A BASIC program for 2-D spectral analysis of gravity data and source-depth estimation (2023)

Cited by (39)

  • The origin of the depressions interrupting the eastern Sierra Pampeanas broken foreland, explained by localized crustal extension through the juan Fernández ridge path, using gravimetric data

    2023, Journal of South American Earth Sciences

    The Salinas Grandes and Salina de Ambargasta salt pans are located in the Eastern Sierras Pampeanas in the Pampean broken foreland zone, a product of the Chilean-Pampean flat subduction zone. This region constitutes an anomalously flat morphology of 350×150km and occupies an extensive area in a northeast-southwest direction.

    These flat depressions abruptly interrupt the eastern Sierras Pampeanas morphology, burying the structure beneath Late Cenozoic-Quaternary deposits of the Andean orogenic front. We obtained from a previous model in spherical harmonics, the geometry of the Moho observing a lower crustal attenuation below the salt pan areas, which is maximum towards the north of the Salina de Ambargasta. From the Bouguer anomaly, calculated by unifying data from different sources, and its residual anomaly, we calculated the analytical signal to adjust the gravimetric anomaly's wavelength, which shows that this flat morphology follows NE to NNE gravity minima. Werner deconvolution and localized Euler deconvolution calculated from the residual Bouguer anomaly indicate that the anomalous masses are located above 15km depth, by the wavelengths of the anomalies, presumably indicating the differential uplift of high-density rocks placed in the lower and mid-crust, following the measured crustal attenuations. Additionally, these solutions show that this NE depression is segmented by NNW structures perpendicular to the main trough. Crustal thickness attenuation is interpreted as related to the track of the Juan Fernandez Ridge beneath the eastern Pampean broken foreland sector, based on plate kinematic reconstructions. As depicted in recently released tomographic data, its tearing and related asthenospheric upwelling could have promoted lithospheric localized extension, explaining the Salinas Grandes-Ambargasta depressions that are segmenting the eastern Sierras Pampeanas at the Andean orogenic front.

  • Role of an advanced gravity data analysis in improving the geologic understanding of the northern Tebessa region, Northeastern Algeria

    2022, Journal of African Earth Sciences

    Citation Excerpt :

    In the present work, we applied the Fast Fourier Transform (FFT) algorithm on the residual gravity field to transform data from the space domain to the frequency domain and to get the energy spectrum (Bath, 1974). The slopes of straight lines fitting to the spectrum curve provide indications and estimations on the distribution and depth of significant density contrasts of shallow and deep-seated anomalous sources (Bhattacharyya and Leu, 1977; Dimitriadis et al., 1987; Tselentis et al., 1988; Sanchez-Rojas and Palma, 2014). Delineating edges of density sources is a common application of gravity data to the geologic interpretation particularly faults and contacts mapping.

    The Tebessa region in the extreme northeastern part of the Saharan Atlas of Algeria is characterized by a succession of anticline and syncline structures, diapirs, grabens, and a thick sedimentary cover underlying the Paleozoic basement. The region constitutes a complex structural system. The extensional structures (i.e., grabens and basins) in the Tebessa region belong to the foreland grabens of the North Africa Alpine Belt and their evolution is related to the tectonic history of the western Mediterranean region. The Tebessa region has been the subject of several geologic studies in the last decades. However, the subsurface structural configuration remains unknown, resulting in uncertainty about the exact origin of the grabens present in the Tebessa region.

    The current research involves gravity data analysis in order to understand the geologic subsurface system. Accordingly, gravity data are used to delineate the fault systems and the tectonic framework by using the 2-D Source Edge Detection (SED) technique, Centre for Exploration Targeting (CET) grid analysis, Euler Deconvolution, multiscale edge detection method (Worming), and 3-D modeling.

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    Results indicate that the main tectonic trends within the study area are NE-SW, E-W, ENE-WNW, and NW-SE directions, making the northern Tebessa a complex structural region. The eastern part of the Tebessa region is characterized by a basement uplift which contributes to the positive gravity anomalies of the diapirs. In addition, the grabens have a sedimentary deposit thickness of about 2–8km and underlain by Triassic evaporite layers that facilitated their opening by providing rheological weakness.

