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Roshan, Ravi, i Upendra Kumar Singh. "Inversion of residual gravity anomalies using tuned PSO". Geoscientific Instrumentation, Methods and Data Systems 6, nr 1 (6.02.2017): 71–79. http://dx.doi.org/10.5194/gi-6-71-2017.
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Abstract. Many kinds of particle swarm optimization (PSO) techniques are now available and various efforts have been made to solve linear and non-linear problems as well as one-dimensional and multi-dimensional problems of geophysical data. Particle swarm optimization is a metaheuristic optimization method that requires intelligent guesswork and a suitable selection of controlling parameters (i.e. inertia weight and acceleration coefficient) for better convergence at global minima. The proposed technique, tuned PSO, is an improved technique of PSO, in which efforts have been made to choose the controlling parameters, and these parameters have been selected after analysing the responses of various possible exercises using synthetic gravity anomalies over various geological sources. The applicability and efficacy of the proposed method is tested and validated using synthetic gravity anomalies over various source geometries. Finally, tuned PSO is applied over field residual gravity anomalies of two different geological terrains to find the model parameters, namely amplitude coefficient factor (A), shape factor (q) and depth (z). The analysed results have been compared with published results obtained by different methods that show a significantly excellent agreement with real model parameters. The results also show that the proposed approach is not only superior to the other methods but also that the strategy has enhanced the exploration capability of the proposed method. Thus tuned PSO is an efficient and more robust technique to achieve an optimal solution with minimal error.
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Al-Garni, Mansour A. "Inversion of residual gravity anomalies using neural network". Arabian Journal of Geosciences 6, nr 5 (22.11.2011): 1509–16. http://dx.doi.org/10.1007/s12517-011-0452-y.
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von Frese, R. R. B., D. N. Ravat, W. J. Hinze i C. A. McGue. "Improved inversion of geopotential field anomalies for lithospheric investigations". GEOPHYSICS 53, nr 3 (marzec 1988): 375–85. http://dx.doi.org/10.1190/1.1442471.
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Instabilities and the large matrices which are common to inversions of regional magnetic and gravity anomalies often complicate the use of efficient least‐squares matrix procedures. Inversion stability profoundly affects anomaly analysis, and hence it must be considered in any application. Wildly varying or unstable solutions are the products of errors in the anomaly observations and the integrated effects of observation spacing, source spacing, elevation differences between sources and observations, geographic coordinate attributes, geomagnetic field attitudes, and other factors which influence the conditioning of inversion. Solution instabilities caused by ill‐posed parameters can be efficiently minimized by ridge regression with a damping factor large enough to stabilize the inversion, but small enough to produce an analytically useful solution. An effective choice for the damping factor is facilitated by plotting damping factors against residuals between observed and modeled anomalies and by then comparing this curve to curves of damping factors plotted against solution variance or the residuals between predicted anomaly maps representing the processing objective (e.g., downward continuation, differential reduction to the radial pole, etc.). To obtain accurate and efficient large‐scale inversions of anomaly data, a procedure based on the superposition principle of potential fields may be used. This method involves successive inversions of residuals between the observations and various stable model fields which can be readily accommodated by available computer memory. Integration of the model fields yields a well‐resolved representation of the observed anomalies corresponding to an integrated model which normally could not be obtained by direct inversion because the memory requirements would be excessive. MAGSAT magnetic anomaly inversions over India demonstrate the utility of these procedures for improving the geologic analysis of potential field anomalies.
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Hamidy, Ahmad Nor, Ashila Juan Fortuna, Imelda Mar'ata Sholih, Revinda Oktavia, Rabiah Al'Adawiyah, Firdha Kusuma Ayu Anggraeni i Sri Astutik. "ANALISIS STRUKTUR BAWAH PERMUKAAN GUNUNG IJEN BANYUWANGI". JURNAL PEMBELAJARAN FISIKA 11, nr 3 (13.10.2022): 93. http://dx.doi.org/10.19184/jpf.v11i3.33323.
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Analysis of the subsurface structure of Mount Ijen, Banyuwangi was carried out based on anomaly data obtained from satellite image data. This research was conducted with the aim of identifying the subsurface structures around the research site. In this study, what is determined is the complete Bouguer anomaly (ABL), regional and residual anomalies based on the ABL, and their inversion modeling. The results of the representation of underground structures based on residual anomalies obtained from complete and regional Bouguer anomaly data. The complete bouguer anomaly values obtained in the Mount Ijen area range from 12.2 to 110.7 mGal. In the process of separating regional and residual anomalies, different anomaly values are produced. The regional anomaly value ranges from +12.2 to +110.7 mGal while the residual anomaly ranges from -4.2 to +2.4 mGal
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Abdelrahman, El-Sayed M., Eid R. Abo-Ezz i Khalid S. Essa. "Parametric inversion of residual magnetic anomalies due to simple geometric bodies". Exploration Geophysics 43, nr 3 (wrzesień 2012): 178–89. http://dx.doi.org/10.1071/eg11026.
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Becht, Andreas, Jens Tronicke, Erwin Appel i Peter Dietrich. "Inversion strategy in crosshole radar tomography using information of data subsets". GEOPHYSICS 69, nr 1 (styczeń 2004): 222–30. http://dx.doi.org/10.1190/1.1649390.
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Detecting discrete anomalies, such as cavities or tunnels, is an important application of crosshole radar tomography. However, crosshole tomographic inversion results are frequently ambiguous, showing smearing effects and inversion artifacts. These ambiguities lead to uncertainties in interpretation; hence, the size and position of anomalies can only be interpreted with limited accuracy and reliability. We present an inversion strategy for investigating discrete anomalies with crosshole radar tomography. In addition to the full traveltime data set, we use subsets of specified ray‐angle intervals for tomographic inversion. By analyzing inversion results from different ray‐angle intervals, a more accurate interpretation of anomalies is possible. The second step of our strategy is to develop a good inhomogeneous starting model from joint interpretation of the inversion results from different subsets. The third step is to invert the full data set using this new starting model and to evaluate the inversion results by analyzing the distributions of mean square traveltime residuals with respect to the ray angles. We use a synthetic model with two discrete anomalies located roughly at the same depth to demonstrate and evaluate our approach. This inversion strategy is also applied to a field data set collected to investigate karst cavities in limestone. From the inversion results of both examples, we show that horizontal smearing of anomalies can be reduced by eliminating near‐horizontal rays. A good starting model can be obtained based on the joint interpretation of the inversion results of the different subsets; it leads to a high‐resolution final image of the full data set.
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Liu, Jie, Jianzhong Zhang, Li Jiang, Qi Lin i Li Wan. "Polynomial-based density inversion of gravity anomalies for concealed iron-deposit exploration in North China". GEOPHYSICS 84, nr 5 (1.09.2019): B325—B334. http://dx.doi.org/10.1190/geo2018-0740.1.
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Inversion of residual gravity anomalies is an important geophysical technique for depicting subsurface density contrasts, for example, for mineral deposits. We have expressed subsurface density variations using depth-variable polynomial functions and developed the polynomial coefficient inversion (PCI) method, which is an alternative method for mapping subsurface density distributions by inverting the coefficients of density-contrast functions. PCI enables the linear inversion of density variations without vertically subdividing the subsurface. Synthetic tests indicate that PCI combines polynomial functions and multiple constraints to highlight the anomalous masses through an iterative process with appropriate weighting parameters. We apply our method to a local investigation of banded iron formation (BIF) deposits in the Hebei Province, North China. The inversion results depict the approximate distribution of the subsurface density contrasts to identify the stratigraphic boundaries of different lithologies and BIF-favorable zones, thus implying that local iron-rich ore bodies may be located at the syncline axis or dip along the faults. The successful application of PCI for the BIF deposits indicates that this method is a promising strategy for density mapping.
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Lv, Qijun, Aiping Zheng, Xiangjin Liang, Hongfei Chen, Shichang Ju, Yanchong Meng, Hongyuan Zhang, Guolin He, Shenshen Deng i Junfang Li. "Research on Remaining Oil Characterization in Superheavy Oil Reservoir by Microgravity Exploration". Geofluids 2022 (18.07.2022): 1–10. http://dx.doi.org/10.1155/2022/1210780.
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Some physical processes such as oil and gas development, metal deposit collection, and groundwater resource migration can cause density changes, for which microgravity monitoring is the most intuitive method to monitor the density change process. Based on the basic principle of microgravity measurement and the idea of multiscale separation, a multiscale, second-order, surface-fitting, residual gravity anomaly extraction method is proposed to separate superimposed microgravity fields. In this method, regional fields of different scales are fitted and calculated successively with the measurement points as the center, so as to separate the gravity anomalies produced by different-depth density bodies. Results from actual data show that this method extracts the reservoir’s residual density characteristics of plane gravity anomaly on the basis of remaining oil distribution characteristics, consistent with reservoir numerical simulation results. A three-dimensional least-squares inversion of the method for extracting residual gravity anomaly was carried out, with the inversion results consistent with the results of vertical remaining oil distribution characteristics and well-test production results.
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Sehah, Sehah, Urip Nurwijayanto Prabowo, Sukmaji Anom Raharjo i Aina Zahra Ikhwana. "Physical modeling of magma chamber of slamet volcano by means of satellite gravimetric data". Communications in Science and Technology 7, nr 2 (28.12.2022): 160–67. http://dx.doi.org/10.21924/cst.7.2.2022.1001.
