing-Tianjin-Tangshan region, which may be caused by intrusion of hot mantle materials. 5, We analyzed the seismogenic structure near Tangshan，Luanxian and Ninghe earthquake region. We infer that these three earthquakes are mainly caused by vertical deformation of upper mantle and material exchange between crust and upper mantle. The magma intrudes the crust along faults near the boundary of crust and upper mantle, which leads to the low velocity anomaly in the uppermost mantle. The magma intrusion heats up the lower crustal material and drops its viscosity. Some minerals are dehydrated. The water moves up and is trapped in the middle crust. The existence of liquid affects the structure and composition of the fault zone, further changes the stress state, weakens the seismotectonic region and triggers the earthquakes.
XXI Ciclo
1981

APA, Harvard, Vancouver, ISO, and other styles

18

Jonsdottir, Frida. "Estimation of Relative Seismic Velocity Changes Around Katla Volcano, Using Coda in Ambient Seismic Noise." Thesis, Uppsala universitet, Institutionen för geovetenskaper, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-353619.

Full text

Abstract:

Relative seismic velocity variations in the Earth’s crust can be estimated by using ambient seismic noise records from a pair of stations. Velocity variations can be caused by stress perturbations in the subsurface. Therefore, information on stress changes in the crust can possibly be retrieved from measured velocity variations in the medium. The measurement is done by comparing the coda part of two cross-correlation functions (CCFs) obtained from ambient noise recordings at two seismic stations; a current CCF that is considered to represent the actual state in the study medium at a specific time and a reference CCF that is considered to represent its average state. Here, the method is applied to the area around Katla volcano in southern Iceland. Katla is an active subglacial volcano and therefore frequent stress changes can be expected to take place there. Long-term changes (of the order of 1-2 months) in relative seismic velocity were estimated over a period of 7 months in 2011. These changes were of the order of about 0.1% for a frequency range of 0.2-1.0 Hz. For this frequency range, surface waves around Katla have been estimated to be most sensitive to velocity changes taking place at depths of about 1-5 km but the sensitivity kernels also have a peak at the surface. The scattering volume (in this case area since we are working with surface waves) depends on both the inter-station distance and how far into the coda the measurements are made. The inter-station distances vary between 5.8 and 23.4 km. Measurements are made 30 s into the coda. This results in scattering areas on the order of 100 km2. The velocity variations have a negative trend over July and over a two month period from the end of August until early November, and a positive trend in August and from early November until the end of the study period in late December. These variations are possibly the results of a combination of changes in the ground water level beneath the glacier, surface load changes and possibly hydrothermal and magmatic pressurization changes. No significant velocity change was estimated in the area associated with the tremor event that took place in early July in 2011.
Seismiska vågor är vibrationer i jordytan som genereras av jordbävningar, explosioner eller andra processer som skakar jorden. Seismiska vågor färdas genom jordens lager och innehåller därför information om jordens inre struktur. Dessa vibrationer kan hämtas med ett känsligt instrument som kallas seismometer. Seismiska vågor färdas med en viss hastighet som beror på hur hård och tung berggrunden är. Förändringar av dessa egenskaper kan därför resultera i förändringar av hastigheten. Dessa förändringar kan orsakas av spänningsförändringar under marken, till exempel trycket i porer eller variationer i vikten ovanför marken, exempelvis från en glaciär. I denna uppsats studeras förändringar av seismiska vågors hastighet kring vulkanen Katla på Island under 7 månader, 2011. Katla är en av Islands mest aktiva vulkaner och är belägen under en glaciär, Mýrdalsjökull. Detta görs genom att använda omgivande seismiskt brus, som består av seismiska vågor. Bruset genereras av tryckvariationer i samband med havsvågor. Bruset analyseras med en korrelationsanalys som bland annat isolerar spridda vågor från detaljer i strukturen och variationer av dessa med tid kan användas til mätningar av hastighets förändringar. Resultaten tyder på förändringar i relativ seismik hastighet avstorleken 0.1% som varar i en till två månader. Hastigheten minskar i juli och över en tvåmånadersperiod från slutet av augusti till början av november, men ökar i augusti och från början av november till slutet av december. Dessa variationer kan ha orsakats av en kombination av förändringar i grundvattennivån under glaciären, förändringar i glaciärens vikt och magmatiska processer. Inga tydliga förändringar i samband med sekvenser av små jordbävningar som ägde rum i början av juli 2011 kunde observeras frånförändringar i relativ seismisk hastighet runt Katla.

APA, Harvard, Vancouver, ISO, and other styles

19

Saygin, Erdinc, and erdinc saygin@anu edu au. "Seismic Receiver and Noise Correlation Based Studies in Australia." The Australian National University. Research School of Earth Sciences, 2007. http://thesis.anu.edu.au./public/adt-ANU20091009.115242.

Full text

Abstract:

This thesis is directed at exploiting information in the coda of seismic phases and the ambient noise field to provide new constraints on the structure of the Australian Continent. ¶ The exploitation of the immediate coda following the onset of P waves from a distant earthquake using radial receiver functions is now a well established method. The 40 sec interval following P contains reverberations and conversions, by deconvolving the radial component trace with the vertical components, the conversions are emphasized by canceling the part of the response that are common to both components. A member of different styles of such deconvolution, are investigated and a variant of the multitaper method is adopted for subsequent applications. The TASMAL experiment 2003-2005 spans the expected location of the transition between Precambrian and Phanerozoic Australia. The 20 portable broadband stations were exploited in receiver function studies to extract S wave crustal structure through the inversion of stacked receiver functions using the Neighbourhood Algorithm. There is no clear crustal transition associated with the presence of Tasman Line. The Precambrian Cratons tend to exhibit crustal thicknesses close to 40 km but such values are also found in some Phanerozoic sites. ¶ The second part of the thesis is directed at the exploitation of ambient noise or seismic coda to gain information on the Green's function between seismic stations. The TASMAL experiment covered a significant fraction of the Australian continent with a simultaneous deployment of portable broadband stations. From these continuous records, it has proved possible to extract very clear Rayleigh wave signals for station separations up to 2000 km, and to demonstrate the frequency dependent variations in group velocity behaviour. The combination of the paths between the 20 stations localize such behaviour, but detailed images needed more data. The entire archive of portable broadband data recorded by RSES was mined, and combined with data from permanent stations to provide more than 1100 estimates of interstation Green's functions within Australia. Group velocity analysis as function of frequency was followed by nonlinear tomography with the Fast Marching Method. The resulting images of group velocity patterns as a function frequency show pronounced regions of lowered group velocities, most of which match regions of thick sediment. The frequency dependence is not consistent with just sedimentary structure and low midcrustal velocities, most likely due to elevated temperatures, are also needed. ¶ The surface wave portion of the interstation Green's function is the most energetic, and is normally all that seen in ambient noise studies. However, in the coda of events record at the broadband Warramunga seismic array in the Northern Territory, the P and S body wave components also emerge. The characteristics of these arrivals match those observed from nearby small earthquakes. The stacked cross-correlation is the normal approach to enhance Green's function information from ambient noise, but a broader spectral band width with the same phase response can be found by spectral division. It appears advantageous to compare both approaches and select the best result, since very little modifications to procedures are needed. ¶ The properties of the ambient noise at a single station have been investigated in the logarithmic spectral domain and a station dependent signal can be extracted by stacking. The signal appears to be related to the local structure beneath the station, and when fully characterized may provide a new means of investigating structure.

APA, Harvard, Vancouver, ISO, and other styles

20

Arosio, Lucia. "Seismic ambient noise tomography of central Italy using a deep learning algorithm." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2022. http://amslaurea.unibo.it/25637/.

Full text

Abstract:

This work aims at testing a convolutional neural network (CNN), developed by Zhang et al. (2020), to measure group velocity of Rayleigh waves, extracted from records of background seismic noise, and at studying the crustal structure of Central Italy. I investigate an area in Central Italy by means of seismograms recorded by 73 seismic stations located approximately in the Central and Northern Apennines. The study is part of a larger project, named MUSE (Multiparametric and mUltiscale Study of Earthquake preparatory phase), that has the goal, among others, of detecting the spatial and temporal evolution of the velocity in the Earth’s crust. In this context, I have obtained the group velocity maps representative of the entire time span, from 01/01/2010 to 01/05/2021. In this study, I employ the technique of seismic ambient noise interferometry to extract Rayleigh wave measurements from the fully diffuse wavefields (Shapiro and Campillo, 2004). I then use CNN and other methods to determine the dispersion characteristics of the Rayleigh wave fundamental mode: a traditional method, requiring operator interaction; a classical automated procedure; and the recent neural network. I then invert each of the three dispersion data sets retrieved with the different approaches, to compute group velocity maps at different periods. I compare the performance of the different methods employed, and thus evaluate the performance of the CNN network, by comparing the maps among them, with geological observations, and also with a pre-existing model from the literature (Molinari et al., 2015). The CNN method shows excellent potential, but – at the current stage of development – it needs more accurate and specific training to reach the precision of manual picks. My maps image the crustal structure of the Northern Apennines area with unprecedented detail. This work poses the basis for further studies, to image the time variations of 3D structure (i.e., 4D tomography).

APA, Harvard, Vancouver, ISO, and other styles

21

Sadeghisorkhani, Hamzeh. "Analyses and Application of Ambient Seismic Noise in Sweden : Source, Interferometry, Tomography." Doctoral thesis, Uppsala universitet, Geofysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-320169.

Full text

Abstract:

Ambient seismic noise from generation to its application for determination of sub-surface velocity structures is analyzed using continuous data recordings from the Swedish National Seismic Network (SNSN). The fundamental aim of the thesis is to investigate the applicability of precise velocity measurements from ambient noise data. In the ambient noise method, a form of interferometry, the seismic signal is constructed from long-term cross correlation of a random noise field. Anisotropy of the source distribution causes apparent time shifts (velocity bias) in the interferometric signals. The velocity bias can be important for the study area (Sweden) which has relatively small velocity variations. This work explores the entire data path, from investigating the noise-source distribution to a tomographic study of southern Sweden. A new method to invert for the azimuthal source distribution from cross-correlation envelopes is introduced. The method provides quantitative estimates of the azimuthal source distribution which can be used for detailed studies of source generation processes. An advantage of the method is that it uses few stations to constrain azimuthal source distributions. The results show that the source distribution is inhomogeneous, with sources concentrated along the western coast of Norway. This leads to an anisotropic noise field, especially for the secondary microseisms. The primary microseismic energy comes mainly from the northeast. The deduced azimuthal source distributions are used to study the level of expected bias invelocity estimates within the SNSN. The results indicate that the phase-velocity bias is less than 1% for most station pairs but can be larger for small values of the ratio of inter-station distance over wavelength. In addition, the nature of velocity bias due to a heterogeneous source field is investigated in terms of high and finite-frequency regimes. Graphical software for phase-velocity dispersion measurements based on new algorithms is presented and validated with synthetic data and by comparisons to other methods. The software is used for phase-velocity measurements, and deduced azimuthal source distributions are used for velocity-bias correction. Derived phase-velocity dispersion curves are used to construct two-dimensional velocity maps of southern Sweden at different periods based on travel-time tomography. The effect of the bias correction is investigated, and velocity maps are interpreted in comparison to previous geological and geophysical information.

