Three-dimensional Magnetotelluric Inversion Research Articles

Three-dimensional Magnetotelluric Inversion and Magnetic for The Characterization of The Geothermal Field Reservoir Zone “X”

Geothermal as an alternative energy source that is renewable and environmentally friendly has an important role in providing domestic energy needs. Exploration is one of the most important stages in the development of geothermal energy because it can minimize the risk at the stage of exploitation and development. Geophysical methods such as geomagnet and magnetotelluric are one of the methods used in exploration. Magnetic method can provide information on the description of rock demagnetization due to the presence of heat source. However, the magnetotelluric method will provide information about rock type resistance on geothermal fields. The existence of heat source using the geomagnetic method is represented by a low anomaly value as an indication of demagnetized rocks. The results of 3D inversion processing show a low value of type resistance in the area and form an updome. The correlation of the two results of processing the data can be seen in the southern part of the "X" geothermal field research area. Based on 3D Inversion modeling it can be seen the depth of the "X" Base of Conductor (BOC) geothermal system ranges from 1000 m to -800 m with resistivity ≤ 10 Ωm which is suspected as volcanic rocks. Geothermal reservoir is at a depth of 1000 m to -2000 m with a moderate resistivity of 40 - 60 Ωm which is suspected as an andesite volcanic rock, with temperature estimates ranging from 218o C to 255o C.

Geothermal Resource Mapping in Northern Botswana Inferred from Three-Dimensional Magnetotelluric Inversion

A set of magnetotelluric (MT) data collected from 28 stations at the Kasane Hot Spring in northern Botswana was used to derive a 3-D model of the electrical resistivity distribution around deep geologic structures that we associated with geothermal resources. The dimensionality analysis results revealed that 3-D modeling should be used to determine the electrical structure. The resistivity models revealed a conductive layer with an average thickness of 200 m, representing the overlying sediments of the Proterozoic volcanic rocks. The thick high-resistivity zone (>100 Ωm) below the conductive layer can be associated with Mesoproterozoic bedrock. The MT measurements in this area show a tube-shaped conductive anomaly that could serve as a fluid pathway feeding the hot spring. A fracture-controlled meteoric fluid circulation presumably determines the existence of the Kasane Hot Spring system.

Three-dimensional magnetotelluric inversion for the characterization of the Sol de Mañana high-enthalpy geothermal field, Bolivia

We describe the three-dimensional magnetotelluric modeling results of Laguna Colorada - Sol de Mañana geothermal field in SW Bolivia. It is located near Laguna Colorada in the foothills of the Western Cordillera of the Andes and at the margin of the Altiplano-Puna Magma Body (APMB), which is characterized by a vast electrical conductivity anomaly and corresponding seismic anomalies in the mid and deep crust. The magnetotelluric (MT) data comprising 70 stations were modeled by a 3-D inversion. The final model shows a broad conductor at a few hundred meters in depth, which we interpret as the confining cap layer. Beneath, equally widespread, a layer of intermediate resistivity marks the high-temperature hydrothermal reservoir. Detailed interpretation of MT data with information from existing wells suggests a potential for a high-enthalpy geothermal field.

Three-dimensional magnetotelluric inversion reveals the typical geothermal structure of Yanggao geothermal field in Datong Basin, northern China

The Yanggao geothermal field, located in the north of the Datong Basin, and close to the Datong Volcanic Group, the geothermal field is affected by deep faults and vigorous tectonic activity. Yanggao has abundant geothermal resources, and contains a large number of hot springs. To explore the geothermal system and characterize the formation mechanism of the Yanggao geothermal field, we deployed 168 magnetotelluric (MT) sitesin the study area. MT phase tensor analysis is used to estimate the dimensionality of the subsurface. Furthermore, we conducted an inversion of the MT data to establish a 3D resistivity model of the study area. The results suggest that the Yanggao geothermal system can be divided into three layers. A low-resistivity layer (C1) exists at a depth of 2 km, which can be interpreted as the clay alteration layer (clay cap) and the reservoir of the geothermal system. Under the reservoir, the model indicates an additional low-resistivity zone (C3) at a depth of 8 km, which is presumed to be a heat source related to the Datong Volcanic Group. Moreover, a vertical low-resistivity zone (C2) between the reservoir and the heat source is related to the Yunmenshan Fault acting as a heat conduction path in the Yanggao geothermal field, controlling the infiltration and recharge of the hot springs.