    The grabens in the Tebessa region have different mechanisms that have been responsible for their formation. However, the Early Mesozoic extensional tectonic regime initiated some grabens by creating the bounding normal faults, followed by Cenozoic strike-slip motions, when grabens opened as pull-apart basins.

  • The buried Variscan granites and associated structures in the High Moulouya basin from gravity data and their role during the Triassic-Liassic extensional tectonics (Moroccan Meseta-Atlas domain).

    2022, Journal of African Earth Sciences

    The High Moulouya Basin (HMB) occupies a key location between the Southern Middle Atlas and the Central High Atlas. Gravity data from the HMB show the presence of three gravity lows. Borehole data show a thin Meso-Cenozoic cover in the HMB that eliminates the possibility that these lows are due to sedimentary rocks. The gravity anomalies may therefore be related to hidden granitic bodies within the Paleozoic basement. In order to map these granite bodies and associated structures, gravity data sets were processed and enhanced using various mathematical techniques, transformations, and 2D gravity modeling. The following were either better defined or revealed: i) The High Moulouya Batholith is a large NE-SW trending batholith that crops out partially in the Aouli-Mibladen and Boumia Paleozoic inliers. Our study shows the continuity of this batholith with a 90km long and 30km wide granitic structure. ii) The Hidden Engil granite is a subcircular, relatively small granite body (20×18km) located at the Middle Atlas's eastern boundary. The hidden granite of Zebzat (30×10km) represents a narrow E-W trending pluton bounded by the High Atlas tectonic structures on its southern boundary. The tectonic influence of these granite bodies may explain, to some extent, the stability of the HMB throughout the development of the Atlasic subsiding basins. The two Atlas basins are separated by the High Moulouya granite-cored intrabasinal high. As shown by the horizontal gradient and Euler deconvolution methods, during Triassic-Liassic NW-SE extensional events, the formation of the Middle and High Atlas basins was guided by Variscan inherited faults bounding the granitic bodies of the HMB. The main factors responsible for the formation of the High Moulouya granite-cored intrabasinal high are the isostatic compensation that follows the granite emplacement during the Variscan orogeny, the distribution of strain and deformation around the granite bodies during the Triassic-Liassic extensional tectonics and their concomitant buoyancy forces.

  • Geophysical modeling of La Primavera caldera and its relation to volcanology activity based on 3D susceptibility inversion and potential data analysis

    2020, Journal of Volcanology and Geothermal Research

    La Primavera (LP) is a rhyolitic caldera located in the western sector of the Trans-Mexican Volcanic Belt (TMVB). LP is located close to the triple point junction of the Chapala, Colima and Tepic-Zacoalco rifts. To understand the internal structure of the LP and its relationship with the Tepic-Zacoalco rift we carried out a geophysical study with different techniques. Satellite gravity, airborne data followed by ground-based gravity and magnetic surveys were used to performed semi-quantitative analysis to understand the structure of the LP. Residual gravity anomalies (22–15 mGal) occuring on the entire volcanic structure are attributed to its rhyolitic nature. Aeromagnetic anomalies ~125 nT occur on the south and western portions of LP. Analyzed lineaments in the area (Tilt Derivative Algorithm) follow predominantly regional NW-SE and W-E trends. Modeled Werner anomalies, identify the presence of numerous contacts and dikes, especially along main faults, such as Rio Caliente, La Gotera, Mesa Nejahuete, and the caldera ring fracture. Strikinlgy, the higher parts of intrusive bodies and dikes appear at variable depths ≤7.3 km beneath San Miguel and Las Planillas domes. These results were replicated by using an Euler's solution map. The deepest parts of these bodies occur at around 7.8 km south of Las Planillas and El Tajo domes. We developed a 3D smooth model of the magnetic susceptibility isosurfaces with five magnetized bodies beneath the LP structure. Depth and geometry of surface volcanic structures were determinate, thus providing a preliminary visualization of the main isosurface of 0.0343 SI located in the southern area of the caldera. Additionally, the upper part of the magnetic source is 5.5 km in depth. The present study, therefore, reveals the presence of various intrusive bodies existing at different depths inside the caldera. Further, structures and lineaments within the caldera provide evidence for understanding the presence of intrusive bodies, geological structures associated with the caldera structure and geothermal activity.