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Slamet Volcano (3,432 m) is the highest volcano in Central Java, Indonesia, with a weak explosive type of eruption compared to other active volcanoes. Designing the magma chamber model may help reveal the characteristics of Slamet Volcano. The modelling uses the gravimetric satellite data from GGMplus, which is best in spatial resolution compared to other satellite data, i.e. 220 m. Data processing begins with Bouguer correction and terrain correction and has resulted in complete Bouguer anomalies data, with values ranging from 11.068 – 117.451 mGal. Further, residual Bouguer anomalies data were obtained after data reduction to the horizontal surface and removal of regional anomalies data, to obtain values ranging from -67.569 – 38.808 mGal. The residual anomaly contour map shows the lowest anomalous value is under the volcanic cone at positions of 109.21967° E and 7.24281° S which is estimated to be the location of the magma chamber of Slamet Volcano. However, the inversion modeling resulting from the residual Bouguer anomalies data shows that the magma chamber of Slamet Volcano can be observed clearly at positions of 109.22053° E and 7.24719° S. The location of the magma chamber is not perfectly vertical under the volcanic cone but has a slight slope. The obtained model of the magma chamber has a relatively small volume and shallow depth, i.e. about 1 – 4 km. The obtained physical parameters of the magma chamber impact the characteristics of the eruption of Slamet Volcano which tend to be weak explosive.
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Eshaghzadeh, Ata, Alireza Dehghanpour i Sanaz Seyedi Sahebari. "Marquardt inverse modeling of the residual gravity anomalies due to simple geometric structures: A case study of chromite deposit". Contributions to Geophysics and Geodesy 49, nr 2 (1.06.2019): 153–80. http://dx.doi.org/10.2478/congeo-2019-0008.
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Abstract In this paper, an inversion method based on the Marquardt’s algorithm is presented to invert the gravity anomaly of the simple geometric shapes. The inversion outputs are the depth and radius parameters. We investigate three different shapes, i.e. the sphere, infinite horizontal cylinder and semi-infinite vertical cylinder for modeling. The proposed method is used for analyzing the gravity anomalies from assumed models with different initial parameters in all cases as the synthetic data are without noise and also corrupted with noise to evaluate the ability of the procedure. We also employ this approach for modeling the gravity anomaly due to a chromite deposit mass, situated east of Sabzevar, Iran. The lowest error between the theoretical anomaly and computed anomaly from inverted parameters, determine the shape of the causative mass. The inversion using different initial models for the theoretical gravity and also for real gravity data yields approximately consistent solutions. According to the interpreted parameters, the best shape that can imagine for the gravity anomaly source is the vertical cylinder with a depth to top of 7.4 m and a radius of 11.7 m.
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Filho, Nelson Ribeiro, Cristiano Mendel Martins i Renata de Sena Santos. "A NOVEL REGIONAL-RESIDUAL SEPARATION APPROACH FOR GRAVITY DATA THROUGH CRUSTAL MODELING". Revista Brasileira de Geofísica 36, nr 4 (21.12.2018): 1. http://dx.doi.org/10.22564/rbgf.v36i4.1980.
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ABSTRACT. Gravity anomalies normally contain information of all sources beneath Earth’s surface. Once residual anomalies exhibit information about the main target, the knowledge of this specific residual signal is extremely important to interpretation. To find this signal, it’s necessary to perform regional-residual separation. We present here a new approach of separation by using gravity crustal modeling. We divide the surface in prisms, with density given by GEMMA. We calculate the regional signal, assuming Earth’s crust can be the source of observed anomaly. This methodology was applied on Barreirinhas basin-Brazil. Its formation is related to geologic events in South America-Africa break. Besides, the complex geology is the main obstacle on finding the residual anomaly. We compare our methodology with robust-polynomial fitting and spectral-analysis. They were not able to identify the residual anomaly. Main trouble relies on absence of crust information. Those kind of environment usually requires forward modeling and/or gravity inversion. On the other hand, our approach considers all crust’s parameters. Then the difficulty on choosing the residual no longer exists. The residual anomaly follows a geologic pattern. The crustal depocenter was mapped between structural faults. Therefore, our results satisfies the main expectation and are extremely linked to Barreirinhas basin’s geological background. We recommend this separation procedure, once Earth’s crustal model and gravity data are available for all planet.Keywords: Gravity modeling; GEMMA model; Barreirinhas basin; residual anomaly. RESUMO. Anomalias gravimétricas contêm informações de todas as fontes na superfície terrestre. Uma vez que anomalias residuais exibem informações sobre alvos principais, o conhecimento desse específico sinal residual é extremamente importante para interpretação. Para encontrá-lo, é necessário realizar separação regional-residual. Apresentamos aqui uma nova abordagem de separação utilizando a modelagem gravimétrica crustal. Discretizamos a superfície em prismas, com densidade fornecida pelo modelo GEMMA. Calculamos o sinal regional, assumindo que a crosta terrestre é a fonte da anomalia observada. Aplicamos esta metodologia na bacia de Barreirinhas - Brasil, que tem sua formação relacionada aos eventos geológicos de separação da América do Sul e África. Além disso, a complexidade geológica é considerada o principal obstáculo para encontrar esta anomalia residual. Comparamos nossa metodologia com Ajuste Polinomial Robusto e Análise Espectral. Essas técnicas não foram capazes de identificar a anomalia residual. O principal problema se dá pela ausência de informações acerca da crosta. Para esse ambiente, geralmente requer modelagem direta e/ou inversão geofísica. Por outro lado, nossa abordagem considera todos os parâmetros crustais e a dificuldade em escolher o residual deixa de existir. A anomalia residual apresenta um padrão geológico. O depocentro crustal foi mapeado entre falhas estruturais. Nossos resultados satisfazem a expectativa principal e estão extremamente ligados ao cenário geológico da bacia. Recomendamos este procedimento de separação, uma vez que os modelos crustais e dados gravimétricos estão disponíveis para todo o planeta.Palavras-chave: Modelagem gravimétrica; modelo GEMMA; bacia de Barreirinhas; anomalia residual
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Greenhalgh, Stewart A., Thomas Gruber i Bing Zhou. "Velocity field imaging with an anomaly recovery algorithm, incorporating later arrivals". GEOPHYSICS 68, nr 2 (marzec 2003): 589–99. http://dx.doi.org/10.1190/1.1567229.
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We have developed a novel velocity‐field recovery algorithm incorporating later arrivals which is aimed at overcoming several drawbacks encountered with standard tomographic inversion schemes. The anomaly recovery algorithm (ARA) places a series of anomalies, each fully described by only a few parameters, into a given velocity‐field estimate, varying the parameters to minimize the residual errors in first‐arrival times and in reflection, refraction, and diffraction times, which are obtained by manual or automatic picks from seismograms. We successfully recover anomalies in synthetic variable velocity fields where standard conjugate gradient‐based tomographic inversion schemes fail, find the ARA to be very robust against errors in traveltimes of up to 8%, and obtain results that are economically more meaningful. A comparison of results of the ARA applied to a mineral field crosshole data set with that of a standard conjugate gradient least‐squares (CGLS) inversion scheme indicates the ARA is a viable option for real‐world applications. It recovers the major velocity features found in subsequent drilling. An examination of the solution space exhibits a generally smooth topography with few significant minima interspersed by a larger number of minor local minima, suggesting the applicability of several standard nonlinear inversion schemes. The central idea underlying the ARA, to represent the velocity‐field inversion problem as low dimensional rather than high dimensional, makes the method generic, highly flexible, and relatively easy to analyze in terms of stability and local minima.
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Lesu, K. K., Jehunias L. Tanesib i Bernandus Bernandus. "PEMODELAN DUA DIMENSI (2D) BAWAH PERMUKAAN PULAU ADONARA DENGAN DATA ANOMALI GRAVITASI". Jurnal Fisika : Fisika Sains dan Aplikasinya 8, nr 1 (27.04.2023): 21–24. http://dx.doi.org/10.35508/fisa.v8i1.11819.
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Geophysical research has been carried out using the gravity method on Adonara island in east Flores Regency, East Nusa Tenggara. This study aims to determine the subsurface rock geological conditions of the research area using free air gravity anomaly data from GGMplus and ERTM2160. The data is reduced by atmospheric correction, simple bouguer correction, curvature correction to produce a complete bouguer anomaly. After correction, regional anomalies and residual anomalies are separated. Then 2-dimensional (2D) modeling is carried out using forward modeling and inversion modeling with Grav2DC software. The results of the study show that the subsurface geological conditions of Adonara island are dominated rock by andesite, granite, diorite, dolomite, gravel, sandstone, basalt, sekis, metamorphic, shale and lavas.
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Simmons, James L., i Milo M. Backus. "Linearized tomographic inversion of first‐arrival times". GEOPHYSICS 57, nr 11 (listopad 1992): 1482–92. http://dx.doi.org/10.1190/1.1443215.
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A linearized tomographic‐inversion algorithm estimates the near‐surface slowness anomalies present in a conventional, shallow‐marine seismic reflection data set. First‐arrival time residuals are the data to be inverted. The anomalies are treated as perturbations relative to a known, laterally‐invariant reference velocity model. Below the sea floor the reference model varies smoothly with depth; consequently the first arrivals are considered to be diving waves. In the offset‐midpoint domain the geometric patterns of traveltime perturbations produced by the anomalies resemble hyperbolas. Based on simple ray theory, these geometric patterns are predictable and can be used to relate the unknown model to the data. The assumption of a laterally‐invariant reference model permits an efficient solution in the offset‐wavenumber domain which is obtained in a single step using conventional least squares. The tomographic image shows the vertical‐traveltime perturbations associated with the anomalies as a function of midpoint at a number of depths. As implemented, the inverse problem is inherently stable. The first arrivals sample the subsurface to a maximum depth of roughly 500 m (≈ one‐fifth of the spread length). The model is parameterized to consist of fifteen 20-m thick layers spanning a depth range of 80–380 m. One‐way vertical‐traveltime delays as large as 10 ms are estimated. Assuming that these time delays are distributed over the entire 20-m thick layers, velocities much slower than water velocity are implied for the anomalies. Maps of the tomographic images show the spatial location and orientation of the anomalies throughout the prospect for the upper 400 m. Each line is processed independently, and the results are corroborated to a high degree at the line intersections.
