Modelling and analysis of electromagnetic fields in 3D inhomogeneous media

The main progress in the 3D modelling of electromagnetic fields, achieved during the last few years, is summarized. Various 3D numerical modelling techniques are described and compared as to their possibilities and efficiency. Numerical studies are complemented by laboratory scale modelling.Conventional methods of the analysis of the magnetotelluric and magnetovariational surface characteristics are confronted with new trends in MT tensor analysis and decomposition. Various approaches to the analysis of simplified 3D structures are also presented.     

Are the crustal and mantle conductive zones isotropic or anisotropic?

One of the significant problems of modern deep magnetotellurics is the recognition of anisotropy in the crustal and mantle conductive zones. In the paper we perform numerical experiment comparing several 2D models of crustal and mantle isotropic and anisotropic prismatic conductors. Anisotropy is modeled by alternating horizontal or vertical thin layers of different resistivities (the vertical layers are parallel to the prism strike). Using these models, we examine conditions under which the magnetotelluric and magnetovariational response functions distinguish between isotropy and anisotropy. The resolution of MT and MV studies depends on the sediments conductance, lithosphere resistance and deep conductor width. Calculations show that the most favorable conditions for anisotropy studies are observed in the active regions characterized by small sediments conductance (10–20 S) and moderate lithosphere resistance (10⁸ Ohm·m²). However, in the stable regions, where sediments conductance exceeds 50–100 S and the lithosphere resistance comes up to 10⁹ Ohm·m², the crustal and mantle anisotropic and isotropic conductors manifest themselves in the equivalent magnetotelluric and magnetovariational functions, which cannot distinguish between anisotropy and isotropy and admit both the interpretations.     

Electrical conductivity investigation of the Corso-Sardinian microplate area

The analysis of magnetovariational data from an array quasi- simultaneously covering Sardinia (Italy) and Corsica (France), from summer 1988 to spring 1991, indicates the existence of two major conducting bodies, one north of Corsica, the other south of Sardinia. Fourier maps also show local conductivity anomalies along the Campidano Graben (Sardinia), across the Bonifacio Straits and in the northernmost part of Corsica. Two-dimensional (2D) modelling and inversion procedures have been carried out in order to find the optimum conductivity and geometrical characteristics of the conductive bodies deduced from Fourier maps, induction arrows and pseudosections studies. As a result of the 2D inversion, two zones with enhanced electrical conductivity are found, respectively, in the Sardinia Channel and in the Bonifacio Straits. The former can be related to the thermal characteristics of the area; the latter, shallower, can be ascribed to an accumulation of sediments. Some short-period magnetovariational anomalies in northern Corsica can be related to recent reheating processes that occurred there, as well as to the crustal thinning in the Ligurian sea.     

Studies on the effects of the assemblages of the oceanic bottom structures on magneto-variational sounding results

The induction interactions between ensembles of secondary oceanic bottom structures were studied by a 2D conductivity model. It was found that frequent intercalation of such structures lead to significant cumulative effects that had to be taken into account when modeling the structures of the first order. The study was performed for the magnetovariational sounding results obtained in the Arctic Ocean.     

Geoelectric section of the Central Tien Shan: Analysis of magnetotelluric and magnetovariational responses along the Naryn geotraverse

During the past two decades, at the Research station (Bishkek) more than a hundred magnetotelluric and magnetovariational soundings were carried out on the Naryn geotraverse that intersects the Tien Shan region from Lake Balkhash to the Tarim Basin along the 76° E meridian. Integration and complex interpretation of the data of these soundings with improved resolution and reliability of the geoelectric model of the Central Tien Shan section became an urgent challenge. Our paper presents a complex of methods for processing and invariant analysis of the electromagnetic data developed for the solution of this problem. Its application allowed us to validate the choice of the 2D interpretation model for the Naryn Line and to form the adequate ensemble of the data to be inverted. The developed approaches will also be useful in similar studies in the other mountain regions.     

Geoelectric section of the Central Tien Shan: Sequential inversion of the magnetovariational and magnetotelluric data along the Naryn Line

The paper presents the results of 2D inversion of deep magnetotelluric (MT) and magnetovariational (MV) soundings along the Naryn Line. The method of partial (sequential) inversions is used. According to this method, at the first stage, magnetovariation responses are used for the localization of deep anomalies of electrical conductivity, and then the magnetotelluric sounding data are invoked to refine the structure of the host medium and the structural details in the upper part of the section. It is shown that this approach enables one to estimate the informativeness of separate components of the electromagnetic field, to reduce the distorting influence of the near-surface geoelectric inhomogeneities, and to increase the stability of the final solution of the inverse problem.     