APA, Harvard, Vancouver, ISO, and other styles

22

Ashruf, Tahira Nicole <1990&gt. "The Moho reflectivity from ambient seismic noise autocorrelations beneath the Western Alps." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2022. http://amsdottorato.unibo.it/10491/1/Thesis30Sept_TahiraNicoleAshruf.pdf.

Full text

Abstract:

The lower crustal structure beneath the Western Alps -- including the Moho -- bears the signature of past and present geodynamic processes. It has been the subject of many studies until now. However, its current knowledge still leaves significant open questions. In order to derive new information, independent from previous determinations, here I wish to address this topic using a different method --- ambient seismic noise autocorrelation --- that is for the first time applied to reveal Moho depth in the Western Alps. Moho reflections are identified by picking reflectivity changes in ambient seismic noise autocorrelations. The seismic data is retrieved from more than 200 broadband seismic stations, from the China--Italy--France Alps (CIFALPS) linear seismic network, and from a subset of the AlpArray Seismic Network (AASN). The automatically-picked reflectivity changes along the CIFALPS transect in the southwestern Alps show the best results in the 0.5--1 Hz frequency band. The autocorrelation reflectivity profile of the CIFALPS transect shows a steeper subduction profile,~55 to ~70 km, of the European Plate underneath the Adriatic Plate. The dense spacing of the CIFALPS network facilitates the detection of lateral continuity of crustal structure, and of the Ivrea mantle wedge reaching shallow crustal depths in the southwestern Alps. The data of the AASN stations are filtered in the 0.4--1 and 0.5--1 Hz frequency bands. Although the majority of the stations give the same Moho depth for the different frequency bands, the few stations with different Moho depths shows the care that has to be taken when choosing the frequency band for filtering the autocorrelation stacks. The new Moho depth maps by using the AASN stations are a compilation of the first and second picked reflectivity changes. The results show the complex crust-mantle structure with clear differences between the northwestern and southwestern Alps.

APA, Harvard, Vancouver, ISO, and other styles

23

Nicolson, Heather Johan. "Exploring the Earth's subsurface with virtual seismic sources and receivers." Thesis, University of Edinburgh, 2011. http://hdl.handle.net/1842/5726.

Full text

Abstract:

Traditional methods of imaging the Earth’s subsurface using seismic waves require an identifiable, impulsive source of seismic energy, for example an earthquake or explosive source. Naturally occurring, ambient seismic waves form an ever-present source of energy that is conventionally regarded as unusable since it is not impulsive. As such it is generally removed from seismic data and subsequent analysis. A new method known as seismic interferometry can be used to extract useful information about the Earth’s subsurface from the ambient noise wavefield. Consequently, seismic interferometry is an important new tool for exploring areas which are otherwise seismically quiet, such as the British Isles in which there are relatively few strong earthquakes. One of the possible applications of seismic interferometry is the ambient noise tomography method (ANT). ANT is a way of using interferometry to image subsurface seismic velocity variations using seismic (surface) waves extracted from the background ambient vibrations of the Earth. To date, ANT has been used to successfully image the Earth’s crust and upper-mantle on regional and continental scales in many locations and has the power to resolve major geological features such as sedimentary basins and igneous and metamorphic cores. In this thesis I provide a review of seismic interferometry and ANT and apply these methods to image the subsurface of north-west Scotland and the British Isles. I show that the seismic interferometry method works well within the British Isles and illustrate the usefulness of the method in seismically quiet areas by presenting the first surface wave group velocity maps of the Scottish Highlands and across the British Isles using only ambient seismic noise. In the Scottish Highlands, these maps show low velocity anomalies in sedimentary basins such as the Moray Firth and high velocity anomalies in igneous and metamorphic centres such as the Lewisian complex. They also suggest that the Moho shallows from south to north across Scotland, which agrees with previous geophysical studies in the region. Rayleigh wave velocity maps from ambient seismic noise across the British Isles for the upper and mid-crust show low velocities in sedimentary basins such as the Midland Valley, the Irish Sea and the Wessex Basin. High velocity anomalies occur predominantly in areas of igneous and metamorphic rock such as the Scottish Highlands, the Southern Uplands, North-West Wales and Cornwall. In the lower crust/upper mantle, the Rayleigh wave maps show higher velocities in the west and lower velocities in the east, suggesting that the Moho shallows generally from east to west across Britain. The extent of the region of higher velocity correlates well with the locations of British earthquakes, agreeing with previous studies that suggest British seismicity might be influenced by a mantle upwelling beneath the west of the British Isles. Until the work described in Chapter 6 of this thesis was undertaken in 2009, seismic interferometry was concerned with cross-correlating recordings at two receivers due to a surrounding boundary of sources, then stacking the cross-correlations to construct the inter-receiver Green’s function. A key element of seismic wave propagation is that of source-receiver reciprocity i.e. the same wavefield will be recorded if its source and receiver locations and component orientations are reversed. By taking the reciprocal of its usual form, in this thesis I show that the impulsive-source form of interferometry can also be used in the opposite sense: to turn any energy source into a virtual sensor. This new method is demonstrated by turning earthquakes in Alaska and south-west USA into virtual seismometers located beneath the Earth’s surface.

APA, Harvard, Vancouver, ISO, and other styles

24

Sabey, Lindsay Erin. "Body and surface wave ambient noise seismic interferometry across the Salton Sea Geothermal Field, California." Thesis, Virginia Tech, 2015. http://hdl.handle.net/10919/51185.

Full text

Abstract:

Virtual source gathers were generated using the principles of seismic interferometry from 135 hours of ambient noise recorded during a controlled-source survey across the Salton Sea Geothermal Field in southern California. The non-uniform nature of the noise sources violated a primary assumption of the method and generated artifacts in the data. The artifacts generated by the high-energy impulsive sources (e.g. earthquakes, shots) were removable using traditional methods of amplitude normalization prior to cross-correlation. The continuous source artifacts generated by the geothermal wells and highways required an unconventional approach of utilizing only normalized impulsive sources to successfully reduce the artifacts. Virtual source gathers were produced successfully that contained strong surface waves at 0.4-2.5 Hz, an order of magnitude below the corner frequency of the geophones, and modest body waves at 22-30 Hz, which are generally more difficult to obtain due to the need for many large, well-distributed subsurface sources. The virtual source gathers compare well to nearby explosive shots and are more densely spaced, but have a much lower signal-to-noise ratio. Analysis of the surface waves was complicated by strong higher-order modes. Spectral analysis of virtual source gathers required utilization of the geothermal plant energy, which produced usable signal at offsets required for mode separation. The virtual source dispersion curve compared well to a dispersion curve from a nearby explosive shot. P-waves were observed on the virtual source gathers. Creation of a low-quality multichannel reflection stack revealed two weak reflectors in the upper 2 km.
Master of Science

APA, Harvard, Vancouver, ISO, and other styles

25

Acarel, Diğdem [Verfasser]. "Characterization of the Crustal Velocity Field in Space and Time Using Ambient Seismic Noise / Digdem Acarel." Berlin : Freie Universität Berlin, 2015. http://d-nb.info/1071547720/34.

Full text

APA, Harvard, Vancouver, ISO, and other styles

26

Noriega, Salmón Raquel. "Seismic Attenuation Analysis using Lg waves and Ambient Noise Recordings: Application to the Iberian Peninsula and Morocco." Doctoral thesis, Universitat de Barcelona, 2016. http://hdl.handle.net/10803/400704.

Full text

Abstract:

In this thesis I have carried out a comprehensive study of the attenuation properties of the Earth´s crust of the Iberia-Morocco region (IMR). I have investigated the crustal attenuation by means of the quality factor Q, which is inversely proportional to attenuation, using both earthquakes and noise-derived measurements. In order to fulfill the thesis objectives a large dataset including earthquake waveforms and seismic noise records has been used. Three different traditional earthquake methods have been implemented to estimate Q in the IMR: the two-station (TS) method, the coda normalization (CN) method and the spectral amplitude decay (SAD) method. For the estimation of Q, these approaches measure the spectral amplitude of the Lg wave (direct and coda) of regional events. Among all the methods evaluated, the TS method allows imaging the spatial variation of the Lg wave attenuation in the Iberian Peninsula whereas the CN and the SAD methods only estimate average attenuation values as well as its frequency dependence. For the Iberian Peninsula, high Lg Q values are observed in the stable Iberian Massif in western Iberia, while lower values are mainly found in the Pyrenean Range and in eastern and southern Iberia. For Morocco, the CN and the SAD methods produce similar results, indicating that the Lg Q models are robust to differences in the methodologies. The frequency-dependent Q estimates represent an average attenuation across a broad region of different structural domains and correlate well with areas of moderate seismicity. Additionally, I have studied the Lg propagation efficiency across the IMR. Results reflect an inefficient or even blocked propagation across the Gulf of Cádiz and for most paths crossing the western Alboran basin. The continental crust of the Iberian Peninsula and Morocco shows efficient Lg propagation. I have also investigated the potential of using ambient noise measurements to retrieve information about the anelastic structure of the Earth´s crust. Since noise preprocessing techniques modify the amplitude of the recovered empirical Green function of the medium, additional tests have to be done in order to verify the reliability of the attenuation results obtained. In this regard, I have carefully examined the influence of the distribution of noise sources and receivers on Q estimates. Azimuthally and spatially averaged Q values derived from noise recordings were further compared with earthquake attenuation measurements. Results reveal that the average Q estimates are in concordance with previous long-period surface-wave measurements from earthquakes in the central part of the Iberian Peninsula. Accurate Q estimates are also found in Morocco. I would like to emphasize that this thesis presents new contributions and improvements to the knowledge of the attenuation structure of the IMR. The first regional map that images the lateral variation of the Lg Q has been estimated for the Iberian Peninsula improving the spatial resolution of earlier studies. The frequency dependence of Lg Q has been also calculated for the first time in Morocco. Furthermore, this work is the first attempt to recover attenuation information from ambient seismic noise measurements in the study area. This novel technique allows us to investigate the attenuation structure of the Earth without the occurrence of earthquakes. Exploiting ambient seismic wavefields for attenuation studies will be a powerful tool to extract information about the anelastic structure and the geodynamics in areas of very low seismicity in the near future. It should be also noticed that recovering crustal attenuation values is important for many reasons. Attenuation estimates can be used to better quantify the hazard associated with earthquake ground shaking. Attenuation is also a valuable property in exploration seismology. For example, the presence of fluids can significantly attenuate the amplitude of the seismic waves.

APA, Harvard, Vancouver, ISO, and other styles

27

Sánchez, Sánchez-Pastor Pilar. "Monitoring the subsurface elastic properties using ambient seismic noise: 2011 El Hierro eruption and Reykjanes geothermal reservoir." Doctoral thesis, Universitat de Barcelona, 2019. http://hdl.handle.net/10803/668464.