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Zhang, Yi, Yixian Xu, Walter D. Mooney i Chao Chen. "Local separation of potential field anomalies using equivalent sources: application for the 3-D structure of mantle uplift beneath Von Kármán crater, the Moon". Geophysical Journal International 227, nr 3 (5.08.2021): 1612–23. http://dx.doi.org/10.1093/gji/ggab307.
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SUMMARY The separation of regional-residual anomalies plays an important role in the processing of potential field anomalies for obtaining better understandings of the nature of the underground sources. Many methods have been developed to achieve the separation of anomalies that are of distinct wavelengths. On the other hand, fewer studies have addressed the separation of local anomalies from the observed potential field anomalies. In this paper, we introduce a new process for separating localized anomalies from the observations under the Cartesian and spherical coordinates. The separation is achieved using the equivalent source technique and an iterative inversion process which is to refine and finalize the separated local anomalies. Additionally, we introduce an inversion method for determining the equivalent sources that are of varying dimensions, as well as a quantitative measurement to assess the accuracy of the separation process. Verified with synthetic examples, the proposed method could extract arbitrary shaped local anomalies from the rest with low error levels. Subsequently, we apply the method to the construction of a 3-D model of the mantle uplift beneath the Von Kármán crater (VKC) on the Moon. The VKC is the landing site of the Chinese lunar exploration mission Chang'e 4, which lies in the northwestern portion of the South-Pole Aitken (SPA) basin on the far side of the Moon. Multiple generations of mare basalts are identified within the VKC, which indicates a complex geological history of the basin. Insights into the evolutionary history of this region can be obtained by investigating the deep crustal structure of the VKC using topographic and gravity data. Processed with the proposed method, the 3-D structure we obtain provides evidence for separated mantle uplifting events triggered by the two impact events that created the VKC and the Von Kármán M crater, respectively.
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Leite, Lourenildo W. B., i Jorge W. D. Leão. "Ridge regression applied to the inversion of two‐dimensional aeromagnetic anomalies". GEOPHYSICS 50, nr 8 (sierpień 1985): 1294–306. http://dx.doi.org/10.1190/1.1442000.
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The methodology presented was developed for processing and mathematical interpretation of aeromagnetic data derived from two‐dimensional sources. The inversion is based on the ridge regression technique as extended in Marquardt (1963) to nonlinear problems. The mathematical formulation provides a statistical approach for study of the efficiency of the solution. A procedure for data preparation is presented which includes a critical evaluation of the digital data obtained during the aerosurvey, joining of flight segments into a single line, and regional‐residual separation for profile analysis to set up the initial model approximation. The forward model is analyzed using predictive diagnostics called sensitivity coefficients and parameter correlation matrix. The parameter confidence intervals take into account the measurements of the nonlinearity of the quadratic approximation to the object function. The initial approximation is determined by profile analysis and by the geologic information that defines the forward model and its constraints. The nonuniqueness of the solution results in an interpretation that can be refined as new geologic information is applied for additional model constraints. Synthetic models are used to test and study the convergence and statistical properties of the solution, and to show the ability of the method to resolve anomalies due to various sources. The procedure is also applied to data obtained from aerosurveys over an oil exploration area of the upper Amazon sedimentary basin of Brazil. The validity of the solution is discussed in terms of its compatibility with the geologic information and constraints, and in terms of the statistical properties of the model parameters used to explain the observed data.
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Clark, Matthew R. "On the variability of near-surface screen temperature anomalies in the 20 March 2015 solar eclipse". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 374, nr 2077 (28.09.2016): 20150213. http://dx.doi.org/10.1098/rsta.2015.0213.
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Near-surface air temperature (NSAT) anomalies during the 20 March 2015 solar eclipse are investigated at 266 UK sites, using operational data. The high density of observing sites, together with the wide range of ambient meteorological conditions, provided an unprecedented opportunity for analysis of the spatial variability of NSAT anomalies under relatively uniform eclipse conditions. Anomalies ranged from −0.03°C to −4.23°C (median −1.02°C). The maximum (negative) anomaly lagged the maximum obscuration by 15 min on average. Cloud cover impacted strongly on NSAT anomalies, with larger anomalies in clear-sky situations ( p <0.0001). Weaker, but statistically significant, correlations were found with wind speed (larger anomalies in weaker winds), proximity to coast (larger anomalies at inland sites), topography (larger anomalies in topographical low points) and land cover (larger anomalies over vegetated surfaces). In this mid-morning eclipse, the topographical influences on NSAT anomalies were apparently dominated by variations in residual nocturnal inversion strength, as suggested by significant correlations between post-sunrise temperature and NSAT anomaly at clear-sky sites (larger negative anomalies with lower post-sunrise temperatures). The largest NSAT anomaly occurred at a coastal site where flow transitioned from onshore to offshore during the eclipse, in a situation with large coastal temperature gradients associated with antecedent nocturnal cooling. This article is part of the themed issue ‘Atmospheric effects of solar eclipses stimulated by the 2015 UK eclipse’.
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Cattaneo, M., i C. Eva. "Propagation anomalies in Northwestern Italy by inversion of teleseismic residuals". Terra Nova 2, nr 6 (listopad 1990): 577–84. http://dx.doi.org/10.1111/j.1365-3121.1990.tb00123.x.
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Liu, Yangjun (Kevin), Michelle Ellis, Mohamed El-Toukhy i Jonathan Hernandez. "Basin-wide rock-physics analysis in Campeche Basin, Gulf of Mexico — Phase II: Reservoir rock and fluid properties". Leading Edge 40, nr 10 (październik 2021): 716–22. http://dx.doi.org/10.1190/tle40100716.1.
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We present a basin-wide rock-physics analysis of reservoir rocks and fluid properties in Campeche Basin. Reservoir data from discovery wells are analyzed in terms of their relationship between P-wave velocity, density, porosity, clay content, Poisson's ratio (PR), and P-impedance (IP). The fluid properties are computed by using in-situ pressure, temperature, American Petroleum Institute gravity, gas-oil ratio, and volume of gas, oil, and water. Oil- and gas-saturated reservoir sands show strong PR anomalies compared to modeled water sand at equivalent depth. This suggests that PR anomalies can be used as a direct hydrocarbon indicator in the Tertiary sands in Campeche Basin. However, false PR anomalies due to residual gas or oil exist and compose about 30% of the total anomalies. The impact of fluid properties on IP and PR is calibrated using more than 30 discovery wells. These calibrated relationships between fluid properties and PR can be used to guide or constrain amplitude variation with offset inversion for better pore fluid discrimination.
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Essa, Khalid S. "A Fast Interpretation Method for Inverse Modeling of Residual Gravity Anomalies Caused by Simple Geometry". Journal of Geological Research 2012 (22.03.2012): 1–10. http://dx.doi.org/10.1155/2012/327037.
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An inversion technique using a fast method is developed to estimate, successively, the depth, the shape factor, and the amplitude coefficient of a buried structure using residual gravity anomalies. By defining the anomaly value at the origin and the anomaly value at different points on the profile, the problem of depth estimation is transformed into a problem of solving a nonlinear equation of the form . Knowing the depth, the shape factor can be estimated and finally the amplitude coefficient can be estimated. This technique is applicable for a class of geometrically simple anomalous bodies, including the semiinfinite vertical cylinder, the infinitely long horizontal cylinder, and the sphere. The efficiency of this technique is demonstrated with gravity anomaly due to a theoretical model, in each case with and without random errors. Finally, the applicability is illustrated using the residual gravity anomaly of Mobrun ore body, situated near Noranda, QC, Canada. The interpreted depth and the other model parameters are in good agreement with the known actual values.
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García-Abdeslem, Juan. "Nonlinear inversion of isostatic residual gravity data from Montage Basin, northern Gulf of California". GEOPHYSICS 82, nr 3 (1.05.2017): G45—G55. http://dx.doi.org/10.1190/geo2016-0144.1.
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The flexural isostatic response to surface loads is used to estimate the crustal thickness in northwestern Mexico and Southwestern USA. This estimate is used to compute an isostatic regional gravity, which, subtracted from Bouguer gravity anomalies, led to the isostatic residual gravity anomaly at Montage Basin. This basin is located between the southern portion of the Mexicali Valley and the northern Gulf of California, it roughly has an extension of [Formula: see text] wide, and it shows a gravity minimum reaching approximately [Formula: see text]. Montage Basin is within the extensional province of the Gulf of California, where rifting is currently an ongoing geologic process, and deep exploratory wells drilled by Petróleos Mexicanos have shown that the basin accommodates thick sedimentary sequences greater than 5 km. The interpretation of the isostatic residual gravity anomaly is considered as a nonlinear inverse problem, constrained using density as a function of depth derived from Gardner’s equation applied to dual time [Formula: see text]-logs, assuming isostatic equilibrium and considering the basin as a subsurface load that is compensated at depth by a mass of unknown shape and density. The outcome of the inverse problem suggests that Montage Basin accommodates as much as 7.5 km thick sedimentary sequences and a compensating mass at a minimum depth of 13 km.
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Singh, Anand, i Arkoprovo Biswas. "Application of Global Particle Swarm Optimization for Inversion of Residual Gravity Anomalies Over Geological Bodies with Idealized Geometries". Natural Resources Research 25, nr 3 (19.10.2015): 297–314. http://dx.doi.org/10.1007/s11053-015-9285-9.
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Ekinci, Yunus Levent, Çağlayan Balkaya, Gökhan Göktürkler i Şenol Özyalın. "Gravity data inversion for the basement relief delineation through global optimization: a case study from the Aegean Graben System, western Anatolia, Turkey". Geophysical Journal International 224, nr 2 (15.10.2020): 923–44. http://dx.doi.org/10.1093/gji/ggaa492.