  • Interpretation of gravity data using 3D inversion and 2D continuous wavelet transform in Hedil deformed structures, northern Tunisia

    2019, Journal of African Earth Sciences

    Citation Excerpt :

    It is mainly used for the separation of source energy after the extraction of different slopes of the spectral analysis curve. Each slope characterizes a source of anomalies at a defined depth (e.g., Dimitriadis et al., 1987; Kivior and Boyd, 1998; Nouck et al., 2006). The radially averaged power spectrum of the Bouguer anomalies of Hedil area (Fig. 5) shows three slopes indicative of three sources of gravity anomalies.

    The Hedil region in northern Tunisia shows several geological structures associated with intense tectonic deformations that induced genesis of overthrusts and tectonic imbrications with rising of Triassic evaporites. This region constitutes a transition zone, between the Atlassic and Tellian domains, and exhibits multi-scale tectonic deformations related to the basins structure and the regional geodynamic of the Northern African margin. Gravity data processing and continuous wavelet transform, coupled with field data, lead to underline the subsurface geometric configuration and to propose an evolutionary structural framework for the studied area.

    The horizontal gradient magnitude (HGM) and its maxima highlight several lineaments trending NE-, E−, NWA BASIC program for 2-D spectral analysis of gravity data and source-depth estimation (1) and NA BASIC program for 2-D spectral analysis of gravity data and source-depth estimation (2) however the NE-SW direction is predominantly expressed. Depths of the main buried structures were estimated using the Euler deconvolution method. The highest values that exceed 3 km seem to underlines the structures associated to NE-SW, NW-SE and rarely E-W inherited features. The NE-SW oriented structures are mainly represented by overthrusts and tectonic imbrication zones that were occurred along inherited faults in the southern part of the area, as a result of the Atlassic compressional tectonics of Late Miocene and Quaternary ages. However the other lineaments correspond mainly to strike-slip and reverse. The application of 3D inversion method allows identification and characterization of geological structures in subsurface. The most deformed structures are marked by positive anomalies and are sometimes underlined by intrusion of Triassic evaporite rocks. However, the collapsed structures are underlined by negative anomalies testifying a material of low density. The extracted inversion sections highlight the deep rooted geometry of the Triassic bodies, locally associated with overturn of pierced strata.

    The continuous wavelet transform method was used to get an idea about the deep structures of the region, to estimate the boundary between the sedimentary filling and the Precambrian basement, and determining the depth and dipping of some identified contacts. Some uplifted structures and Triassic evaporite intrusions especially that of the Bazina are marked by risings of pre-Triassic deep causative sources. These zones of significant anomalies in the Atlassic domain are probably areas of intense deformation related to the ancient Hercynian tectonics that affected the North African margin. The identified structures should be related to the regional tectonic inversion, related to the African and European plates subduction.

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  • Internal architecture of the Tuxtla volcanic field, Veracruz, Mexico, inferred from gravity and magnetic data

    2016, Journal of Volcanology and Geothermal Research

    The Tuxtla Volcanic Field (TVF) is a basaltic volcanic field emerging from the plains of the western margin of the Gulf of Mexico in the Mexican State of Veracruz. Separated by hundreds of kilometers from the Trans-Mexican Volcanic Belt to the NW and the Chiapanecan Volcanic Arc to the SE, it stands detached not only in location but also in the composition of its rocks, which are predominantly alkaline. These characteristics make its origin somewhat puzzling. Furthermore, one of the large volcanoes of the field, San Martin Tuxtla, underwent an eruptive period in historical times (CE 1793). Such volcanic activity conveys particular importance to the study of the TVF from the perspective of volcanology and hazard assessment. Despite the above circumstances, few investigations about its internal structure have been reported. In this work, we present analyses of gravity and aeromagnetic data obtained from different sources. We present the complete Bouguer anomaly of the area and its separation into regional and residual components. The aeromagnetic data were processed to yield the reduction to the pole, the analytic signal, and the upward continuation to complete the interpretation of the gravity analyses. Three-dimensional density models of the regional and residual anomalies were obtained by inversion of the gravity signal adding the response of rectangular prisms at the nodes of a regular grid. We obtained a body with a somewhat flattened top at 16km below sea level from the inversion of the regional. Three separate slender bodies with tops 6km deep were obtained from the inversion of the residual. The gravity and magnetic anomalies, as well as the inferred source bodies that produce those geophysical anomalies, lie between the Sontecomapan and Catemaco faults, which are proposed as flower structures associated with an inferred deep-seated fault termed the Veracruz Fault. These fault systems along with magma intrusion at the lower crust are necessary features to understand the origin and structure of the TVF.

