Interpretation Of Magnetotelluric Data Research Articles

Magnetotelluric Observations in the Caspian Sea

Abstract —To study the geological structure in the search for raw hydrocarbons within the Volga delta and the Caspian Sea, two intersecting profiles of magnetotelluric sounding were performed. The analysis of geoelectric sections based on the one-dimensional inversion of the initial and normalized invariant curves of apparent resistance showed that it is necessary to use methods of three-dimensional mathematical modeling to form reliable geoelectric models. Their initial construction necessary for the three-dimensional interpretation of invariant apparent resistance curves was carried out taking into account their one-dimensional inversion. The resulting model, including the lower structural part, is constructed by the method of interactive matching to the apparent resistance curves of model curves calculated according to the program of three-dimensional mathematical modeling. This approach made it possible to take into account the influence of local galvanic distortions on the apparent resistance curves when evaluating the distribution of electrical conductivity in the lower parts of the subsea deposits. As a result of the integrated interpretation of magnetotelluric data, blocks with increased conductivity have been identified in the subsea deposits of the Northern Caspian Sea, which are most likely associated with high fluid saturation of Cretaceous and Neogene sediments. Their position correlates with the regional discontinuous structures of the region.

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.

Geoelectric Model of the Central Part of the North Caucasus: Three-Dimensional Inversion

Abstract ––Interpretation of magnetotelluric data within the North Caucasus marginal massif, the uplift of the Main Range, and the Stavropol Arch, performed on the basis of one- and two-dimensional inversions of the impedance tensor components and three-dimensional mathematical modeling, makes it possible to compile the starting and test models that are necessary for a three-dimensional inversion. It is shown by the three-dimensional inversion of all the impedance tensor components, carried out on a test three-dimensional model, that it is possible to estimate the parameters of conducting model blocks. The three-dimensional inversion of all the experimental impedance matrix components, performed with account for the results obtained on the test three-dimensional model, significantly corrects the parameters of the conducting blocks identified at the stages of one- and two-dimensional inversions, as well as the three-dimensional mathematical modeling of magnetotelluric fields in the central part of the North Caucasus. In the resulting three-dimensional geoelectrical model of the region, the position of low-resistance blocks correlates with the location of suture zones, deep faults, volcanic chambers, and domains with converted earthquake waves, whose velocities are reduced and whose absorption is increased. This can be explained by the dependence of the conductivity of crustal blocks on water saturation. Earthquake hypocenters are grouped near low-resistance anomalies.

Geophysical data interpretation technologies in the study and exploration of oil-and-gas deposits

The article presents the results of scientific research on the creation of computer technology for the interpretation of geophysical field data in the exploration of oil-and-gas deposits of Ukraine. The theoretical, informational, technological and methodical foundations for increasing the efficiency of geological exploration work due to the in-depth extraction of information from geological and geophysical data based on their complex interpretation within the framework of new mathematical models have been developed. Three-dimensional gravity and magnetometric modeling can be directed to the detection of densification zones and the tracing of tectonic disturbances in the consolidated crust, without which the existence of hydrocarbon transportation channels is impossible. The practical application of examples of quantitative interpretation of three-component magnetic survey data is given, which will significantly help in the detection and localization of hydrocarbon deposits. Development and expansion of the software complex for the interpretation of magnetotelluric data based on the use of impedance-type boundary conditions. The technique is intended for visualization of MTS data at the stage of qualitative interpretation in parallel with the method of the impedance tensor and Wiese vectors. The absolute advantage of this approach is its independence from the condition of a plane incident wave and the use of all six components of the electromagnetic field (including the Z component of the electric component of the MT field). The integration of statistical and deterministic methods during the inversion of geophysical data will increase the re-liability of the obtained geological results. The relevance and importance of the results presented in the work is determined by the conceptual novelty of methods and tools for forecasting new promising areas, as well as the reassessment of reserves of known deposits.

