Three-dimensional Inversion Of Magnetotelluric Data Research Articles

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.

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.

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.

Consideration on three-dimensional inversion of magnetotelluric data by finite-difference modeling including topography: Data interpretation at a northern Hakkoda area

Consideration on three-dimensional inversion of magnetotelluric data by finite-difference modeling including topography: Data interpretation at a northern Hakkoda area

Electrical structure of the eastern segment of the Qilian orogenic belt revealed by 3-D inversion of magnetotelluric data: New insights into the evolution of the northeastern margin of the Qinghai-Tibet Plateau

Abstract The eastern segment of the Qilian orogenic belt, composed of the Linxia block and the Longzhong block, is located at the northeastern margin of the Qinghai–Tibet Plateau. The area has experienced multiple tectonic events, such as the closure of the Qilian Ocean, the convergence of the North China block and the Qilian terrane, and the collision of the Indian and Eurasian plates, forming a complex tectonic framework. To investigate the geological evolution of the area and the current location of the suture between the plates, we performed three-dimensional inversion of magnetotelluric data acquired along two profiles in the eastern segment of the Qilian orogenic belt using a nonlinear conjugate gradient algorithm to obtain electrical resistivity structure models to 60-km depth. The results include three key findings. (1) The Linxia block was originally a Japanese-type island arc with a mainly low-resistivity structure, which corresponds to deformed, fractured middle and lower crust that contains saline fluids. (2) The Longzhong block was originally a Mariana-type island arc and has a resistive structure with good integrity, consistent with crust composed of ocean island basalt. (3) The Maxianshan fault, shown as a belt with a steep gradient in electrical resistivity model, is a supracrustal fault that acts as a suture between the Japanese-type island arc (now the Linxia block) and the Mariana-type island arc (now the Longzhong block).

Geothermal potential of the eastern end of the Gediz basin, western Anatolia, Turkey revealed by three-dimensional inversion of magnetotelluric data

Three-dimensional models of the crustal electrical resistivity were derived from the inversion of magnetotelluric data in the eastern end of the Gediz basin, western Anatolia. In total, 179 MT sites measured in three different areas have been used in modeling. The study aims at providing information about the electrical resistivity distribution in the subsurface and its relations to structural setting and fluid pathways mainly controlling the development of geothermal reservoirs in the upper crust. The electrical resistivity models reveal a surficial low-medium resistivity layer associated with Neogene-Quaternary supradetachment sedimentary succession and a deeper high resistivity layer related to the Central Menderes Massif. The sedimentary cover over the metamorphic basement includes undulations, presenting an asymmetric extensional basin. The thickness of the sedimentary layer varies along strike from west to east and reaches approximately 3 km (max) in the graben. An anomalously conductive zone associated with a clay alteration zone was observed in this layer. The resistivity models also bring out two deep conductive zones along the southern margin of the basin: a conductive zone that could be interpreted as a geothermal reservoir at the easternmost end of the basin, however instead described as a part of a deep low resistivity zone with regional extent and another conductive zone interpreted as a region possessing an interconnected fracture network developed mainly by the extensional tectonic regime and that probably contains geothermal fluids. Moreover, normal faults exposing along the basin margins characterize the geometry of the basin, and particularly the main graben-bounding fault and Gediz detachment fault provide possible fluid pathways in the upper crust.

Ore-controlling structures of the Xiangshan volcanic Basin, SE China: Revealed from three-dimensional inversion of Magnetotelluric data

We report a high-resolution three-dimensional (3D) Magnetotelluric (MT) study to define the mineralizing system of the Xiangshan (XS) volcanic Basin, which hosts the largest volcanic-related uranium deposits in China. MT data from 81 stations were collected and inverted, using a 3D Truncated Newton (TRN) inversion algorithm. The generated electrical resistivity model reveals a high resistivity layer in the shallow, followed by a conductivity layer, and they were formed respectively by the Cretaceous Ehuling Formation and Daguding Formation. Under these Cretaceous formations, a series of high resistivity features delineate the Precambrian metamorphic basement. The contact zone between the high resistivity and conductivity layer corresponds to the inter-formational surface in the Cretaceous volcanic cover, whereas the contact zone between the volcanic cover and metamorphic basement represents the unconformity surface which was supported by gravity data. The identified faults in the 3D resistivity model coincide with the NE- and NS-trending faults observed on the surface. These faults are the crucial ore-controlling structures with high permeability allowing the ore-forming fluids to migrate upwards. We reveal a possible genetic model for the uranium mineralization in the XS volcanic Basin. The mixture of the descending meteoric water and ascending magmatic fluids along the deep faults formed the ore-forming fluids. The precipitation and enrichment of uranium were prone to occur in the ring faults, interfaces, unconformity surfaces and those geologic contacts intersecting with faults, where the physical and chemical properties of the ore-forming fluids were changed. Given that the mafic magmatism generally was coeval with the uranium mineralization, the deep-seated magmatism is supposed to be the force that drove the fluids upwards and governed the circulative convection of ore-forming fluids in the XS volcanic Basin.