Full text

Abstract:

Monitoring the elastic properties of the subsurface is of a special interest to mitigate the associated risk with natural and artificial hazards. In the last decade, coda-wave interferometry has become an excellent tool to characterize the subsurface in a large variety of environments. Additionally, coda-wave interferometry applied to the ever-present seismic ambient noise enables a continuous retrieval of virtual-source responses that allows monitoring structural and mechanical changes of media. Throughout this thesis, we perform a comprehensive study of seismic noise interferometry employing all the currently used methodologies to observe lag-time changes: Time evolution of waveform similarity, waveform stretching and moving window cross-spectrum technique. Furthermore, we introduce some improvements in order to increase the temporal accuracy and sensitivity of said methodologies to detect tiny medium changes. In particular, we carry out the study in two scenarios with very di fferent tectonic settings: the 2011 El Hierro eruption and the Reykjanes geothermal system (RGS). We compute the phase auto- and cross-correlation (PCC) of 1.5 years of continuous seismic noise records of all available seismic stations in both cases. The PCC provides an accurate and amplitude-unbiased measure of coherence between two seismic traces and allows obtaining detailed daily seismic response of media. In the case of the 2011 El Hierro volcanic eruption, through the analysis of waveform similarity time series of auto-correlations we clearly identify different pre-eruptive phases, as well as the end of the main magmatic emission and three magmatic intrusions that occurred in 2012. We use probabilistic sensitivity kernels to locate the places of the highest dynamics, providing that the ected areas correspond to the magmatic accumulation zone and the extinct volcanic area of Tiñor. In this study, we also introduce the change point analysis approach in order to automatically detect significant changes in time series. The second scenario consists in studying stress changes and potential deformations of the subsurface caused by geothermal well operations at RGS. For that purpose, we retrieve and analyse time series of waveform similarity values and seismic velocity variations. However, the continuous production over a large area and various injection wells make challenging the detection of time-lag changes in the coda. To tackle this issue, we decompose the similarity time series into the time-frequency domain through the S- transform, which allows us to discriminate fluctuations associated to injection and production rate drops. Furthermore, we observe a slow seismic velocity decrease in the reservoir due to the water deficit as well as seasonal variations associated with the energy production demand.
El monitoreo de las propiedades elásticas del subsuelo es de especial interés para mitigar el riesgo asociado con peligros naturales y artificiales. En la última década, la interferometría de ondas de coda (coda-wave interferometry) se ha convertido en una excelente herramienta para caracterizar el subsuelo en una gran variedad de escenarios. Además, esa metodología aplicada al ruido sísmico ambiental, el cual está siempre presente en los registros sísmicos, permite calcular de forma continua las respuestas sísmicas del medio (o funciones de Green) y así monitorear los cambios estructurales y mecánicos de los medios. En esta tesis, hemos realizado un estudio exhaustivo de la interferometría de ruido sísmico empleando todas las metodologías utilizadas actualmente para observar cambios temporales: similitud de la forma de onda, estiramiento de la forma de onda (stretching) y la técnica de moving window cross-spectrum. Además, introducimos algunas mejoras para aumentar la precisión temporal y la sensibilidad de dichas metodologías para detectar pequeños cambios en el medio. En particular, hemos llevado a cabo el estudio en dos escenarios con marcos tectónicos muy diferentes: la erupción de El Hierro de 2011 y el sistema geotérmico de Reykjanes (RGS). Calculamos auto-correlaciones y correlaciones cruzadas de fase (phase cross-correlation) de 1,5 años de registros continuos de ruido sísmico de todas las estaciones sísmicas disponibles en ambos casos. La PCC proporciona una medida precisa de la coherencia entre dos trazas sísmicas la cual no está condicionada por la amplitud de dichas trazas. De esta forma, hemos obtenido una detallada y diaria respuesta sísmica del medio. En el caso de la erupción volcánica de El Hierro de 2011, a través del análisis de la evolución temporal de las similitudes de forma de onda de auto-correlaciones, identificamos las diferentes fases pre-eruptivas de la erupción, así como el final de la emisión magmática y las tres intrusiones magmáticas ocurridas en 2012. Utilizamos sensitivity kernels para localizar los lugares de mayor dinamismo durante la crisis volcánica. Resultando que las áreas más afectadas son la zona de acumulación magmática alrededor del volcán Tanganasoga y al área volcánica extinta de Tiñor. En este estudio, presentamos el change-point analysis para detectar automáticamente cambios significativos en las series de tiempo y así poder automatizar su búsqueda con un bajo coste computacional. La segunda parte de la tesis consiste en estudiar cambios de esfuerzos y posibles deformaciones del subsuelo causadas por las operaciones de los pozos geotérmicos en RGS. Con este fin, calculamos y analizamos las series temporales de los valores de similitud de forma de onda y variaciones de velocidad sísmica. Sin embargo, la producción de energía se produce de forma continua y hay varios pozos de inyección de fluidos distribuidos por la zona de estudio, esta perturbación continuada del medio dificulta la detección de cambios temporales en la coda. Por ello, abordamos el estudio descomponiendo las series de tiempo de similitud en el dominio de tiempo-frecuencia a través de la transformada S. Esto nos permite discriminar las fluctuaciones en la coda que están asociadas a las variaciones bruscas que se producen en la actividad de la planta geotérmica. Por otro lado, observamos una disminución lenta de la velocidad sísmica en el reservorio debido al déficit de agua que está causando el sistema geotérmico. Por último, observamos variaciones estacionales en el subsuelo que acompañan a la demanda de producción de energía de la población islandesa.

APA, Harvard, Vancouver, ISO, and other styles

28

Mosher, Stephen. "P-Wave Study of the San Andreas Fault Near Parkfield, CA, from Ambient Noise Interferometry of Borehole Seismic Data." Thesis, Université d'Ottawa / University of Ottawa, 2016. http://hdl.handle.net/10393/35375.

Full text

Abstract:

In this thesis, we investigate and develop the optimal data processing procedures necessary to recover Green’s functions for body waves propagating among a network of borehole seismometers near Parkfield, CA. Applying these procedures, we detect P-waves propagating among these stations, which allows us to produce a first-order crustal velocity model for the San Andreas Fault in the Parkfield region. We also discuss under what conditions body wave phenomena such as reflections and mode conversions (P to S) may be observed, as further observing these would provide a dramatic improvement in our ability to characterize seismic velocity structures. Finally, we discuss the potential of seismic interferometry to produce time-lapse body wave characterizations of the San Andreas Fault, in which properties of the fault can be seen to change in time.

APA, Harvard, Vancouver, ISO, and other styles

29

Heath, Ben. "New Constraints on the Magmatic System beneath Newberry Volcano from the Analysis of Active and Passive Source Seismic Data and Ambient Noise." Thesis, University of Oregon, 2015. http://hdl.handle.net/1794/18744.

Full text

Abstract:

Using joint P-wave seismic tomography, receiver functions, and ambient noise we image the magmatic structure beneath Newberry Volcano, located near Bend, Oregon. Use of active source and teleseismic events in a joint tomographic inversion provides the ray crossings necessary to resolve a low velocity body around 4 km depth. Receiver functions show large lateral heterogeneity and are consistent with the location of a low velocity body derived from the tomography but require a larger low velocity anomaly. Ambient noise autocorrelations are used to image a low velocity reflector, located at ~3 km depth, shallower than the imaged low velocity body recovered using tomography and receiver functions. Ultimately, our results reveal a magma chamber at 3-4 km depth beneath Newberry caldera, with an overlying partially molten sill at ~3 km depth. These results show the usefulness of dense seismometer deployments over volcanoes.

APA, Harvard, Vancouver, ISO, and other styles

30

Käestle, Emanuel David. "High-resolution ambient-noise and earthquake surface-wave tomography of the Alps Apennines and Dinarides." Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066307/document.

Full text

Abstract:

La collision alpine a créé des structures complexes comme des chaînes de montagnes très arqués et des interactions compliquées entre les slabs subduits. La polarité de subduction est inversée à la transition entre les Alpes et les Apennins et les Alpes et les Dinarides. Le fait que la plaque Adria subducte en même temps vers l'ouest et vers l'est avec un fort pendage, presque verticalement, suggèrent une flexion importante de cette plaque. Notamment, si on considère de plus la proposition qu'Adria subducte aussi vers le nord sous les Alpes de l'est, ce qui est toujours sujet de discussion. Des déchirures dans le slab adriatique sous les Dinarides du nord à plus de 150 km de profondeur et sous les Apennins à moins de 200 km, pourraient être des signes d'une forte tension et, en conséquence, un détachement de la plaque adriatique. La plaque européenne pourrait aussi avoir subi plusieurs déchirures le long des Alpes. Cette hypothèse sujette à débat nécessite de nouveaux modèles tomographiques. Le modèle tomographique présenté dans cette thèse se base sur les vitesses de phase des ondes de surface pour donner un modèle 3-D à haute résolution des vitesses de cisaillement de la surface jusqu'à 200 km de profondeur. Ce modèle est unique de par sa haute résolution dans le manteau lithosphérique où des modèles antérieurs montrent de fortes incertitudes. Afin d'imager la croûte et le manteau supérieur en même temps, une combinaison des données de vitesses de phase des ondes de surface mesurés à partir des bruits ambiants ainsi que des séismes est utilisée dans cette thèse.Pour tester la validité de cette procédure, une comparaison détaillée des mesures de vitesses de phases et des structures imagées avec les deux méthodes est présentée. De l’analyse résulte un faible biais qui montre des vitesses plus élevées avec les données se basant sur des séismes par rapport aux données se basant sur le bruit ambiant. En comparant avec des travaux antérieurs, il est apparu que ce biais est dû à une différence méthodologique. Plusieurs paramètres qui pourraient influencer les mesures du bruit ambiant sont testés numériquement. Une cause unique n'a pu être identifiée. L'explication la plus probable pour le biais est une combinaison entre différentes sensibilités des méthodes aux structures et l'influence des modes supérieurs. Néanmoins, l'écart est suffisamment faible par rapport aux variations structurales pour être négligé.Un modèle final de vitesse de cisaillement de la région alpine est obtenu avec une résolution latérale d'environ 25 km dans la croûte peu profonde. Les tests synthétiques donnent une résolution approximative de profondeur estimée à 2 km près de la surface et de 5 km à la profondeur du Moho. Dans le manteau supérieur, la résolution baisse rapidement mais les structures principales des panneaux plongeants restent bien imagées jusqu'à une profondeur de 200 km le long des Alpes et des Apennins.La partie crustale du modèle donne des informations à haute résolution sur la taille et la profondeur des bassins sédimentaires et du corps d’Ivrée ainsi que sur la profondeur et la structure du Moho. Ce modèle de vitesses de cisaillement est le premier montrant autant de détails et couvrant les Alpes entières, il est proposé que le modèle pourrait servir comme référence pour la région.Le modèle montre les limites des zones de subduction et les régions de basses vitesses asthénosphèriques montants sous les bassins Ligure et pannonien. Des structures connues comme les déchirures de slabs sous les Apennins et les Dinarides sont imagés. Des découvertes supplémentaires ont été mises en évidence : une petite zone de faible vitesse qui coupe la lithosphère au nord des Dinarides est interprété comme l'expression d'une grande faille décrochante
The plate collision in the Alps and adjacent orogens has created a complex picture of highly arcuate mountain belts and complicated interactions of subduction slabs. The subduction polarity is reversed from European to Adriatic subduction in the transition of the Alps to the Apennines and to the Dinarides. The subduction of Adria both to the west and east and the almost vertical dip of the slabs implies an important flexure of this plate. Even more so if one considers the proposed subduction of Adria also to the north under the eastern Alps, which is still a matter of discussion. Gaps in the Adriatic slab under the northern Dinarides, below 150~km depth and in the southern Apennines above 200~km may be signs of the stresses and the consequent tearing that the Adriatic plate is exposed to.Also the European plate has supposedly undergone one or several break-offs all along the Alpine arc. Especially in the eastern and western Alps it is still an open question whether the European slab is detached below the lithosphere. New tomographic models are thus needed.The herein presented tomographic model is based on surface-wave phase velocities and gives a picture of the shear-velocity structure from the surface to 200 km depth. It is the first high-resolution shear-velocity model of the entire Alpine crust and upper mantle. It is also unique in its good resolution in the lithospheric mantle, where previous body-wave models are subject to high uncertainties. In order to be able to image both crust and upper mantle, a combination of ambient-noise and earthquake-based phase-velocity measurements is used in the present thesis.The validity of this approach is tested by a detailed comparison of the phase-velocity measurements and the structures that are imaged from each method individually. A small bias between the methods results in slightly elevated velocities from earthquake measurements. By comparison with earlier works it appears that this bias is due to methodological differences. Several effects that may influence the ambient-noise records are tested with synthetic experiments, but no unique cause is found. The most likely explanation for the bias between the two methods is a combination of different structural sensitivities and the influence of higher modes. Nevertheless, the discrepancy is sufficiently small with respect to the structural variations that the bias can be neglected.A final shear-velocity model of the Alpine region is obtained which has a lateral resolution in the shallow crust of approximately 25 km. From synthetic tests, the average depth resolution is estimated to be 2~km close to the surface and 5 km for the Moho depth. In the upper mantle the resolution decreases significantly, but main slab structures are well imaged in the central Alps and the Apennines down to the bottom of the model at 200 km depth.Highlights of the crustal part of the model are size and depth of sedimentary basins, the Ivrea body and the Moho structure. Being the first shear-velocity model of this detail and extend it is proposed to serve as reference for the Alps