Analysis of sensitivity patterns for characteristics of magnetotelluric (MT) response functions in inversion

SUMMARYBecause the magnetotelluric (MT) method uses natural sources, the electric and magnetic fields recorded in the field acquisition are not directly used but usually converted into other MT response functions for interpretation such as inversion. Considering that inversion results are dependent on types of input data, it can be helpful to analyse different characteristics of MT response functions for inversion. In this study, we examine sensitivity patterns of MT response functions used commonly in MT inversion, which are the impedance tensor, apparent resistivity, phase, tipper, effective impedance and phase tensor; and investigate how their sensitivity patterns affect inversion results. We first describe overall tendencies of 3-D sensitivity patterns of the MT response functions, and then classify the MT response functions into six groups based on 2-D sensitivity patterns computed at the surface, which are briefly called ‘surface-sensitivity patterns’ in this study. The ’diagonal components of the impedance’ and ‘off-diagonal components of the phase tensor’, which have four petals-shaped surface-sensitivity patterns along the diagonal directions, belong to Group 1, and contribute to imaging 3-D subsurface structures from receivers installed evenly at the surface. Group 2 contains the ‘xy-components of the impedance, apparent resistivity and phase’ and ‘yy-component of the phase tensor’ whose surface-sensitivity patterns are linear in the y-axis. The ‘yx-components of the impedance, apparent resistivity and phase’ and ‘xx-component of the phase tensor’ that have strong linear surface-sensitivity patterns along the x-axis are classified into Group 3. The MT response functions of Groups 2 and 3 are useful for inversion of structures close to 2-D, whose strike extends along the y- and x-axes, respectively. Groups 4 and 5 include the ‘x- and y-components of tipper’ that possess linearly aligned two petals-shaped surface-sensitivity patterns in the x- and y-axes, respectively. The tipper can be helpful in imaging both 2-D and 3-D structures. The ‘effective impedance’ belongs to Group 6, whose surface-sensitivity patterns appear as a small circle. The surface-sensitivity patterns allow the effective impedance to have an advantage in interpretation of 1-D structures. By using several MT response functions for specific cases of 1-D, 2-D and 3-D interpretation of MT data, we investigate whether characteristics of the sensitivity patterns are reflected in modelling (simulating field data) and inversion results, and then suggest optimal MT response functions for those cases. In doing so, we show how to utilize the characteristics of the sensitivity patterns in inversion, and recommend the input MT response functions for inversion according to MT exploration situations. Our study provides basic information on similarities and differences of major MT response functions for inversion and insights on which MT response functions are suitable to increase the feasibility of MT inversion for different field situations based on the sensitivity patterns.

Determination of Geoelectrical Strike To Investigate The Geological Structures In Surabaya And Gresik Areas

The Magnetotelluric data acquisition is performed with a certain coordinate system orientation, thus the possibility of the field measurement data would be not proper with the geological strike direction. Therefore, the information of geoelectrical strike is needed to enhance the quality of Magnetotelluric data interpretation where if the data had the similar direction with the geological strike thus it will be properly sufficient with the actual subsurface condition. A magnetotelluric study by utilized polar diagram was conducted to analyze the orientation of geoelectrical strike that are correlated with the geological structures in the Surabaya and Gresik areas. However, polar diagram analysis using phase tensor data has an ambiguity of +90° therefore additional analysis is carried out using tipper data. The results showed the geoelectrical strike direction is N315°E and it is slightly similar with the strike trend of geological structures anomaly. Meanwhile, the geological structures is generally characterized by low to medium value of resistivity contrasts around 30-500 ohm.m consisting of the Surabaya Kendeng Fault, Kendeng Kedung Waru Fault, anticline, and other expected faults.