The 3D profile inversion of the MTS–MVS data by the example of the Shiveluch volcanic zone in the Kamchatka Peninsula

The joint use of the magnetovariational (MV) and magnetotelluric (MT) data and the a priori information about the areal distribution of the deep electrical conductivity within the studied area and the integral conductance of the sedimentary cover enabled us to conduct a three-dimensional (3D) inversion of these data and construct the 3D model of the northeastern part of Kamchatka. The results show that within the Shiveluch volcano, north and south of it, there are northwesterly striking conductive faults penetrating up to the mantle which ascends from a depth of 41 to 35 km. The electrical conductivity of these crustal structures which accommodate high seismic and volcanic activity is probably accounted for by fluid saturation.     

Electromagnetic measurements in the vicinity of the KTB drill site. Part II. Magnetotelluric results

Summary In addition to the magnetovariational measurements across an array in Western Bohemia, close to the KTB ultradeep borehole (Germany), discussed in part I of this paper [1], magnetotelluric results from pivot sounding point Ostrvek within the array are presented here. Good quality of long-period magnetotelluric data (period range from 30 s to about 1 hour) allowed structural dimensionality of the medium to be analysed in detail. The geoelectrical structure was identified as a slightly distorted two-dimensional regional substratum, with dominating E - W strike, overlaid by a heterogeneous subsurface layer with extremely strong and anisotropic galvanic distortion effect on the magnetotelluric data. Estimating the total static shift distortion tensor by fitting the local magnetotelluric curve to the curve of the global magnetovariational soundings (for the European continent), the static distortions were identified as of generally multidirectional origin. The resulting telluric ellipse is, however, strongly anisotropic, indicating an approximately SW - NE apparent local strike, which is in the approximate agreement with remote reference magnetovariational results. Finally, the magnetotelluric results from the station Ostrvek are compared with long-period data from the immediate neighbourhood of the KTB borehole on the German territory.     

Experiments on the dynamic correction of magnetotelluric and magnetovariational data

Pathways of further development of the dynamic correction technique suggested by E. Fainberg are discussed. This technique allows the correction of magnetotelluric (MT) and magnetovariational (MV) data distorted by subsurface irregularities within a broad range of frequencies beyond the static shift interval. This paper addresses new algorithms for the dynamic correction of tensor MT and MV response functions. Moreover, experiments on the dynamic correction of MT and MV data calculated for models containing heterogeneities in sedimentary cover and infinite or half-infinite conductive crustal prism are described. Model experiments show that the dynamic correction can ensure sufficient accuracy of the input data for the reliable solution of the inverse problem of magnetotellurics.     

Crustal electrical conductivity of the Indian continental subduction zone: New data from the profile in the Garhwal Himalaya

We present the results of studying the geoelectrical structure of the zone of continental subduction of the Indian lithospheric plate within the Gahrwal Himalaya. In the framework of the Russian–Indian project, the data of the broadband magnetotelluric soundings conducted by the Indian Institute of Technology Roorkee on the regional profile across the structures of the orogen were expanded, processed, and interpreted by the new program tools adapted for the measurements in the mountain conditions and for the presence of industrial noise. The constructed model of the deep electrical conductivity cross section for Garhwal revealed its two-dimensional (2D) features and more accurately delineated the location of the midcrustal conductor associated with the ramp structure of the detachment plane. The correlations with the regional distribution of the earthquake hypocenters and the seismotomographic images suggest a common, fluid-related nature of the seismic and geoelectrical anomalies in the crust of the Garhwal Tectonic Corridor and enabled the identification of the seismogenerating zones. Among the data of the expanded profile set of magnetotelluric and magnetovariational transfer functions, the response of a poorly explored deep conductive body is revealed. This object is located east of the profile and is probably associated with the activation of the ancient trans-Himalayan cratonic structures which prepares the segmentation of the Himalayan arc.     

Method of pseudotopography: A new approach to analysis of magnetovariational and magnetotelluric data

The method of pseudotopography is a new mode of representing magnetotelluric and magnetovariational data based on a 3-D generalization of geoelectric pseudosections widely used at the stage of qualitative interpretation. The paper considers a synthetic model whose layers, imitating the sedimentary cover, crust, and upper mantle, contain a poorly conductive and a few highly conductive prisms differing in strike. The pseudotopography of apparent resistivities demonstrates the static superposition effect: the effects of near-surface chaotic heterogeneities (geoelectric noise) together with the near-surface and crustal prisms are superimposed on the effect of the mantle prism, distorting the information on the deep structure of the Earth. However, the pseudotopographies of the Wiese-Parkinson matrix, horizontal magnetic tensor, phase tensor, and phases of the impedance tensor are free from the superposition effect: arising at definite frequencies, geoelectric noise and the effects of the shallower prisms decay with decreasing frequency, making the effects of deeper prisms recognizable. Thus, it is clearly demonstrated that magnetovariational and phase estimates, being independent of static distortion, can provide reliable information on the deep structure of the Earth and significantly enhance the effectiveness of magnetotellurics.     