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SUMMARY Aegean Graben System is a significant member of the complex geology of western Turkey. The depths to the metamorphic basement reliefs in two major grabens have been reported by many geophysical studies. However, the sediment thicknesses of these graben basins still remain controversial due to the findings differing from each other. Thus, we have inverted the gravity data of the sedimentary cover–metamorphic basement using a stochastic derivative-free vector-based metaheuristic named differential evolution algorithm (DEA). This is the first application of DEA adapted to the basement relief depth problem. Model parametrizations have been achieved by discretizing the basins using a group of juxtaposed vertical blocks. Before the inversion studies, mathematical nature of the inverse problem has been investigated via prediction cost function/error energy maps for some block pairs using a hypothetical basin model. These maps have shown the resolvability characteristic of the block thicknesses on such inversion problem. Parameter tuning studies for the optimum mutation constant/weighting factor have been performed to increase the efficiency of the algorithm. The synthetic data have been successfully inverted via the tuned control parameter and some smoothing operators. Probability density function (PDF) analyses have shown that the best solutions are within the confidence interval limits without uncertainties. In the field data case, long-wavelength anomalies caused by both crustal and deeper effects have been removed from the complete Bouguer anomalies through 2-D finite element method using the element shape functions. Some profiles extracted from the residual gravity anomaly map have been used for the inversion and obtained results have shown that the maximum depths to the metamorphic basement reliefs in the grabens are shallower than the findings of the previous studies. Information obtained from the lithological logs drilled in the grabens has supported our results. Moreover, PDF analyses have indicated the reliability of the obtained solutions without uncertainties.
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Minardi, Suhayat, Ahmad Mudyanto i Hiden Hiden. "Subsurface Structure Models Of Sumbawa Island And Flores Back Arc Thrust Based On Gravity Data". Jurnal Penelitian Pendidikan IPA 7, nr 3 (8.07.2021): 414. http://dx.doi.org/10.29303/jppipa.v7i3.826.
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Based on the BMKG report on Wednesday, July 29, 2018, at 05.47 WIB, an earthquake with a magnitude of 6.4 SR occurred at the epicenter at a depth of 24 km and was 47 km northeast of the island of Lombok. They are allegedly originating from the Back Arc Thrust Flores, the interaction between the Indo-Australian Plate and the Eurasian plate. This study aims to analyze the subsurface structure of the island of Sumbawa and Flores sea based on the distribution of density values. The analysis was carried out by modeling the subsurface structure based on regional gravity field anomaly data. The data used is secondary data downloaded from the topex.ucsd.edu page 18,400 measuring points. Data processing consists of data correction, separation of anomalies, determination of the Second Vertical Derivative (SVD) value, and 3D and 2D inversion modeling. The data corrections performed are Bouguer correction and terrain correction to produce the total gravity anomaly value. Upward continuation is used to separate regional anomalies and residual anomalies. Analysis of the Second Vertical Derivative (SVD) value was carried out to identify rock contact positions and fault structures. The 3D inversion modeling is done by making a mesh model, and to get a 2D cross-section, six slices are used in the 3D model. The results of the 3D model and 2D cross-section, namely the island of Sumbawa to the Back Arc Thrust zone of Flores, consists of 3 main layers comprised of the upper crust with a density of 2.29 gr/cc - 2.63 gr/cc, the middle crust with a density of 2.64 gr/cc - 2.90 gr/cc, and the lower crust with a density of 2.91 gr/cc - 3.14 gr/cc, and based on the model and SVD value, it shows that the Flores back fault, the reverse fault type, starts at a depth of ± 26 km
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Guspí, Fernando, i Beatriz Introcaso. "A sparse spectrum technique for gridding and separating potential field anomalies". GEOPHYSICS 65, nr 4 (lipiec 2000): 1154–61. http://dx.doi.org/10.1190/1.1444808.
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The separation of regional and residual potential field anomalies, regarded as a spectral problem, can be greatly facilitated when a spectrum estimate shows a clear break between low‐ and high‐frequency components, a feature that normal fast‐Fourier‐transform (FFT) methods fail to present. In this work, we model the discrete Fourier transform of a potential field, measured at stations irregularly distributed on a surface, by means of a high‐resolution sparse estimate derived originally for seismic signal processing. The coefficients of this estimate, which are distributed according to the Cauchy probability law, produce a model with only few components having a significant value. A steepest‐descent algorithm gives a computing alternative to large matrix multiplications and inversions. Advantages of taking this approach are twofold. First, the high‐resolution transform can be used as a gridding tool to evaluate the potential field either on a horizontal plane or on the topographic surface. The enhancement of the spectral peaks and the virtual absence of sidelobes prevents oscillations and edge effects in the result. Secondly, the sparse distribution of the spectral elements allows the interpreter to locate clearly the low‐frequency components related to the regional field. After a second and faster pass, the values of those coefficients can be redefined in order to obtain a more robust separation, ajusting the residuals by the Cauchy criterion. A theoretical noise‐free example to separate the magnetic anomaly of a prism from a polynomial background illustrates well the difference between sparse and FFT spectra. An example with real Bouguer anomalies in the Interserrana basin, Argentina, shows that gridding results, in this case reduced to sea level, compare well with those obtained by other gridding methods, and that the separation procedure is able to outline well defined areas of positive and negative residual anomalies.
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Al-Rahim, Ali M. "Gravity Study using Multi-2.5D Modeling and 3D Presentation for Al Ma'aniyah Depression, Southwest of Iraq". Iraqi Geological Journal 54, nr 2D (31.10.2021): 113–24. http://dx.doi.org/10.46717/igj.54.2d.9ms-2021-10-28.
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Tectonic depression area within and/or beside widespread basin is regarded as an important location for sub-basin sedimentary sequence of Iraq which may represent an excellent accumulation of bounded sediments. Al-Ma'aniyah depression, southwest Iraq is one of such type of sub-basin. Free-air gravity data show a NS extend of this depression inside Saudi Arabia. This work focuses on studying and multi-2.5D model creation for the depression in the Iraqi territory part using Bouguer gravity data and mapping its basement relief. Firstly, the exact boundary of the depression was outlined utilizing the Free-Air gravity data. Then, a precise selection of regional field for the study area was determined by using the power spectrum method, which accordingly defines the residual anomalies that could represent structural enclosures. Many positive anomalies were assigned and enhanced using vertical and total horizontal derivatives, where they were interpreted as basement-related features. Subsequently, a 2.5D multi modeling and depth inversion for the Bouguer gravity data were accomplished by converting the gravity map to a stacked profiles depth map. A nineteen gravity profiles, which cover the study area, were modeled by assuming 2D intra-sedimentary bodies. These bodies were best presented by a 3D view that clarifies the nature of the subsurface modeled structures. The modeling shows an extra density at the northern part of the depression, in contrast, it suggests low density bodies at its southern part, the case that appears inconsistent with a previously performed magnetic interpretation. The inversion of gravity data shows that the basement depth at Al-Ma'aniyah depression ranges from 7.5 to10 km.
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Setiadi, I., Marjiyono i T. B. Nainggolan. "Gravity Data Analysis Based on Optimum Upward Continuation Filter and 3D Inverse Modelling (Case Study at Sedimentary Basin in Volcanic Region Malang and Its Surrounding Area, East Java)". IOP Conference Series: Earth and Environmental Science 873, nr 1 (1.10.2021): 012008. http://dx.doi.org/10.1088/1755-1315/873/1/012008.
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Abstract The study on the fore-arc sedimentary basin for hydrocarbon exploration is rare because of the more complicated geological structures, and conventional seismic methods cannot optimally penetrate the rock layers as there are many volcanic and limestone rocks. One of the natural resources potential in the Southern part of the East Java region, especially in Malang and its surrounding areas is the possibility of hydrocarbons in the fore-arc basin, so research is needed to know the existence of these sedimentary basins. The gravity method is one of the geophysical methods used to assess sedimentary basins based on physical parameters of mass density. The aims of this research are to delineate the sedimentary sub-basin, to find out its structure pattern, interpret subsurface geological and basement configuration. The data analysis approach used in this study involves spectral analysis, upward continuation filter, and 3D inverse modeling. The maximum height for the optimum upward continuation filter is 3000 m, which results in regional and residual anomalies. There were five sedimentary sub-basins identified based on residual gravity anomaly, and the gravity anomalies can also detect structure patterns such as basement high, lineament, and fault pattern. The bedrock is supposed as an intermediate igneous rock with a mass density of around 2.7 gr/cc according to the results of 3D inverse modeling. Deposition from bottom to upward is Mandalika, Nampol, and Wonosari Formations and completed by the uppermost are quaternary volcanic rocks. The inversion modeling results show that the Malang and surrounding areas have thick sedimentary rocks covered by volcanic deposits, which is impressive for further investigation to explore the possibility of the hydrocarbon existence in these areas.
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Fullea, J., S. Lebedev, Z. Martinec i N. L. Celli. "WINTERC-G: mapping the upper mantle thermochemical heterogeneity from coupled geophysical–petrological inversion of seismic waveforms, heat flow, surface elevation and gravity satellite data". Geophysical Journal International 226, nr 1 (10.03.2021): 146–91. http://dx.doi.org/10.1093/gji/ggab094.
