Two-dimensional Forward Modeling Research Articles

Two dimensional forward simulation of magnetotelluric waves based on fourier neural network

Abstract Magnetotelluric method is a geophysical exploration technology with large detection depth, high efficiency and low cost. Its theory, method and data processing and interpretation technology are relatively mature, so it has been increasingly widely applied in many industries in recent years. Accurate and efficient forward modeling is the basis for magnetotelluric method to be developed. Machine learning is a popular research topic in many fields. It can be used to simulate magnetotelluric two-dimensional forward modeling accurately and quickly, and Fourier neural network is a typical deep learning network in machine learning. In theory, after inputting the training model set, we use Fourier neural network to conduct network training, and the purpose of two-dimensional magnetotelluric forward modeling can be achieved quickly by using the trained network. In this paper, two-dimensional magnetotelluric forward modeling will be conducted based on Fourier neural network, and the generated neural network will be tested to verify whether the constructed network fits the training model and whether its forward prediction results are accurate.

Insights into the crustal and the magmatic feeding structure at the Payunia Volcanic Province highlighted by geophysical methods, in the retroarc of the Southern Central Andes

The Payunia Volcanic Province is a Quaternary volcanic plateau emplaced in the retroarc area in the northern Neuquén Mesozoic Basin, associated with a hydrocarbon system. At deeper levels, this basin is linked to different intrusive systems that developed in the retroarc region at different times, during the Jurassic, Cretaceous, Eocene, Miocene, and even the Quaternary which influenced the hydrocarbon system maturity. We analyzed the Moho structure through this retroarc region, as well as the crustal structure affected by different stages of regional extension. From continuous seismic noise data, we calculated the autocorrelograms to obtain the reflection response below each seismological station. This allowed imaging the surface of primary crustal reflectors and in a few stations the top of an asthenospheric anomaly (SWAP) found by magnetotelluric survey and in concordance with satellite magnetic data. The crustal reflectors were identified in all stations at a mean two-way travel time of about ∼8.5 s and ∼12.5 s using frequency bands of about 1.0–2.4 Hz. Therefore, this is the first geophysical research that estimates the depth of the magmatic system, hosted at the top of the lower crust and the Moho discontinuity. The deepest reflector, only recognized in 4 stations, was observed with a two-way travel time of 17.2 s to 19.6 s. We used a mean one-dimensional Vp model to obtain the corresponding reflector depths which constrain the two-dimensional forward gravity model that fits with the observed regional anomaly for the region. We finally established a relationship between the shallowest sublithospheric electrical conductivity anomalies determined in previous researches and the strong deep reflections observed in some of the seismological stations. This information may help to constrain geochemical and petrological models and re-evaluate the hydrocarbon system maturity of the northern Neuquén basin.

Understanding the crustal architecture beneath the Bangui magnetic anomaly and its interactions with central African tectonic megastructures based gravity and magnetic analysis

The Bangui magnetic anomaly (BMA) in Central Africa is one of the largest continental magnetic anomalies on Earth in terms of amplitude and lateral size. Determining the sources of the BMA can lead to an increased understanding of the crustal dynamic in the Central African sub-region and the African continent as a whole. Magnetic and gravity analysis-based derivative, two-dimensional forward modelling and a Curie isothermal depth, showed that (a) the bottoms of the magnetic sources were between 15 and 35 km; (b) the BMA is a coalescence of several anomalies that trend E-W and roughly NE-SW. These directions coincide with regional Pan African-aged shear zones along the Central African orogenic belt and to thrust sheets at the northern edge of the Congo Craton. The depth of magnetization does not exceed 35 km with the amplitude of magnetization becoming smaller in the Central African Republic. The potential magnetic susceptibility sources have an average density of 2850 kg/m3 and magnetic susceptibilities between 0.06 and 0.25 SI. The BMA is interpreted to be a combination of middle and lower crustal bodies that are not continuous and consist of magnetic mineral rich granulites and banded iron formations. The gravity and magnetic modelling indicate that the entire crust was involved in the Pan African collisional event similar to what is seen in the Mozambique belt in East Africa. Combined with geological and geochemical studies, the models add evidence that one or two subduction zones were involved in accreting terranes on the northern edge of the Congo Craton. The tectonic accretions caused a crustal remobilization along major shear zones that has locally contributed to a probable circulation of fluids enriched in ferromagnesian minerals during late Neoproterozoic magmatism that created the BMA sources.

