Dissertations / Theses on the topic 'Wide Angle Reflection/Refraction'

The Ontario-New York-New England seismic refraction/wide-angle reflection profile was acquired to investigate the deep structural interrelationships between the southeastern Grenville province and the western New England Appalachians. The Grenville province is characterized by 45 km thick crust, with an average crustal seismic velocity of 6.6 km/s and a Poisson's ratio of 0.28+0.01. In the mid-crust a laminated dome-like body is inferred to be composed of mafic cumulate sills on the basis of its high velocity (7.1 km/s) and Poisson's ratio (0.27). The lower crust is characterized by a velocity of 7.0 km/s which suggests a strongly mafic composition such as garnet pyroxene granulite. The Moho is a variable feature, characterized by en-echelon reflections suggestive of compositional interlayering. An anomalous mantle layer with a velocity of 8.6 km/s is proposed to represent an eclogized basaltic layer added to the lithosphere during Grenvillian orogenesis. The boundary between the Grenvillian craton and the western New England Appalachians is marked by an eastward dipping ramp structure which penetrates to a depth of 25 km where it soles out above a transitional mid-lower crustal interface. The New England Appalachians are characterized by an average crustal velocity of 6.4 km/s and a sharply reflective Moho delineating crustal thinning from 41 km to 37 km towards the Atlantic margin. The lower crustal velocity is 6.8 km/s, with a Poisson's ratio of 0.26+0.01. In contrast to the Grenvillian craton the seismic properties of the Appalachian lower crust are consistent with an intermediate composition interlaced with mafic sills related to extensional underplating and intrusion during the rifting of the Atlantic Ocean.

This dissertation is devoted to seismic tomography. I have implemented a new modelling tool for 3-D joint refraction and reflection travel-time tomography of wide-angle seismic data (TOMO3D). The reason behind this central objective is the evidence that the information based on 2-D seismic data does not allow to capture the structural complexity of many 3-D targets, and in particular that of the seismogenic zone in subduction margins. The scientific rationale for this statement, which justifies the central part of my thesis work, is based on the analysis of 2-D models obtained in the convergent margin of Nicaragua, a seismically active area where a textbook example of tsunami earthquake took place in 1992. In this application I modelled two perpendicular wide-angle seismic profiles for the characterisation of the overriding plate and the interplate fault. To do this, I applied TOMO2D, a state-of-the-art joint refraction and reflection 2-D travel-time tomography code. The inversion outcomes are two 2-D velocity models along both profiles, together with the 1-D geometry of the interplate boundary. In combination with other geophysical data measurements, namely coincident multichannel seismic profiles and gravity data, these models provide new constraints on the nature and structure of the margin, and in particular add new insights on the nucleation and propagation of the said earthquake and its tsunamigenic behaviour. Ultimately, this case study evidences the aforementioned limitations of 2-D modelling in the investigation of 3-D geological structures and phenomena. Following from this first application and with the idea of increasing the amount of data used in travel-time tomography, I focused on an a priori paradoxical phenomenon related to water-layer multiple phases, that under certain circumstances, is observed on wide-angle record sections. The interest of this study lies in the fact that this phenomenon can provide additional constraints on travel-time tomography models. First, I propose and corroborate the hypothesis explaining the apparent paradox, and then derive the most favourable geological conditions for the phenomenon to occur. Subsequently, the possibility to model this multiple-like phases is introduced in TOMO3D. The development of TOMO3D, which constitutes the core of my work, is founded on TOMO2D, from which it inherits the numerical methods for solving the forward and inverse problems. Source files have been rewritten, redefining and introducing the necessary variables and functions to handle 3-D data inversion. The tests made with the sequential version of the code emphasise the need of parallelisation for practicality reasons. Indeed, the increasing size of data sets along with the modelling of the additional spatial dimension results in computationally demanding inversions. Hence, I parallelised the forward modelling part of the code, which takes up to 90% of the computing time, with a combination of multiprocessing and message-passing interface extensions. Subsequently, the parallel version of TOMO3D is applied to a complex synthetic case simulating a subduction zone. This first 3-D application serves to evaluate the correctness of the code's programming, and as step-by-step description of the modelling procedure, with particular attention on the layer-stripping strategy used to successively model several reflectors. The outcomes demonstrate the ability of the code and the chosen inversion strategy to accurately recover the velocity distribution and the geometry of the two reflectors. Finally, TOMO3D is applied to a real 3-D wide-angle seismic data set acquired at the Pacific margin of Ecuador and Colombia to extract a 3-D velocity model of the overriding and incoming plates, which is then compared to previous results obtained with an extensively tested and used 3-D refraction travel-time tomography code (FAST). The comparison indicates that TOMO3D is more accurate than FAST but at the same time it is computationally more demanding. However, the parallelisation of TOMO3D allows using high-performance computing facilities, which is not the case of FAST or most of the existing codes.
Aquesta tesi està dedicada a la tomografia sísmica. Concretament, he implementat una eina de modelització 3D per a la tomografia conjunta de temps de trajecte de refraccions i reflexions (TOMO3D). La raó darrere d'aquest objectiu és l'evidència de que la informació basada en dades sísmiques 2D no permet copsar la complexitat de gran part dels cossos geològics, i en particular de la zona sismogènica en marges de subducció. El desenvolupament del TOMO3D es basa en el TOMO2D, un codi d'avantguarda per a la tomografia conjunta de refraccions i reflexions en 2D. Els arxius de codi han estat reescrits, redefinint i introduint les funcions necessàries per dur a terme la inversió 3D. Els testos fets amb la versió seqüencial del codi posen de manifest la necessitat de paral·lelització ja que l'increment de la mida dels conjunts de dades així com la modelització de la dimensió espacial afegida fan que les inversions siguin computacionalment exigents. La versió paral·lelitzada del TOMO3D ha sigut aplicada a un cas sintètic complex que simula una zona de subducció. Aquesta primera aplicació 3D serveix per avaluar la correcció de la programació del codi, i com a descripció pas a pas del procediment de modelització. Els resultats demostren la capacitat del codi per recuperar acuradament la distribució de velocitat i la geometria dels dos reflectors. Finalment, el TOMO3D és aplicat a un conjunt 3D de dades de sísmica de gran angle adquirit al marge pacífic d'Equador i Colòmbia per extreure'n un model 3D de la velocitat de les plaques cavalcant i subduïda, que és comparat amb el resultat obtingut amb un codi 3D de tomografia de temps de trajecte de refraccions (FAST). La comparació indica que el TOMO3D és més acurat que el FAST però al mateix temps és computacionalment més exigent. Tot i així, la paral·lelització del TOMO3D permet utilitzar plataformes de supercomputació, a diferència del que passa amb el FAST i la majoria de codis existents.