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    Multiple successions of buried fluvial channel systems were identified in the Quaternary section of the mid-shelf region of the northern South China Sea, providing a new case study for understanding the interplay between sea level variations and climate change. Using three commercial 3D seismic surveys, accompanied by several 2D lines and a few shallow boreholes, the sequence stratigraphy, seismic geomorphology and stratal architecture of these fluvial channels were carefully investigated. Based on their origin, dimensions, planform geometries and infill architectures, six classes of channel systems, from Class 1 to Class 6, were recognized within five sequences of Quaternary section (SQ1 to SQ5). Three types of fluvial systems among them are incised in their nature, including the trunk incised valleys (Class 1), medium incised valleys (Class 2) and incised tributaries (Class 3). The other three types are unincised, which comprise the trunk channels (Class 4), lateral migrating channels (Class 5) and the stable channels (Class 6). The trunk channels and/or the major valleys that contain braided channels at their base are hypothesized to be a product of deposition from the “big rivers” that have puzzled the sedimentologists for the last decade, providing evidence for the existence of such rivers in the ancient record.

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    Absolute age dates from a few shallow boreholes indicate that the landscapes that were associated with these fluvial systems changed significantly near the completion of the mid-Pleistocene climate transition (MPT), which approximately corresponds to horizon SB2 with an age of ∼0.6 Ma BP. Below SB2, the Early Pleistocene sequence (SQ1) is dominated by a range of different types of unincised fluvial systems. Evidence of incised valleys is absent in SQ1. In contrast, extensive fluvial incision occurred in the successions above horizon SB2 (within SQ2–SQ5). Although recent studies call for increased incision being a product of climate-controlled increase in river discharge, the down-dip location of our study area suggests that relative sea level change was the most important control of the evolution of fluvial systems. However, it is acknowledged that climate change was also important through its role in regulating glacio-eustasy. We speculate that the small amplitude and periodicity of sea level cycles before and during the MPT were not sufficient to fully expose the shelf and cause extensive fluvial incisions. Completion of the MPT as well as the onset of 100ky climate cycles at ∼0.6 Ma, during which the duration of cycles and magnitude of sea level change both increased, are considered to be triggering event for extensive development of incised fluvial systems. In addition to the eustatically driven causes of enhanced incision, the intensification of the East Asia monsoon at 0.9 Ma and 0.6 Ma driven by the episodic uplift of the Tibetan Plateau may have also significantly enhanced the amplitude of sea level falls and thus the fluvial incisions of the northern shelf of the South China Sea.

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    3D gravity inversion and uncertainty assessment of basement relief via Particle Swarm Optimization

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    Nonlinear gravity inversion in sedimentary basins is a classical problem in applied geophysics. Although a 2D approximation is widely used, 3D models have been also proposed to better take into account the basin geometry. A common nonlinear approach to this 3D problem consists in modeling the basin as a set of right rectangular prisms with prescribed density contrast, whose depths are the unknowns. Then, the problem is iteratively solved via local optimization techniques from an initial model computed using some simplifications or being estimated using prior geophysical models. Nevertheless, this kind of approach is highly dependent on the prior information that is used, and lacks from a correct solution appraisal (nonlinear uncertainty analysis). In this paper, we use the family of global Particle Swarm Optimization (PSO) optimizers for the 3D gravity inversion and model appraisal of the solution that is adopted for basement relief estimation in sedimentary basins. Synthetic and real cases are illustrated, showing that robust results are obtained. Therefore, PSO seems to be a very good alternative for 3D gravity inversion and uncertainty assessment of basement relief when used in a sampling while optimizing approach. That way important geological questions can be answered probabilistically in order to perform risk assessment in the decisions that are made.

