Inversion Of Magnetotelluric Data Research Articles

Deep heat source detection using the magnetotelluric method and geothermal assessment of the Farafra Oasis, Western Desert, Egypt

Nonrenewable hydrocarbon energy sources provide the bulk of Egypt's energy. However, the nation needs to diversify its energy portfolio and use conventional and renewable energy in tandem to ensure its long-term economic growth; therefore, harvesting untapped geothermal resources might help Egypt meet its energy needs in a sustainable and effective manner. The Farafra Oasis is located in the midst of Egypt's Western Desert and has been selected as the focal point for sustainable long-term development in the Western Desert.Using a magnetotelluric (MT) survey, a deep exploration of the Farafra region has been conducted to explore the underlying geological state and, as a result, explain the origin of several thermal wells around the oasis. None of the prior studies reached the necessary depths to offer information on the surface of deep igneous rocks, which may be the source of heat in the study area. Two field excursions were conducted to measure 19 stations along a line crossing the Farafra Oasis and extending more than 130 km. The MT instrument measured periods greater than 1000 s to gather low-frequency data with great efficiency.On the basis of 2D inversion of MT data, a mathematical model of the Farafra region's geothermal system was built to visualize and characterize the heat sources underneath the Farafra Oasis. It was shown that the existence of a high-temperature pluton at considerable depth is the primary cause of the temperature rise in the aquifer under the Farafra Oasis, as the water mass builds up on top of plutonic rocks and flows upwards via fractures and faults.

Flexible and Accurate Prior Model Construction Based on Deep Learning for 2-D Magnetotelluric Data Inversion

The conventional magnetotelluric (MT) data inversion methods, such as the nonlinear conjugate gradient method, quasi-Newton method, and Gauss–Newton method and so on, can converge robustly, but their results are easily affected by the initial model and regularization term. Although supervised learning can break through the resolution limitation by directly learning the nonlinear relationship between the model and the data, it cannot guarantee the data fitting without considering physical constraints. Here, we propose a novel prior model generation method using deep learning for conventional inversion to jointly take advantages of the two techniques. We first combine Gaussian random rough surface scheme and random polygon generation algorithm to construct practical 2-D geoelectric models, in which the prior information on the geoelectric structure can be flexibly integrated. Then, a fast 2-D MT forward modeling method is applied to calculate the forward responses and establish the training set. Finally, we use the training set to complete the parameter optimization of U-shaped network (U-NET) and run the conventional inversion with prior model generated by the trained U-NET. Numerical experiments with synthetic data show that the proposed method can effectively integrate the advantages of conventional inversion and supervised learning, and remarkably improve the resolution in the inversion if proper training sets are used. The inversions of the USArray data also prove that our method can retain high-resolution structures predicted by the U-NET in the final inversion results with a data fitting as good as the traditional Gauss–Newton method.

An Intelligent MT Data Inversion Method With Seismic Attribute Enhancement

Magnetotelluric data inversion reconstructs a electrical resistivity structure most compatible with the observed magnetotelluric data, and static correction can remove the undesired static shift effect in magnetotelluric data. Conventional magnetotelluric data static shift correction often face the challenge of demanding requirements, such as large data amount, additional types of data, or a deep understanding of the research area. Magnetotelluric inversion constrained by seismic data often has better resolution and model consistency compared with independent magnetotelluric inversion. However, valuable inversion knowledge contained in geophysicists’ expertise is not effectively incorporated. In this work, we present an intelligent magnetotelluric data inversion method leveraging data-driven and physics-driven techniques based on deep learning. A novel magnetotelluric data static shift correction method is introduced based on a neural network. A magnetotelluric data inversion method is formulated with the constraint of the extracted seismic reflection image based on two different neural networks. Experiments on synthetic and field data verify the effectiveness of the proposed method.

A fully finite-element based model-space algorithm for three-dimensional inversion of magnetotelluric data