Crustal Electrical Structure of the Ganzi Fault on the Eastern Tibetan Plateau: Implications for the Role of Fluids in Earthquakes

The initiation and evolution of seismic activity in intraplate regions are controlled by heterogeneous stress and highly fractured rocks within the rock mass triggered by fluid migration. In this study, we imaged the electrical structure of the crust beneath the Ganzi fault using a three-dimensional magnetotelluric inversion technique, which is host to an assemblage of resistive and conductive features extending into the lower crust. It presents a near-vertical low-resistance zone that cuts through the brittle ductile transition zone, extends to the lower crust, and acts as a pathway for fluid migration from the crustal flow to the upper crustal depths. Conductors in the upper and lower crust are associated with saline fluids and 7% to 16% partial melting, respectively. The relationship between the earthquake epicenter and the surrounding electrical structure suggests that the intraplate seismicity is triggered by overpressure fluids, which are dependent on fluid volume changes generated by the decompression dehydration of partially molten material during upwelling and native fluid within the crustal flow.

Three-dimensional magnetotelluric inversion considering electrical anisotropy with synthetic and real data

Electrical anisotropy increases the non-uniqueness of magnetotelluric (MT) inversions. However, it is still useful to develop MT anisotropic inversion to provide new knowledge for this field. In this study, we developed a three-dimensional (3-D) MT inversion code considering electrical anisotropy. The forward procedure is based on a staggered-grid finite-difference method and solves the governing equation for the electric fields with a direct method. The values of a traditional objective function and its gradient could then be calculated. The inversion procedure was completed with a well-developed subroutine of non-linear conjugate gradient method. Synthetic tests for different isotropic models indicate the validity of this inversion code. Furthermore, we performed 3-D anisotropic inversions for synthetic and real data. The results indicate that vertical resistivity is still indistinguishable in the anisotropic inversion results. In our synthetic tests, it becomes more difficult to recover low-resistivity components of anisotropic anomalies as the depth increases, and the vertical sizes of anisotropic anomalies are reduced, as characterized by a much shallower bottom boundary. The anisotropic strike angle is not as stable as the principal resistivities in the inversion process. Under the same changes in model parameters, the strike angle may cause larger deviations of the responses than the principal horizontal resistivities. With this code, we revisited the field data collected at the northern margin of the Tibetan Plateau. The inversion results indicate a possible anisotropic anomaly exists below the boundary between the plateau and its northern neighbor. This anisotropic anomaly could represent an area of concentration of shear deformation.

The electrical conductivity structure of the Tarim basin in NW China as revealed by three-dimensional magnetotelluric inversion

The Tarim basin, located in northwestern China, is the largest inland basin with oil, gas, and mineral resources in China. To study the deep structure of the Tarim basin, we conducted a magnetotelluric (MT) survey of the entire basin using 756 datasets to build a three-dimensional (3-D) electrical conductivity model. Based on the initial 3-D model, which was derived from a two-dimensional (2-D) minimum support gradient (MSG) inversion, the electrical conductivity structure of the Tarim basin was imaged by a 3-D MT regularized inversion using the contracting integral equation method (CIE) and quasi-Newton optimization. Based on simulation results, a thick resistive anomaly exists near the center of the Tarim basin, which suggests that the basin has the properties of a cratonic basin. The basin is also surrounded by resistive lithosphere faults, which indicates that it has the properties of a fault basin. The crystalline basement of the Tarim basin is divided into a resistive craton basement, high resistance basalt basement, and conductive crevice water metamorphic basement based on the shallow resistivity structure. In addition, the lithospheric thickness was estimated using the deeper resistive structure.