Geodesic equations for guided wave helical path separation for a pipe bend

The sections of pipe bends are susceptible to wall thinning by flow-accelerated corrosion due to the sudden change in the fluid flow direction and velocity. Compared to existing screening approaches, Guided Wave Tomography (GWT) has been demonstrated to provide more accurate information about the defect. In GWT an inversion problem is solved, in which a forward model is used to predict a synthetic dataset for a given physical model. Two-dimensional (2D) forward models are preferred for the inversion scheme due to the feasibility of combining these with tomographic algorithms. Since 2D models resemble the GW propagation of a plate, they do not consider the cyclic nature of the pipe. To overcome this limitation, a robust helical path separation was introduced in Huthwaite and Seher (2015) for a straight pipe. However, it relies on time-reversing the measured wave field ϕ(t) as a function of the propagation distance xs,r from the source to the receiver. Although ray-tracing and grid-based methods are capable of computing the distance xs,r for complex geometries such as the pipe bend, they are complex and time-consuming. Hence, in this paper a straight forward approach is proposed to compute xs,r based on the pipe geometry and the geodesic equations of a torus. Then the computed distance xs,r is used for removing the helical path trajectories. Compared to the distance obtained by using the Hilbert envelope, the results show that the geodesic distances preserve the information of the wavefront of interest, including the focusing effect and the scattered wave field from a Hann-shaped defect. The technique is applied to experimental data and demonstrated to separate the wavefront of interest.

Application of enhanced methods of gravity data analysis for mapping the subsurface structure of the bahira basin in Morocco

Gravity-based imaging of the subsurface has increased worldwide recently. Improvements in the processing and analysis of gravity data have allowed us to locate the basement surface, map geologic basins, and define structural patterns. In this study, gravity data were analyzed to study the Bahira basin’s underlying geology. The Bahira basin is very important economically. The Ganntour plateau is distinctive due to the importance of the phosphate mining resources. Using gravity data, we mapped the subsurface and determined the underlying structural patterns that affect the study area. In this study, we used several techniques to edge detection including Total horizontal derivative (THDR), first vertical derivative (FVD), tilt derivative (TDR), and its horizontal derivative (THDR_TDR) methods. Accordingly, the geological history of the Bahira basin suggests that the main lineaments/faults trends are NE-SW, NW-SE, ENE-WSW, and WNW-ESE. The 3D Euler deconvolution showed the depth and location of lineaments/faults, and matched edge detection results. The eastern Bahira basin’s sedimentary layer is 2–8 km deeper according to the Euler technique. Two-dimensional forward modeling along three profiles in the Bahira basin revealed a horst-graben basement structure. The outcomes of this study improved the subsurface topographical variations of the Bahira Basin. The information collected so far can help future studies in the area.

Red Sea rifting in central Egypt: constraints from the offshore Quseir province

The formation of oceanic crust in the southern and central Red Sea is generally accepted to have started at c. 5 Ma. However, the nature of the crust in the northern Red Sea is still debated. This paper describes the rift architecture, dynamics and evolution of the northern Red Sea and identifies domains that relate to first-order geodynamic processes. The proximal margin domain is located onshore and is characterized by latest Oligocene–Miocene half-graben basins. New seismic interpretations show that the offshore region is a necking domain dominated by low-angle, high-offset extensional faults, which led to the exhumation of lower crustal gabbros at Brothers Islands. Two-dimensional forward models suggest that the necking domain passes into a distal margin domain, where the continental crust thins to <10 km at 120 km from the coast. Sensitivity testing of interpretations for the distal domain indicates a probable scenario where exhumed lower continental crust or serpentinized mantle is present. A comprehensive rift model for the northern Red Sea in the Quseir province accounts for magmatic underplating accompanied by half-graben development at c. 25 Ma, followed by Early Miocene crustal thinning accommodated by an east-dipping detachment fault. A Late Miocene phase with a flip of the detachment geometry led to the present day configuration.