APA, Harvard, Vancouver, ISO, and other styles

31

Lontsi, Agostiny Marrios [Verfasser], and Frank [Akademischer Betreuer] Krüger. "1D shallow sedimentary subsurface imaging using ambient noise and active seismic data / Agostiny Marrios Lontsi ; Betreuer: Frank Krüger." Potsdam : Universität Potsdam, 2016. http://d-nb.info/1218401591/34.

Full text

APA, Harvard, Vancouver, ISO, and other styles

32

THAPA, HARI RAM. "Crustal structure and discontinuities beneath the Nepal Himalaya using seismic ambient noise and teleseismic P wave coda autocorrelation." Doctoral thesis, Università degli Studi di Trieste, 2021. http://hdl.handle.net/11368/2996096.

Full text

Abstract:

Nepal is an actively deforming region because of its tectonic setting as demon-strated by many destructive seismic events it hosted in the past with the most re-cent Gorkha 2015 magnitude 7.8 earthquake. For a better understanding of the crustal structure, of the physics of earthquakes as well as their detailed high-resolution location and monitoring, and to mitigate the real-time seismic hazard, an adequate 3-D regional velocity model is needed. So far, a country-wide 3-D velocity structure is unavailable. Here, we present a new high-resolution 3-D shear-wave velocity structure down to 60 km depth beneath the Nepal Himalaya using ambient noise cross-correlations. Our results show significant lateral varia-tion in crustal structure and thus, correlate well with the known geological and tectonic features of the study area. The shear wave velocity structure of shallow depth (top 5 km) shows the low shear wave velocity beneath the epicenter of 1934, 1833, 2015 earthquakes, and beneath the region which were hardly hit by 2015 Gorkha Earthquakes (for example, Sindhuplachok, Kathmandu valley, etc.). Mapping of crustal discontinuities is crucial to decipher the dynamics of the continental deformation as well as better constrain the earthquake potential. Ac-cordingly, we also use autocorrelation of teleseismic P coda to recover the shal-low and deeper interfaces in the crust beneath Nepal. The results show the high-resolution images of four major seismic reflectors in the crust beneath Central Nepal. The shallow discontinuity recovered from our study is well supported by the geology. The result obtained from our study shows the Moho depth variation from 40 km beneath South Nepal to 60 km beneath High Himalaya. Our results provide for the first-time good constraint on both the seismogenesis and on the earthquake hazard in the Nepal Himalaya.
Nepal is an actively deforming region because of its tectonic setting as demon-strated by many destructive seismic events it hosted in the past with the most re-cent Gorkha 2015 magnitude 7.8 earthquake. For a better understanding of the crustal structure, of the physics of earthquakes as well as their detailed high-resolution location and monitoring, and to mitigate the real-time seismic hazard, an adequate 3-D regional velocity model is needed. So far, a country-wide 3-D velocity structure is unavailable. Here, we present a new high-resolution 3-D shear-wave velocity structure down to 60 km depth beneath the Nepal Himalaya using ambient noise cross-correlations. Our results show significant lateral varia-tion in crustal structure and thus, correlate well with the known geological and tectonic features of the study area. The shear wave velocity structure of shallow depth (top 5 km) shows the low shear wave velocity beneath the epicenter of 1934, 1833, 2015 earthquakes, and beneath the region which were hardly hit by 2015 Gorkha Earthquakes (for example, Sindhuplachok, Kathmandu valley, etc.). Mapping of crustal discontinuities is crucial to decipher the dynamics of the continental deformation as well as better constrain the earthquake potential. Ac-cordingly, we also use autocorrelation of teleseismic P coda to recover the shal-low and deeper interfaces in the crust beneath Nepal. The results show the high-resolution images of four major seismic reflectors in the crust beneath Central Nepal. The shallow discontinuity recovered from our study is well supported by the geology. The result obtained from our study shows the Moho depth variation from 40 km beneath South Nepal to 60 km beneath High Himalaya. Our results provide for the first-time good constraint on both the seismogenesis and on the earthquake hazard in the Nepal Himalaya.

APA, Harvard, Vancouver, ISO, and other styles

33

Galetti, Erica. "Seismic interferometry and non-linear tomography." Thesis, University of Edinburgh, 2015. http://hdl.handle.net/1842/10506.

Full text

Abstract:

Seismic records contain information that allows geoscientists to make inferences about the structure and properties of the Earth’s interior. Traditionally, seismic imaging and tomography methods require wavefields to be generated and recorded by identifiable sources and receivers, and use these directly-recorded signals to create models of the Earth’s subsurface. However, in recent years the method of seismic interferometry has revolutionised earthquake seismology by allowing unrecorded signals between pairs of receivers, pairs of sources, and source-receiver pairs to be constructed as Green’s functions using either cross-correlation, convolution or deconvolution of wavefields. In all of these formulations, seismic energy is recorded and emitted by surrounding boundaries of receivers and sources, which need not be active and impulsive but may even constitute continuous, naturally-occurring seismic ambient noise. In the first part of this thesis, I provide a comprehensive overview of seismic interferometry, its background theory, and examples of its application. I then test the theory and evaluate the effects of approximations that are commonly made when the interferometric formulae are applied to real datasets. Since errors resulting from some approximations can be subtle, these tests must be performed using almost error-free synthetic data produced with an exact waveform modelling method. To make such tests challenging the method and associated code must be applicable to multiply-scattering media. I developed such a modelling code specifically for interferometric tests and applications. Since virtually no errors are introduced into the results from modelling, any difference between the true and interferometric waveforms can safely be attributed to specific origins in interferometric theory. I show that this is not possible when using other, previously available methods: for example, the errors introduced into waveforms synthesised by finite-difference methods due to the modelling method itself, are larger than the errors incurred due to some (still significant) interferometric approximations; hence that modelling method can not be used to test these commonly-applied approximations. I then discuss the ability of interferometry to redatum seismic energy in both space and time, allowing virtual seismograms to be constructed at new locations where receivers may not have been present at the time of occurrence of the associated seismic source. I present the first successful application of this method to real datasets at multiple length scales. Although the results are restricted to limited bandwidths, this study demonstrates that the technique is a powerful tool in seismologists’ arsenal, paving the way for a new type of ‘retrospective’ seismology where sensors may be installed at any desired location at any time, and recordings of seismic events occurring at any other time can be constructed retrospectively – even long after their energy has dissipated. Within crustal seismology, a very common application of seismic interferometry is ambient-noise tomography (ANT). ANT is an Earth imaging method which makes use of inter-station Green’s functions constructed from cross-correlation of seismic ambient noise records. It is particularly useful in seismically quiescent areas where traditional tomography methods that rely on local earthquake sources would fail to produce interpretable results due to the lack of available data. Once constructed, interferometric Green’s functions can be analysed using standard waveform analysis techniques, and inverted for subsurface structure using more or less traditional imaging methods. In the second part of this thesis, I discuss the development and implementation of a fully non-linear inversion method which I use to perform Love-wave ANT across the British Isles. Full non-linearity is achieved by allowing both raypaths and model parametrisation to vary freely during inversion in Bayesian, Markov chain Monte Carlo tomography, the first time that this has been attempted. Since the inversion produces not only one, but a large ensemble of models, all of which fit the data to within the noise level, statistical moments of different order such as the mean or average model, or the standard deviation of seismic velocity structures across the ensemble, may be calculated: while the ensemble average map provides a smooth representation of the velocity field, a measure of model uncertainty can be obtained from the standard deviation map. In a number of real-data and synthetic examples, I show that the combination of variable raypaths and model parametrisation is key to the emergence of previously-unobserved, loop-like uncertainty topologies in the standard deviation maps. These uncertainty loops surround low- or high-velocity anomalies. They indicate that, while the velocity of each anomaly may be fairly well reconstructed, its exact location and size tend to remain uncertain; loops parametrise this location uncertainty, and hence constitute a fully non-linearised, Bayesian measure of spatial resolution. The uncertainty in anomaly location is shown to be due mainly to the location of the raypaths that were used to constrain the anomaly also only being known approximately. The emergence of loops is therefore related to the variation in raypaths with velocity structure, and hence to 2nd and higher order wave-physics. Thus, loops can only be observed using non-linear inversion methods such as the one described herein, explaining why these topologies have never been observed previously. I then present the results of fully non-linearised Love-wave group-velocity tomography of the British Isles in different frequency bands. At all of the analysed periods, the group-velocity maps show a good correlation with known geology of the region, and also robustly detect novel features. The shear-velocity structure with depth across the Irish Sea sedimentary basin is then investigated by inverting the Love-wave group-velocity maps, again fully non-linearly using Markov chain Monte Carlo inversion, showing an approximate depth to basement of 5 km. Finally, I discuss the advantages and current limitations of the fully non-linear tomography method implemented in this project, and provide guidelines and suggestions for its improvement.