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    Gravity analysis of the Precambrian basement topography associated with the northern boundary of Ghadames Basin (southern Tunisia)

    Journal of Applied Geophysics, Volume 111, 2014, pp. 299-311

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    Marine and Petroleum Geology, Volume 120, 2020, Article 104562

    Recognition of stratigraphic patterns or boundaries of any formation through well log data is a very decisive and critical part of hydrocarbon exploration in the petroleum industry. In the conventional method, the stratigraphic boundaries are not clearly distinguishable due to interference of low and high frequency noise in well log data. The Fourier transform and wavelet transform are spectrum based filtering method to delineate the stratigraphic boundaries using well log data, but it has some disadvantage of introducing the undesirable component of noise from log data into result. In the present study, we used the combined application of wavelet transform and Fourier transform to demarcate the stratigraphic boundaries. This two-fold frequency filtering operation is applied to natural gamma ray log data. Apart from this, we also demarcated these boundaries by wavelet based fractal dimension value and made a comparison with the previous method. We applied the proposed method on the natural gamma-ray log because it is sensitive to only the rock matrix of formation and indicates lithology better than other logs. In a comparative study of the result of both methods, it is clear that the wavelet-Fourier method fails to identify some boundaries in cases of the thin layers while wavelet based fractal of logging signal (WBFA) is able to resolve all boundaries and well correlated with the boundary identified by the conventional method. We have tested and validate the method on three synthetic signals including a thin layer in one signal and then demonstrated to the natural gamma ray well log data of the Limbodara oil field, Cambay basin.

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    The paper deals with an attempt to draw a blue print of ‘Fast Track Mineral Targeting’ at a reasonably cheap cost around Odisha and adjoining regions, India.

    Taking clues from known structural controls of ore deposits, mineralogical, lithological, metamorphic histories, actual as well as proven mineral deposits, occurrences and assuming probable shape and size of ore deposits, we have used 3D Euler’s deconvolution solutions on gravity data for locating different ore groups, viz ferrous group, base and noble metals, diamond and gemstones, over a virgin and buried terrain.

    3D Euler’s deconvolution solutions indicate that there are four sets of apparently unknown faults aligned in E-W, N-S, NW-SE and NE-SW directions, related to different tectonic events. In addition to the above, we mapped several circular to elliptical shaped structures. Taking guidelines from different controls of ore deposits for various kinds, as mentioned above, we have prognosticated several hitherto unknown areas for the search of the above minerals from the 3D Euler’s deconvolution solutions and known mineralized zones. This is primarily an attempt to explore those areas which are otherwise written off forever, as there are not sufficient clues available from either geological or geochemical data set or their synergy.

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    We use gravity information obtained from the XGM2016 global gravitational model together with topographic, bathymetric and seismic data to interpret the crustal structure beneath Cameroon and adjoined geological regions. For this purpose, we apply the regularized non-linear gravity inversion for a gravimetric determination of the Moho depth utilizing existing results of seismic data analysis as constraints. The estimated Moho model reflects regional tectonic configuration and geological structure of this region, mainly consisting of two major geological units, i.e. the Cameroon Volcanic Line and the Congo Craton. A validation of gravimetric result at sites of the Cameroon Broadband Seismic Experiment (CBSE) reveals overall similarities between gravimetric and seismic estimates. A comparison of our result is also conducted with previously published results. The cross-comparison of these results reveals a good agreement between them, particularly beneath the Cameroon Volcanic Line, the Adamawa Plateau and the Garoua Rift. Nevertheless, some relatively large inconsistencies roughly reaching ±10 km in estimated values of the Moho depth are identified in geological regions of the Congo Craton and the Yaoundé domain. The spatial correlation analysis between the Moho geometry and the topography indicates an isostatic state of particular geological units, suggesting their compensation stage. Our result closely agrees with the assumption that most of isostatically over compensated geological structures were formed during a compressional tectonic regime, except for the Garoua Rift that was likely formed during an extensional regime. We also computed the Bouguer gravity data at different constant elevations above sea level in order to supress a gravitational signature of shallower sources, while enhancing a gravitational signature from deeper crustal and lithospheric structures, focusing primarily on cores of major cratonic formations. The Bouguer gravity maps indicate that the Yaoundé domain likely represents the crustal manifestation of the suture zone between the Congo Craton and the Adamawa-Yadé domain, acting as a micro-continent.

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