Constrained 3D gravity modelling of a multilayered aquifer system in an arid region: Case of Sebkhat El Bhira basin (Central Tunisia)

In arid regions, the significant and increasing demand for water has caused intensive exploitation of groundwater resources. The Sebkhat El Bhira basin, shared between Kairouan and Sidi Bouzid governorates in Central Tunisia, presents a multi-layer aquifer system composed of four main reservoirs: the sandy Mio-Plio-Quaternary (MPQ), the Paleogene, the Upper Cretaceous limestone and the Lower Cretaceous limestone, sand and dolomite reservoirs. The intensive exploitation of the shallower aquifer has led to a drawdown of the water table and a deterioration of water quality. The purpose of this work is to characterize the geometry of these potential reservoirs for sustainable water management, by applying geophysical methods.We used land gravity data covering 1387 Km2, twelve seismic reflection profiles with a total length of approximately 308 Km, two petroleum wells and 39 water wells.The processing of the gravity data shows the subdivision of Sebkhat El Bhira basin into four sub-basins and the identification of principal and minor lineaments oriented N-S, NE-SW, NNW-SSE, and E-W. The interpretation of reflection seismic lines allowed the identification of five main horizons: the bottom of MPQ, the bottom of Paleogene, the bottom of Upper Cretaceous, the bottom of Lower Cretaceous and the bottom of Jurassic (Top Triassic). For a better understanding of the subsurface structuring, we built twelve two-dimensional (2D) forward models of density. These latter are constrained in the area inside the plain by the converted time-to-depth seismic reflection data and wells. The interpolation of the 2D models allowed us to visualize the lateral and vertical thickness variations of each of the chrono-stratigraphic units and to produce a three-dimensional (3D) density model. The geophysical modelling allowed us to highlight the post-Jurassic tectonic and the Triassic halokinesis activities responsible for the actual subsurface geometry.The Triassic evaporites, in the study area, flow and rise through the fault's subvertical branches. It outcrops locally at the North-South Axis mountains range (NOSA). Inside the plain, the evaporitic uprising shows different schemes like diapir, wall and pillow. This Triassic halokinesis, with the tectonics, has shaped the deep morphology of the basin and managed the available space for the deposition of the overlying stratigraphic formations.The general structure of the potential aquifers is characterized by a depth which increases from the East to the West and towards the central part of the plain. Inside the basin, the structure of these aquifers is complex.

Geophysical and geological investigation of the spatial and subsurface extent of the Segwagwa and Masoke igneous ring complexes in southeast Botswana: Geotectonic implications

The Segwagwa-Masoke Igneous Ring Complex (SMIRC) occurs within the Kanye Basin and is part of a suite of elliptical igneous complexes in SE Botswana. We used map analyses and three-dimensional (3D) joint inversion modelling of airborne magnetic and ground gravity data to examine the internal structure and configuration of the SMIRC. Our results show the SMIRC as nested ring-within-ring structures characterized by high magnetic intensity and high gravity anomalies. The Segwagwa and Masoke Complexes are both elliptical in shape and cover surface areas of 127 km2 and 26 km2, respectively. Additionally, our results reveal two new intrusions, termed here Selokolela A and B, which form part of the SMIRC, bringing the total number of individual intrusions in the SMIRC to four. The 3D models suggest that the Segwagwa and Masoke Complexes are vertically oriented and steep-sided, extending to 12 km and ∼5 km depths, respectively. Two-dimensional (2D) forward modelling of the gravity and magnetic data confirms that the Segwagwa Complex is steep-sided and has a depth extent of 13 km. We interpret the ring-shaped magnetic anomalies to represent magma crystallization along concentric fracture zones that developed in association with the collapse of calderas.