A geoelectric model for the Uzon caldera,Kamchatka

The results of magnetotelluric and magnetovariational studies in the Uzon caldera are considered. An analysis of magnetotelluric parameters yielded the required method of interpretation. The MTS curves were interpreted in the framework of a 2D model using the REBOCC program. Geoelectric cross sections of the caldera were constructed along two orthogonal lines. Anomalies of high electrical conductivity were identified in the sediments and in the basement and were found to be confined to the locations of geothermal springs. The higher conductivity of these anomalies is here related to the presence of highly mineralized hydrothermal solutions. Electrical conductivity was used for an approximate estimation of porosity in the sediments and basement. A subvertical zone of higher porosity was identified at depths of 1.5–3.5 km in the caldera with a connection to the channelways of fluids rising into the sediments. It is hypothesized that highly mineralized solutions are diluted with vadose water in that zone and come through fissures onto the ground surface in the form of hot springs. The totality of these data suggested a conceptual model to characterize the main features in the generation of hydrothermal springs in the Uzon caldera.     

Use of practical aspects of soil behaviour to evaluate different methods to generate soil hydraulic functions

Soil hydraulic functions can be obtained with methods that range from complex and costly to simple and cheap. Decisions as to which is the most appropriate method for a specific application have to be based on a comparison of generated hydraulic functions. This comparison should preferably be based on a statistical comparison of practical applications calculated with the different hydraulic functions rather than on a statistical comparison of the functions themselves. In this study four different methods were used to generate hydraulic functions: (A) direct on-site measurement, (B) measurement in soil horizons in the area, (C) use of a national data set, and (D) use of Van Genuchten parameters correlated with soil texture and organic matter content. The four methods were compared by their effect on two practical aspects of soil behaviour: (1) evapotranspiration deficit and (2) flux through a plane at 30 cm below soil surface. These two aspects are highly relevant for agricultural and environmental use. However, direct measurement is not feasible. A validated simulation model was used for the calculations and results obtained with method A were taken as a reference. Calculations were performed for three soil profiles for a period of seven years. Deficits and fluxes, calculated with the four methods to generate hydraulic functions, were not significantly different using the data of the seven-year period. However, methods were significantly different when rainfall deficits were used as a covariable. This is true with the exception of downward fluxes in the period October until March which are most important for leaching of pollutants. The user has to decide whether differences between methods are sufficiently large to justify repeated, expensive on-site measurements (method A) or whether an investment will be made to make standard series of curves to be used everywhere (methods C and D).     

Electromagnetic measurements in the vicinity of the KTB drill site. Part I: The MV results across a 2-D array

Summary We have analysed magnetovariational data of ten temporary field stations deployed as a 2-D array covering about 50 km square of a SFR/FRG border area adjacent to the KTB superdeep drill site. Single- and inter-station induction vectors were computed by frequency and time domain analyses. The distribution of induction vector characteristics was analysed, interpreted in terms of the electrical conductivity distribution across the array, and correlated with geological phenomena.     

The Experience of Magnetovariational Sounding in the Arctic: the Laptev Sea Region

Izvestiya, Physics of the Solid Earth - Abstract—We present the results of magnetovariational soundings at two sites (Tiksi Observatory and Kotelny Island in the Laptev Sea region of the...     

Magnetovariational information to improve distortion diagnostics in deep magnetotelluric soundings