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SUMMARY We present a new global thermochemical model of the lithosphere and underlying upper mantle constrained by state of the art seismic waveform inversion, satellite gravity (geoid and gravity anomalies and gradiometric measurements from ESA's GOCE mission), surface elevation and heat flow data: WINTERC-G. The model is based upon an integrated geophysical–petrological approach where seismic velocities and density in the mantle are computed within a thermodynamically self-consistent framework, allowing for a direct parametrization in terms of the temperature and composition variables. The complementary sensitivities of the data sets allow us to constrain the geometry of the lithosphere–asthenosphere boundary, to separate thermal and compositional anomalies in the mantle, and to obtain a proxy for dynamic surface topography. At long spatial wavelengths, our model is generally consistent with previous seismic (or seismically derived) global models and earlier integrated studies incorporating surface wave data at lower lateral resolution. At finer scales, the temperature, composition and density distributions in WINTERC-G offer a new state of the art image at a high resolution globally (225 km average interknot spacing). Our model shows that the deepest lithosphere–asthenosphere boundary is associated with cratons and, also, some tectonically active areas (Andes, Persian Gulf). Among cratons we identify considerable differences in temperature and composition. The North American and Siberian Cratons are thick (>260 km) and compositionally refractory, whereas the Sino-Korean, Aldan and Tanzanian Cratons have a thinner, fertile lithosphere, similar to younger continental lithosphere elsewhere. WINTERC-G shows progressive thickening of oceanic lithosphere with age, but with significant regional differences: the lithospheric mantle beneath the Atlantic and Indian Oceans is, on average, colder, more fertile and denser than that beneath the Pacific Ocean. Our results suggest that the composition, temperature and density of the oceanic mantle lithosphere are related to the spreading rate for the rates up to 50–60 mm yr–1: the lower spreading rate, the higher the mantle fertility and density, and the lower the temperature. At greater spreading rates, the relationship disappears. The 1-D radial average of WINTERC-G displays a mantle geothermal gradient of 0.55–0.6 K km–1 and a potential temperature of 1300–1320 °C for depths >200 km. At the top of the mantle transition zone the amplitude of the maximum lateral temperature variations (cratons versus hotspots) is about 120 K. The isostatic residual topography values, a proxy for dynamic topography, are large (>1 km) mostly in active subduction settings. The residual isostatic bathymetry from WINTERC-G is remarkably similar to the pattern independently determined based on oceanic crustal data compilations. The amplitude of the continental residual topography is relatively large and positive (>600 m) in the East European Craton, Greenland, and the Andes and Himalayas. By contrast, central Asia, most of Antarctica, southern South America and, to a lesser extent, central Africa are characterized by negative residual topography values (>–400 m). Our results show that a substantial part of the topography signal previously identified as residual (or dynamic) is accounted for, isostatically, by lithospheric density variations.
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Luo, Song, Huajian Yao, Jiannan Wang, Kangdong Wang i Bin Liu. "Direct inversion of surface wave dispersion data with multiple-grid parametrizations and its application to a dense array in Chao Lake, eastern China". Geophysical Journal International 225, nr 2 (25.01.2021): 1432–52. http://dx.doi.org/10.1093/gji/ggab036.
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SUMMARY The direct surface wave tomography has become an efficient tool in imaging 3-D shallow Earth structure. However, some fundamental problems still exist in selecting the grids to parametrize the model space. This study proposes to implement a model parametrization approach with multiple grids to the direct surface wave tomography. These multiple grids represent several overlapping collocated grids with the same or different grid spacings, such as staggered grids, multiscale grids and multiscale-staggered grids. At each iteration, direct inversion is applied to each individual set of collocated grids to invert for the shear wave velocity (Vs) model; the models are then projected onto a set of predefined base grids (usually the finest grids) using 3-D B-spline interpolation. At the end of each iteration, we average the Vs models of all sets of collocated grids to obtain the average 3-D Vs model, which is then used as the initial model for the next iteration. The properties of this approach are explored by applying it to a newly deployed dense array in Chao Lake (CL), eastern China. Synthetic and field data tests demonstrate that the method using multiple grids recovers anomaly patterns better than that using the individual set of collocated grids, though it does not necessarily achieve the smallest traveltime residual. We then obtain a high-resolution 3-D shallow crustal Vs model beneath the CL. The 3-D Vs model reveals two prominent features: (1) a stripe-like structural pattern of velocity variations, where the Hefei basin and eastern CL display low-velocity anomalies while the Tanlu fault zone (TFZ), Zhangbaling uplift and Yinping mountain present high-velocity anomalies and (2) north-shifted low-velocity anomalies beneath the eastern CL as depths go shallow. The shallow Vs features are consistent well with the local geological units and topography. We suggest that the two main features could be associated with the multistage tectonic activities of the Tanlu fault. The multiple-grid scheme proposed in this study could be conveniently extended to other 3-D direct inversion approaches in the near future.
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Martin, Tina, Kerstin Kuhn, Thomas Günther i Rudolf Kniess. "Geophysical Exploration of a Historical Stamp Mill Dump for the Volume Estimation of Valuable Residues". Journal of Environmental and Engineering Geophysics 25, nr 2 (czerwiec 2020): 275–86. http://dx.doi.org/10.2113/jeeg19-080.
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We present an approach for the estimation of ore processing residue volumes in historical mine waste dumps by the use of different geophysical methods in combination with mineralogical investigations. The stamp mill dump in the Harz mountains, Germany was examined with the methods electrical resistivity tomography (ERT), ground penetrating radar (GPR) and spectral induced polarization (SIP) flanked by mineralogical studies at many drilling points. The mineralogical results were used to calibrate the geophysical results and to distinguish between valuable and non-valuable waste material. With SIP we investigated individual profiles and took lab samples. These lab results emphasize the differences between the fine-grained tailings of clayey silt to silty sand in the top layer and the sandy tailings underneath in both resistivity and phase. From the GPR results we can distinguish between different layers and various backfillings in the first two meters due to the much higher resolution than the other methods. From ERT we achieved an overview about the dimension and inner structure of the dump and the boundary between the sandy residual material and the host rock. To estimate the volume of the residual body we carried out 2D inversion of all ERT profiles followed interpolation between the inverted profiles. From the drilling interpretation, the SIP lab results and the ERT field measurements we defined a resistivity threshold of 350 ohm-m for the ore processing residues to achieve a 3-dimensional body of the dump. The volume of this body was then corrected by a factor due to consideration of uncertainties, e.g., forest areas, inaccessible dump sections, small-scale anomalies (geological or different anthropogenic nature) and inversion coverage. As a result, we were able to calculate the volume of the ore processing residues which can be used further for the determination of the economic potential (remaining metal content).
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Liu, Yan-Xu, Wen-Yong Li, Zhi-Yuan Liu, Jia-Wei Zhao, An-Qi Cao, Shan Gao, Li-Jie Wang i Cheng Yang. "Occurrence Characteristics of Magnetite and Aeromagnetic Prospecting Northeast of Hebei Province". Minerals 12, nr 9 (14.09.2022): 1158. http://dx.doi.org/10.3390/min12091158.
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The occurrence characteristics of magnetite and the methods to quickly and effectively explore are important topics for ore prospecting in the new era. Taking northeast of Hebei Province of China as an example, this article aimed at an important strategic mineral of magnetite, then discussed its distribution characteristics and aeromagnetic exploration methods of it. First of all, we discuss the occurrence characteristics of sedimentary metamorphic and magmatic magnetite. Then, using the latest high-precision aeromagnetic data, combined with the geological outcrops, known iron deposits, ground magnetic surveys, and verification, we studied the relationship between the aeromagnetic anomalies and iron deposits through potential field conversion processing of the reduction to the pole, vertical derivative, upward continuation and residual anomaly, and the forward modeling and inversion methods of 2.5 D optimization fitting. Next, we summarize the metallogenic conditions and attributes of aeromagnetic prospecting and make magnetite predictions. In addition, it has suitable magnetite prospecting potential in the Laochenjia, Dabai, Jiuwuying, Beierying, Sidaogoumen, and Wuyingzi aeromagnetic anomaly regions. In conclusion, these regions have aeromagnetic anomalies with high amplitudes, large scales, and favorable metallogenic backgrounds for magmatic rocks, strata, and structures caused by concealed magnetite. In addition, they have great prospecting potential. Eventually, we hope this research method in this article can provide a reference for magnetite exploration in other areas with similar geological conditions.
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McLaughlin, K. L., J. R. Murphy i B. W. Barker. "A lithospheric velocity anomaly beneath the Shagan river Test Site. Part 2. Imaging and inversion with amplitude transmission tomography". Bulletin of the Seismological Society of America 82, nr 2 (1.04.1992): 999–1017. http://dx.doi.org/10.1785/bssa0820020999.
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Abstract A linear inversion procedure is introduced that images weak velocity anomalies using amplitudes of transmitted seismic waves. Using projection operators from geometrical ray theory, an image of an anomaly is constructed from amplitudes recorded at arrays of receivers using arrays of sources. The image is related to the velocity anomaly by a second-order partial-differential equation that is inverted using 2-D discrete Fourier transforms. As an example of the inversion procedure, magnitude residuals for European stations recording Shagan River explosions are used to image the deep lithospheric anomaly beneath the Shagan River test site described in Part 1. This formal inversion analysis confirms the existence of a small-scale lateral heterogeneity located 50 km west-northwest of the test site at a probable depth between 80 and 100 km and indicates that it is consistent with a deterministic 1.5% peak-to-peak (or 0.5% rms) velocity anomaly with a scale length of about 3 km. 3-D dynamic raytracing is then used to verify that the inferred laterally varying structure produces amplitude fluctuations consistent with observations.
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Khesin, Boris, Yevgeny Vapnik i Sonya Itkis. "Geophysical evidence of deep hydrocarbon flow in Mottled Zone areas, Dead Sea Transform zone". GEOPHYSICS 75, nr 3 (maj 2010): B91—B101. http://dx.doi.org/10.1190/1.3375236.
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The origin of unusual magnetic initially sedimentary rocks of the Mottled Zone (MZ) in Israel and Jordan remained enigmatic for several decades until integrated characterization of the MZ area was achieved by ground magnetic measurements and geologic observations along representative profiles in parallel with reprocessing and reinterpretation of available aeromagnetic and gravity data. Micromagnetic profiling was combined with gamma-radioactivity measurements, and representative samples were selected to determine rock physical properties. Results of field measurements, reduction and transformation of geophysical fields, anomaly inversion, and forward modeling accompanied by geologic analysis suggest that two types of magnetic anomalies and local gravity minima in the MZ areas are related to the same event, i.e., deep hydrocarbon flow associated with fossil mud volcanism. Physicochemical interaction of deep hydrocarbon flow and surrounding sedimentary rocks caused widespread weak magnetization and corresponding aeromagnetic anomalies. Other scattered heterogeneous magnetization and linked ground magnetic anomalies are common for surface/near-surface local sources and are caused by the burning of combustible gases ejected by mud volcanoes; such origin of the magnetization is confirmed by magnetic measurements of burned rocks in mud volcano areas of the Caucasus. Increased radioactivity of the lower part of the MZ likely indicates mud ejection from a deep uranium-enriched source. Locations of MZ outcrops commonly coincide with residual gravity minima, showing zones of disintegration and possible hydrocarbon accumulations. Seismic prospecting data support the existence of disintegration zones at depth. Results show the need for additional geophysical studies and a potential for revealing hydrocarbon accumulations in the MZ areas, particularly at the Halamish, Nevatim, Ma’ale Adumim, and Nabi Musa sites.