A prediction method of suspended natural gas hydrate saturation in sea area based on P-wave impedance

A large amount of suspended natural gas hydrate (NGH) has been found in Shenhu sea area of China. At present, Wood method is considered to be a suitable method for predicting the saturation of suspended NGH. However, there are two problems in the application of this method in Shenhu sea area of China. First, this method is difficult to directly predict NGH saturation based on post-stack seismic data inversion, since it does not clarify the relationship between NGH saturation and P-wave impedance. Second, the prediction results of this method are quite different from the actual values in Shenhu sea area of China, which makes it difficult to obtain reliable resource estimation in this sea area. To clarify the relationship between NGH saturation and P-wave impedance, this paper proposes an improved Wood equation called Wood-Imp equation, which lays a foundation for predicting NGH saturation by using post-stack wave impedance data. For the second problem, the main reason for the large prediction error of Wood method is that the existing measurement technology cannot avoid the measurement errors in sediment composition, pore sizes, etc. Thus, Wood-Imp method is proposed based on non-NGH layer to reduce the of NGH saturation prediction error. Three comparison experiments are used to demonstrate the effectiveness of our proposed Wood-Imp method, including simulation data, well S data in Shenhu sea area, two-dimensional forward modeling of Shenhu sea area data. (1) Wood-Imp method has much higher accuracy in predicting NGH saturation than Wood method; (2) The predicted NGH saturation in Shenhu well S by Wood-Imp method is closer to that calculated by chloride ion method than that of Wood method; (3) Wood-Imp method has obtained better NGH saturation prediction accuracy based on P-wave impedance data obtained by post-stack inversion in two-dimensional forward modeling. Therefore, the Wood-Imp method proposed in this paper is more suitable for predicting suspended NGH saturation than the existing Wood method in Shenhu sea area.

Crustal configuration of the Naama and El Bayadh region of northwest Algeria: Inferences from gravity and magnetic analysis

Northwestern Algeria is located where the Saharan Atlas borders the Saharan Platform along the South Atlas Front. The origin of the South Atlas Front is controversial, thus a detailed gravity and magnetic analysis constrained by seismic reflection profiles and well data were conducted to determine the structural configuration of this region. A residual gravity anomaly map created using upward continuation and a reduced to the pole magnetic anomaly map indicated a series of northwest-trending maxima anomalies parallel to the Atlassic orogeny folds and faults and east-trending maxima within the Benoud foreland basin. These maxima mostly coincide with Paleozoic basement uplifts based on seismic reflection profiles. Depth analyses based on upward continuation, and two-dimensional forward modeling of the gravity and magnetic data indicated that the source of the maxima are mainly 5 km in depth with the magnetic sources being approximately 0.5 km deeper than the gravity sources. The gravity and magnetic models indicate that the source bodies are steep-sided and coincide with interpreted faults from the seismic reflection profiles. The maxima anomalies are mainly caused by basement uplifts instead of variations in density and/or magnetic susceptibility in the Paleozoic or older basement lithologies. The South Atlas Front and the Saharan Atlas on the central and eastern portions of Algeria is governed by a thin-skinned tectonics style involving the Mesozoic-Cenozoic sedimentary cover. While the western part was probably controlled by a mixed thin- and thick-skinned tectonic style with the reactivation of deeply rooted Triassic and Jurassic faults during Tertiary compressive events in the interpreted Paleozoic basement.Our gravity and magnetic models illustrate a crustal architecture model of the Paleozoic basement which is consistent with a north-dipping basement normal fault between less deformed Saharan Platform than the more deformed Atlassic domain. Such a crustal model aids in determining the nature of the Atlassic orogeny as geological and geophysical studies have been determining the variations of structural styles along its entire length.