Three-Dimensional Inversion of Magnetotelluric Data in Studying the Source Zones of the Earthquakes in the Altai–Sayan Region

A three-dimensional (3D) inversion of the components of magnetotelluric impedance matrix is used for constructing a geoelectric model of the Altai-Sayan region. Based on the model and empirical data, the possibilities of three-dimensional inversion of all the four components of the impedance matrix and its invariant values ​​are studied. The model values ​​of the impedance matrices obtained by solving the inverse problem by interactive fitting the 3D model invariant curves of apparent electrical resistivity to the experimental curves for the central part of the Altai–Sayan region were used as the test data. The high conductivity anomalies obtained by 3D inversion are correlated with the locations of earthquake sources, deep faults, domains of increased attenuation of the converted waves from the earthquakes, and low velocities of P-waves in the deep seismic sounding and earthquake converted waves methods.

Three-Dimensional Inversion of Magnetotelluric Data in Study of Focal Zones of Earthquakes in the South of the Altai–Sayan Region

Abstract —The potentialities of the 3D inversion program WSINV3DMT were estimated on the model [Zm] and observed [Zob] components of the impedance tensor and its invariants during the construction of geoelectrical models for the southern part of the Altai–Sayan region characterized by a three-dimensional distribution of electrical conductivity. These components were obtained for a 3D model constructed by the method of interactive selection of the 3D model induction curves for the experimental ones. Testing of the WSINV3DMT program on the model magnetotelluric data showed the possibility of not only isolation of blocks of high electrical conductivity but also of the appearance of conductivity anomalies different from the model ones. The paper presents a 3D geoelectrical model for the southern focal zones of the region, constructed with the use of the WSINV3DMT program and the proposed method of interpretation. The isolated blocks of low electrical resistivity are correlated with the location of earthquake foci, deep faults, and regions of high absorption of earthquake exchange waves.

3D inversion of MT data from northern Mexico for geothermal exploration using TEM data as constraints

San Felipe's area has geothermal surface manifestations such as hot springs and high temperature in drinking water wells. As a first stage of the geothermal exploration program, a regional geophysical survey has been carried out where 17 MT (Magnetotelluric) and TEM (Transient Electromagnetic) soundings were recorded along 4 different profiles. Both techniques were applied in the same spots and the TEM data are used to correct for static shift in MT data. Transient responses were measured from 1 to 70 ms, using a 50 x 50m2 single-loop configuration. MT signals were recorded with Metronix systems in the period range from 0.001 to 100 s. A robust processing scheme is applied to the MT time series and estimated Transfer Functions are appraised. The dimensionality analysis of MT data suggests a 1D and 3D subsurface structure. Thereby, a three-dimensional inversion of MT data is later carried out. The information obtained from the inversion modeling of TEM data is incorporated into the 3D MT inversion scheme. Thus, unconstrained and constrained inverse models are achieved and appraised. The preferred 3D inversion model is correlated to geological information of the survey area and a conductive structure is interpreted as a sedimentary basin at its shallow part and as a fault zone with geothermal fluids at depths >1.5 km.

The conductivity structure of the Gediz Graben geothermal area extracted from 2D and 3D magnetotelluric inversion: Synthetic and field data applications

In this study, we have created a virtual three-dimensional conductivity model of the Gediz Graben geothermal area using real topographical variations and geological settings as prelude to the interpretation of acquired MT field data. The thicknesses of the geological units were obtained from previously studied seismic sections and boreholes drilled inside the study area. A large magnetotelluric grid was created using 234 sites with 1km spacing between each site. The three-dimensional magnetotelluric response of the model was calculated at each site and used as an input for two- and three-dimensional inversion to assess resolving capability for subsurface features. The inversion results were compared to the virtual graben model, and the efficiency of the inversion methods was examined. Comparisons show that the three-dimensional inversion of magnetotelluric data has many advantages with respect to the two-dimensional inversion results for the interpretation of the three-dimensional geological structure of the graben fill. Although the two-dimensional inversion is a rapid tool for exploration of the conductive anomalies, it is inadequate for interpreting the resistive basement rock structure under the sedimentary cover. We also applied three-dimensional inversion to the field data previously recorded inside the study area. The target borehole locations were defined according to the interpretation of the three-dimensional inversion model. A resistivity cross-section extracted from the three-dimensional model of the field data is presented with the borehole observations. The lithological boundaries of the actual borehole data and the resistivity transactions of the three-dimensional model show close correlation. The synthetic data examples show that the virtual graben model is a practical tool for understanding the advantages and disadvantages of the magnetotelluric inversion methods on extensional tectonic basins. Additionally, the field data experiments show that the three-dimensional inversion of magnetotelluric data is an efficient tool for interpreting the geological structure of extensional graben systems.