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Lakshmanan, J. "The generalized gravity anomaly: Endoscopic microgravity". GEOPHYSICS 56, nr 5 (maj 1991): 712–23. http://dx.doi.org/10.1190/1.1443090.
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Various underground 3-D gravity surveys have necessitated a generalization of the usual gravity corrections and of the Bouguer anomaly. The method presented here compares raw, time‐dependent gravity measurements, to a model’s total theoretical field, including known fields: moon, sun, 1967 Reference Ellipsoid, oceans; partially known fields: due to a single digital terrain model of known geometry but of unknown densities; and unknown fields due to underground structures of unknown shapes and of unknown densities. For a single‐density model, the corresponding first‐degree residual is close in concept to the Bouguer anomaly. To best determine underground structure, generalized inversion then leads to determination of the one or several densities and of one or several “regional” parameters, which minimize residuals. The suggested method is mainly advantageous in special types of gravity surveys, such as rugged terrain, or in the case of underground surveys, where conventional corrections, with a preset terrain density can possibly lead to substantial errors. Two field examples are developed (1) the Cheops pyramid survey, where the processing of gravity measurements inside, above, and around the pyramid led to an evaluation of the structure’s overall density and of density changes in the structure; and (2) the Coche hydroelectric tunnel in the Alps, where the method leads to a 3-D model explaining the very strong gravity anomalies observed in the tunnel and on the mountain above it.
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Boddupalli, Bhargav, Tim A. Minshull, Joanna Morgan, Gaye Bayrakci i Dirk Klaeschen. "Comparison of 2-D and 3-D full waveform inversion imaging using wide-angle seismic data from the Deep Galicia Margin". Geophysical Journal International 227, nr 1 (11.05.2021): 228–56. http://dx.doi.org/10.1093/gji/ggab164.
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SUMMARY Full waveform inversion (FWI) is a data-fitting technique capable of generating high-resolution velocity models with a resolution down to half the seismic wavelength. FWI is applied typically to densely sampled seismic data. In this study, we applied FWI to 3-D wide-angle seismic data acquired using sparsely spaced ocean bottom seismometers (OBSs) from the Deep Galicia Margin west of Iberia. Our data set samples the S-reflector, a low-angle detachment present in this area. Here we highlight differences between 2-D, 2.5-D and 3-D-FWI performances using a real sparsely spaced data set. We performed 3-D FWI in the time domain and compared the results with 2-D and 2.5-D FWI results from a profile through the 3-D model. When overlaid on multichannel seismic images, the 3-D FWI results constrain better the complex faulting within the pre- and syn-rift sediments and crystalline crust compared to the 2-D result. Furthermore, we estimate variable serpentinization of the upper mantle below the S-reflector along the profile using 3-D FWI, reaching a maximum of 45 per cent. Differences in the data residuals of the 2-D, 2.5-D and 3-D inversions suggest that 2-D inversion can be prone to overfitting when using a sparse data set. To validate our results, we performed tests to recover the anomalies introduced by the inversions in the final models using synthetic data sets. Based on our comparison of the velocity models, we conclude that the use of 3-D data can partially mitigate the problem of receiver sparsity in FWI.
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Takanashi, Mamoru, i Ilya Tsvankin. "Correction for the influence of velocity lenses on nonhyperbolic moveout inversion for VTI media". GEOPHYSICS 76, nr 3 (maj 2011): WA13—WA21. http://dx.doi.org/10.1190/1.3569799.
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Nonhyperbolic moveout analysis plays an increasingly important role in velocity model building because it provides valuable information for anisotropic parameter estimation. However, lateral heterogeneity associated with stratigraphic lenses such as channels and reefs can significantly distort the moveout parameters, even when the structure is relatively simple. We analyze the influence of a low-velocity isotropic lens on nonhyperbolic moveout inversion for horizontally layered VTI (transversely isotropic with a vertical symmetry axis) models. Synthetic tests demonstrate that a lens can cause substantial, laterally varying errors in the normal-moveout velocity [Formula: see text] and the anellipticity parameter [Formula: see text]. The area influenced by the lens can be identified using the residual moveout after the nonhyperbolic moveout correction as well as the dependence of errors in [Formula: see text] and [Formula: see text] on spreadlength. To remove such errors in [Formula: see text] and [Formula: see text], we propose a correction algorithm designed for a lens embedded in a horizontally layered overburden. This algorithm involves estimation of the incidence angle of the ray passing through the lens for each recorded trace. With the assumption that lens-related perturbation of the raypath is negligible, the lens-induced traveltime shifts are computed from the corresponding zero-offset time distortion (i.e., from “pull-up” or “push-down” anomalies). Synthetic tests demonstrate that this algorithm substantially reduces the errors in the effective and interval parameters [Formula: see text] and [Formula: see text]. The corrected traces and reconstructed “background” values of [Formula: see text] and [Formula: see text] are suitable for anisotropic time imaging and producing a high-quality stack.
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Lewerissa, Richard. "Overview of Geothermal Potential in Momiwaren, South Manokwari Regency using Earth Gravity Field Analysis". Kasuari: Physics Education Journal (KPEJ) 3, nr 2 (2.01.2021): 97–109. http://dx.doi.org/10.37891/kpej.v3i2.142.
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West Papua province has a geothermal resource that can be established as renewable electricity energy or tourism. One of the locations for geothermal prospects is in the Momiwaren district of South Manokwari regency. A preliminary study in form of a qualitative interpretation has been conducted through the analysis of high-resolution of earth gravity fields from the Global Gravity Map developed by Curtin University, Australia. This research was performed to increase the boundary of the geological structure which indicates the existence of a source of manifestation of hotsprings and the fault structures that control it. The study begins with the reduction of the gravity field data to obtain a complete Bouguer anomaly based on the SRTM2gravity terrain model correction. Furthermore, regional and residual gravity anomalies are separated, and vertical and horizontal gravitational gradient analysis using 2-D fast Fourier transform. Qualitative interpretation produces models and boundaries of the main structure and the corresponding Demini and Gaya Baru faults and the distribution of the geological rock formations in the study area. This interpretation provides useful information as a constraint for quantitative interpretation through subsurface inversion modeling to obtain detailed geothermal models in Momiwaren.
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Liu, Bin, Wenkun Yu, Wujiao Dai, Xuemin Xing i Cuilin Kuang. "Estimation of Terrestrial Water Storage Variations in Sichuan-Yunnan Region from GPS Observations Using Independent Component Analysis". Remote Sensing 14, nr 2 (8.01.2022): 282. http://dx.doi.org/10.3390/rs14020282.
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GPS can be used to measure land motions induced by mass loading variations on the Earth’s surface. This paper presents an independent component analysis (ICA)-based inversion method that uses vertical GPS coordinate time series to estimate the change of terrestrial water storage (TWS) in the Sichuan-Yunnan region in China. The ICA method was applied to extract the hydrological deformation signals from the vertical coordinate time series of GPS stations in the Sichuan-Yunnan region from the Crustal Movement Observation Network of China (CMONC). These vertical deformation signals were then inverted to TWS variations. Comparative experiments were conducted based on Gravity Recovery and Climate Experiment (GRACE) data and a hydrological model for validation. The results demonstrate that the TWS changes estimated from GPS(ICA) deformations are highly correlated with the water variations derived from the GRACE data and hydrological model in Sichuan-Yunnan region. The TWS variations are overestimated by the vertical GPS observations the northwestern Sichuan-Yunnan region. The anomalies are likely caused by inaccurate atmospheric loading correction models or residual tropospheric errors in the region with high topographic variability and can be reduced by ICA preprocessing.
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Ibe Alexander Omenikolo, Terhemba Theophilus Emberga i Alexander Iheanyichukwu Opara. "Basement depth re-valuation of anomalous magnetic bodies in the lower and middle Benue trough using Euler deconvolution and spectral inversion techniques". World Journal of Advanced Research and Reviews 14, nr 2 (30.05.2022): 129–45. http://dx.doi.org/10.30574/wjarr.2022.14.2.0356.
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Spectral technique and Euler Deconvolution were applied to high-resolution aeromagnetic data of parts of Benue trough to estimate the depth of anomalous magnetic sources within the study area; Data enhancement techniques such as total magnetic intensity map, reduction to pole, regional-residual separation and upward continuation maps were employed to identify different magnetic anomalies, structural trends representing the tectonics of the location were observed trending NE-SW and N-S directions; The result of 3D Euler deconvolution for the structural index (SI) = 0, 1, 2, 3 gave depths to magnetic sources that range from -589;3 m to -2678;8 m, -459;0 m to -2691;9 m, -294;6 m to -2817;5 m, - 430;2 m to -2780;6 m respectively; The depth estimates from 2-D spectral revealed a two-layer model; The shallow magnetic depth ranges between 0;135 km to 0;200 km with a mean depth of 0;158 km and the depth to magnetic basement vary between 2;585 km to 4;878 km with a mean depth of 3;415 km; This result, therefore, indicates that the average basement depth of the study area obtained from the spectral analysis is 3;415km; This investigation, therefore provides appropriate sedimentary thickness for suitable hydrocarbon prospecting within the study area.