A Comprehensive Evaluation for the Tunnel Conditions with Ground Penetrating Radar Measurements

We aim to develop a comprehensive tunnel lining detection method and clustering technique for semi-automatic rebar identification in order to investigate the ten tunnels along the South-link Line Railway of Taiwan (SLRT). We used the Ground Penetrating Radar (GPR) instrument with a 1000 MHz antenna frequency, which was placed on a versatile antenna holder that is flexible to the tunnel’s condition. We called it a Vehicle-mounted Ground Penetrating Radar (VMGPR) system. We detected the tunnel lining boundary according to the Fresnel Reflection Coefficient (FRC) in both A-scan and B-scan data, then estimated the thinning lining of the tunnels. By applying the Hilbert Transform (HT), we extracted the envelope to see the overview of the energy distribution in our data. Once we obtained the filtered radargram, we used it to estimate the Two-dimensional Forward Modeling (TDFM) simulation parameters. Specifically, we produced the TDFM model with different random noise (0–30%) for the rebar model. The rebar model and the field data were identified with the Hierarchical Agglomerative Clustering (HAC) in machine learning and evaluated using the Silhouette Index (SI). Taken together, these results suggest three boundaries of the tunnel lining i.e., the air–second lining boundary, the second–first lining boundary, and the first–wall rock boundary. Among the tunnels that we scanned, the Fangye 1 tunnel is the only one in category B, with the highest percentage of the thinning lining, i.e., 13.39%, whereas the other tunnels are in category A, with a percentage of the thinning lining of 0–1.71%. Based on the clustered radargram, the TDFM model for rebar identification is consistent with the field data, where k = 2 is the best choice to represent our data set. It is interesting to observe in the clustered radargram that the TDFM model can mimic the field data. The most striking result is that the TDFM model with 30% random noise seems to describe our data well, where the rebar response is rough due to the high noise level on the radargram.

Insights on the tectonic styles of the Red Sea rift using gravity and magnetic data

Global topography, bathymetry, magnetic, and gravity data were analyzed to study the nature of the Red Sea crust to infer its tectonic regime. The Red Sea is a classic example of incipient ocean basin formation after the breakup of a continent. We used spectral analysis of the magnetic and gravity data and two-dimensional (2D) forward modelling of the gravity data to image crustal sources within the Red Sea. The magnetic data were also analyzed to determine the Curie point depth, which was then used to estimate heat flow values within the Red Sea. Profiles of bathymetry, gravity, magnetic, and heat flow data were constructed across the southern, central and northern portions of the Red Sea to determine crustal structural differences between these regions. Residual and regional gravity anomaly maps determined from the spectral analysis and 2D gravity forward models indicate that there is a large amplitude gravity maxima over the main axial trough within the central and southern Red Sea caused by a combination of partially molten material intruding into the crust and the formation of a new oceanic crust. However, the central portion of the Red Sea based on the residual gravity anomaly map is a transition zone where the oceanic crust is discontinuous and is separated by inter-trough zones. In the northern portion of the Red Sea, the discontinuous residual gravity maxima and high frequency magnetic anomalies can be correlated with isolated deeps implying the last stage of the continental rifting and beginning of seafloor spreading.