A Study on Magnetotelluric Characteristics of Magmatic Geothermal Systems

The Magnetotelluric (MT) method is a widely used and effective method of exploring geothermal resources because it can reveal geological information at a great depth and is cost effective. In order to further improve the reliability and rationality of MT data interpretation, MT responses for a typical hydrothermal system and a hot dry rock (HDR) and partial melting system are investigated by a finite-element (FE) forward modeling approach based on unstructured tetrahedral grids that can handle with complex-shaped geothermal systems. These two geothermal models, designed by the 3ds Max software, are comprised of a clay cap, a reservoir, and a heat source, and are discretized into tetrahedral elements by TetGen software. The results show that the apparent resistivities at the broadband of frequencies are mainly affected by the shallow low-resistivity clay cap due to its strong shielding effects, and the induction arrows effectively reflect the boundary of the clay cap. The conductive heat-conducting path and heat source (1 S/m) considered in the models of the HDR and partial melting system could cause significant changes in the apparent resistivities and induction arrows at low frequencies. It suggests that in addition to the apparent resistivities, the induction arrows should be taken into consideration in MT data processing and inversion for better lateral resolution when exploring geothermal resources.

Understanding the effect of 1-D dipping anisotropic conductivity on the response and interpretation of magnetotelluric data

SUMMARY Anisotropy in geological settings has been shown to be able to have a significant effect on data and to result in different interpretations when anisotropy is not taken into account. Recent work in inversion of electromagnetic data has emphasized the need for greater accuracy in interpretations where anisotropic effects dominate in data. We focus on how effectively inversions can extract anisotropic characteristics from magnetotelluric (MT) data for the 1-D dipping anisotropy case. We assess the sensitivity of 1-D MT data to each electrical conductivity component by examining the variability of polar plot eccentricity between analytic cases and by examining how sensitivity varies with increasing anisotropic dip angle. We apply a Tikhonov regularized inversion to both synthetic and experimental data using both an L2 norm and an Ekblom norm to approximate an L1 norm to identify the extent to which all model parameters associated with the 1-D dipping anisotropy case are recovered. We find that the application of an L1 norm aids in recovering the true depth extent and horizontal conductivities of an anisotropic layer when compared to the results from a standard L2 norm.

Comparison of 3D, 2D, and 1D Magnetotelluric Inversion Results on the Example of Data from Fore-Sudetic Monocline

This study’s main objective is to better define and understand results for the most commonly used inversion algorithms in magnetotelluric data interpretation as part of geological exploration of the region of the Dolsk fault and the Odra fault. The data obtained from the eastern part of Fore-Sudetic Monocline measurements were used to describe the boundaries of lithospheric blocks (terranes) and recognize their origin. The magnetotelluric (MT) soundings were carried out to achieve this goal. There were conducted 51 soundings on five quasiparallel profiles. That allows constructing a quasiregular mesh in the area of the Fore-Sudetic Monocline. This arrangement of the measuring grid allowed reducing the influence of the largest sources of disturbances on MT data. 1D and 2D models were created by using the inverse algorithms. The models were prepared for each profile separately. Further, parallel (ModEM) 3D inversion codes were applied. The area where the investigation was done involves the region of the Dolsk fault and the Odra fault. These zones are essential geologic borders of a regional nature, and they pull apart the crust blocks with different origins. It was vitally needed to correctly identify the crust and upper mantle structure around a part of the Fore-Sudetic Monocline. The paper shows how these key features of the geological structures are revealed using 1D, 2D, and 3D algorithms.

Structural and geological studies in the Naryn and Atbashi depressions (Tien Shan) and geological interpretation of magnetotelluric data

The article presents the results of complex geological and geophysical studies in the Naryn depression and Atbashi depression in the Middle Tien Shan. They included the geological interpretation of new magnetotelluric data along the detailed profile crossing the key segment of the Tien Shan, and the study of the morphology and spatial position of the sedimentary cover and basement structures. The compilation of the results of structural-geological and geophysical studies makes it possible to create a 2D model of the upper-crust geological structure, consistent with the structure of the electrical conductivity to depths of about 10 km and to analyze the structural features of deeper horizons. Two types of structural patterns of the electric conductivity, corresponding to the sedimentary complexes of the cover and the folded-metamorphic complexes of the basement, have been identified. Sedimentary rock complexes in depressions have a high electrical conductivity and subhorizontal structure. The upper crust above the K2 density layer is characterized by an alternation of rocks volumes with contrasting conductivity, elongated vertically. The recorded structure of the field confirms the presence of steep zones of fluid permeability and fragmentation, noted earlier in seismic profiles and probably corresponding to the Paleozoic structures of fragmentation of the Earth’s crust, activated during Alpine orogeny. Comprehensive research allow to characterize the deformations of the Cenozoic sedimentary complex and the surface of the Paleozoic basement associated with the Alpine activation of the key segment of the Tien Shan.