SUMMARY We present a fully finite-element based inversion methodology for imaging 3-D magnetotelluric impedance data on unstructured meshes. The inverse problem is formulated using a minimum-structure Gauss–Newton type optimization scheme that minimizes an objective function with respect to the model perturbation. By introducing a rigorous regularization scheme, we derived a Ritz-type variational formulation of the model objective function and designed a face-based finite-element basis function to discretize the model gradient across tetrahedron’s inter-element boundaries. The forward modelling engine of our optimization scheme is based on a finite-element solution of the E-field Helmholtz equation that is enforced for the magnetotelluric simulation problem using the appropriate edge-based basis functions and 3D boundary conditions. The optimization algorithm developed here utilizes a message passing interface scheme and uses a direct solver to factorize and store both the regularization matrix and the forward modelling coefficient matrix on the processes working in parallel. Having to do this only once within each Gauss–Newton optimization cycle facilitates both the calculation of the dot product of the model regularization terms with the evolving model perturbation, and computing implicitly the sensitivity-vector products. We validated the methodology and the correctness of the developed algorithm for two test examples (COMMEMI 3Ds) from the literature. Also, by comparing the performance between classes of iterative solvers we demonstrated the superior performance of generalized minimum residual solver in reducing the residual norm of the iterative solver during model updates. Using the algorithm in a geologically realistic scenario, we imaged the anticipated geometry of the Lalor volcanogenic massive sulphide deposit in Canada. The feasibility of the imaging methodology is further evaluated with the survey data, for which, again the algorithm converged to the anticipated model solution reproducing the lithostratigraphic sequence of the ore deposit.

Seismogenic structure of the 2014 M6.5 Ludian earthquake from three-dimensional joint inversion of magnetotelluric data and seismic arrival times

Seismogenic structure of the 2014 M6.5 Ludian earthquake from three-dimensional joint inversion of magnetotelluric data and seismic arrival times

Deep Electrical Structure of the Qilian Orogenic Belt with Dynamic Implications for the Northeastern Margin of the Tibetan Plateau: Revealed by 3D Magnetotelluric Inversion Using Unstructured Tetrahedral Elements

Abstract We present the results of a magnetotelluric (MT) array across the Qilian orogenic belt to elucidate the uplift mechanism of the northeastern Tibetan Plateau (Qinghai-Tibet Plateau). The array extends from the Qaidam basin in the south to the southern edge of the Alxa block in the north. Using the three-dimensional (3D) inversion of MT data based on unstructured tetrahedral elements, the electrical structure 100 km below the orogenic belt is obtained. The results show that there are high-resistivity bodies in the lithospheric mantle of the North Qilian and Hexi Corridor, which may represent the trace of southward subduction of the Asian lithosphere. Besides, there are partially molten bodies with low resistivity in the middle and lower crust below the Qilian orogenic belt, which may be caused by tectonic heat. The melt fraction of low-resistivity bodies is 2-5%, which indicates that the crustal flow from the Qiangtang and Songpan-Ganzi blocks is unable to penetrate beneath the Qilian orogenic belt. The low-resistivity bodies beneath the Qilian orogenic belt decouples the upper crust from the middle-lower crust. Owing to the continuous compression, the decoupled middle-lower crust has subsequently driven the northward movement of the upper crust, resulting in the uplift of the Qilian orogenic belt.

Modeling the crustal electrical structure of the Zhangye Basin via three-dimensional inversion of magnetotelluric data – Implications for future geothermal development

Because of its unique geographical location, the Tibetan Plateau has been widely studied in geological research. Several studies revealed the geological structure and evolutionary process of the lithosphere deep beneath the northeastern Tibetan Plateau; however, the deep structural characteristics of the Zhangye Basin, which is in the middle of the Hexi Corridor at the junction of the northeastern Tibetan Plateau and Alxa Block, have not been widely examined. To better understand the geodynamics of the Zhangye Basin, studies are needed to investigate the tectonic mechanism of the northeastern Tibetan Plateau. To determine its subsurface structure, 46 broadband magnetotelluric survey stations were deployed across the basin in 2020. The crustal electrical structure of the basin was modeled via three-dimensional nonlinear conjugate gradient inversion. The results of this inversion revealed a low-resistivity zone in the middle-lower crust of the basin, which was characterized as an intracrustal decollement, thus suggesting decoupling deformation of the crust interior. This conductive body may have formed during continuous growth of the Tibetan Plateau, which may have expanded northeastward below the Hexi Corridor and Helishan Fault. The latter may have partly underthrust southwestward in the lower part of the Hexi Corridor and Tibetan Plateau. These results regarding geothermal formation of the Zhangye Basin improve the understanding of its underground structure and may guide the future development of geothermal resources in this area.