2008/2009
Gli OBS (Ocean Bottom Seismometer) sono ampiamente utilizzati negli studi sismici crostali attraverso l’utilizzo delle registrazioni di onde rifratte e riflesse a grande offset (Wide Angle Reflection/Refraction, WAR/R). L'analisi di questi dati comporta, solitamente, l'impiego di modellistica diretta e/o inversa al fine di giungere alle velocità dei singoli strati e alle profondità delle interfacce. In questa tesi, invece, si presenta un approccio diverso che prevede: 1) costruzione di modelli di velocità dall’analisi dei segnali rifratti; 2) elaborazione dei dati OBS al fine di ottenere immagini sismiche a riflessione. In particolare, gli OBS e gli scoppi sono stati riportati ad uno stesso datum (superficie del mare), utilizzando il Wave Equation Datuming (WED). Dopo il WED è stato possibile applicare l'elaborazione tipica della sismica a riflessione. I profili OBS analizzati sono stati acquisiti nell'area dell'Arco Ellenico e, i risultati ottenuti, hanno permesso di riconoscere le strutture geologiche principali e i meccanismi dell’evoluzione tettonica. L'utilizzo del WED ha migliorato la risoluzione delle sezioni finali rispetto a quanto ottenibile con le correzioni statiche classiche. Per confronto è stata applicata la metodologia WED anche a dati simici ad alta risoluzione a terra, risolvendo problemi di correzioni statiche e di strutturazioni tettoniche apparenti nell'area della Bassa Pianura Friulana.
XXII Ciclo
1979

The Chesapeake Bay impact structure is one of the largest and most well preserved impact structures on Earth. It has a unique morphology composed of an inner crater penetrating crystalline basement surrounded by a wider crater in the overlying sediments. In 2004, the U.S. Geological Survey conducted a seismic survey with the goals of constraining crater structure and in support of the drilling of a borehole into the deepest part of the crater. Travel-time and waveform inversion were applied to the data to produce a high-resolution velocity model of the crater. Low-fold reflection processing was also applied. Northeast of the crystalline crater, undeformed, eastward-sloping crystalline basement is ~1.5 km deep. The edge of the inner crater is at ~ 15 km radius and slopes gradually down to a depth of 1.5 - 1.8 km. A central peak of 4-5 km radius rises to a depth of ~0.8 km. Basement velocity in the crystalline crater is much lower than undeformed basement, which suggests ~10% fracturing of the crater floor, and up to 20% fracturing of the central uplift. A basement uplift and lateral change of velocity, interpreted as the edge of the transient crater, occurs at a radius of ~ 11 km. Assuming a 22 km diameter transient crater, scaling laws predict a ~30 km diameter crater and central peak diameter of 8-10 km. This indicates that post-impact collapse processes that created the ~ 30 km diameter crystalline crater were unaffected by the much weaker rheology of the overlying sediments.
Master of Science

High quality, coincident near-vertical incidence (NVI) and refraction/wide-angle seismic reflection data (R/WAR) acquired along a profile in the Northwest Territories are used to study the nature of the Moho and subcrustal reflectors. First, we re-examine distinct subhorizontal reflections on NVI data in the uppermost mantle that were interpreted previously as a separate feature from a relict subducted slab. Using forward and inverse traveltime modeling of both data sets along the crooked line, we investigate the origin of the reflections. Our results demonstrate that the subhorizontal reflectors are the continuation of the relict subducted slab, which extends laterally for 300 km at depths from 35 to 90 km. Its base is the source of the R/WAR reflections. The apparent flattening is an artifact of projecting a 3-D geometry onto a 2-D cross section. The shallowly subducted slab probably contributed to the thickening and stabilization of the sub-crustal lithosphere in the region. Second, we examine the detailed structure of the Moho and propose a possible scenario for its formation and evolution. Strong Moho reflections are observed on the NVI data (shot gathers and stacked section). The WA data are characterized by a ~0.5 s coda trailing the PmP (Moho) phase. For analysis of these observed data, we follow two approaches, forward and inverse modeling. In forward modeling, we calculate wide-angle and near-vertical synthetic seismograms using 1- and 2-D wave propagation algorithms. Comparison between synthetic and observed data for shot gathers was made possible through development of a novel noise-removal technique using the curvelet transform. For the inverse method, we use a statistical analysis approach based on the von Karman autocorrelation function. Our results indicate that the Moho is a finite-thickness (~3 km), heterogeneous transition zone. The heterogeneities can be described by laterally discontinuous layering, lamellae structure with randomly distributed ellipses or a von Karman distribution with a lateral correlation length of 936 m. The transition zone separates the lower crust with a lateral correlation length of 732 m from the uppermost mantle with a correlation length of 261 m. The Moho is interpreted as a thermal/metamorphic front, a regional décollement, or both.