An integrative 3-D model of the deep lithospheric structure beneath Dolsk and Odra fault zones as a result of magnetotelluric data interpretation

SUMMARY Long-period magnetotelluric soundings were carried out in the eastern part of the Fore-Sudetic Monocline in central Europe to determine the deep geoelectrical structure. It is an important area in the contact zone of the Palaeozoic terranes of Western Europe and the Proterozoic East European Craton. The research area includes the Dolsk fault zone and the Odra fault zone, which are essential geological boundaries of a regional nature separating crustal blocks of various origins. There were conducted 51 soundings on the quasi-regular mesh 150 km in SE and 225 km in NE. The research region is highly urbanized, and hence some of the data were strongly disturbed. Careful processing of data was required, and sometimes measurements had to be repeated at other locations. The collected data allow constructing 3-D models of the electrical conductivity distribution. A parallel version of the ModEM 3-D inversion code was used for modelling. The information contained in each used transfer function was also examined by performing a separate inversion of these functions. The obtained resistivity distribution models confirm the Dolsk and Odra faults' location as postulated previously by geologists. They also show that these very deep faults are reaching the lower crust. However, they do not clearly state how far the contemporary lowering of the Baltica reaches under cover of younger Palaeozoic sediments.

2-D versus 3-D Magnetotelluric Data Interpretation: A case study from the Dharwar craton, India

The resistivity structure of the lithosphere in the Dharwar craton is imaged using 17 magnetotelluric stations along a ~280 km long profile with an inter-station spacing of ~15 km. Since the adjacent two profiles have shown 2-D nature, initially 2-D inversion is carried out. But the resulting structures are completely different. WALDIM dimensionality analysis shows the 3-D nature of the subsurface structures for most of the frequencies beneath the given profile. 3-D inversion was done by using the WSINV3DMT inversion code. 3-D modelling results imaged several conductors in the crust and uppermost mantle region. The lithosphere can be divided into shallow and deeper lithospheres based on the conductivity anomalies present in the model. In the crustal region, vertical conductive features (~10 Ω-m) are present in the western and central part of the profile. A new finding of this 3-D study is the presence of an uppermost mantle conductor at the depth range of 100–200 km. The electrical properties of the conductors are comparable with the northward situated adjacent profiles conductors. The conductors present in the crust are due to sulphides and carbon-rich fluids. The low electrical resistivity in the deeper lithosphere is due to the refertilization of the mantle scar in the Cretaceous age by the passage of several hotspots. The conductor could be interpreted in the terms of the melt present in the region. In general, 2-D interpretation of 3-D magnetotelluric data acts as a guide to geological interpretation, but not in this case of the Dharwar craton. The concluding remark is that the 3-D image of the single profile data gives a better understanding of the subsurface structures as compared to 2-D inversion results.

Three-dimensional edge-based finite element modeling of magnetotelluric data in anisotropic media with a divergence correction

When simulating a 3-D magnetotelluric (MT) response, the convergence rate slows at low frequencies. Previous works have shown that by implementing divergence correction, the convergence rate of the iterative solution can be accelerated, the computational accuracy can be enhanced. However, traditional correction schemes of MT are mostly based on isotropic modeling. Electrical anisotropy strongly affects the interpretation of MT data and anisotropic influence cannot be ignored. To our best knowledge, there has been no open literature to apply divergence correction in 3-D MT anisotropic modeling. In this study, based on the divergence-free property of the current density, a divergence correction technique is incorporated into the edge-based finite element (FE) method to speed up the modeling process for 3-D MT problems with axial anisotropic earth. By applying the divergence scheme, the computation time and iteration number for MT numerical simulation are largely reduced. The effectiveness of the anisotropic divergence correction technique on MT modeling is verified by numerical experiments.