Large-Scale Inversion of Magnetotelluric Data Using Regularized Gauss–Newton Method in the Data Space

Large-Scale Inversion of Magnetotelluric Data Using Regularized Gauss–Newton Method in the Data Space

A Geoelectrical Model of the Southern Ladoga Region on the Basis of 3D Magnetotelluric Data Inversion

A Geoelectrical Model of the Southern Ladoga Region on the Basis of 3D Magnetotelluric Data Inversion

Deep Geological Controls on Formation of the Highest‐Grade Uranium Deposits in the World: Magnetotelluric Imaging of Unconformity‐Related Systems From the Athabasca Basin, Canada

AbstractUnconformity‐related uranium (URU) deposits in the Athabasca Basin, Canada, include the highest grade, large tonnage deposits in the world. Recent studies of the Patterson Lake corridor uranium deposits suggest a deep heat source enhances hydrothermal fluid flow and incursion of basinal brines along brittlely reactivated ductile shear zones. However, the spatial extent of the ore systems and lower crustal features capable of driving the hydrothermal cells remained largely unresolved. Three‐dimensional electrical conductivity models derived from inversion of magnetotelluric data and coincident gravity modeling identified a voluminous conductivity anomaly and corresponding gravity low related to a belt of high‐heat producing intrusions that dips below the deposit. Shallow conductive bodies are identified below the deposits and similarly prospective corridors. The volume and location of the intrusive bodies may have provided a source of radiogenic heat that helped drive remobilization of fluids along reactivated structures, redefining the spatial extent of URU systems.

Joint one‐dimensional inversion of magnetotelluric data and surface‐wave dispersion curves using correspondence maps

ABSTRACTWe use a correspondence map to jointly invert surface‐wave dispersion curves and magnetotelluric data for subsurface shear velocity and resistivity but also for a possible relationship between them. Our first experiments consist of inversions of synthetic data computed from models linked by first‐ and second‐order polynomial relationships. Our methodology produces joint inversion model pairs from which 100% fit the ‘observed’ parameter relationship within a 5% error vs only 15% of the separate inversion pairs for the degree 1 relationship experiment. For the degree 2 relationship synthetic test, 80% of the joint inversion model pairs fit the ‘observed’ relationship within a 5% error while 45% of the separate inversion pairs. This reduces the number of acceptable models without compromising the data fit (‘reduction of non‐uniqueness'). The next experiment involves synthetic data from models of known physical properties, taken from well logs, but without a known relationship. We show how to select an appropriate polynomial degree for joint inversion when the relationship is unknown. Having validated the approach with synthetic cases, we apply our methodology to field data. We compare separate and joint inversions, and we find that the one‐dimensional subsurface models retrieved from joint inversions are more similar to previous models documented in the area than the separate inversion models.

Deep Electrical Resistivity Structure of the European Lithosphere in Poland Derived from 3-D Inversion of Magnetotelluric Data

Deep Electrical Resistivity Structure of the European Lithosphere in Poland Derived from 3-D Inversion of Magnetotelluric Data

Geophysical extent of the Wyoming Province, western USA: Insights into ancient subduction and craton stability

AbstractA new 3-D resistivity model, estimated from inversion of magnetotelluric data, images crustal and upper-mantle structure of the Wyoming Province and adjacent areas. The Archean province is imaged as a coherent resistive domain, in sharp contrast to active tectonic domains of the western U.S. Prominent high-conductivity belts define the northern, eastern, and southern margins of the Wyoming Province and are interpreted as sutures marking the remnants of Paleoproterozoic orogens. The model results suggest the northern boundary of the Wyoming Province is located 150 km south of its traditional placement and adjacent to a composite orogen separating the Wyoming Province and Medicine Hat block. The eastern province boundary is clearly imaged along the Black Hills, whereas the western margin is obscured by Cenozoic extension and magmatism. An internal boundary within the Wyoming Province is interpreted to represent a Neoarchean suture; in stark contrast to Proterozoic sutures, though, it is not marked by a high-conductivity belt. This difference in conductivity is speculated to reflect changes in the subduction process through time. The absence of high-conductivity along Archean sutures appears to be global in nature and related to reduced continental freeboard in the Archean which limited continental weathering and the delivery of carbon-rich sediments to the seafloor. Although the entire Wyoming Province has been proposed to have undergone lithospheric modification that lessened its stability, the resistivity model suggests a thick lithospheric root remains in place except along its western margin. These results suggest that Archean cratons may be more resistant to lithospheric modification by influx of heat and fluids associated with extension and plumes than previously thought, and that metasomatism does not necessarily weaken the lithosphere and set a craton on the path to destruction.