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Eshaghzadeh, Ata, i Alireza Hajian. "Multivariable Modified Teaching Learning Based Optimization (MM-TLBO) Algorithm for Inverse Modeling of Residual Gravity Anomaly Generated by Simple Geometric Shapes". Journal of Environmental and Engineering Geophysics 25, nr 4 (grudzień 2020): 463–76. http://dx.doi.org/10.32389/jeeg20-003.
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This paper presents an improved nature-based algorithm, namely multivariable modified teaching learning based optimization (MM-TLBO) algorithm, as in an iterative process can estimates the best values for the model parameters in a multi-objective problem. The algorithm works in two computational phases: the teacher phase and the learner phase. The major purpose of the MM-TLBO algorithm is to improve the value of the learners and thus, improving the value of the model parameters which leads to the optimal solution. The variables of each learner (model) are the radius ( R), depth ( h), shape factor ( q), density contrast ( ρ) and axis location ( x0) parameters. We apply MM-TLBO and TLBO methods for the residual gravity anomalies caused by the buried masses with a simple geometry such as spheres, horizontal and vertical cylinders. The efficiency of these methods are also tested by noise corruption synthetic data, as the acceptable results were obtained. The obtained results indicate the better performance the MM-TLBO algorithm than the TLBO algorithm. We have utilized the MM-TLBO for the interpretation of the six residual gravity anomaly profiles from Iran, USA, Sweden and Senegal. The advantage of the MM-TLBO inversion is that it can estimates the best solutions very fast without falling into local minimum and reaches to a premature convergence. The considered primary population for the synthetic and real gravity data are thirty and fifty models. The results show which this method is able to achieve the optimal responses even if a small population of learners had been considered.
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Zhang, Pingchuan, Changqing Yu i Xiangzhi Zeng. "Crustal Electrical Structure of the Zhaheba Complex Imaged by Magnetotelluric Data and Its Tectonic Implications". Applied Sciences 11, nr 21 (26.10.2021): 10013. http://dx.doi.org/10.3390/app112110013.
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A Magnetotelluric profile stretching northward from the Wulungu Depression (on the northern margin of the Junggar Basin) to the Dulate arc (crossing the Zhaheba–Aermantai ophiolite belt) was carried out in an attempt to probe the crustal structure and properties of the East Junggar, NW China. Along the profile, the inversion model was used to determine the electrical structure of the crust and uppermost mantle. The results revealed that the crust of the eastern Junggar Basin is composed of the shallow low resistivity layer and underlying high resistivity bodies. There is a crustal detachment in the basement: the upper layer is a Hercynian folded basement and the lower is a Precambrian basement. The Zhaheba complex is characterized by relatively high resistivity, with a thickness of ~5 km, the bottom controlled by the Zhaheba–Aermantai fault. The crust of the Yemaquan arc is composed of the residual continental crust, characterized by stable resistance. The exposed intrusive rocks are characterized by irregular resistors. The crust of the Dulate arc is characterized by relatively low resistivity. The shallow low resistivity layers represent the Zhaheba depression composed of the Devonian-Permian volcanic and sedimentary rocks. The crustal conductive anomalies are related to the magmatism and mechanism of metal deposits in the post-collision period.
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Trojanowicz, Marek, Magdalena Owczarek-Wesołowska, Lubomil Pospíšil i Olgierd Jamroz. "Determination of the Selected Gravity Field Functionals by the GGI Method: A Case Study of the Western Carpathians Area". Applied Sciences 10, nr 21 (6.11.2020): 7892. http://dx.doi.org/10.3390/app10217892.
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In this paper, some features of the local disturbing potential model developed by the GGI method (based on Geophysical Gravity Inversion) were analyzed. The model was developed for the area of the Western Carpathians covering the Polish–Slovak border. A detailed assessment of the model’s property was made regarding the accuracy of the disturbing potential values (height anomalies), gravity values, complete Bouguer anomalies (CBA), and differences between geoid undulations and height anomalies (N−ζ). Obtained accuracies of the GGI quasigeoid model (in terms of standard deviation of the residuals to the reference quasigeoid models) were at the level of ±2.2 cm for Poland and ±0.9 cm for the Slovak area. In terms of gravity, there was shown dependence of the accuracy of the GGI model on the digital elevation model (DEM) resolution, the point height, the density of gravity data used, and used reference density of topography model. The best obtained results of gravity prediction were characterized by an error of approximately 1 mGal. The GGI approach were compared with classical gravity prediction methods (using CBA and topographic-isostatic anomalies supported by Kriging prediction), getting very similar results. On the basis of the GGI model, CBA and differences (N−ζ) were also determined. The strong dependence of resolution of the CBA model obtained by GGI approach, on the size of the constant density zones, has been demonstrated. This significantly reduces the quality of such a model. The crucial importance of the topographic masses density model for both determined values (CBA and (N−ζ)) was also indicated. Therefore, for determining these quantities, all available information on topographic mass densities should be used in modelling.
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Vasco, Don W., John E. Peterson i Ernest L. Majer. "Beyond ray tomography: Wavepaths and Fresnel volumes". GEOPHYSICS 60, nr 6 (listopad 1995): 1790–804. http://dx.doi.org/10.1190/1.1443912.
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Two techniques that account for the band‐limited nature of seismic data are incorporated into tomographic traveltime inversion schemes. The first technique, the wavepath algorithm, is based upon the wave equation, the Born approximation, and an adjoint method for computing Frechet derivatives. Computation of a single wavepath requires the forward propagation of the seismic wavefield, as well as the reverse propagation of a residual wavefield. The second technique, the Fresnel volume approach, is based upon the paraxial ray approximation. The Fresnel volume algorithm requires little more computation than does conventional ray tracing and an order of magnitude less computer time than our calculation of wavepaths. When the Fresnel volume sensitivity functions are normalized by the area of the Fresnel ellipse perpendicular to the ray, the sensitivity estimates are very similar to the wavepaths. In particular, there is heightened sensitivity to velocity structure near the source and receiver locations. The normalization by the Fresnel ellipse area is necessary to ensure ray theoretical results in the limit of infinite frequency. Tomographic inversion based upon wavepaths or Fresnel volumes is more appropriate when considering the arrival time of the peak of the initial pulse rather than the first‐arrival time. Furthermore, using the traveltime of the peak instead of the first‐arrival time reduces the bias of tomograms to high velocity anomalies. The raypath, wavepath, and Fresnel volume techniques were applied to a set of cross‐borehole traveltime observations gathered at the Grimsel Rock Laboratory. All methods imaged a low velocity fracture zone in the granitic site, in agreement with independent well information. Estimates of model parameter resolution are similar for the wavepath and Fresnel volume schemes. The source‐receiver regions are the most well resolved areas. However, the model parameter resolution computed using a conventional ray‐based formalism is more evenly distributed over the cross‐borehole area.
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Tschirhart, Peter, Bill Morris i Greg Hodges. "A new regional/residual separation for magnetic data sets using susceptibility from frequency-domain electromagnetic data". GEOPHYSICS 78, nr 6 (1.11.2013): B351—B359. http://dx.doi.org/10.1190/geo2012-0506.1.
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Regional-residual separation is a fundamental processing step required before interpreting any magnetic anomaly data. Numerous methods have been devised to separate deep-seated long-wavelength (regional) anomalies from the near-surface high-frequency (residual) content. Such methods range in complexity from simple wavelength filtering to full 3D inversions, but most procedures rely on the assumption that all long-wavelength anomalies are associated with deep source bodies: an incorrect assumption in some geologic environments. We evaluated a new method for determining the contributions of near-surface magnetic sources using frequency-domain helicopter-borne electromagnetic (HFEM) data. We inverted the in-phase and quadrature components of the HFEM data to produce an estimate of the spatial variation of magnetic susceptibility. Using this susceptibility information along with known topography and original survey flight path data, we calculated a magnetic intensity grid by forward modeling. There are two immediate benefits to this approach. First, HFEM systems have a limited effective depth of penetration, within the first hundred meters from the surface, so any magnetic sources detected by this method must be located in the near surface. Second, the HFEM-derived susceptibility is completely independent of magnetic remanence. In contrast, apparent susceptibility computed from the original magnetic intensity data incorporates all magnetic signal sources in its derivation. Crossplotting of [Formula: see text] versus [Formula: see text] served to reveal areas where the observed magnetic field was dominated by magnetic remanence and provided an estimate of the polarity of the remanence contribution. We evaluated an example, and discussed the limitations of this method using data from an area in the Bathurst Mining Camp, New Brunswick. Though it is broadly successful, caution is needed when using this method because near-surface conductive bodies and anthropogenic sources can cause erroneous HFEM susceptibility values, which in turn produce invalid magnetic field estimates in the forward modeling exercise.
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Shi, Huiyan, Tonglin Li, Rui Sun, Gongbo Zhang, Rongzhe Zhang i Xinze Kang. "Insights from the P Wave Travel Time Tomography in the Upper Mantle Beneath the Central Philippines". Remote Sensing 13, nr 13 (23.06.2021): 2449. http://dx.doi.org/10.3390/rs13132449.
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In this paper, we present a high resolution 3-D tomographic model of the upper mantle obtained from a large number of teleseismic travel time data from the ISC in the central Philippines. There are 2921 teleseismic events and 32,224 useful relative travel time residuals picked to compute the velocity structure in the upper mantle, which was recorded by 87 receivers and satisfied the requirements of teleseismic tomography. Crustal correction was conducted to these data before inversion. The fast-marching method (FMM) and a subspace method were adopted in the forward step and inversion step, respectively. The present tomographic model clearly images steeply subducting high velocity anomalies along the Manila trench in the South China Sea (SCS), which reveals a gradual changing of the subduction angle and a gradual shallowing of the subduction depth from the north to the south. It is speculated that the change in its subduction depth and angle indicates the cessation of the SCS spreading from the north to the south, which also implies that the northern part of the SCS opened earlier than the southern part. Subduction of the Philippine Sea (PS) plate is exhibited between 14° N and 9° N, with its subduction direction changing from westward to eastward near 13° N. In the range of 11° N–9° N, the subduction of the Sulu Sea (SS) lies on the west side of PS plate. It is notable that obvious high velocity anomalies are imaged in the mantle transition zone (MTZ) between 14° N and 9° N, which are identified as the proto-SCS (PSCS) slabs and paleo-Pacific (PP) plate. It extends the location of the paleo-suture of PSCS-PP eastward from Borneo to the Philippines, which should be considered in studying the mechanism of the SCS and the tectonic evolution in SE Asia.