La marge NE Américaine est une des marges les mieux étudiées au monde, elle a fait l’objet de plusieurs études géophysiques. En comparaison, la marge africaine reste peu étudiée car uniquement deux campagnes océanographiques y ont été menées : la campagne Sismar (2001) au large de la Meseta et la campagne Dakhla (2002) au large du Sahara. La structure profonde de la marge canadienne est connue grâce aux profils de sismique grand-angle SMART-1, 2 et 3. Le premier objectif du projet MIRROR était d’acquérir des profils combinant sismique grand-angle et sismique réflexion sur un segment homologue au profil SMART-1. La comparaison entre les segments homologues de ces deux marges ayant pour but de mieux comprendre le mode d’ouverture de l'océan Atlantique Central. Une comparaison entre les modèles Sismar, Dakhla et Mirror montre que la croûte continentale est plus épaisse au nord et s'amincit vers le sud. La largeur de la zone de transition est plus étroite au sud et les profils Sismar sont localisés sur un bassin sédimentaire posé sur une croûte continentale très amincie. La comparaison avec la marge homologue montre que l'épaisseur, la structure de la croûte continentale et la zone d'amincissement sont très semblables. Par contre, il existe une zone de manteau exhumé et serpentinisé sur le profil Canadien qui n'a pas d’homologue sur la marge africaine. De plus, l'épaisseur de la croûte océanique est différente avec 8 km sur la côte africaine et seulement 3-4 km sur la marge canadienne. Plusieurs hypothèses ont été proposées pour expliquer cette différence (a) une différence d’âge entre les deux croûtes (b) un épaississement lié au passage du point chaud des Canaries (c) une accrétion asymétrique
The NE American margin represents one of the best studied margins in the world, it was the subject of several scientific programs. In comparison, the conjugate NW African margin remains fairly unknown, only two deep seismic cruises were acquired: the SISMAR cruise (2001) offshore the Meseta and the DAKHLA cruise (2002) offshore the Sahara. The deep structure of the Canadian margin is known due to the SMART wide-angle seismic profiles 1, 2 and 3. The first objective of the MIRROR project was to acquire combined wide-angle and deep reflection seismic data offshore a segment conjugate to the SMART-1 profile. The comparison between the homologous segments of these two margins aimed to better understand the opening mechanism of the Central Atlantic Ocean. A comparison between Sismar, Dakhla and Mirror models shows that the continental crust is thicker in the north and thins toward the south. The width of the transition zone is narrower south and Sismar profiles are located on a sedimentary basin placed on a very thinned continental crust. Comparing the Mirror profile with that of the Canadian conjugate margin (Smart 1) shows that the thickness, the structure of the continental crust and the thinning is very similar. However, zones of exhumed and serpentinized mantle were imaged along the Canadian profile that have no conjugate on the African margin. Moreover, the thickness of the oceanic crust is variable with 8 km on the African side and only 3-4 km on the Canadian margin. Several hypotheses have been proposed to explain this difference (a) an age difference between the two types of crust (b) thickening associated with the passage of the Canary hotspot (c) an asymmetric accretion or (d) an accretion at slow to ultra-slow speading centers

Dans ce travail de thèse, nous analysons la structure crustale de la marge est-algérienne et du bassin adjacent (région d’Annaba), à partir d’un ensemble de nouvelles données acquises durant la Campagne SPIRAL’2009 incluant un profil sismique terre-mer de ~240 km de long, des lignes sismiques réflexion pénétrante 360-traces, et des profils gravimétriques et magnétiques. Nous avons par ailleurs disposé pour cette étude de données complémentaires incluant notamment un ensemble de profils de sismique réflexion offrant des résolutions complémentaires. La structure crustale ainsi établie nous permet de discuter les nombreux modèles cinématiques d’ouverture du bassin est-algérien proposés dans la littérature, afin de caler dans le temps la formation du bassin par rapport à la collision. Elle permet également de discuter la localisation de la déformation liée à la réactivation de la marge, par rapport aux grands domaines lithosphériques du système marge-bassin, afin de mieux comprendre les modalités de l’inversion. Dans le bassin profond, la modélisation directe des temps d’arrivée et des amplitudes des ondes réfractées et réfléchies met en évidence une croûte océanique anormalement mince de 5-5.5 km d’épaisseur, composée de deux couches. La première, de 2.2 km d’épaisseur, montre des vitesses comprises entre 4.8 à 6.0 km/s impliquant un fort gradient; la seconde de 3.3 km d’épaisseur, présente des vitesses comprises entre 6.0 à 7.1 km/s et un plus faible gradient de vitesse. La modélisation des temps d’arrivées des ondes S fourni pour cette couche un coefficient de Poisson de 0.28, indiquant qu’elle est majoritairement constituée de gabbros
In this study, we determine the deep structure of the eastern Algerian basin and its southern margin in the Annaba region (easternmost Algeria), to better constrain the plate kinematic reconstruction in this region. This study is based on new geophysical data collected during the SPIRAL cruise in 2009 that included a wide-angle, 240-km-long, onshore-offshore seismic profile, multichannel seismic reflection lines, and gravity and magnetic data, which was complemented by the available geophysical data for the study area. The analysis and modeling of the wide-angle seismic data using travel-times and amplitudes, and integrated with the multichannel seismic lines, reveal the detailed structure of an ocean-to-continent transition. In the deep basin, there is an ~5.5-km-thick oceanic crust that is composed of two layers. The upper layer of the crust is defined by a high velocity gradient and P-wave velocities between 4.8 km/s and 6.0 km/s from the top to the bottom. The lower crust is defined by a lower velocity gradient and P-wave velocity between 6.0 km/s and 7.1 km/s. The Poisson ratio in the lower crust deduced from S-wave modeling is 0.28, which indicates that the lower crust is composed mainly of gabbros. Below the continental edge, a typical continental crust with P-wave velocities between 5.2 km/s and 7.0 km/s from the top to the bottom shows a gradual seaward thinning of ~15 km over an ~35-km distance