Shallow crustal electrical structure of the Qingchengzi orefield in Liaodong area revealed by three-dimensional inversion of Magnetotelluric data

Dense array Magnetotelluric sounding (MT) was applied in the study of the emergence of massive gold deposits relating to the tectonic activity responsible for the creation of the North China Craton. This study focuses on understanding the shallow crustal electrical structure characteristics (>1.5 km) of PbZn and AuAg deposits in the Qingchengzi orefield, through the analysis of shallow electrical resistivity distribution at depths less than 5 km, obtained by 3D inversion of full impedance at a frequency range of 320–0.107 Hz. The results showed that the electrical characteristics of Xiaotongjiapuzi and Taoyuan deposits in the 3D resistivity model are consistent with the known 2D resistivity model, and the Jianshanzi fault plays an important role in the gold mineralization, acting as a migration channel and precipitation zone for ore-forming materials. It is inferred that the fault extends at least 5 km deep with deep mineralization prospects. The strata of Liaohe Group in the north of the orefield, the intrusive granite in the south, and the NW-trending Jianshanzi and Yushanggou faults show distinguishable electrical properties, which are either high resistance, low resistance, or high and low resistance transition zone on the depth slice map. Furthermore, the Xinling rock mass has a NE trending extension, which confirms the inference that the Jianshanzi fault extends to a depth of 5 km into the crust.

2D inversion of magnetotelluric data using deep learning technology

2D inversion of magnetotelluric data using deep learning technology

An efficient cascadic multigrid solver for 3-D magnetotelluric forward modelling problems using potentials

SUMMARY The fast and accurate 3-D magnetotelluric (MT) forward modelling is core engine of the interpretation and inversion of MT data. In this study, we develop an improved extrapolation cascadic multigrid method (EXCMG) to solve the large sparse complex linear system arising from the finite-element (FE) discretization on non-uniform orthogonal grids of the Maxwell’s equations using potentials. First, the vector Helmholtz equation and the scalar auxiliary equation are derived from the Maxwell’s equations using Coulomb-gauged potentials. The weighted residual method is adopted to discretize the weak formulation and assemble the FE equation. Secondly, carefully choosing the preconditioned complex stable bi-conjugate gradient method (BiCGStab) as multigrid smoother, we develop an improved EXCMG method on non-uniform grids to solve the resulting large sparse complex non-Hermitian linear systems. Finally, several examples including three standard testing models (COMMEMI3D-1, COMMEMI3D-2 and DTM1.0) and a topographic model are used to validate the accuracy and efficiency of the proposed multigrid solver. Numerical results show that the proposed EXCMG algorithm greatly improves the efficiency of 3-D MT forward modelling, is more efficient than some existing solvers, such as Pardiso, incomplete LU factorization preconditioned biconjugate gradients stabilized method (ILU-BiCGStab) and flexible generalized minimum residual method with auxiliary space Maxwell preconditioner (FGMRES-AMS), and capable to simulate large-scale problems with more than 100 million unknowns.

Lithospheric electrical structure and its implications for the evolution of the middle Qinling Orogenic Belt, Central China: Constraints from 3-D magnetotelluric imaging

The Qinling Orogenic Belt is located at the central part of the China Central Orogenic Belt and links the Qilian and Kunlun Orogenic Belts to the west with the Dabie-Sulu Orogenic Belt to the east, which is sandwiched between the South China Block and North China Block. There have been several geodynamic models developed to explain the evolution process of the Qinling Orogenic Belt, but they remain controversial. The magnetotelluric (MT) method can study subsurface electrical structures. In order to have a better understanding of the tectonic evolution of the Qinling Orogenic Belt, a study of broadband and long period MT profile across the middle Qinling Orogenic Belt is carried out. The MT profile starts from the northeastern margin of the Sichuan basin, crosses the Qinling, and finally reaches the Weihe basin at about 108.5°E. Based on data analysis results, we have finally obtained a lithospheric electrical resistivity model of the middle Qinling Orogenic Belt through three-dimensional (3-D) inversion of magnetotelluric data. The MT result reveals that the Sichuan and Weihe basins may have remnants of the ancient craton basement in our study area. The electrical structure characteristics of the South Qinling Belt and North Qinling Belt are obviously different, the South Qinling Belt is relatively low resistivity, while the North Qinling Belt is relatively high resistivity. This may indicate that the Southern Qinling Belt is affected by the later strike slip extrusion and the Northern Qinling Belt is a relatively stable block. The lithospheric mantle of middle Qinling Orogenic Belt is characterized by overall high resistivity, which may indicate that the lateral extrusion effect of the Tibetan Plateau material is negligible in this region.