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Abtahi, Sayyed Mohammad, Laust Börsting Pedersen, Jochen Kamm i Thomas Kalscheuer. "Consistency investigation, vertical gravity estimation, and inversion of airborne gravity gradient data — A case study from northern Sweden". GEOPHYSICS 81, nr 3 (maj 2016): B65—B76. http://dx.doi.org/10.1190/geo2014-0428.1.
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For airborne gravity gradient data, it is a challenge to distinguish between high-frequency intrinsic and dynamically produced noise caused by the aircraft and small-scale effects from shallow density variations. To facilitate consistent interpretation, techniques that include all of the measured gravity gradient components are particularly promising. We represented the measurements by a common potential function accounting for lateral and height variations. Thus, it was possible to evaluate the internal consistency between the measured components and to identify components with bias or particularly strong noise. As an extra benefit for data sets that contain terrain-corrected and nonterrain-corrected gravity gradient measurements at flight altitude, we estimated terrain-corrected anomalies on the topographic relief using downward continuation and retrieved nonterrain-corrected gravity gradient data suitable for inversion using upward continuation. For a field data set from northern Sweden, the largest differences (up to 50 eotvos) between the measured and estimated components of the gravity gradient data were found in areas of high topographical relief. But the average residual standard deviations of the individual components were between 3.6 and 7.4 eotvos, indicating that the components were consistent in an average sense. We have determined the successful conversion of terrain-corrected airborne gravity gradient data to Bouguer gravity data on the topographic relief using ground-based vertical gravity data as a reference. A 3D inverse model computed from the nonterrain-corrected data clearly showed the depth extent of the geologic structures observed at the surface, but it only produced a weak representation of the shallow structure. In contrast, a 2D surface density model in which only lateral variations of density in the topographic relief was allowed exhibited more realistic density distributions in fair correlation with geology.
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Sarkowi, Muh, Rahmat Catur Wibowo, Regina Febryzha Sawitri i Bagus Sapto Mulyanto. "Wai Selabung geothermal reservoir analysis based on gravity method". Jurnal Ilmiah Pendidikan Fisika Al-Biruni 10, nr 2 (30.10.2021): 211–29. http://dx.doi.org/10.24042/jipfalbiruni.v10i2.9705.
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Research has been conducted using the gravity method in the Wai Selabung area, South Ogan Kemiring Ulu Regency, South Sumatra Province, correlated with geological data, magnetotellurics, and geochemical data. This research aims to get structural patterns, subsurface models and identify the heat source and reservoir areas of the Wai Selabung geothermal system. This study uses the gravity method to model the subsurface, which is correlated with magnetotelluric and geochemical data to identify reservoir prospect areas. The results obtained from this research include residual anomalies in the research area showing the presence of a northwest-southeast trending fault structure by the main fault structure of this area trending northwest-southeast and slightly southwest-northeast. Analysis of the Second Vertical Derivative value of zero indicates the boundaries of the geothermal reservoir in the middle of the research area. The results of the 3D inversion modeling of the research area show that low density (2 to 2.15 g/cm3) indicates the location of the reservoir, medium-density values (2.2 to 2.4 g/cm3) are tertiary sandstone sedimentary. The high-density distribution value (2.5 to 2.9 g/cm3) indicates a potential heat source. And based on the analysis of the gravity method correlated with geological data, magnetotelluric, and geochemical data, the prospect area for the Wai Selabung geothermal reservoir, is around Teluk Agung, Perekan, and Talang Tebat.
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Lanz, Eva, Hansruedi Maurer i Alan G. Green. "Refraction tomography over a buried waste disposal site". GEOPHYSICS 63, nr 4 (lipiec 1998): 1414–33. http://dx.doi.org/10.1190/1.1444443.
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Determining the depth and geometry of a landfill’s lower boundary is a difficult task. Potential field methods generally lack the necessary depth resolution, and seismic reflection data are usually contaminated by source‐generated noise in the time range of interest (<50 ms). To address this problem, we have developed a surface 2-D tomographic refraction scheme that is based on a fast finite‐difference eikonal solver and an inversion method that incorporates appropriate damping and smoothing constraints. This new scheme has been applied to a first‐arrival traveltime data set collected across adjacent landfills in northern Switzerland. High‐quality seismic data were collected along five profiles that crossed the landfills and two that sampled undisturbed natural sediments. Seismic waves generated from multiple shots were recorded on large numbers of closely spaced receivers during quiet evening periods. Reliability of the resultant velocity tomograms was estimated on the basis of (1) ray diagrams, (2) plots of synthetic and observed traveltimes, (3) traveltime residual analyses, (4) comparisons of coincident velocity‐depth profiles computed from intersecting profiles, (5) inversions with diverse input models, and (6) quantitative error analyses using a bootstrap technique. At our study site, the base of the near‐surface natural layer and the lower boundaries of the landfills were defined by rapid increases in velocity from <1000 m/s to >1500 m/s, with velocities in the upper parts of the models determined to within about ±100 m/s. The thickness of the near‐surface natural layer varied between 2 and 6 m, with occasional thickening to ∼7 m. In contrast, low velocities associated with the landfills could be traced to 9 to 11 m depth. Although our results have demonstrated that the tomographic refraction scheme may be an efficient and cost‐effective means of studying the very shallow subsurface (<20 m depth), complementary geological and other geophysical data were required to discriminate between velocity anomalies attributed to the landfills and those attributed to natural variations in the near‐surface geology.
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Bédard, Jean H. "Parental magmas of Grenville Province massif-type anorthosites, and conjectures about why massif anorthosites are restricted to the Proterozoic". Earth and Environmental Science Transactions of the Royal Society of Edinburgh 100, nr 1-2 (marzec 2009): 77–103. http://dx.doi.org/10.1017/s1755691009016016.
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ABSTRACTTrace element inversion modelling of Grenvillean anorthosite massifs and associated rocks yield NMORB-normalised trace element profiles enriched in highly incompatible elements; commonly with negative Nb and Th anomalies. Model melts can be divided into subtypes that cannot be linked through fractional crystallisation processes. Most model melts are depleted in the heavy rare-earth elements and can be explained by partial melting of arc basaltic sources (5–60 melting ) with garnet-bearing residues. Some of the model melts have flat NMORB-normalised profiles (for rare-earth elements), have high compatible element contents, and might have been derived from mantle fertilised by arc magmatism, followed by low-pressure fractional crystallisation. Intermediate Ce/Yb types may represent mixtures of these end-members, or less probably, variations in the crustal source composition and residual assemblage. The active tectonic context now favoured for the Grenville Province appears to be inconsistent with plume or thermal insulation models. The heat source for crustal and mantle melting could record either post-orogenic thermal relaxation of a tectonically-thickened arc crust, or basaltic underplating caused by delamination of a mantle root or subduction slab beneath this arc crust. In this context, pre-Proterozoic anorthosites may be lacking, because prior to ca. 2·5 Ga, the crust may have been too weak to be thickened tectonically. The absence of post-Proterozoic anorthosites may be due to the secular decrease in radiogenic heating and cooling of the mantle and crust.
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Eitzen, Zachary A., Kuan-Man Xu i Takmeng Wong. "An Estimate of Low-Cloud Feedbacks from Variations of Cloud Radiative and Physical Properties with Sea Surface Temperature on Interannual Time Scales". Journal of Climate 24, nr 4 (15.02.2011): 1106–21. http://dx.doi.org/10.1175/2010jcli3670.1.
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Abstract Simulations of climate change have yet to reach a consensus on the sign and magnitude of the changes in physical properties of marine boundary layer clouds. In this study, the authors analyze how cloud and radiative properties vary with SST anomaly in low-cloud regions, based on five years (March 2000–February 2005) of Clouds and the Earth’s Radiant Energy System (CERES)–Terra monthly gridded data and matched European Centre for Medium-Range Weather Forecasts (ECMWF) meteorological reanalaysis data. In particular, this study focuses on the changes in cloud radiative effect, cloud fraction, and cloud optical depth with SST anomaly. The major findings are as follows. First, the low-cloud amount (−1.9% to −3.4% K−1) and the logarithm of low-cloud optical depth (−0.085 to −0.100 K−1) tend to decrease while the net cloud radiative effect (3.86 W m−2 K−1) becomes less negative as SST anomalies increase. These results are broadly consistent with previous observational studies. Second, after the changes in cloud and radiative properties with SST anomaly are separated into dynamic, thermodynamic, and residual components, changes in the dynamic component (taken as the vertical velocity at 700 hPa) have relatively little effect on cloud and radiative properties. However, the estimated inversion strength decreases with increasing SST, accounting for a large portion of the measured decreases in cloud fraction and cloud optical depth. The residual positive change in net cloud radiative effect (1.48 W m−2 K−1) and small changes in low-cloud amount (−0.81% to 0.22% K−1) and decrease in the logarithm of optical depth (–0.035 to –0.046 K−1) with SST are interpreted as a positive cloud feedback, with cloud optical depth feedback being the dominant contributor. Last, the magnitudes of the residual changes differ greatly among the six low-cloud regions examined in this study, with the largest positive feedbacks (∼4 W m−2 K−1) in the southeast and northeast Atlantic regions and a slightly negative feedback (−0.2 W m−2 K−1) in the south-central Pacific region. Because the retrievals of cloud optical depth and/or cloud fraction are difficult in the presence of aerosols, the transport of heavy African continental aerosols may contribute to the large magnitudes of estimated cloud feedback in the two Atlantic regions.