Analysis of the features of the spatio-temporal distribution of geoelectric inhomogeneities in the Earth’s crust and seismic events

The paper presents the results of experiments carried out at the regime points of magnetotelluric monitoring both on the territory of the Bishkek geodynamic test site (Northern Tien Shan) and on a series of monitoring profiles in various geological conditions. Previous studies indicate the relationship of variations in the electromagnetic and seismic fields, lunisolar tidal effects, seismic regime with the processes of fracturing. The purpose of this work is to establish the features of the relationship between the spatio-temporal distribution of seismicity and the distribution of geoelectric inhomogeneities in the Earth’s crust (fault-block tectonics of the region). Based on the analysis of the results of the interpretation of magnetotelluric data (2D inversion) and new detailed seismotomographic constructions, the verification of geoelectric models was carried out, the analysis of the distribution of hypocenters of seismic events was carried out. Special attention was paid to the confinement of earthquakes to listric fault structures. The relationship between the distribution of the hypocenters of seismic events and the spatial position of the electrical conductivity anomalies is confirmed by the authors explanation of the physical nature of the identified conducting structures, based on hypotheses of fluidization and partial melt of the Earth’s crust.

Application of magnetotelluric (MT) study for the identification of shallow and deep aquifers in Dholera geothermal region

In India geothermal energy is still in nascent stage and there are several geothermal regions which are not explored. By exploring these fields India will be able to understand the potential of geothermal energy and its application for power generation and development in other industrial and agricultural sectors. As world is switching from conventional to renewable energy sector, geothermal energy will be one of the most promising alternative for fossil fuels. The Cambay region of West Coast continental margin has enormous geothermal potentials. In this paper one such geothermal zone of Cambay Basin namely Dholera has been explored to understand the potential of the region. An extensive MT survey has been carried out in this region along 6 profile lines and 66 station points. The main aim of the paper is to identify the shallow and deep aquifer zones in the study area for exploitation of geothermal energy which can be used for various societal benefits. In order to explore aquifer potentials MT data was acquired using System2000.net Phoenix Geophysics Ltd. equipment. This is the first high resolution 3D seismic survey was carried out in the study area in order to identify the geothermal sweet spots. Skew, Tipper and Polar diagrams were prepared to understand the dimension of the identified conductive bodies. The interpretation of MT data in the form of geoelectrical maps indicate presence of high conductive zones at 5 km depth extending up to 7 km. This high conductive bodies indicates the presence of aquifers which can be the zone of interest. Drilling will target these zones to exploite geothermal water.

Magnetotelluric Apparent Resistivity Tensors for Improved Interpretations and 3‐D Inversions

AbstractThe complex magnetotelluric (MT) apparent resistivity tensor can be decomposed into two real tensors, the apparent resistivity and the resistivity phase tensors, which represent relationships between the observed electric field at a point on the Earth's surface and an associated apparent current density. We explain the differences between these tensors and conventional estimates of apparent resistivity and phase for simple resistivity environments and demonstrate, using canonical models in 1‐D and 2‐D environments, that both tensors are more sensitive to vertical and horizontal resistivity gradients than their conventional counterparts. The properties of the new tensors are explained using electromagnetic induction theory and the effects of associated charges at resistivity boundaries. We introduce a new way to plot tensor ellipses, which brings significant improvements to the interpretation of MT data, using appropriate visualization software. The apparent resistivity tensor gives information about the magnitude and direction of apparent resistivity subsurface structures and has a strong response to vertical resistivity contrasts. The resistivity phase tensor is highly sensitive to vertical boundaries and the associated fields in the TM mode. It is also free from static distortions under the same conditions implied for the conventional phase tensor. These findings have prompted a study in the potential of the new tensors for 3‐D inversions. The results from a 3‐D inversion of a canonical oblique conductor straddling two quarter spaces show distinct improvements in resolving the boundaries of the conductor and open a promising field for future studies.