An interpretative experience from nine magnetotelluric soundings was accomplished in the central region of Argentina (32°S?C34°S; 63°W?C69°W), from the Andean region in the west to the platform zone in the east. To do this, magnetovariational information was used to improve the distortion diagnostics in magnetotelluric curves. Using Pilar Geomagnetic Observatory as a reference site, horizontal magnetic transfer functions were estimated, which were compared with the integrate conductivity at each location in field. As a result, a rather simple methodology is proposed to better approach the accurate positions of normal curves. Results suggest that, in this way, better formal interpretations of soundings may be reached. In addition, a more clear and comprehensible knowledge about the nature of lateral in-homogeneities is obtained; e.g., discovering 3-D effects no suspected from tectonic maps. This methodology seems to be particularly useful when-as in the present case-magnetotelluric soundings are far away each others; i.e., when effective volumes of soundings are not interpenetrated. Horizontal magnetovariational information suggests two elongate conductivity anomalies (about N25°?C30°E), possibly associated with deep seated faults belonging to the South American regmatic network. These anomalies would be produced by partial melting in lower crust and possibly in the asthenospheric zone next to Andean Range. Another elongate anomaly (possibly of graphitic nature) is shown in the study region. It seems to be a marginal fault following the border between The Sierras Pampeanas dynamic zone and the South American craton. Magnetotelluric results indicate the study region can be considered as divided in a dynamic belt next to Andean Range and a cratonic zone eastward. The dynamic zone presents a well developed lower crust, with conductances ranging 300?C4300 Siemens and depths of about 20?C30 km. An asthenosphere close to the Andes with 1000 Siemens of conductance at 74 km depth is also observable. Heat flows of 63?C70 mW/m² are estimated next to Andes and 48 mW/m², eastward, close to South American platform. The cratonic zone presents a first conductive layer with a conductance of 270 Siemens underneath BUL sounding site, but it does not seem to present lateral development. An intermediate conductive layer is also present in this region, but it does not have so much development. Therefore, this layer would not have asthenospheric character; so, the lithosphere would be tied to upper mantle. Heat flows ranging 40?C35 mW/m² were estimated for this cratonic region.     

The results of marine electromagnetic sounding with a high-power remote source in the Kola Bay in the Barents Sea

The first Russian six-component seafloor electromagnetic (EM) receivers were tested in an experiment carried out in Kola Bay in the Barents Sea. The signals transmitted by a remote high-power ELF source at several frequencies in the decahertz range were recorded by six receivers deployed on the seafloor along the profile crossing the Kola Bay. Although not all the stations successfully recorded all the six components due to technical failures, the quality of the data overall is quite suitable for interpretation. The interpretation was carried out by the three-dimensional (3D) modeling of an electromagnetic field with neural network inversion. The a priori geoelectrical model of Kola Bay, which was reconstructed by generalizing the previous geological and geophysical data, including the data of the ground magnetotelluric sounding and magnetovariational profiling, provided the EM fields that are far from those measured in the experiment. However, by a step-by-step modification of the initial model, we achieved quite a satisfactory fit. The resulting model provides the basis for introducing the corrections into the previous notions concerning the regional geological and geophysical structure of the region and particularly the features associated with fault tectonics.     

Quasi-2D hybrid joint inversion of seismic and geoelectric data

Many joint inversion schemes use 1D forward modelling in the integrated interpretation of various geophysical data. In extending the joint inversion approach to the investigation of 2D structures, the discretization of the model parameters and the appropriate choice of the forward‐modelling procedure play a very important role. In this paper, a hybrid seismic–geoelectric joint inversion method is proposed for the investigation of 2D near‐surface geological structures. The electric and seismic models are coupled together through the use of common boundaries between the adjacent layers. Assuming a 2D model composed of homogeneous layers with curved boundaries, a fast ray‐tracing algorithm is used for the calculation of refraction seismic traveltime data. In the geoelectric forward modelling, a locally 1D approximation is used. The boundary surfaces are written in the form of series expansion; the inversion algorithms are formulated for the expansion coefficients and the petrophysical parameters as unknowns. Two versions of the inversion method are proposed: in versions A and B, interval‐wise constant functions and Chebyshev polynomials are, respectively, used as basis functions of the series expansion. The versions are tested by means of synthetic and in situ measured data. The tests show that both methods are stable and accurate.     

Electromagnetic soundings of the sedimentary cover and consolidated crust in the transition zone from the Moscow syneclise to the Voronezh anteclise: Problems and prospects

The methods and results of electromagnetic soundings (EMS) performed in the transition zone from the Moscow syneclise to the Voronezh anteclise in the vicinity of the MSU geophysical base are considered. This base is located in the village of Aleksandrovka in the Yukhnov district of Kaluga area. The composite EMS curves characterizing rock complexes composing the sedimentary cover are constructed, and changes in these complexes within the specified transition zone are traced. The standard curves of magnetotelluric (MT) and magnetovariational (MV) soundings are constructed from the results of long-term measurements at the ALX observation point located at the Moscow State University’s (MSU) geophysical base. The maps of thickness and total longitudinal conductance of the sedimentary cover are constructed from the results of interpretation of MT data obtained in the region. A conductor in the consolidated Earth’s crust is identified within the Voronezh anteclise. Prospects for further investigations of the region are associated with the tracing of the crustal conductor within the Voronezh anteclise, as well as with the organization of an observatory at the MSU’s geophysical base in order to perform long-term measurements of the electromagnetic (EM) and other geophysical fields.