Crustal Structure of the Nile Delta: Interpretation of Seismic-Constrained Satellite-Based Gravity Data

Interpretations of the tectonic setting of the Nile Delta of Egypt and its offshore extension are challenged by the thick sedimentary cover that conceals the underlying structures and by the paucity of deep seismic data and boreholes. A crustal thickness model, constrained by available seismic and geological data, was constructed for the Nile Delta by inversion of satellite gravity data (GOCO06s), and a two-dimensional (2D) forward density model was generated along the Delta’s entire length. Modelling results reveal the following: (1) the Nile Delta is formed of two distinctive crustal units: the Southern Delta Block (SDB) and the Northern Delta Basin (NDB) separated by a hinge zone, a feature widely reported from passive margin settings; (2) the SDB is characterized by an east–west-trending low-gravity (~−40 mGal) anomaly indicative of continental crust characteristics (depth to Moho (DTM): 36–38 km); (3) the NDB and its offshore extension are characterized by high gravity anomalies (hinge zone: ~10 mGal; Delta shore line: >40 mGal; south Herodotus Basin: ~140 mGal) that are here attributed to crustal thinning and stretching and decrease in DTM, which is ~35 km at the hinge zone, 30–32 km at the shoreline, and 22–20 km south of the Herodotus Basin; and (4) an apparent continuation of the east-northeast–west-southwest transitional crust of the Nile Delta towards the north-northeast–south-southwest-trending Levant margin in the east. These observations together with the reported extensional tectonics along the hinge zone, NDB and its offshore, the low to moderate seismic activity, and the absence of volcanic eruptions in the Nile Delta are all consistent with the NDB being a non-volcanic passive margin transition zone between the North African continental crust (SDB) and the Mediterranean oceanic crust (Herodotus Basin), with the NDB representing a westward extension of the Levant margin extensional transition zone.

Structural trends and basement depths across Nigeria from analysis of aeromagnetic data

This study analyses Aeromagnetic Survey Data to delineate magnetic lineaments associated with fault system trends across Nigeria. A High-Pass filter is used to separate the Total Magnetic Intensity's regional and residual components. A filter like Total Horizontal Gradient, Grid Analysis and Tilt Angle derivative are used to enhance the structural trends and fault systems that predominantly trend Northeast-Southwest and East-West. Some of the magnetic signatures on the southern and northeastern Nigeria follow the rift system trend that connects the South Atlantic and inland West Africa. The Euler deconvolution depth estimate of faults range between 1840 and 6321 m (meters) across the region. The Werner deconvolution and two-dimensional forward modelling are used to infer sedimentary thicknesses beneath ten profiles (each profile traverses a major river basin in the country). The estimated depths to basement range between 1500 and 7500 m beneath the Cross-River Basin (in the south) and the Gongola Basin (in the northeast). The Hadejia River Basin in northeast Nigeria is characterised by distinct magnetic anomaly lows and a shallow depth to the basement. Further investigation of this basin is recommended.

Basement Structure and Styles of Active Tectonic Deformation in Central Interior Alaska

Central Interior Alaska is one of the most seismically active regions in North America, exhibiting a high concentration of intraplate earthquakes approximately 700 km away from the southern Alaska subduction zone. Seismological evidence suggests that intraplate seismicity in the region is not uniformly distributed, but concentrated in several discrete seismic zones, including the Nenana basin and the adjacent Tanana basin. Although the location and magnitude of the seismic activity in both basins are well defined by a network of seismic stations in the region, the tectonic controls on these intraplate earthquakes and the heterogeneous nature of Alaska’s continental interior remain poorly understood. We investigated the crustal structure of the Nenana and Tanana basins using available seismic reflection, aeromagnetic and gravity anomaly data, supplemented by geophysical well logs and outcrop data. We developed nine new two-dimensional forward models to delineate internal geometries and the crustal structure of Alaska’s interior. The results of our study demonstrates a strong crustal heterogeneity beneath both basins. The Tanana basin is a relatively shallow (up to 2 km) asymmetrical foreland basin with its southern, deeper side controlled by the northern foothills of the Central Alaska Range. Northeast-trending left lateral strike-slip faults within the Tanana basin are interpreted as a zone of clockwise crustal block rotation. The Nenana basin has a fundamentally different geometry. It is a deep (up to 8 km), narrow transtensional pull-apart basin that is deforming along the left-lateral Minto Fault. This study identifies two distinct modes of current tectonic deformation in Central Interior Alaska and provides a basis for modeling the interplay between intraplate stress fields and major structural features that potentially influence the generation of intraplate earthquakes in the region.