Crustal structure and magma plumbing system beneath the Puer Basin, southwest China: Insights from three-dimensional magnetotelluric imaging

The Puer Basin region, where Yangtze continental block and the Tethyan orogen collide, provides a favorable area for studying the processes of magmatic evolution, intracontinental earthquakes and hydrothermal systems. Focused on these issues, we have acquired a three-dimensional electrical model of the Puer Basin in southern Yunnan, using broadband magnetotelluric data extending from 320 to 0.001 Hz along eight profiles covering the Puer Basin. The complex subsurface model is filled with zones of fluid-filled fracture, partial melt networks, regional faults, and cold plutons. There are two highly conductive zones (3–10 Ωm) beneath the Puer Basin from 10 km to 35 km in depth. The estimated melt fractions are in the range of 4–26 vol%, which is required to account for the observed high conductivities in the crust based on the formulation of the multiple conduction models and resistivity from the three-dimensional magnetotelluric inversion. The partial melt in the middle and lower crust may have been derived from either magma intrusion of the mantle sources or the dehydration processes in the slab subducting of Yangtze continental block beneath the Tethyan orogen in southern Yunnan. Based on the three-dimensional electrical model, integrating with crustal anisotropy, thermal structure, potential field and seismic survey results, we have a more comprehensive understanding of the mechanism of the hydrothermal system, seismotectonics, and deep seismogenic background.

Varying Indian crustal front in the southern Tibetan Plateau as revealed by magnetotelluric data

In the southern Tibetan plateau, which is considered to be the ongoing India–Eurasia continental collision zone, tracing of the Indian crustal front beneath Tibet is still controversial. We conducted deep subsurface electrical modeling in southern Tibet and discuss the geometry of the front of the Indian crust. Three areas along the Yarlung-Zangbo river zone for which previous magnetotelluric (MT) data are available were inverted independently using a three-dimensional MT inversion algorithm ModEM. Electrical horizontal slices at different depths and north–south oriented cross sections at different longitudes were obtained to provide a geoelectrical perspective for deep processes beneath the Tethyan Himalaya and Lhasa terrane. Horizontal slices at depths greater than − 15 km show that the upper crust is covered with resistive layers. Below a depth of − 20 km, discontinuous conductive distributions are primarily concentrated north of the Yarlung-Zangbo sutures (YZS) and could be imaged from mid- to lower crust. The results show that the maximum depth to which the resistive layers extend is over − 20 km, while the mid- to lower crustal conductive zones extend to depths greater than − 50 km. The results indicate that the conductive region in the mid- to lower crust can be imaged primarily from the YZS to south of the Bangong-Nujiang sutures in western Tibet and to ~ 31°N in eastern Tibet. The northern front of the conductive zones appears as an irregular barrier to the Indian crust from west to east. We suggest that a relatively less conductive subsurface in the northern portion of the barrier indicates a relatively cold and strong crust and that the front of the Indian crust might be halted in the south of the barrier. We suggest that the Indian crustal front varies from west to east and has at least reached: ~ 33.5°N at ~ 80°E, ~ 31°N at ~ 85°E, and ~ 30.5°N at ~ 87°E and ~ 92°E.

Parallel three-dimensional magnetotelluric inversion using adaptive finite-element method. Part I: theory and synthetic study

Parallel three-dimensional magnetotelluric inversion using adaptive finite-element method. Part I: theory and synthetic study

Erratum to: A regularized three-dimensional magnetotelluric inversion with a minimum gradient support constraint

Erratum to: A regularized three-dimensional magnetotelluric inversion with a minimum gradient support constraint

Three-dimensional magnetotelluric inversion using a heterogeneous smoothness-constrained least-squares method