Dans les zones de subduction, la géométrie du slab est l'un des paramètres qui contrôle les déformations tectoniques de la marge, le couplage et le potentiel sismogénique. La subduction des Antilles résulte du chevauchement de la plaque Caraïbe vers le NE sur les plaques Amériques, à une vitesse de convergence de 2 cm/an. La sismicité est hétérogène et augmente localement sous la plateforme des Iles Vierges. D’une obliquité croissante vers le nord, cette zone est favorable à un partitionnement dont les déformations sont très peu observées. Ces travaux ont permis de confirmer un substratum d’une affinité de croûte océanique épaissie par un panache mantellique et faiblement épaissi par le magmatisme formant les arcs volcaniques Deux structures accommodant le partitionnement de la déformation ont été mis en évidence. La faille de Bunce est une faille décrochante sénestre de 850 km s’étendant d’Hispaniola jusqu’à Barbuda découplant le prisme d’accrétion du substratum. Le Passage d’Anegada, dont nous avons découvert son extension vers le NE, entaille profondément l’avant-arc. Formé par extension due à la collision du Banc des Bahamas sur le nord de la marge caribéenne, ces structures sont réactivées en décrochement sénestre compatible avec du partitionnement. Les caractéristiques d’une dorsale lente (core complexe et grain océanique) entrant en subduction sont imagées pour la première fois. Elles affectent probablement la morphologie du prisme d’accrétion et le potentiel sismogène. Là où les séismes et les déformations de partitionnement (Passage d’Anegada) s'accentuent localement, le slab est moins profond. Ainsi, le couplage interplaque pourrait augmenter localement et favoriser l'activité sismique et le partitionnement tectonique sous le Passage d'Anegada
In subduction zones, the 3D geometry of the plate interface is one of the key parameters that controls margin tectonic deformation, interplate coupling and seismogenic behavior. The North American plate subducts below the Caribbean plate with a convergence rate of 20 mm/y. The seismic activity is heterogeneous and increases locally under the Virgin Islands platform. The northward increasing convergence obliquity is favorable in partitioning which deformations were not really observed. This PhD confirms that the forearc crust is a crust of oceanic affinity thickened by hotspot magmatism and poorly affected by subduction magmatism. Two structures accommodating the partitioning of the deformation were identified. The Bunce Fault is a 850-kmlength sinistral strike-slip fault extending from Hispaniola to the east of Barbuda decoupling the accretionary prism from the Caribbean substratum. The Anegada Passage, whose extension towards the NE is highlighted, entailed deeply the forearc. The structures are reactivated in sinistral strike-slip faults compatible with the partitioning of the deformation after formation in extension due to the collision of the Bahamas Bank with the northern margin. We image for the first time the characteristics of a slow ridge formation (partly complex core and partly oceanic grain) entering in subduction. It affects the morphology of the accretion prism and probably the seismogenic potential of the subduction interface. We have highlighted a shallower slab which is located under the NE Anegada Passage and where earthquakes and partitioning deformations increase locally. Thus, the shallowing slab might results in a local greater interplate coupling favoring seismic activity and tectonic partitioning beneath the Anegada Passage