Analysis for Geoelectric Strike Direction of Magnetotelluric Data from a Geothermal Area

Magnetotelluric (MT) method plays an important role in geothermal exploration. For MT data interpretation, 2D modelling is still performed in a routine basis since 3D MT modelling is computationally intensive and needs sigificant computational resources. In 2D modelling, information on geoelectric strike direction is a requirement. In this study, analysis of geoelectric strike was performed on MT data from a geothermal prospect using phase tensor and tipper strike. The results from phase tensor analysis and verified by tipper strike show that the geoelectric strike of the study area is about N300°E that agrees well with the regional structure of the study area. The MT tensor data were rotated to the dominant geoelectric strike and 2D inversion modelling was performed. The subsurface resistivity model reveals the geothermal system of the study area.

Analysis and interpretation of magnetotelluric data in characterization of geothermal resource in Eburru geothermal field, Kenya

The magnetotelluric method (MT) is an essential geophysical method for the exploration of geothermal systems. In this study, the MT method was used to assess the extent of the geothermal resource in Eburru geothermal field, Kenya, with the aim of delineating the electrical conductivity structure of the area. Dimensionality analyses demonstrated that the MT data could be interpreted using two-dimensional approaches, but some localized 3-D effects were detected. A 2-D MT inversion was performed to generate resistivity models of Eburru geothermal field. Given its ability to recover complex resistivity models for the ground, three dimensional (3-D) MT inversion was also carried out, and a joint interpretation made from the 2-D and 3-D models. Both inversion approaches gave similar results and revealed a low resistivity layer (<10 Ωm) interpreted as clay cap, and an intermediate resistivity beneath interpreted as a geothermal reservoir immediately below the low resistivity. The sequence here infers the presence of geological structures controlling the geothermal system. The resistivity profiles analyzed revealed a structure of low resistivity (<10 Ωm) interpreted as the fluid pathway. This structure trend an S-N direction which is consistent with the faults orientation in the field and serves as a conducting channel for transporting heat from the heat source to the shallow region approximately 2 km above sea level.

Direct Method Solution of 3-D Magnetotelluric Modeling Using Vector Finite Element Method

As exploration is forced into more difficult areas with complex three-dimensional (3-D) structural environments, the importance of 3-D magnetotelluric (MT) modeling is essential for the correct interpretation of MT data. To reduce the complexity of the calculations introduced by 3-D models, iterative forward calculation of MT response functions is used as basis for inversion of 3-D MT data. This paper proposes an alternative procedure for making reliable 3-D MT modeling codes for forward calculation that is not only effective but also accurate. This is accomplished by using a direct method to solve the linear systems arising from the discretization process in the vector finite element approach. The vector finite element method is known for its capability of overcoming difficulties in modeling caused by possible jumps of normal components across discontinuities of material properties. Meanwhile, by using a direct method rather than an iterative method, the process of solving the linear equations does not suffer from slow convergence. Here, we present a comparison between our modeling codes and codes based on a different approach. In the resulting 3-D MT responses it was found that the proposed method has high accuracy.

Three dimensional topography correction applied to magnetotelluric data from Sikkim Himalayas

Magnetotelluric (MT) method is one of the powerful tools to investigate the deep crustal image of mountainous regions such as Himalayas. Topographic variations due to irregular surface terrain distort the resistivity curves and hence may not give accurate interpretation of magnetotelluric data. The two-dimensional (2-D) topographic effects in Transverse Magnetic (TM) mode is only galvanic whereas inductive in Transverse Electric (TE) mode, thus TM mode responses is much more important than TE mode responses in 2-D. In three-dimensional (3-D), the topography effect is both galvanic and inductive in each element of impedance tensor and hence the interpretation is complicated. In the present work, we investigate the effects of three-dimensional (3-D) topography for a hill model. This paper presents the impedance tensor correction algorithm to reduce the topographic effects in MT data. The distortion caused by surface topography effectively decreases by using homogeneous background resistivity in impedance correction method. In this study, we analyze the response of ramp, distance from topographic edges, conductive and resistive dykes. The new correction method is applied to the real data from Sikkim Himalayas, which brought out the true nature of the basement in this region.