Analysis of gravity and aeromagnetic data to determine structural trend and basement depth beneath the Ajdabiya Trough in northeastern Libya

In this study, we analyse both ground gravity and aeromagnetic data in order to delineate structural trends, fault systems and deduce sedimentary thicknesses within the Ajdabiya Trough in Libya’s northeast. A high-pass filter and a reduced-to-the-pole (RTP) transformation are applied to the gravity and aeromagnetic data respectively. Different filters are used to enhance the structural signatures and fault trends within the study area. The Werner deconvolution and source parameter imaging (SPI) techniques are applied to the RTP magnetic data for source depth estimation. Four well-data within the area are used as constraints in the two-dimensional forward modelling process. The results show that the Ajdabiya Trough is characterised by gravity anomaly highs and magnetic anomaly lows. The analysis of gravity data shows predominant Northeast–Southwest structural trends, whereas the analysis of magnetic data shows predominant North–South magnetic lineaments within the Ajdabiya Trough. The Euler deconvolution depth estimates of faults depths range between 1500 and 9500 m. The SPI estimates of the magnetic basement range between 2500 and 11,500 m beneath the study area (deepest beneath the Ajdabiya Trough). Constrained by the well-data, six major layers characterize the four profiles that are taken within the area. One of the profiles shows a high-density intrusion (about 4 km from the surface) within the sedimentary sequence. The intrusion may be the result of the rifting Sirt Basin which caused a weakening of the crust to allow for mantle intrusion.

Crustal and thermal structure of the Permian – Jurassic Luangwa – Lukusashi – Luano Rift, Zambia: Implications for strain localization in magma – Poor continental rifts

This work uses aeromagnetic, satellite gravity, and ground gravity data to image the thermal and crustal structures beneath the NE-trending Paleozoic - Mesozoic Luangwa – Lukusashi – Luano (LLL) rift in southeastern Zambia. This rift extends between the Precambrian Irumide metacraton in the northwest and the Niassa craton to the southeast and is partially controlled by the NE-trending Mwembeshi suture zone between the two blocks. Although the LLL rift is suggested to have been reactivated by the Southwestern Branch (SWB) of the East African Rift System (EARS), this work did not find shallowing of Curie Point Depth (estimated from the spectral analysis of aeromagnetic data) that would be indicative of elevated heat flow beneath the rift. Three-dimensional inversion of the aeromagnetic data shows a number of magnetized bodies, the largest occurring beneath the central part of the LLL rift and extending to a depth of ~35 km. Spectral analysis of the satellite gravity data indicates a ~45 km thick crust beneath the southwestern and central parts and the northeastern tip of the LLL rift. Two-dimensional (2D) forward modeling of the ground gravity data indicates a ~200 km wide and ~12 km thick under-plated mafic body (UPMB). These results support limited reactivation of the LLL rift by the SWB of the EARS. Further, extensional strain localization during the onset of the LLL rifting is attributed to the accretion of the UPMB beneath the crust.

Gravity contribution to the Mejerda foreland basin, Northwestern region of Tunisia