L'inversion des marges passives apparaît comme le premier stade vers l'initiation de nouvelles zones de subduction. Cette étape cruciale dans la tectonique des plaques soulève néanmoins encore de nombreuses questions. L'étude des marges actuellement réactivées en compression apparaît ainsi comme essentielle pour mieux comprendre ce processus. Ces marges sont peu nombreuses, situées dans des contextes géodynamiques variés, et les facteurs déterminant leur évolution mal contraints. Située au nord de l'Afrique, la marge algérienne fait partie de ces rares exemples potentiels à travers le monde. L'évolution de cette marge formée au Miocène en contexte d'arrière-arc s'intègre dans le puzzle complexe de l'histoire de la Méditerranée occidentale. Elle est depuis quelques millions d'années réactivée en compression dans le cadre de la convergence lente entre les plaques européenne et africaine, générant un potentiel sismogène fort au nord de l'Algérie. La relative jeunesse du bassin algérien, la charge sédimentaire, les forces aux limites compressives, constituent des conditions favorables à la formation d'une future subduction. A la suite des travaux menés depuis une dizaine d'années, les principales lacunes de connaissances identifiées portent sur (1) la structuration profonde du bassin algérien et de sa marge sud (type de marge, nature du socle,dimension et nature de la transition océan-continent, style et distribution de la déformation compressive), et (2) l'histoire de l'évolution cinématique et géodynamique du bassin, ce qui limite à l'heure actuelle une analyse approfondie des modalités d'inversion de cette marge. L'étude menée se focalise sur la marge centre-algérienne, dans le secteur de Tipaza (à l'ouest d'Alger), un endroit clé pour la compréhension des mécanismes d'ouverture du bassin algérien. Le traitement et l'analyse de nouvelles données de sismique profonde grand-angle et multitraces acquises dans le cadre du projet franco-algérien SPIRAL (Sismique Profonde et Investigations Régionales en Algérie, 2009) ont notamment permis de déterminer la structure crustale du bassin algérien et de sa marge sud, ainsi que la structuration pseudo-3D d'une structure spécifique au secteur d'étude constituée par le haut topographique sous-marin de Khayr-al-Din. L'analyse de la structure profonde de la marge indique un certain nombre de structures héritées de son évolution complexe : (1) une croûte de nature continentale de plus de 15 km d'épaisseur sur le haut de marge (banc de Khayr-al-Din), (2)une croûte fine de nature océanique de 5-6 km d'épaisseur dans le bassin incluant des vitesses légèrement élevées à sa base (7,2 km/s - 7,3 km/s), (3) des similitudes avec des marges formées dans des contextes de déformation transformante, (4) un approfondissement progressif de l'ensemble de la pile sédimentaire et l'épaississement des sédiments Plio-Quaternaire, depuis le bassin profond distal vers le pied de marge,coïncidant avec (5) une flexuration à grande longueur d'onde du socle. Les résultats obtenus apportent de nouvelles contraintes sur (1) la géométrie et la nature de la marge et du bassin, (2) l'évolution de la marge,suggérant une histoire multiphasée comprenant un stade de rifting et/ou d'accrétion océanique, suivi d'un épisode de déformation coulissante tardive liée à la migration du bloc Alboran vers l'ouest, et d'une reprise en compression distribuée du bassin profond au haut de la marge au Plio-Quaternaire; (3) les modalités de réactivation qui se traduisent par des chevauchements aveugles néoformés à pendages sud, notamment au pied du banc de Khayr-al-Din, suggérant un soulèvement du banc de 0,2 mm/an à 0,75 mm/an au Plio-Quaternaire et un début d'écaillage crustal
The inversion of passive margins appears to be one of the first steps towards the initiation of new subduction zones. This crucial step in plate tectonics nevertheless still raises many questions. The study of margins currently reactivated by compressional tectonics is thus essential to better understand this process. These margins are uncommon, located in different geodynamic settings, and the factors determining their evolution are poorly constrained. The Algerian margin, located in North Africa, is one of handful of modern examples worldwide. The evolution of this margin, rifted during the Miocene, in a back-arc setting, is part ofthe complex puzzle of the western Mediterranean. Since a few million years, the margin has suffered inversion and compression in the framework of slow on going convergence between the European and African plates. This convergence generates moderate to strong earthquakes in North Algeria. The relatively young age of the Algerian basin, the large sediment load, and the compressive forces, constitute favorable conditions to the formation of a future subduction zone. Studies from the past ten years indicate, that themain unresolved questions are related to (1) the deep structure of the Algerian basin and its southern margin (the type of margin, the nature of the basement, the dimension and nature of the ocean-continent transition, the style and the distribution of the compressional deformation), and (2) the history of the kinematic and geodynamic evolution of the basin. All of these unknowns have prevented a complete and thorough analysis of modalities of the Algerian margin inversion. This study focuses on the Central Algerian margin, in the area of Tipaza (West of Algiers), a key region to understand the mechanism of the opening of the Algerian basin. Processing and analysis of a deep wide-angle and multichannel seismic new data set acquired in the context of the French-Algerian project SPIRAL (Sismique profonde et Investigation Régionales en Algérie, 2009)have enabled us to determine the crustal structure of the Algerian basin and its southern continental margin,as well as the pseudo-3D structure of a specific feature in the study area: the submarine topographic highformed by the Khayr-al-Din bank. The analysis of the deep structure of the margin reveals features inherited from its complex evolution: (1) a crust of continental nature of more than 15 km thick at the upper margin(Khayr-al-Din Bank), (2) a thin crust of oceanic nature, 5-6 thick in the deep basin, including slightly high velocities at its base (7.2 km/s - 7.3 km/s), (3) similarities with margins formed in context of transform deformation, (4) a progressive deepening of the whole sedimentary cover and the thickening of the Plio-Quaternary sediments, from the distal deep basin towards the margin foot, coeval with (5) a long wavelengthflexuration of the basement in the basin. Results from this study provide new constraints on (1) the geometryand nature of the margin and the basin, (2) the evolution of the margin, suggesting a multiphased history including a stage of rifting and/or oceanic spreading, a transcurrent episode due to the westward migration of the Alboran block, and a diffuse Plio-Quaternary compressional reactivation distributed from the deep basinto the upper margin; (3) the mechanisms of the reactivation marked by newly formed south-dipping blind-thrusts, especially at the foot of the Khayr-al-Din bank, and suggesting a Plio-Quaternary uplift of the bankof 0.2 mm/y to 0.75 mm/y and the early stages of imbricate thrusting of crustal scales