This paper presents a fast algorithm for electromagnetic data inversion to three-dimensional (3D) resistivity models. The algorithm is distinctive for the level of accuracy it attains while bypassing the sensitivity matrix update. A common sensitivity matrix for homogeneous half-space is used in all iterations. Instead of updating the sensitivity matrix, the smoothness filter coefficients at each model element are updated, based on the spatial variations in resistivity in the model derived from the latest iteration. This substitution is expected not only to reduce the computation time required for large-scale inversions, such as those for 3D surveys, but also to allow the resolution of sharp boundaries in resistivity structures. Our algorithm was applied to 3D magnetotelluric inversion in order to confirm its effectiveness. Using synthetic examples under several conditions, we demonstrated that the method can reduce the number of forward calculations required to reduce data misfits to noise level, and that the method is robust for constructing target models even with sharp boundaries without generating fatally false resistivity structures or boundaries under noisy conditions.

Three-dimensional magnetotelluric inversion in practice—the electrical conductivity structure of the San Andreas Fault in Central California

Three-dimensional magnetotelluric inversion in practice—the electrical conductivity structure of the San Andreas Fault in Central California

A regularized three-dimensional magnetotelluric inversion with a minimum gradient support constraint

SUMMARY Most 3-D magnetotelluric (MT) inversions are classified as a regularized inversion with a smoothness constraint. These inverse algorithms provide smooth solutions but cannot clearly image sharp geo-electrical interfaces. In this paper, we introduce the minimum gradient support (MGS) functional to regularize the 3-D MT inverse problem. This functional has a property whereby the functional seeks a structure with minimum volume containing large conductivity gradients. Therefore, the MGS functional can be used to search for a model with a sharp boundary. We apply the MGS functional to 3-D MT inversion to obtain a clear and accurate image of geo-electrical interfaces. In addition, the modified scattering equation approach introduced in the modified iterative dissipative method (MIDM) is applied to forward calculation, which is based on integral equation (IE) formulation and allows us to efficiently reduce the time required for forward calculation with high accuracy. The quasi-Newton iterative method is used to optimize the objective functional. It is a kind of iterative method with simplified calculation of the inverse Hessian matrix using Broyden–Fletcher–Goldfarb–Shanno (BFGS) update. The convergence of this iterative method is guaranteed with inexact line searches. We also modify the adaptive approach for optimum selection of the regularization parameter so as to fit the inverse algorithm of this study. Three synthetic models are investigated, and the obtained results are compared with those obtained by a smoothing inversion. Based on the comparison, we confirm that the MGS inversion can provide higher resolution when geo-electrical interfaces are sharp. This property will help us to determine reliable electrical structures by the MT exploration method.

An efficient data space conjugate gradient Occam's method for three-dimensional magnetotelluric inversion

SUMMARY In this paper, we start with the implementation of a data space conjugate gradient (DCG) method for 3-D magnetotelluric (MT) data. This code will be referred to as WSDCG3DMT. It is an extension of the 2-D method previously developed. Several experiments on both synthetic and real data sets show that WSDCG3DMT usually needs more computational time than the data space Occam's inversion (OCCAM) for which the corresponding code is referred to as WSINV3DMT. However, the memory requirement of WSDCG3DMT is only a fraction of that of WSINV3DMT. Based on the knowledge gained from several studies of both codes, we have created a new hybrid scheme called the DCG Occam's inversion (DCGOCC) and the corresponding code, WSDCGOCC3DMT, from combining aspects of the OCCAM and DCG methods. As with OCCAM, the DCGOCC method divides the inversion into two phases. In Phase I the misfit is brought down to a desired level. In Phase II unnecessary structures are smoothed out. Because its mathematical basis is of a similar form to that of DCG, its memory requirement is similarly low but more stable. However, DCGOCC is significantly faster than both methods. We demonstrate the computational performances with comparisons of all three methods with both synthetic and EXTECH field data sets.