APA, Harvard, Vancouver, ISO, and other styles

34

Käestle, Emanuel David. "High-resolution ambient-noise and earthquake surface-wave tomography of the Alps Apennines and Dinarides." Electronic Thesis or Diss., Paris 6, 2017. http://www.theses.fr/2017PA066307.

Full text

Abstract:

La collision alpine a créé des structures complexes comme des chaînes de montagnes très arqués et des interactions compliquées entre les slabs subduits. La polarité de subduction est inversée à la transition entre les Alpes et les Apennins et les Alpes et les Dinarides. Le fait que la plaque Adria subducte en même temps vers l'ouest et vers l'est avec un fort pendage, presque verticalement, suggèrent une flexion importante de cette plaque. Notamment, si on considère de plus la proposition qu'Adria subducte aussi vers le nord sous les Alpes de l'est, ce qui est toujours sujet de discussion. Des déchirures dans le slab adriatique sous les Dinarides du nord à plus de 150 km de profondeur et sous les Apennins à moins de 200 km, pourraient être des signes d'une forte tension et, en conséquence, un détachement de la plaque adriatique. La plaque européenne pourrait aussi avoir subi plusieurs déchirures le long des Alpes. Cette hypothèse sujette à débat nécessite de nouveaux modèles tomographiques. Le modèle tomographique présenté dans cette thèse se base sur les vitesses de phase des ondes de surface pour donner un modèle 3-D à haute résolution des vitesses de cisaillement de la surface jusqu'à 200 km de profondeur. Ce modèle est unique de par sa haute résolution dans le manteau lithosphérique où des modèles antérieurs montrent de fortes incertitudes. Afin d'imager la croûte et le manteau supérieur en même temps, une combinaison des données de vitesses de phase des ondes de surface mesurés à partir des bruits ambiants ainsi que des séismes est utilisée dans cette thèse.Pour tester la validité de cette procédure, une comparaison détaillée des mesures de vitesses de phases et des structures imagées avec les deux méthodes est présentée. De l’analyse résulte un faible biais qui montre des vitesses plus élevées avec les données se basant sur des séismes par rapport aux données se basant sur le bruit ambiant. En comparant avec des travaux antérieurs, il est apparu que ce biais est dû à une différence méthodologique. Plusieurs paramètres qui pourraient influencer les mesures du bruit ambiant sont testés numériquement. Une cause unique n'a pu être identifiée. L'explication la plus probable pour le biais est une combinaison entre différentes sensibilités des méthodes aux structures et l'influence des modes supérieurs. Néanmoins, l'écart est suffisamment faible par rapport aux variations structurales pour être négligé.Un modèle final de vitesse de cisaillement de la région alpine est obtenu avec une résolution latérale d'environ 25 km dans la croûte peu profonde. Les tests synthétiques donnent une résolution approximative de profondeur estimée à 2 km près de la surface et de 5 km à la profondeur du Moho. Dans le manteau supérieur, la résolution baisse rapidement mais les structures principales des panneaux plongeants restent bien imagées jusqu'à une profondeur de 200 km le long des Alpes et des Apennins.La partie crustale du modèle donne des informations à haute résolution sur la taille et la profondeur des bassins sédimentaires et du corps d’Ivrée ainsi que sur la profondeur et la structure du Moho. Ce modèle de vitesses de cisaillement est le premier montrant autant de détails et couvrant les Alpes entières, il est proposé que le modèle pourrait servir comme référence pour la région.Le modèle montre les limites des zones de subduction et les régions de basses vitesses asthénosphèriques montants sous les bassins Ligure et pannonien. Des structures connues comme les déchirures de slabs sous les Apennins et les Dinarides sont imagés. Des découvertes supplémentaires ont été mises en évidence : une petite zone de faible vitesse qui coupe la lithosphère au nord des Dinarides est interprété comme l'expression d'une grande faille décrochante
The plate collision in the Alps and adjacent orogens has created a complex picture of highly arcuate mountain belts and complicated interactions of subduction slabs. The subduction polarity is reversed from European to Adriatic subduction in the transition of the Alps to the Apennines and to the Dinarides. The subduction of Adria both to the west and east and the almost vertical dip of the slabs implies an important flexure of this plate. Even more so if one considers the proposed subduction of Adria also to the north under the eastern Alps, which is still a matter of discussion. Gaps in the Adriatic slab under the northern Dinarides, below 150~km depth and in the southern Apennines above 200~km may be signs of the stresses and the consequent tearing that the Adriatic plate is exposed to.Also the European plate has supposedly undergone one or several break-offs all along the Alpine arc. Especially in the eastern and western Alps it is still an open question whether the European slab is detached below the lithosphere. New tomographic models are thus needed.The herein presented tomographic model is based on surface-wave phase velocities and gives a picture of the shear-velocity structure from the surface to 200 km depth. It is the first high-resolution shear-velocity model of the entire Alpine crust and upper mantle. It is also unique in its good resolution in the lithospheric mantle, where previous body-wave models are subject to high uncertainties. In order to be able to image both crust and upper mantle, a combination of ambient-noise and earthquake-based phase-velocity measurements is used in the present thesis.The validity of this approach is tested by a detailed comparison of the phase-velocity measurements and the structures that are imaged from each method individually. A small bias between the methods results in slightly elevated velocities from earthquake measurements. By comparison with earlier works it appears that this bias is due to methodological differences. Several effects that may influence the ambient-noise records are tested with synthetic experiments, but no unique cause is found. The most likely explanation for the bias between the two methods is a combination of different structural sensitivities and the influence of higher modes. Nevertheless, the discrepancy is sufficiently small with respect to the structural variations that the bias can be neglected.A final shear-velocity model of the Alpine region is obtained which has a lateral resolution in the shallow crust of approximately 25 km. From synthetic tests, the average depth resolution is estimated to be 2~km close to the surface and 5 km for the Moho depth. In the upper mantle the resolution decreases significantly, but main slab structures are well imaged in the central Alps and the Apennines down to the bottom of the model at 200 km depth.Highlights of the crustal part of the model are size and depth of sedimentary basins, the Ivrea body and the Moho structure. Being the first shear-velocity model of this detail and extend it is proposed to serve as reference for the Alps

APA, Harvard, Vancouver, ISO, and other styles

35

Liu, Jin-Yuan. "Seismo-acoustic rough interface scattering of surface-generated ambient noise in a stratified ocean." Thesis, Massachusetts Institute of Technology, 1992. http://hdl.handle.net/1721.1/44398.

Full text

APA, Harvard, Vancouver, ISO, and other styles

36

Collaço, Bruno de Barros. "Tomografia de ruído ambiental na Bacia do Paraná." Universidade de São Paulo, 2014. http://www.teses.usp.br/teses/disponiveis/14/14132/tde-04062018-171929/.

Full text

Abstract:

A tomografia sísmica convencional apresenta baixa resolução em regiões de baixa sismicidade, como consequência, estudos prévios realizados na América do Sul não mapearam com clareza áreas menores do continente, como a região da bacia do Paraná. Para descrever a estrutura da crosta na região da bacia do Paraná, além de dados obtidos através da análise de terremotos andinos, foram utilizadas curvas de dispersão provenientes da correlação do ruído ambiental entre pares de estações sismográficas, técnica conhecida com Ambient Noise Tomography (ANT). Os mapas de velocidade de grupo obtidos correspondem bem com as principais províncias geológicas já conhecidas na América do Sul: baixas velocidades sob os Andes e bacias sedimentares e altas velocidades sob regiões cratônicas. As anomalias de alta velocidade encontradas para o manto superior, concordam com trabalhos prévios que confirmam a presença de um núcleo cratônico sob a bacia. Contudo, não é possível afirmar se tal núcleo está intacto ou não, principalmente devido resultados anteriores que mostram evidências da existência de um núcleo dividido por zonas de sutura. No entanto, a tomografia de ondas de superfície não possui resolução para confirmar este modelo. Como as etapas do processamento de dados são bem definidas e independentes, à medida que novas estações forem instaladas com o avanço do projeto BRASIS, novos caminhos serão adicionadas à base inicial, aumentando a resolução e confiabilidade dos resultados futuros.
Conventional seismic tomography is known to have poor resolution in regions of low seismicity, therefore, studies carried out in South America did not mapped clearly smaller areas of the continent, for example the region of the Parana basin. To describe the of the crustal structure of the Parana basin, in addition to data obtained by analysis of Andean earthquakes, we used dispersion curves from ambient noise correlation between pairs of seismographic stations, a technique known by Ambient Noise Tomography (ANT). The obtained group velocity maps correspond well with the main geological provinces already known in South America: low velocities under the Andes and sedimentary basins and high velocities in cratonic regions. The high velocity anomalies encountered in the upper mantle, agree with previous studies that confirm the presence of a nucleus in the cratonic basin. However, it is not possible to say whether such a nucleus is intact or not, mainly because previous results showing evidence of the existence of a nucleus divided by suture zones. Nevertheless, surface wave tomography has no resolution to confirm this model. The steps of data processing of this work are well defined and independent, thus, as new stations will being deployed with the advance of BRASIS Project, new paths will be added to the database, increasing the resolution and reliability of the future results.

APA, Harvard, Vancouver, ISO, and other styles

37

D'hour, Virginie. "Medium change monitoring using ambient seismic noise and coda wave interferometry: examples from intraplate NE Brazil and the Mid-Atlantic Ridge." Universidade Federal do Rio Grande do Norte, 2015. http://repositorio.ufrn.br/handle/123456789/21148.

Full text

Abstract:

Submitted by Automa??o e Estat?stica (sst@bczm.ufrn.br) on 2016-08-09T23:00:00Z No. of bitstreams: 1 VirginieD'hour_TESE.pdf: 8753386 bytes, checksum: 2e3e6a55a73239c9eeac2c8def85eb39 (MD5)
Approved for entry into archive by Arlan Eloi Leite Silva (eloihistoriador@yahoo.com.br) on 2016-08-13T00:23:12Z (GMT) No. of bitstreams: 1 VirginieD'hour_TESE.pdf: 8753386 bytes, checksum: 2e3e6a55a73239c9eeac2c8def85eb39 (MD5)
Made available in DSpace on 2016-08-13T00:23:12Z (GMT). No. of bitstreams: 1 VirginieD'hour_TESE.pdf: 8753386 bytes, checksum: 2e3e6a55a73239c9eeac2c8def85eb39 (MD5) Previous issue date: 2015-09-01
Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico (CNPq)
Nesta tese s?o apresentados e discutidos os resultados de correla??o do ru?do s?smico em dois contextos: regi?o intraplaca e Dorsal Meso-oce?nica. O m?todo de interferometria de cauda de onda (coda wave interferometry?CWI) tamb?m foi utilizado para os dados da regi?o intraplaca. A correla??o do ru?do permite recuperar a fun??o de Green emp?rica entre dois receptores , como se uma das esta??es atuasse como uma fonte (virtual). Esta t?cnica ?amplamente utilizado em sismologia para a imagem do subsolo e para monitorar mudan?as estruturais associadas principalmente com erup??es vulc?nicas e terremotos grandes (mb > 6.0). No estudo da regi?o intraplaca, fomos capazes de detectar mudan?as estruturais localizadas relacionadas com esta pequena sequ?ncia de terremotos, cujo evento principal ? de mR 3.7, no Nordeste do Brasil. N?s tamb?m mostramos que a normaliza??o de 1-bit de e o branqueamento spectral provoca perdas de detalhes na forma de onda e que a auto- correla??o de fase, que ? pouco sens?vel ? amplitude , parece ser mais sens?vel e robusta para a nossa an?lise. A an?lise de 6 meses de dados usando correla??es cruzadas detecta claramente altera??es do meio logo ap?s do evento principal, enquanto que as auto- correla??es essencialmente detectam altera??es ap?s 1 m?s. Estas mudan?as na correla??o cruzada e na auto-correla??o podem serexplicadas pela redistribui??o da press?o do fluido ocasionadas mudan?as hidromec?nicas e novos caminhos preferenciais para difus?o de press?o e fuidos , devido a terramotos que ocorrem mais tarde. No estudo da Dorsal Meso-oce?nica, investigamos as mudan?as estruturais associadas a um terremoto de mb 4,9 aolongo da falha transformante de S?o Paulo. Os dados foram registrados por a ?nica esta??o s?smica localizada a menos de 200 km da Dorsal Meso-oce?nica. Os resultados da auto-correla??o de fase por um per?odo de 5 meses, mostram uma forte mudan?a de meio co-s?smica seguido por uma recupera??o p?s-s?smica relativamente r?pida. Esta mudan?a do meio provavelmente est? relacionada aos danos causados pelo terremoto de mb 4.9. O processo de cicatriza??o (enchimento das novas fissuras) que durou 60 dias pode ser decomposto em duas fases, uma recupera??o r?pida na fase p?s-s?smica (de 70% em ~ 30 dias) precoce e uma recupera??o relativamente lenta depois (de 30% em ~ 30 dias) No estudo de interferometria de cauda de onda, monitoramos mudan?as temporais da subsuperf?cie causada pela sequ?ncia de pequenos terremotos intraplaca mencionado anteriormente. O m?todo foi validado com dados sint?ticos. Fomos capazes de detectar uma mudan?a da fonte de 2.5% e uma redu??o de 15% da quantidade dos espalhadores. A partir dos dados reais, observamos uma r?pida diminui??o da correla??o da cauda da onda ap?s do evento s?smico mR 3.7. Isso indica uma mudan?a r?pida do subsolo na regi?o da falha induzida pelo terremoto.
This thesis presents and discusses the results of ambient seismic noise correlation for two different environments: intraplate and Mid-Atlantic Ridge. The coda wave interferometry method has also been tested for the intraplate data. Ambient noise correlation is a method that allows to retrieve the structural response between two receivers from ambient noise records, as if one of the station was a virtual source. It has been largely used in seismology to image the subsurface and to monitor structural changes associated mostly with volcanic eruptions and large earthquakes. In the intraplate study, we were able to detect localized structural changes related to a small earthquake swarm, which main event is mR 3.7, North-East of Brazil. We also showed that the 1-bit normalization and spectral whitening result on the loss of waveform details and that the phase auto-correlation, which is amplitude unbiased, seems to be more sensitive and robust for our analysis of a small earthquake swarm. The analysis of 6 months of data using cross-correlations detect clear medium changes soon after the main event while the auto-correlations detect changes essentially after 1 month. It could be explained by fluid pressure redistribution which can be initiated by hydromechanical changes and opened path ways to shallower depth levels due to later occurring earthquakes. In the Mid-Atlantic Ridge study, we investigate structural changes associated with a mb 4.9 earthquake in the region of the Saint Paul transform fault. The data have been recorded by a single broadband seismic station located at less than 200 km from the Mid-Atlantic ridge. The results of the phase auto-correlation for a 5-month period, show a strong co-seismic medium change followed by a relatively fast post-seismic recovery. This medium change is likely related to the damages caused by the earthquake?s ground shaking. The healing process (filling of the new cracks) that lasted 60 days can be decomposed in two phases, a fast recovery (70% in ~30 days) in the early post-seismic stage and a relatively slow recovery later (30% in ~30 days). In the coda wave interferometry study, we monitor temporal changes of the subsurface caused by the small intraplate earthquake swarm mentioned previously. The method was first validated with synthetics data. We were able to detect a change of 2.5% in the source position and a 15% decrease of the scatterers? amount. Then, from the real data, we observed a rapid decorrelation of the seismic coda after the mR 3.7 seismic event. This indicates a rapid change of the subsurface in the fault?s region induced by the earthquake.

APA, Harvard, Vancouver, ISO, and other styles

38

Obermann, Anne-Christine. "Etude des propriétés élastiques de la croûte : analyse numérique et applications au bruit de fond sismique." Thesis, Grenoble, 2013. http://www.theses.fr/2013GRENU028/document.

Full text

Abstract:

Durant mes trois années de thèse, j'ai été amenée à travailler sur différents aspects de la surveillance des propriétés élastiques de la croûte, à la fois par analyses numériques, mais aussi par des applications d'utilisation du bruit ambiant. Ma thèse s'articule autour de deux parties principales: Les propriétés des ondes de la coda Le libre parcours moyen est un paramètre crucial rencontré continuellement dans les différentes parties de mon travail. Dans cette partie, je présente une nouvelle façon de déterminer ce libre parcours moyen à partir des statistiques de phase. Cette méthode a été testée avec succès dans l'étude d'un volcan situé en Auvergne (France). Je présente ensuite une analyse numérique de la sensibilité que les ondes de la coda manifestent en fonction de la profondeur. Ce travail nous a permis de relier cette sensibilité à la profondeur observée dans les ondes de la coda, à une combinaison de la sensibilité des ondes de volume et de celle des ondes de surface. En effet, cette sensibilité des ondes de volume et de surface dépend directement du temps considéré dans la coda et du libre parcours moyen du milieu étudié. Nous avons été capable de montrer que le changement de vitesse relatif dans la coda était lié à une dépendance en temps. Cette importante observation nous permet de pouvoir établir une distinction entre un changement qui se produit en surface et un changement ayant lieu en profondeur. Localisation des changements dans un milieu diffusif Nous avons développé une méthode d'inversion basée sur des approches probabilistes de la propagation des ondes, afin de pouvoir localiser les changements dans le milieu. Nous avons également étudié la décohérence de la forme d'onde, ce qui constitue un aspect additionnel des techniques de surveillance du bruit sismique, qui traditionnellement sont basés sur l'évaluation du temps de retard dans la coda. Au cours de cette thèse, nous avons affiné notre méthode d'inversion en l'appliquant à trois cas d'études. Dans un premier cas, nous avons étudié les changements ayant eu lieu avant et pendant l'éruption du volcan du Piton de la Fournaise situé sur l'Ile de la Réunion. Le challenge ici est de parvenir à localiser correctement la prochaine éruption. La seconde application a concerné le séisme de Mw7,9 de Wenchuan (Chine), sur lequel nous avons obtenu de nombreuses données. Dans ce cas, nous avons pris en compte l'évolution de la coda au cours du temps et nous avons conjointement inversé les données à différents temps dans la coda pour étudier les changements induits dans la croûte, avant et pendant le séisme. A partir de ces résultats, nous avons pu clairement voir que la décohérence de la forme d'onde et les variations de vitesse n'étaient pas sensibles aux mêmes propriétés physiques. Dans le troisième cas d'étude, nous avons testé la procédure d'inversion avec un noyau 3D pour étudier la fracturation dans un bloc de béton soumis à une contrainte croissante
During my thesis, I worked on different aspects of monitoring the elastic properties of the crust, with both numerical analysis and applications with ambient noise. The main body of my thesis consists of two main parts: 1. Properties of coda waves A very important parameter that we continuously encounter in the different parts of my thesis work is the scattering mean free path. I present an original way to determine the scattering mean free path from phase statistics that was successfully tested on a volcano in Auvergne. Then I discuss an intensive numerical analysis of the depth sensitivity of coda waves. This work allowed us to relate the depth sensitivity of coda waves to a combination of bulk wave sensitivity and surface wave sensitivity that depends on the time in the coda and on the scattering mean free path of the medium. We were able to show a time dependence of the relative velocity change in the coda that allows us to discriminate a change that occurs at the surface from a change that occurs at depth. 2. Locating changes in multiply scattering media We developed an inversion method based on probabilistic approaches of the wave propagation to locate changes in the medium. As an additional aspect to seismic noise monitoring techniques that are based on the evaluation of time delays in the coda, we also study the waveform decoherence. Within this thesis, we apply and refine the inversion method with three case studies. We study pre-and co-eruptive changes at Piton de la Fournaise volcano in La Réunion Island. The challenge here is to correctly locate forthcoming eruptions. A second data set comes from the Mw7.9 Wenchuan earthquake in China. In this study, we take the time evolution of the coda into account and jointly invert the data at different times in the coda to study changes in the crust during and after the earthquake. From the results, we can clearly see that waveform decoherence and velocity variations are not sensitive to the same physical properties. In the third study, we test the inversion procedure with a 3D sensitivity kernel to study the fracturing of concrete blocks under increased tension

APA, Harvard, Vancouver, ISO, and other styles

39

Hable, Sarah [Verfasser], and Heiner [Akademischer Betreuer] Igel. "Applications of ambient seismic noise: clock error detection and group velocity estimation in land and ocean bottom seismograms / Sarah Hable ; Betreuer: Heiner Igel." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2019. http://d-nb.info/1191691616/34.

Full text

APA, Harvard, Vancouver, ISO, and other styles

40

Ma, Ning. "On the seismic response in a large deep-seated landslide in southwest Japan-with special focus on the topographic and geological effects-." Kyoto University, 2019. http://hdl.handle.net/2433/242901.

Full text

APA, Harvard, Vancouver, ISO, and other styles

41

Furtado, José Augusto Mendes. "Confirmação do modelo da estrutura 3D do vale inferior do Tejo a partir de dados de ruído sísmico ambiente." Master's thesis, Universidade de Évora, 2010. http://hdl.handle.net/10174/19553.

Full text

Abstract:

Ao longo de sua história a região do Vale Inferior do Tejo VIT foi abalada por vários sismos consideravelmente destrutivas, tendo alguns deles produzido significativas deformações nas estruturas marítimas localizadas no litoral a sudoeste do território Português; outros, moderados, foram produzidos por fontes locais, como os de 1344, 1531 e 1909. Nos últimos anos, devido à melhoria dos modelos de estrutura 3D e o desenvolvimento dos métodos numéricos, foram elaborados vários estudos de síntese de movimento forte do solo para a região do Baixo Tejo utilizando o método de diferenças finitas. Para confirmar o modelo de velocidades desta bacia usámos medidas de ruído sísmico, aplicámos um método baseado na razão espectral H/V, e, a partir destas curvas, por inversão, obtivemos um modelo de velocidades para a região estudada. Os resultados revelam uma boa concordância entre o modelo obtido e os dados geofísicos e geológicos recolhidos na mesma área._ ABSTRACT: Along his history the Lower Tagus Valley (LTV) area was shaken by several earthquakes. The largest reported had their origin in the southwestern part of Iberia. Other moderate earthquakes were produced by local sources such as the 1344, 1531 and the 1909. ln the last years, due to 3D structural model improvement and development in numerical methods, several studies have successful obtained strong-ground motion synthesis for the LVT region using finite difference method. To confirm the velocity model of the LTV sedimentary basin obtained by geophysical and geological data, we use broad-band microtremor measurements and application of the horizontal to vertical (H/V) spectral ratio method. We have obtained a velocity model for the studied region by inversion of the H/V curve. The results have good agreement geological and geophysical data.