Cette thèse a été conduite dans le cadre du programme de coopération de recherche Algéro-française SPIRAL (Sismique Profonde et Investigations Régionales du Nord de l’Algérie). Ce projet vise à étudier la structure profonde de la marge algérienne par une approche combinée des techniques sismiques ; grand-angle et multi-canal. Le domaine couvert par la présente étude se concentre dans la région de Jijel dans la marge algérienne orientale. L’objectif principal de notre thèse est d'améliorer en profondeur l'imagerie de la marge algérienne en utilisant une combinaison de données sismiques grand-angle (OBS, sismomètres de fond de l'océan) et multi-canal (MCS). Le but de cette thèse est d'apporter de nouvelles connaissances pour répondre à quelques questions sur la nature de la croûte terrestre, la zone de transition continentale-océanique, la présence du sel messénien, sa distribution et sa relation entre les formations sédimentaires superficielles et les structures crustales. Dans cette étude, notre approche est une inversion jointe des enregistrements grand-angle et des données sismiques multi-canal. Nous avons conduit une série de tomographie des premières arrivées, une inversion jointe des arrivées réfractées et réfléchies ainsi qu’une modélisation gravimétrique. Etant donné que la solution du problème inverse n’est pas unique, deux programmes de tomographie ont été utilisés sur les mêmes données pour la même région d’étude à savoir : FAST (First Arrival Seismic Tomography) et Tomo2D. La tomographie a été suivie par une inversion jointe des arrivées réfractées et réfléchies suivant une approche basée sur la combinaison de la migration en profondeur « Kirchhoff » avant sommation (PSDM) des données de sismique réflexion multi-canal (MCS) et la modélisation directe des enregistrements grand-angle sur le fonds marin (OBS). Afin de vérifier la consistance du modèle de la vitesse avec les données gravimétriques, l’anomalie à l'air libre a été modélisée. Les résultats de l’imagerie conduite dans ce travail montrent la structure de la marge, la croûte continentale, la zone de transition continent-océan et la croûte océanique de la Méditerranée. La structure du modèle confirme les études antérieures basées sur des données bathymétriques, gravimétriques et magnétiques. Cette structure montre essentiellement : - un plateau continental étroit et pente continentale une très raide.- l’Expulsion du sel vers le nord impliquant la formation de diapirs au-dessus du flanc nord du bassin (plaine abyssale).- L’approfondissement et l’épaississement des séquences sédimentaires (bassin sédimentaire) près de la marge algérienne. Le modèle de vitesses obtenu et l’épaisseur des différentes unités structurales formant ce modèle apportent des arguments quantitatifs pour enrichir la connaissance de cette partie de la Méditerranée occidentale. Les couches sédimentaires dans le bassin montrent des vitesses sismiques allant de 1,9 km / s à 3,8 km / s. Les formations messéniennes ont été modélisées en utilisant une vitesse située entre 3,7 km / s à 3,8 km / s. La croûte continentale s’amincit sur une bande étroite de la marge dont la distance est d'environ 15 km. La vitesse de la croûte océanique dans cette région présente deux couches distinctes : l’une caractérisée par des vitesses variant de 4,7 km / s à 6.1 et l’autre de 6.2 à 7.1 km / s. La vitesse du manteau supérieur quant à elle a été modélisée par 7,9 km / s
This thesis has been conducted within the framework of the Algerian-French research cooperation program SPIRAL (Sismique Profonde et Investigations Régionales du Nord de l’Algérie). This project aims to study the deep structure of the Algerian margin. The area covered by this study focuses in the region of Jijel in eastern Algerian margin.The main objective of our thesis is to improve depth imaging of the Algerian margin using a combined approach of seismic techniques; wide-angle and multi- channel seismic data. The purpose of this thesis is to bring new knowledge to answer some questions about the nature of the crust, the area of continental -oceanic transition, the presence of Messinian salt, its distribution and relationship between surface sedimentary formations and crustal structures.This study presents the results of a deep seismic survey across the north Algerian margin, based on the combination of 2D multi-channel and wide-angle seismic data simultaneously recorded by 41 ocean bottom seismometers deployed along a North-South line extending 180 km off Jijel into the Algerian offshore basin, and 25 land stations deployed along a 100 km-long line, cutting through the Lesser Kabylia and the Tellian thrust-belt.In this study, our approach is a joint inversion of wide-angle seismic recordings (OBS, ocean bottom seismometers) and multi- channel seismic data (MCS). We conducted a series of first arrivals tomography, a joint inversion of reflected and refracted arrivals and gravity modelling. Since the solution of the inverse problem is not unique, two tomography programs were applied using the same data for the same study area; FAST (First Arrival Seismic Tomography) and Tomo2D. Tomography was followed by a joint inversion of reflected and refracted arrivals following an approach based on the combination of Kirchhoff prestack depth migration (PSDM) for MCS data and forward modelling of OBS. To check the consistency of the velocity model with gravity data, the free air anomaly was modeled.The final model obtained using forward modelling of the wide-angle data and pre-stack depth migration of the seismic reflection data provides an unprecedented view of the sedimentary and crustal structure of the margin. The sedimentary layers in the Algerian basin are 3.75 km thick to the north and up to 4.5 to 5 km thick at the foot of the margin. They are characterised by seismic velocities from 1.9 km/s to 3.8 km/s. Messinian salt formations are about 1 km thick in the study area, and are modelled and imaged using a velocity between 3.7 km/s to 3.8 km/s. The crust in the deep sea basin is about 4.5 km thick and of oceanic origin, presenting two distinct layers with a high gradient upper crust (4.7 km/s - 6.1 km) and a low gradient lower crust (6.2 km/s - 7.1 km/s). The upper mantle velocity is constrained to 7.9 km/s. The ocean-continent transition zone is very narrow between 15 km to 20 km wide. The continental crust reaches 25 km thickness as imaged from the most landward station and thins to 5 km over a less than 70 km distance. The continental crust presents steep and asymmetric upper and lower crustal geometry, possibly due to either asymmetric rifting of the margin, an underplated body, or flow of lower crustal material towards the ocean basin. Present-time deformation, as imaged from 3 additional seismic profiles, is characterized by an interplay of gravity-driven mobile-salt creep and active thrusting at the foot of the tectonically inverted Algerian margin