Three-Dimensional Magnetotelluric Inversion: An Introductory Guide for Developers and Users

In the last few decades, the demand for three-dimensional (3-D) inversions for magnetotelluric data has significantly driven the progress of 3-D codes. There are currently a lot of new 3-D inversion and forward modeling codes. Some, such as the WSINV3DMT code of the author, are available to the academic community. The goal of this paper is to summarize all the important issues involving 3-D inversions. It aims to show how inversion works and how to use it properly. In this paper, I start by describing several good reasons for doing 3-D inversion instead of 2-D inversion. The main algorithms for 3-D inversion are reviewed along with some comparisons of their advantages and disadvantages. These algorithms are the classical Occam’s inversion, the data space Occam’s inversion, the Gauss–Newton method, the Gauss–Newton with the conjugate gradient method, the non-linear conjugate gradient method, and the quasi-Newton method. Other variants are based on these main algorithms. Forward modeling, sensitivity calculations, model covariance and its parallel implementation are all necessary components of inversions and are reviewed here. Rules of thumb for performing 3-D inversion are proposed for the benefit of the 3-D inversion novice. Problems regarding 3-D inversions are discussed along with suggested topics for future research for the developers of the next decades.

WSINV3DMT: Vertical magnetic field transfer function inversion and parallel implementation

We describe two extensions to the three-dimensional magnetotelluric inversion program WSINV3DMT (Siripunvaraporn, W., Egbert, G., Lenbury, Y., Uyeshima, M., 2005, Three-dimensional magnetotelluric inversion: data-space method. Phys. Earth Planet. Interiors 150, 3–14), including modifications to allow inversion of the vertical magnetic transfer functions (VTFs), and parallelization of the code. The parallel implementation, which is most appropriate for small clusters, uses MPI to distribute forward solutions for different frequencies, as well as some linear algebraic computations, over multiple processors. In addition to reducing run times, the parallelization reduces memory requirements by distributing storage of the sensitivity matrix. Both new features are tested on synthetic and real datasets, revealing nearly linear speedup for a small number of processors (up to 8). Experiments on synthetic examples show that the horizontal position and lateral conductivity contrasts of anomalies can be recovered by inverting VTFs alone. However, vertical positions and absolute amplitudes are not well constrained unless an accurate host resistivity is imposed a priori. On very simple synthetic models including VTFs in a joint inversion had little impact on the inverse solution computed with impedances alone. However, in experiments with real data, inverse solutions obtained from joint inversion of VTF and impedances, and from impedances alone, differed in important ways, suggesting that for structures with more realistic levels of complexity the VTFs will in general provide useful additional constraints.

Three-dimensional magnetotelluric inversion: data-space method

A three-dimensional magnetotelluric (MT) minimum structure inversion algorithm has been developed based on a data-space variant of the Occam approach. Computational costs associated with construction and inversion of model-space matrices make a model-space Occam approach to 3D MT inversion impractical. These difficulties are overcome with a data-space approach, where matrix dimensions depend on the size of the data set, rather than the number of model parameters. With the transformation to data space it becomes feasible to invert modest 3D MT data sets on a PC. To reduce computational time, a relaxed convergence criterion is used for the iterative forward modeling code used to compute the sensitivity matrix. This allows reduction in computational time by more than 70%, without affecting the inversion results. Numerical experiments with synthetic data show that reasonable fits can be obtained within a small number of iterations, with a few additional iterations required to eliminate unnecessary structure and find the model with minimum norm.

Magnetotelluric inversion using regularized Hopfield neural networks

Hopfield neural networks are massive parallel automata that support specific models and are adept in solving optimization problems. They suffer from a ‘rough’ solution space and convergence properties that are highly dependent on the starting model or prior. These detractions may be overcome by introducing regularization into the network in the form of local feedback smoothing. Application of regularized Hopfield networks to over 50 optimization test cases has yielded successful results, even with uniform (minimal information) priors. In particular, the non‐linear, one‐ and two‐dimensional magnetotelluric inverse problems have been solved by means of these regularized networks. The solutions compare favourably with those produced by other methods. Such regularized networks, with either hardware or programmed, parallel‐computer implementation, can be extended to the problem of three‐dimensional magnetotelluric inversion. Because neural networks are natural analog‐to‐digital converters, it is predicted that they will be the basic building blocks of future magnetotelluric instrumentation.