Three-Dimensional Inversion of Magnetotelluric Data for the Sediment–Basement Interface

Determining the sediment–basement interface is the major step in evaluating the mineral resource potential of a region. The magnetotelluric (MT) method can be effectively used for solving this problem because there exists a strong contrast in resistivity between a conductive sedimentary basin and a resistive basement. Conventional inversions of MT data are aimed at determining the volumetric distribution of the conductivity within the inversion domain. The recovered distribution of the subsurface conductivity is typically diffusive, which makes it difficult to select the sediment–basement interface. This letter develops a novel approach to 3-D MT inversion for the depth-to-basement estimate. The key to this approach is selection of the model parameterization, with the depth to basement being the major unknown parameter. In order to estimate the depth to the basement, the inversion algorithm recovers both the thickness and the conductivities of the sedimentary basin. The forward modeling is based on the integral equation approach. The inverse problem is solved using a regularized conjugate gradient method. The Frechet derivative matrix is calculated based on quasi-Born approximation. The developed method and the algorithm for MT inversion for the depth-to-basement estimate are illustrated on several realistic geoelectrical models.

Electrical conductivity structure of north-west Fennoscandia from three-dimensional inversion of magnetotelluric data

New magnetotelluric (MT) data in north-west Fennoscandia were acquired within the framework of the project “Magnetotellurics in the Scandes” (MaSca). The project focuses on the investigation of the crustal and upper mantle lithospheric structure in the transition zone from stable Precambrian cratonic interior to passive continental margin beneath the Caledonian orogen and the Scandinavian Mountains in western Fennoscandia. An array of 59 simultaneous long period and 220 broad-band MT sites were occupied in the summers of 2011 to 2013.The 3-D inversion of the MaSca data was obtained using the ModEM 3-D code. The full impedance and tipper data were used for the inversion. The rocks of Archaean and Proterozoic basement towards east and the Caledonian nappes towards west are modelled as resistive structures. In the central and southern parts, the whole crust is resistive and reflects the Trans-Scandinavian Igneous Belt granitoids. The middle to lower crust of the Svecofennian province is conductive. An uppermost crustal conductor is revealed in the Skellefteå Ore District. The south end of the Kittilä Greenstone Belt is seen in the models as a strong upper to middle crustal conductor. In the Caledonides, the highly conductive alum shales are observed along the Caledonian Thrust Front. A map of the crustal conductance for the north-west Fennoscandian Shield is presented.

Three-dimensional inversion of magnetotelluric data from the Central Andean continental margin

Abstract Magnetotelluric data were collected in the late 1990s in the Central Andes of Chile and Bolivia, with the aim to delineate the electrical conductivity distribution in the subsurface and its relations to subduction processes. In previous studies, these data were interpreted based on 2-D models. The principal result was a vast conductivity zone beneath the Altiplano high plateau at mid and lower crustal depths and a much smaller, though significant conductor associated with the Precordillera Fault System. However, there are some significant 3-D effects in the investigation area, in particular near the coast and on the eastern Altiplano. The aim of this work is to give a reinterpretation based on new 3-D inversion of these data. The 3-D inversion not only provides a better fit to the data compared to 2-D results but furthermore allows to include sites with strong telluric distortion which were ignored in previous studies. We are now able to explain anomalous phases above 90° and induction arrows pointing subparallel to the coast as observed at several sites in the Coastal Cordillera. These strongly distorted data are caused by highly conductive near-surface structures that are partly connected to the Pacific Ocean, forcing currents to flow around the sites. The lower crust beneath the Coastal Cordillera resembles a poorly conductive, nearly homogeneous half-space and is electrically unremarkable. Besides, we can now image the Precordillera conductor as a continuous, elongated feature. The volcanic arc of the Western Cordillera is highly resistive with the exception of a few conductive spots which may be associated with certain individual volcanoes or geothermal resources, respectively. The Altiplano conductor is again the dominant electrical feature in the Central Andes, indicating widespread melting of the middle and lower back-arc crust.