The Mejerda basin in northwestern Tunisia is located between the Tell and Atlas Mountains and contains structures related to the complicated tectonic history of the study region. The Mejerda contains compressional structures related to the Cenozoic collision between Europe and Africa, and strike-slip faults related to extensional tectonics. As a result of the varied tectonic history, there has been numerous models of the basin formation ranging from simple grabens, pull-apart basins and foreland basins. In order to constrain the subsurface structure of the basin, a gravity analysis consisting of the construction of residual gravity anomaly maps, estimated depths from upward continuation and Euler deconvolution, and detailed two-dimensional forward models. The gravity analysis was constrained by a seismic reflection profile and a deep well. The map analysis indicated two separate basins, one called the Quaternary basin and another called the Pliocene basin. Based on the gravity depth analyses and the gravity forward modeling, and the residual gravity anomaly map, these basins were between 2.5 and 3 km thick, and separated by gravity maxima probably due to uplift of Triassic and Cretaceous sediments related to compressional tectonics and evaporitic movement. The Euler deconvolution analysis imaged a number of lineaments, interpreted to be faults, that trend in the NE and NW directions and these faults basically delineate the basins. Results confirm that the Mejerda basin is a wedge-top basin located in the foreland side of the Tellian thrust belt and consists of two separate basins whose origin was mostly determined by thin-skin compressional tectonics.

Surface and Crustal Study Based on Digital Elevation Modeling and 2-D Gravity Forward Modeling in Thandiani to Boi Areas of Hazara Region, Pakistan

Gravity data indicates that there is a regular relation between crustal structure, crustal density (composition), and surface ascension. In order to delineate surface and subsurface geological structure features, and to calculate the thickness variation of the crust and sedimentary/metasedimentary wedges, integrated approach of Geographic Information System (GIS) i.e. digital elevation models (DEMs) and two-dimensional forward modeling of gravity data were utilized, which provide the best results for the primary objectives. Tectonically, the study area lies in the Lesser Himalayas as well as to an extent in the sub-Himalaya, more concretely in the western limb of Hazara Kashmir Syntaxis. Topographic data was accumulated in XYZ coordinates utilizing point heights method, and DEMs generation, manipulation, interpretation, and visualization process were directed to surfer-15 and ArcGIS software. Determinately the visualization of surface geological structure in the form of DEMs were proposed. The gravity stations in single contour mode have been quantified by using Scintrex CG-5 gravity meter. The collected gravity data was processed by standardizing corrections, two-dimensional forward modeling along with gravity profile were utilized and bouguer anomaly map and gravity model was computed utilizing bouguer density of 2.4 g/cm3, where the subsurface structures are demarcated by the bouguer anomaly and gravity model. In summary this research has allowed the validation of surface and subsurface geological structure visualization. Digital elevation models provide a defensive prediction of the geological structure of the regional surface. The gravity model demarcated a series of stratigraphic units with density boundaries within the basement. The gravity model also suggests that the thickness of sedimentary/metasedimentary wedge in Thandiani area is 11.48 km and in Boi area, the thickness elongates to about 14.43 km. The total thickness of crust in Thandiani and Boi area is 49.53 km and 52.43 km respectively.

Gaussian Process Methodology for Multi-Frequency Marine Controlled-Source Electromagnetic Profile Estimation in Isotropic Medium

The marine controlled-source electromagnetic (CSEM) technique is an application of electromagnetic (EM) waves to image the electrical resistivity of the subsurface underneath the seabed. The modeling of marine CSEM is a crucial and time-consuming task due to the complexity of its mathematical equations. Hence, high computational cost is incurred to solve the linear systems, especially for high-dimensional models. Addressing these problems, we propose Gaussian process (GP) calibrated with computer experiment outputs to estimate multi-frequency marine CSEM profiles at various hydrocarbon depths. This methodology utilizes prior information to provide beneficial EM profiles with uncertainty quantification in terms of variance (95% confidence interval). In this paper, prior marine CSEM information was generated through Computer Simulation Technology (CST) software at various observed hydrocarbon depths (250–2750 m with an increment of 250 m each) and different transmission frequencies (0.125, 0.25, and 0.5 Hz). A two-dimensional (2D) forward GP model was developed for every frequency by utilizing the marine CSEM information. From the results, the uncertainty measurement showed that the estimates were close to the mean. For model validation, the calculated root mean square error (RMSE) and coefficient of variation (CV) proved in good agreement between the computer output and the estimated EM profile at unobserved hydrocarbon depths.