碩士
國立中央大學
地球科學學系
106
Surface and body waves generated by near-surface explosions and recorded along T4b line is investigated for imaging subsurface velocity structure in Southern Taiwan. Surface waves propagate only at the Earth’s surface meanwhile body waves propagate deep through the body/interior of the Earth. For crustal-scale velocity structure investigation, wave-field processing and inversion of both body and surface waves records can be very useful to image Earth’s subsurface structures without picking travel-time. Seismic dataset used in this research is a crustal-scale Wide Angle Refraction/Reflection (WARR) survey under TAiwan Integrated GEodynamics Research (TAIGER) project conducted in 2008 in Taiwan. The south main array consist of four shot points (S1, S2, S3, S4) across southern Taiwan from west to east. A total of 609 geophones was deployed with receiver interval of about 200m. We use two approaches to estimate separate Vp and Vs velocity models along T4b line. The first method is through implementation of Multichannel Analysis of Surface Wave (MASW) technique to determine near-surface velocity structure. Shear wave velocity is the best indicator of material stiffness for engineering related applications. The near-surface soil or formation shear wave velocity (Vs) characteristics can be retrieved by analyzing the dispersion curve associated with the fundamental mode of surface waves in heterogeneous media. MASW method is only applicable for shot number 1 (S1) and shot number 2(S1) that possess significant amount of ground-rolls records. The second method is based on tau-p wave-field inversion. Through wave-field transformation and downward continuation approach, application of long-offset (or local short-offset) seismic data to estimate compressional wave velocity (Vp) structures can be achieved via direct global (or localized) search algorithm. Through wave-field processing strategy, independent Vp and Vs velocity models can be obtained. From inverted shear and body wave velocity structure, we can see obvious velocity gradient exist in southwestern Taiwan Coastal Plain with the bedrock boundary is slightly west-dipping. Assume the shear wave velocity of Pliocene to Miocene bedrock is 1.5 km/s, the thickness of alluvial sediments at the west-end of western Coastal Plain is ~1.8 km and become thinner toward the east with depth ~0.9 km. The Vs30 site classification for shot S1 is Class D1 and for shot S2 is Class D3 which all corresponds to “stiff soil”. From body wave wave-field inversion, subsurface structure can be imaged up to depth of approximately 20 km. In the Western Coastal Plain, a thick sediment layer (~ 3 km) with fairly low velocity (Vp= 1.5-4 km/s and Vs=250-1500 km/s) and high gradient changes produce significant seismic reverberations (refracted free-surface multiples) shown in data S1 and S2. Evidence of strong lateral velocity changes marked the transition between Coastal Plain and Western Foothills near east side of shot point S2. Strong lateral velocity transition also can be observed between Central Range and Coastal Range at shot point S4. The contour lines for Vp values between 5.0 to 6.5 km/s highlight the general feature of crust layer which thicken in the west (depth=8 and 16 km respectively) and thinned toward the east (depth =4 and 9 km respectively). By examining the contour value of 6.0 and 6.5 km/s, a relatively flat structure feature extend from Coastal Plain to Western Foothills; thickening at the distance between 40-78 km and thinning toward Central Range and Coastal Range provide a vivid view of crustal thickening and thinning feature associate with tectonic structure along the T4b line. The 6 km/s contour shown at the depth of 13 km from Coastal Plain and reaches the depth of ~7 km at Coastal Range may highlight the potential boundary between upper and lower crust (Conrad discontinuity?). The extreme thinning (necking) of the continental crust can be occurred by a typical rift zone at continental margin. The 6.5 km/s and a high velocity lower crust layer with velocity ranging from 6.9 (or 7.0) to 7.5 km/s may show similar features and may highlight the existing boundary between lower crust and upper mantle (Moho discontinuity?) which can be identified from wavefield processing and velocity inversion studies. With more careful investigate near-surface velocity distribution within 2.5-5 km, crust thickness can be thinner than the result from tomography studies. Picking the first arrival time with trial-and-error processes are somehow tedious, not efficient and time-consuming may lead to overly smoothed model. The advantages of the proposed method are no need to manually pick travel time and automatically and stable update of velocity model. The quality of the derived model is effectively checked by the consistency between computed travel-time with seismic records. Keywords: Tau-p Transform, Tau-p Inversion, MASW, Surface Waves, Ground-roll, TAIGER Project

The deep structure of the Bohemian Massif, the largest stable outcrop of the Variscan rocks in central Europe, was studied using the data of the international seismic refraction and wide- angle reflection experiments CELEBRATION 2000, ALP 2002 and SUDETES 2003. The data were interpreted by seismic tomographic inversion and by 2-D trial-and-error forward modelling of the P and S waves. Above, additional constraints on the crustal structure were imposed by reflectivity or gravity modelling, and by receiver function interpretation. Knowledge of the crustal velocity structure in the Bohemian Massif was complemented by its azimuthal variation. Though consolidated, the Bohemian Massif can be subdivided into several tectonic units separated by faults, shear zones, or thrusts reflecting varying influence of the crust forming processes. The resultant velocity models determined different types of the crust-mantle transition reflecting variable crustal thickness and delimiting contacts of these tectonic units at depth.