Joint Interpretation of Magnetotelluric and Potential Field Data From North-Eastern Norrbotten, Sweden

Potential field data in databases of the Geological Survey of Sweden (SGU) combined with newly acquired broadband magnetotelluric data are used to map and interpret geological units and structures of a 200 km by 250 km area in the Paleoproterozoic Norrbotten ore province (northern Sweden, latitudes 66°–68.5° and longitudes 19°–24°). In order to achieve this, a new approach is proposed with respect to extracting and analysing the possible correlation between modelled physical properties as well as their patterns with respect to depth variation within the crust. In this study, we propose the use of a neural net self-organising map procedure (SOM) for simplification, data reduction, and domain classification of the models derived from independent 3-D geophysical inversion of magnetotelluric, gravity, and magnetic data. The crustal model of the electrical conductivity structure was obtained from previous 3-D inversion of the magnetotelluric data. Processing and 3-D inversion of the regional magnetic and gravity field data were performed using an open-source object-oriented code called SimPEG. The input data to the SOM analysis contain resistivity, magnetic susceptibility, and density model values within the Norrbotten area for some selected depth levels of the entire crust. The domain classification is discussed with respect to the geological boundaries and composition of the crust. Consistency between model domain classification and geological boundaries is observed in general but an apparent discrepancy is noted for some areas. The reason for the apparent discrepancy is likely related to that most geological boundaries represent surface features whereas the geophysical data includes information at depth.

Lithospheric Conductors Shed Light on the Non‐Uniform Destruction of North China Craton

AbstractLow resistivity in the continental lower crust and uppermost mantle has been commonly observed in both Phanerozoic and Precambrian areas. Given different tectonic history, these conductors could be attributed to different conductive mechanisms. The North China Craton (NCC) is one of typical craton that have suffered from extensive destruction, part of which was modified by Phanerozoic tectonic‐thermal events and the others remain stable cratonic lithosphere. We here present a high‐resolution 3‐D electrical resistivity model from the inversion of magnetotelluric data collected in 183 sites of NCC. Beneath the stable craton in the north part of Ordos Block, an E‐W trending conductor extends from lower crust to uppermost mantle, which can be attributed to the concentration of sulfides/graphite emplaced through oceanic subduction during the Paleoproterozoic assembly of Columbia. The conductors beneath the Taihang Mountains and the south part of Ordos Block also reflect ancient sutures. In contrast, the conductor in mantle beneath the south part of North China Plain (SNCP) reflects an on‐going process of craton destruction relating to the subduction of the Pacific Plate in Cenozoic. The MT data, combined with seismic radial anisotropy model suggest the lower crust and uppermost mantle beneath SNCP is anisotropic. And a significant electrical anisotropic layer with the most conductive direction along N70°W and the anisotropy factor of 6.3 is revealed in the 20–70 km depth by 2‐D anisotropic inversion. We attribute the enhanced conductivity along anisotropy direction to the accumulation of melts in sills, resulting from the upwelling of asthenosphere in Cenozoic.

Three-dimensional regularized inversion of magnetotelluric data with a minimum support gradient constraint

Regularization is an effective technique to obtain stable solutions for 3D magnetotelluric (MT) inversion. Smooth constraints are commonly used in traditional inversion algorithms. It is unsuitable if anomalies occur in block patterns. In some cases, it is necessary to obtain sharp boundaries. Some studies have proposed stabilizing functionals to obtain clear and sharp boundaries, such as minimum support (MS) and minimum gradient support (MGS) functionals; however, the optimal value of the focusing parameter should be selected to produce reliable inversion results. In this study, we expanded a new stabilizing functional that we called minimum support gradient (MSG) stabilizing functional as a stabilizer in 3D MT inversion. We developed a Gauss-Newton (GN) algorithm based on the edge-based finite-element method (FEM) to invert the MT data. In the inversion process, the forward modeling and the adjoint problem are solved by the Math Kernel Library (MKL) PARDISO parallel direct solver. we also parallelized the inversion program over frequencies by using Message Passing Interface (MPI) to further speed up the inversion process. Two synthetic models are tested to compare the inversion results for different stabilizing functionals. Compared to traditional smooth inversion, the MSG inversion can clearly distinguish the boundaries of the anomalies. The inverted resistive targets are closer to the true test model. Additionally, the MSG inversion results are stable and robust for a wide range of focusing parameter values, indicating that our method is effective and easy to implement. Finally, the proposed algorithm is applied to the long MT data from the USArray component of EarthScope project in the United States.