APA, Harvard, Vancouver, ISO, and other styles

42

Bottelin, Pierre. "Caractérisation des phases pré-et post-rupture d'éboulements rocheux de taille intermédiaire : apport des enregistrements sismiques." Thesis, Grenoble, 2014. http://www.theses.fr/2014GRENU015/document.

Full text

Abstract:

Les éboulements rocheux de volume intermédiaire (103-105 m3) posent un problème sérieux dans les régions montagneuses en raison de leur fréquence d'occurrence relativement élevée et de leur fort pouvoir destructeur. En conséquence, il est difficile de mener des travaux de protection pour réduire l'aléa, ce qui souligne le besoin de techniques de suivi et d'alerte précoce avant la rupture. Après l'éboulement, peu d'informations quantitatives sont disponibles sur la phase de propagation en raison de la soudaineté du phénomène et de sa localisation dans des pentes raides et difficiles d'accès. Dans ce travail de thèse, une approche expérimentale est proposée pour extraire des informations à partir d'enregistrements de signaux sismiques durant les phases pré- et post-rupture d'éboulements.La première partie de ce travail vise à tester la pertinence des vibrations ambiantes pour le suivi temporel de la réponse dynamique de compartiments rocheux lors de la phase pré-rupture. Cette technique (couramment employée en génie civil pour le suivi de l'intégrité des structures) permet d'extraire les fréquences de résonance d'une structure, dont la décroissance peut traduire l'endommagement. Une étude antérieure menée sur une colonne calcaire instable a montré une décroissance d'environ 30% de la fréquence fondamentale de résonance (f1) environ deux semaines avant la rupture, interprétée comme une diminution de la rigidité du contact avec le massif stable adjacent
Mid-size rockfalls (103-105 m3) represent a substantial hazard in mountainous areas, because of relative high rate of occurrence and destructive power. Consequently, few protection means can be applied, emphasizing the need for monitoring techniques and early warning prior to the collapse. After the rupture, quantitative information on the rockfall propagation phase is scarce, owing to their suddenness and location in steep and rugged slopes. In this thesis work, an experimental approach is proposed to derive valuable information from seismic records during rockfall pre-rupture and post-rupture phases. The first part of this work aims at testing the applicability of the ambient vibration technique to monitor unstable rock compartments dynamic response in the pre-rupture phase. This technique (commonly employed in civil engineering for structural health monitoring) reveals the resonant frequencies of a structure, a decrease in frequency revealing potential damage. A previous case study of an unstable limestone compartment brought to light a #30% decrease in fundamental resonant frequency (f1) two weeks before the collapse. Following this innovative work, we selected and instrumented four prone-to-fall medium-size rock compartments located in the Occidental Alps, showing various geological contexts (limestones, argillite and shale-sandstone series), deformation patterns and failure mechnisms. Ambient vibrations recorded on-site revealed caracteristic seismic noise features. Spectral peaks were observed and attributed to resonant frequencies of the rock compartments, the fundamental resonant frequency (f1) showing clear polarization parallel to the line of maximum slope gradient, and perpendicular to the main bounding fracture observed at most of the sites. Similar findings were made for an unstable rock compartment located in a volcanic caldera, characterized by rapid morphological changes and intense rockfall activity. The dynamic response of the rear fracture network was explored, showing that spectral content of seismic noise is controlled by the caldera structure in the 0.5-5 Hz range. The direction of vibration is polarized perpendicularly to the fractures, while vibration amplitudes are linked to compartment uncoupling from the rock massif. In this case, the physical origin of seismic noise amplification may be due to complex 2D or 3D resonance effects. For the four alpine sites, the fundamental frequency f1 was monitored over more than one year, showing fluctuations clearly correlated with temperature oscillations. The thermal control over f1 is highly complex, showing both positive and negative correlations, depending on site morphology and destructuration, as well as on the studied oscillations periods (daily or seasonal). No change in fundamental frequency resulting from damage was observed over this time span. One site, characterized by intense rock fracturing and a deep-open rear fracture, showed high f1 sensitivity to temperature changes. Thermo-mechanical numerical simulations revealed that both material contraction-dilation and thermal dependancy of the elastic modulus control f1 fluctuations. In addition, high amplitude seasonal f1 oscillations were explained by ice formation in the rear fracture. A criterion was developped to separate thermal-induced f1 fluctuations from damage effects, under the hypothesis that thermal sensitivity of a rock compartment increases towards failure. The second part of this work relates to the post-rupture phase of rockfalls. The seismic records generated by two mid-size rockfalls {one natural, one provoked{ that occured in the same place were analyzed, showing complex enveloppe and spectrogram features. Both events showed close magnitude, duration and spectral content. The seismic signals of the provoked event were calibrated using video shots, allowing estimation of fallen material velocity during the successive propagation phases

APA, Harvard, Vancouver, ISO, and other styles

43

Caudron, Corentin. "Multi-disciplinary continuous monitoring of Kawah Ijen volcano, East Java, Indonesia." Doctoral thesis, Universite Libre de Bruxelles, 2013. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/209416.

Full text

Abstract:

Kawah Ijen (2386 m) is a stratovolcano located within Ijen Caldera, at the easternmost

part of Java island in Indonesia. Since 2010, the volcano has been equipped with seismometers

and several sensors (temperature and level) have been immersed in its acidic lake waters and in the acidic river seeping on the volcano flanks. While finding instruments capable of resisting to such extreme conditions (pH~0) has been challenging, the coupling of lake monitoring techniques with seismic data improves the knowledge of the volcanic-hydrothermal dynamics. Moreover, the monitoring capabilities have been considerably

enhanced supporting the decision-making of the authorities in case of emergency.

Several methods and processing techniques were used to analyze the seismic data. Much effort has been given to implement the seismic velocities (Moving Window Cross Spectral Analysis (MWCSA)) calculations. At Kawah Ijen, the frequency band that is less affected by the volcanic tremor and the seasonal fluctuations at the source ranges between 0.5-1.0 Hz. Moreover, a stack of 5 days for the current CCF gives reliable results with low errors and allows to detect fluctuations which are missed using a 10-day stack.

The background seismic activity mostly consists in low frequency events and a continuous tremor of low amplitude. Fluctuations of the lake temperature and level result from the recharge of the hydrothermal system during the rainy season. Kawah Ijen lake waters are not perfectly mixed and a shallow stratification occurs during the rainy season, because meteoric waters are less dense than the lake fluids.

Different unrest occurred during our study. Some of them strongly affected the volcanic lake, while others did only weakly. In the first category, a strong unrest commenced in October 2011 with heightened VT (Volcano Tectonic) earthquakes and low frequency events activity, which culminated mid-December 2011. This unrest was correlated with an enhanced heat and hydrothermal fluids discharge to the crater and significant variations of the relative velocities (~1%). This suggests an important build-up of stress into the system. VT earthquakes opened pathways for the fluids to ascend, by increasing the permeability of the system, which latter allowed the initiation of monochromatic tremor (MT) when the steam/gases interacted with the shallow portions of the aquifer. Our calculations evidence a higher contribution of steam in March 2012 that might explain the increase of the MT frequency when bubbles were observed at the lake surface. This period was also characterized by short-lived but strong velocity variations, related to water level

rises containing important amount of bubbles, and important heat and mass discharges

into the lake. On the contrary, the second category of unrest did only slightly affect the

lake system. This could be explained by a dryer hydrothermal system and/or locations of

the seismic sources, which were not directly linked to the lake.

While a magmatic eruption will likely be preceded by a strong seismic activity, the major challenges remain to understand why the unrest we studied did not lead to an eruption and to identify precursory signs of a phreatic eruption. Even a small phreatic eruption would be devastating for the people working everyday in the crater and the ones

who live nearby the voluminous acidic lake.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

APA, Harvard, Vancouver, ISO, and other styles

44

LeticiaLi, Nancy, and 李知穎. "Taiwan seismic ambient noise levels and their implications." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/20973697951213062468.

Full text

Abstract:

碩士
國立成功大學
地球科學系碩博士班
100
To characterize the ambient noise levels in the Taiwan region and to understand its relationship with climate parameters, we used two methods to analyze the continuous waveforms recorded at Taiwan broadband seismic stations. On the basis of these results, we are able not only to evaluate the station performance and provide crucial information for site selection, but also to realize the controlling factors of noise. First, we extract the noise amplitude of 9 frequency bands in 2-18 Hz from seismic data collected at 18 BATS stations between 2006 and 2009. Then we compare these high-frequency noise amplitudes with ambient temperature, atmospheric pressure, rainfall and wind-speed, respectively. The results show that the high-frequency noise is affected by rainfall and wind-speed. However, 2 Hz noise exhibits seasonal variation for the stations in western Taiwan, which may reflect the seasonal change of wave climate. High-frequency noises in western Taiwan, Kenting and Taitung are larger than elsewhere, implying intensive human activities in these areas. Stations that are close to river are characterized by a delay of returning to background high-frequency noise level after typhoons. Secondly, we follow the method proposed by MaNamara and Buland (2004) to compute the probability density functions of power spectral density (PDFs) of ambient noise. The data is collected at 95 broadband stations in BATS, TAIGER, and CWBSN from July 2006 to June 2008. The result shows that the high-frequency noise (〉 1 Hz) is much higher than the global low noise model. But the long-period noise is within the global high and low noise models. The spatial variation shows that noise level in the plain area is 20-50 dB higher than the mountainous area, and the noise level along the western coast is 10-30 dB higher than the eastern coast. This implies that the excitation of the secondary microseism is more effective in the shallower Taiwan Strait than in the deep Pacific. Noise amplitude in 1-10 Hz presents significant diurnal and weekly variation. Significant annual variation that is associated with the wave climate can be seen at temporal variation of the 2-8 s noise amplitude. We also verify that the borehole stations present lower noise level by about 20 dB when compared with corresponding surface stations. It means that the ambient noise could be much reduced by installing borehole seismic stations and therefore enhance the ability of detecting smaller earthquakes.

APA, Harvard, Vancouver, ISO, and other styles

45

Zhan, Zhongwen. "Exploiting Seismic Waveforms of Ambient Noise and Earthquakes." Thesis, 2014. https://thesis.library.caltech.edu/7965/61/ZhongwenZhan_PhD_Thesis_FinalVersion.pdf.

Full text

Abstract:

In this thesis, I apply detailed waveform modeling to study noise correlations in different environments, and earthquake waveforms for source parameters and velocity structure.