碩士
國立清華大學
材料科學工程學系
91
Researches of AR coating had been studied for a long time, but most of them focused on vertical-incident part. In this paper, we would find out the kind of ideal AR coating which could be used for every incident angle. Based on the characteristic matrix theory, we wrote the computer program to calculate the average reflectance for p- and s-polarization of different AR coating structures on the glass for visible spectral. We designed the ideal AR coating by using of the inhomogeneous structure with refractive index changing from air to substrate. The numerical results were discussed by discussing the distribution of index change, the numbers of layers and the thickness of coatings to obtain the most effective AR coating. After series of discussion, we obtain the best refractive index distribution for 10-layers. The average reflectance was less than 0.01 over the whole spectral region up to 80°, and its result was close to that of the ideal AR coating. we also develop the best index distribution for 5-layers structure. For practical manufacturing, we decreased numbers of layers and raised the initial index of coating, however the effects were better than that of the traditional 4-layers AR coating. At last we brought up some methods to fabricate ideal AR coating for the future work.

碩士
中華大學
機械工程學系碩士班
100
Traditional wide-angle refractive mirror is not transparent for visible light, it is not possible to take photography day or night behind the mirror. This study aims to design reflective cut-off filter for wide angle mirror with day and night photography function. The Essential Macleod software was utilized to design the cut-off filter with day and night photography features. The practical cut-off filter multilayer film was carried out by sputtering and sol-gel processes. The PC (Polycarbonate) was used as the substrate material. The alternated TiO2 and SiO2 dielectric films were selected to be deposited on the PC substrate acted as the cut-off filter. It is expected that the ideal cut-off filter with a high reflectivity in visible light region (~ 80% reflectivity or 20% transmittance), and a high transmittance of 90% in infrared wavelength range. The optimal modeling results show that the transmittance of visible light is in the range of 20 to 40 % and infrared transmittance achieves 95%. The practical measured results for sputtered cut filter film show that the average transmittance of visible light approach 25% and the infrared transmittance reaches 95% at wavelength above 750 nm. The sol-gel process was also utilized to deposit alternated TiO2 and SiO2 dielectric films on glass plate as a cut-off filter. The simulation results show that the transmittance of visible light is in the range of 10~20% and infrared transmittance reaches 80%. The practical sol-gel coated results are not satisfied due to the difficulty to control the thickness and uniformity of TiO2 and SiO2 layers.

碩士
國立臺灣海洋大學
應用地球科學研究所
100
In 2009, more than 250 stations of OBS (Ocean Bottom Seismometer) were deployed in the TAIGER research area. This is the co-operation among Taiwan, America, and France, collecting geological and geophysical data. Under this objective, we are aiming to have a better understanding of the Taiwan orogeny zone and the deep crust structure around the Manila Subduction Zone and Luzon Arc. The OBSs refraction data, T1 line, which is collected in TAIGER, is used to analysis the deep crustal velocity structure. Because of the weather, the T1 was separated into two profiles, consisted of MGL0908_05 and MGL0908_07. The T1 line, a 438-km-long horizontal line, begins in the South China Sea (SCS), and across the Manila Trench, Luzon Arc, Huatung Basin, Gagua Ridge, and ends in West Philippine Basin. In the preliminary result of P-wave velocity model, the crust thickness of the northeastmost SCS is about 11 km, which is slightly thicker than the typical oceanic crust. However, the crust thins east-ward along the T1 line, and 8-9 km thick between the LRTPB (Luzon-Ryukyu Transform Plate Boundary) and Manila Trench. The depth of both side of the Gagua Ridge is about 8.5 km in the east side, and 7.5 km, which is shallower in west side. In addition, the crust thickness on both side of the Gagua Ridge is 6 km in the east and 8 km in the west. The compression between the Huatung Basin and West Philippine Basin is not apparent from theT1 line in this study. However, it is much more clear in the north side of the Gagua Ridge. The earthquake distribution from CWB and IRIS shows that there is a group of the events between LRTPB and Manila Trench. This result probably is from the trend of Manila Trench and the basement depth of SCS crust in this area. However, the compress stress in this area could be changed either by a double subduction zone or by a seamount subduction in the seismogenic zone. If the stress is accumulated and over the friction of the asperity, it will probably reactivate the splay fault or out-of-sequence thrust in the region. Because the length of the destructive faults in the Manila Subduction Zone could extend to 500 km long, it may cause the mega earthquake and tsunami, like that in Indonesia, Chile, and Japan.

In the BATHOLITHSonland seismic project, a refraction - wide-angle reflection survey was shot in 2009 across the Coast Mountains and Interior Plateau of central British Columbia. Part of the seismic profile crossed the Nechako Basin, a Jurassic-Cretaceous basin with potential for hydrocarbons within sedimentary rocks that underlie widespread volcanics. Along this 205-km-long line segment, eight explosive shots averaging 750 kg were fired and recorded on 980 seismometers. Forward and inverse modelling of the traveltime data were conducted with two independent methods: ray-tracing based modelling of first and secondary arrivals, and a higher resolution wavefront-based first-arrival seismic tomography. Gravity modelling was utilized as a means of evaluating the density structure corresponding to the final velocity model. Material with velocities less than 5.0 km/s is interpreted as sedimentary rocks of the Nechako Basin, while velocities from 5.0-6.0 km/s may correspond to interlayered sediments and volcanics. The greatest thickness of sedimentary rocks in the basin is found in the central 110 km of the profile. Two sub-basins were identified in this region, with widths of 20-50 km and maximum sedimentary depths of 2.5 km and 3.3 km. Such features are well-defined in the velocity model, since resolution tests indicate that features with widths greater than ~13 km are reliable. Beneath the sedimentary rocks, seismic velocities increase more slowly with depth – from 6.0 km/s just below the basin to 6.3 km/s at ~17 km depth, and then to 6.8-7.0 km/s at the base of the crust. The Moho is interpreted at a depth of 33.5-35 km along the profile, and mantle velocities are high at 8.05-8.10 km/s.