Three-dimensional inversion of magnetotelluric data including sea effects obtained in Pohang, Korea

Magnetotelluric (MT) surveys were conducted in Pohang, Korea, for low-temperature geothermal exploration in 2002 and 2003. Pohang is located in the southeastern part of the Korean Peninsula and close to the East Sea. In the interpretation of MT data from a coastal environment, sea effects must be correctly included because seawater is a strong conductor. We first constructed a five-layered earth model with a realistic coastline and bathymetry to investigate sea effects on MT data measured in Pohang. This model clearly shows that the Pohang data are significantly influenced by sea water at frequencies blow 1 Hz at the whole measurement sites. Next, we utilized a three-dimensional inversion algorithm based on the Gauss–Newton approach to produce a reliable resistivity model. Seawater is excluded from the inversion domain to fix the resistivity, while included in the modeling domain to simulate sea effects on MT responses. Blocks for the sub-seafloor are included in unknown parameters since they are sufficiently close to the survey area to affect MT responses in Pohang. Static shifts are also considered in inversion for more accurate interpretation. The rms data misfit is smoothly reduced from 11.2 to 1.87 after 7 iterations. The resulting resistivity model shows a pattern of low–high–low resistivity with depth. The model is compatible with resistivity logs obtained from four boreholes in the survey area, and can explain major geological features in Pohang.

Efficient three-dimensional inversion of magnetotelluric data using approximate sensitivities

SUMMARY A method for accurately approximating sensitivities is introduced for the efficient 3-D inversion of static-shifted magnetotelluric (MT) data. Approximate sensitivities are derived by replacing adjoint secondary electric fields with those computed in the previous iteration. These sensitivities can reduce the computation time, without significant loss of accuracy when constructing a full sensitivity matrix for 3-D inversion, based on the Gauss–Newton method. Additional reduction of computational cost can be attained by modifying the inversion scheme to run on a parallel computing platform. The effectiveness of approximate sensitivities is tested by inverting both synthetic and field data obtained in Pohang, Korea, and Bajawa, Indonesia. The accuracy of approximate sensitivities is validated by sensitivity analysis of synthetic data. To make the inversion of static-shifted MT data more stable, a weighting coefficient for static-shift parameters is added to the objective function and is updated at each iteration. Approximate sensitivities are calculated much faster than exact sensitivities, and are accurate enough to drive an iterative inversion algorithm.

Three-dimensional inversion of magnetotelluric data for mineral exploration: An example from the McArthur River uranium deposit, Saskatchewan, Canada

Shallow exploration targets are becoming scarce, meaning interest is turning towards deeper targets. The magnetotelluric method has the necessary depth capability, unlike many of the controlled-source electromagnetic prospecting techniques traditionally used. The geological setting of ore deposits is usually complex, requiring three-dimensional Earth models for their representation. An example of the applicability of three-dimensional inversion of magnetotelluric data to mineral exploration is presented here. Inversions of an audio-magnetotelluric data-set from the McArthur River uranium mine in the Athabasca Basin were carried out. A sub-set comprising data from eleven frequencies distributed over almost three decades was inverted. The form of the data used in the inversion was impedance. All four elements of the tensor were included. No decompositions of the data were done, nor rotation to a preferred strike direction, nor correction for static shifts. The inversions were successful: the observations were adequately reproduced and the main features in the conductivity model corresponded to known geological features. These included the graphitic basement fault along which the McArthur River uranium deposit is located.

Three-dimensional inversion of magnetotelluric data in complex geological structures

Interpretation of magnetotelluric data over inhomogeneous geological structures is still a challenging problem in geophysical exploration. We have developed a new 3-D MT inversion method and a computer code based on full nonlinear conjugate gradient inversion and quasi-analytical approximation for forward modeling solution. Application of the QA approximation to forward modeling and Frechet derivative computations speeds up the calculation dramatically. However, in order to control the accuracy of the inversion, our method allows application of the rigorous forward modeling in the intermediate steps of the inversion procedure and for the final inverse model. The 3-D magnetotelluric inversion code QAINV3D based on QA approximation, has been tested on synthetic models and applied to the practical MT data collected in an area with complex geology. The inversion of 3-D MT data can be done within a few minutes on a PC to generate a 3-D image of subsurface formations on a large grid with tens of thousands of cells.

Three-dimensional inversion of magnetotelluric data with static shifts

The static shifts caused by small-scale shallow resistivity anomalies hamper the interpretation of magnetotelluric (MT) data. In this paper, an approximate three-dimensional inversion scheme is developed that inverts jointly for resistivities and static-shift values. The unknown static-shift parameters are incorporated into the inverse problem based upon an assumption that they are Gaussianly distributed. A staggered-grid finite-difference method is used to calculate the MT responses in three dimensions. The sensitivities of the MT responses to subsurface resistivity changes are calculated from the analytic solutions of the electric field for a homogeneous half-space model using the adjoint-equation approach. These approximate sensitivities are used throughout iterations, which results in a substantial saving in the computation time over the full inversions. A synthetic example indicates that the inversion scheme recovers the resistivity structure from the data contaminated heavily with static shifts.