Green's functions from ambient noise correlations have primarily been used for travel-time measurement. In Part I of this thesis, by detailed waveform modeling of noise correlation functions, I retrieve both surface waves and crustal body waves from noise, and use them in improving earthquake centroid locations and regional crustal structures. I also present examples in which the noise correlations do not yield Green's functions, yet the results are still interesting and useful after case-by-case analyses, including non-uniform distribution of noise sources, spurious velocity changes, and noise correlations on the Amery Ice Shelf.

In Part II of this thesis, I study teleseismic body waves of earthquakes for source parameters or near-source structure. With the dense modern global network and improved methodologies, I obtain high-resolution earthquake locations, focal mechanisms and rupture processes, which provide critical insights to earthquake faulting processes in shallow and deep parts of subduction zones. Waveform modeling of relatively simple subduction zone events also displays new constraints on the structure of subducted slabs.

In summary, behind my approaches to the relatively independent problems, the philosophy is to bring observational insights from seismic waveforms in critical and simple ways.

APA, Harvard, Vancouver, ISO, and other styles

46

Chien, Chih-Chieh, and 簡志傑. "Shallow Seismic Structure of Taiwan Constrained by Ambient Seismic Noises." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/h83dze.

Full text

Abstract:

碩士
國立臺灣大學
地質科學研究所
105
In this study, we aim to better constrain the shallow seismic structure (<10 km) of Taiwan by utilizing the short period (3-7 seconds) empirical Green’s functions (EGFs) of Rayleigh waves extracted from ambient seismic noise. To improve the data coverage, continuous data recorded by 6 seismic networks during the time period from 2006 to 2007 are used to derive the EGFs between all the cross-network station pairs. With this unprecedented data set, we are able to build up high resolution 3-D Vs and Vs azimuthal anisotropy models for the shallow crust of Taiwan, using a wavelet-based multi-scale inversion technique. Robust features both in Vs and Vs anisotropy variations of the shallow crust are obtained in the resulting 3-D models. The strong Vs lateral variations are closely related to the surface geology, and the patterns of Vs anisotropy are mostly dominated by structure-related anisotropy, with fast polarization direction generally parallel to the mountain ranges. With the new results, and our recent studies with broad-band EGFs and borehole noise interferometry, we conclude that the stress-aligned anisotropy is likely confined in the near-surface depths (< ~1 km).

APA, Harvard, Vancouver, ISO, and other styles

47

You, Shuei-Huei, and 尤水輝. "Ambient Seismic Noise Rayleigh Wave Tomography across Northern Taiwan." Thesis, 2008. http://ndltd.ncl.edu.tw/handle/69826042341885294604.

Full text

Abstract:

碩士
國立臺灣大學
地質科學研究所
96
We apply the ambient noise cross-correlation technique to the vertical component of the continuous short-period waveforms recorded by three seismic arrays, including Tatun Volcanic Area (TVA) array, Hsinchu array and stations in the northern portion of Central Weather Bureau Seismic Network (CWBSN). The noise-derived Green’s functions are treated as fundamental mode Rayleigh waves. With adequate data selection and quality control, the group velocity of each traces is analyzed, and the qualified results at about 0.35Hz are used to invert for a tomographic map of short period Rayleigh wave group velocity in northern Taiwan with multi-scale parameterization. The main feature of the tomographic result shows very good fit to the surface geological units. In addition to tomographic study, we demonstrate how the retrieved surface waves are affected by various factors, such as seasonal change, site location, and path azimuth. In particular, we notice an evident path-azimuth-dependent asymmetry between causal and acausal signals in most paths. The asymmetric properties imply the correlated ambient noise in northern Taiwan is mainly contributed from the north. Two unexpected while useful by-products are observed, the waveform polarity reversal and time shift of the stations, both present typical features on the retrieved surface waves. Thus, the ambient noise cross-correlation technique provides an alternative method to examine the station condition and secure the data quality from instrument errors.

APA, Harvard, Vancouver, ISO, and other styles

48

Müller, Jozef. "Studium horninových struktur v Českém ráji pomocí seismického šumu." Master's thesis, 2021. http://www.nusl.cz/ntk/nusl-446555.

Full text

Abstract:

We carry out non-invasive ambient noise investigation of rock structures in Bohemian Paradise (Bohemian Cretaceous Basin, Czech Republic). The study is focused on two key topics: 1) An in-situ elastic moduli estimate of competent, horizontally deposited sandstone layers. This is done by performing an ambient noise array measurement. The recording is processed with f-k array analysis, from which frequency-dependent Love and Rayleigh wave dispersion curves as well as the Rayleigh wave ellipticity are retrieved. The data are inverted for P- and S-wave velocity profiles, from which the Young's and shear modulus are successfully estimated. 2) Study of local response of Kapelník rock tower. We analyse a dataset of ambient noise recordings from the top of the tower and from its foot. Information about tower oscillation frequencies and directions, together with amplification ratios, are retrieved from particle motion polarisation analysis and from site-to-reference spectral ratios. The Euler-Bernoulli beam theory is finally used to interpret the measured data using the elastic moduli estimated from the noise array measurement. Keywords: Bohemian Paradise, rock tower, seismic ambient noise, seismic surface waves

APA, Harvard, Vancouver, ISO, and other styles

49

Kuponiyi, Ayodeji Paul. "Imaging major Canadian sedimentary basins and their adjacent structures using ambient seismic noise (and other applications of seismic noise)." Thesis, 2021. http://hdl.handle.net/1828/12947.

Full text

Abstract:

Over a decade ago, it was discovered that the earth’s natural seismic wavefields, propagating as seismic noise, can be processed using correlation methods to produce surface waves, similar to those generated by earthquakes. This discovery represents a paradigm shift in seismology and has led to several tomographic studies of earth structures, at different scales and resolutions, in previously difficult-to-study areas around the world. This PhD dissertation presents research results on multi-scale and multi-purpose applications of ambient seismic noise wavefields under three topics: (1) Imaging of sedimentary basins and sub-basin structures in eastern and western Canada using ambient seismic noise, (2) Combining measurements from ambient seismic noise with earthquake datasets for imaging crustal and mantle structures, and (3) Temporal variation in cultural seismic noise and noise correlation functions (NCFs) during the COVID-19 lockdown in Canada. The first topic involved imaging the sedimentary basins in eastern and western Canada using shear wave velocities derived from ambient noise group velocities. The results show that the basins are characterized by varying depths, with maximums along the studied cross-sections in excess of 10 km, in eastern and western Canada. Characteristics of accreted terranes in eastern and western Canada are also revealed in the results. A seismically distinct basement is imaged in eastern Canada and is interpreted to be a vestige of the western African crust trapped beneath eastern Canada at the opening of the Atlantic Ocean. In western Canada, the 3D variation of the Moho and sedimentary basin depths is imaged. The thickest sediments in eastern Canada are found beneath the Queen Charlotte, Williston and the Alberta Deep basins, while the Moho is the deepest beneath the Williston basin and parts of Alberta basin and northern British Columbia. For the second topic, I worked on improving the seismological methodology to construct broadband (period from 2 to 220 s) dispersion curves by combining the dispersion measurements derived from ambient seismic noise with those from earthquakes. The broadband dispersion curves allow for imaging earth structures spanning the shallow crust to the upper mantle. For the third topic, I used ambient seismic data from the earlier stages of the COVID-19 pandemic to study the temporal variation of seismic power spectra and the potential impacts of COVID-19 lockdown on ambient NCFs in four cities in eastern and western Canada. The results show mean seismic power drops of 24% and 17% during the lockdown in eastern Canada, near Montreal and Ottawa respectively and reductions of 27% and 17% near Victoria and Sidney respectively. NCF signal quality within the secondary microseism band reached maximum before the lockdown, minimum during lockdown and at intermediate levels during the gradual reopening phase for the western Canada station pair.
Graduate

APA, Harvard, Vancouver, ISO, and other styles

50

Huang, Yu-chih, and 黃有志. "Multi-scale ambient seismic noise study in the Taiwan region." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/77148208506562709533.

Full text

Abstract:

博士
國立中央大學
地球物理研究所
99
Over the past 5 years, it has been demonstrated that the Time Domain Empirical Green’s Function (TDEGF) from ambient seismic noise cross-correlation can be used to investigate crustal and uppermost mantle velocity structure from many studies around the world. To decipher subsurface structures in various scales, researchers can utilize some existing continuous-recording seismic stations and/or deploy a new dense receiver array in the study region. In this thesis, we perform tomographic applications of ambient seismic noise analysis in the Taipei Basin, Taiwan Strait, and Taiwan Region for three arrays with very different scales. The Taipei Basin is a high-level artificial noise metropolis and requires detailed shallow crustal structure. The high levels of ambient seismic noise and the low levels of regional seismicity of this region complicate investigations of crustal structure with traditional seismic exploration or earthquake tomography methods. Analysis of the TDEGF amplitudes suggests that the dominant sources of ambient seismic noise are generated from the coastlines and shallow continental shelf of the Taiwan Strait, northwest of the study region. The ambient seismic noise tomography is feasible at periods 0.5-3 s, which is much shorter than most other studies. The lateral variation in Rayleigh wave phase velocities correlates well with surface geology and suggest that faults play an important role in the regional tectonic setting. High phase velocities mark the Tatun volcanic area, the Kuanyin Mountain dominated by Quaternary igneous rock, and the Miocene Western Foothills south of the Taipei Fault. Low phase velocities are along western and southeastern edges of the Taipei basin and the Pleistocene Linkou Tableland. Main faults in the region are either marked by low phase velocities or define transitions between regions of high and low velocity anomalies. The Taiwan Region is located at a complex convergent plate boundary zone where the Philippine Sea plate interacts with the Eurasian plate. As a result, the lateral velocity variations show dramatic patterns among different geologic provinces. Rather than only focusing on the Taiwan Island as most previous studies, this thesis includes broader regions also with the Taiwan Strait and the eastern sea area. The 5-120 s phase velocity maps are constructed from analysis of ambient seismic noise and teleseismic Rayleigh waves. At 5-12 s, phase velocity distribution can compare well with surface geology. At 16-19 s, there is a saxophone shape low velocity zone beneath the Taiwan Island. At 20-24 s, phase velocity patterns beneath the Taiwan Island are almost contrary to 5-12 s. At 30-50 s, the Taiwan Island is a high velocity zone surrounded by low velocity zone. At 60-120 s, the Taiwan Strait shows low velocity and east of it shows high velocity. The Taiwan Strait is located on the passive margin of collision zone and is also adjacent to South China Sea opening to the southwest. Despite that in the Taiwan Strait the tectonic structure is assumed relatively simple and seismicity is much lower than the Taiwan Island, intraplate earthquakes sometimes occurred but the mechanisms are not well constrained. Limited studies have focused on this region before and most previous ambient seismic noise studies focused on continental regions rather than across a strait. The data are from 50 broadband stations on both sides of the Taiwan Strait, in mainland China and the Taiwan region. The preliminary 5-30 s Rayleigh wave phase velocity dispersion curves show high velocities along mainland China coastlines and southern part of the Taiwan Strait while low velocities appear in the central and northern parts of the Taiwan Strait.

APA, Harvard, Vancouver, ISO, and other styles

Dissertations / Theses on the topic 'Seismic ambiant noise'

Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles