Magnetovariational Data Research Articles

Geoelectric model of the Earth’s crust and upper mantle of the Dniester-Bug megablock of the Ukrainian Shield

A network of long-period magnetotelluric and magnetovariational data (124 sites) in the period range of 9—16 to 2500—6400 s made it possible to explore the geoelectric structure of the Erth’s crust and upper mantle of most of the Dniester-Bug and adjacent megablocks of the Ukrainian shield. Based on the resistivity cross-sections along the profiles (with 2D inversion), a three-dimensional matrix was created for the territory limited by coordinates 27.7—30.4° E and 47.7—49.4° N, which included the spatial coordinates of each grid node on each profile, the power of the model cells, and the resistance value in the cell. As a result, geoelectrical anomalous structures were identified at different depths from 3 to 100 km.
 The entire block of rocks 200×200 km down to a depth of 100 km is characterized by high resistivity, against which objects of reduced resistivity (LRO) are identified.
 The resulting distribution of high-resistivity rocks over the entire depth of the model is in good agreement with the laboratory dependencies obtained both for the rocks of the Ukrainian Shield and other data. Model data show a significant difference in resistivities in the upper 14—16 km (above 105 ohm), lower crust (about 104 ohm), and upper mantle (103 ohm). Against a general decrease in resistance with depth in the Earth’s crust, three regions were identified in which anomalously high (for a given depth) resistances extend to the entire thickness of the crust. These high-resistivity objects are consistent with positive Bouguer anomalies.
 Against the background of high-resistivity rocks, LROs stand out, the resistance of which does not exceed 120 ohm·m. The spatial dimensions of the LRO zones indicate their locality and do not form a continuous layer. An analysis of the distribution of LROs in space and depth suggests a genetic relationship between mantle LROs and crustal LROs. Comparison of mantle LROs with the Beltska zone of modern activation on the territory of Ukraine shows their good agreement both vertically and horizontally. To explain the lower LRO resistivity in the upper mantle, overheating of the rocks to solidus and 2—3 % melting and/or the presence of fluids is necessary [Gordienko, 2017]. In recent studies discussing the influence of thermobaric conditions and the fluid content necessary to explain the presence of increased conductivity in the upper mantle, the authors of [Blatter et al., 2022] concluded that an anomalously large amount of volatiles is needed with small amounts of melt. The assumption that mantle LROs are related to crustal LROs has been tested by comparing LROs with fault zones.
 The presence of LROs in the mantle, their vertical extent, and their connection with rejuvenated fault systems can serve as a basis for the deep migration of fluids enriched in volatiles.

Analysis of synchronous magnetotelluric and magnetovariational regime observations for the Kentor mini test polygon

The paper discusses various approaches for processing and analysis of synchronous magnetotelluric and magnetovariational data obtained at the Kentor mini test polygon, which is located in the Baytic basin of the Chui region of Kyrgyzstan. The versions of noise reduction of received electromagnetic signals at the stage of robust processing are considered. The materials were processed using a remote reference technique for possible noise detection and calculate the additional components, such as a horizontal magnetic tensor for further interpretation of the obtained data. The dynamics of changes in the geoelectric cross-section for 2 sessions of researches by the tested profiles is considered.

Geomagnetic evidence for a continuously connected plume conduit extending to at least the 660-km discontinuity below Hawaii

Do plumes exist? If so, do plume tails connect hotspots to the core-mantle boundary (CMB), or only to the 660-km discontinuity (between the upper and the lower mantle)? Are they continuous or fragmented? Here, we demonstrate quantitatively that the Hawaiian plume distorts 24-h-period magnetovariational data by providing a continuous electrical connection between the Earth's surface (via the ocean) and the 660-km mid-mantle discontinuity. We extend our analysis to predict how a plume ascending from the CMB should distort magnetic secular variation observed locally at the surface. Hence, the Hawaiian plume is demonstrated to be a continuous feature with a deep-seated (at least mid-mantle) source. Our analysis provides a novel technique for determining whether the Hawaiian plume originates at the mid-mantle transition zone or CMB subject to magnetic secular variation data being acquired locally, as well as allowing the depth of genesis of other mantle plumes to be constrained should further geomagnetic data become available in their vicinity.

Comparison of the 2‐D and Quasi‐3‐D Geoelectric Models of the Ukrainian Eastern Carpathians and Their Link to the Tectonic Structure

AbstractWe present results of a study of the electrical conductivity distribution beneath the Ukrainian Eastern Carpathians where the Carpathian arc turns to the southeast and reaches the maximum curvature further in the Romanian territory. We have used the magnetotelluric and magnetovariational data in the period range of 1–6,000 s collected by geoelectric groups from several countries during deep induction experiments, which covered the studied area by a number of field stations clustered along profiles crossing the Carpathian arc. In this work, we compare the 2‐D and quasi‐3‐D thin sheet inverse models, which allow us to constrain both the geometry and electrical properties of the anomalous conductivity structures beneath the Carpathians. Comparison of the inverse results with the Carpathians tectonics indicates that the Carpathian conductivity anomaly within the Polish and Slovak territory is located approximately between the Magura nappe front and the Pieniny Klippen Belt. In Ukraine, where the Magura nappe narrows and disappears, the anomaly reaches the front of the Outer Carpathians. The resulting quasi‐3‐D model of the conductivity structure beneath the Carpathians is compared with other geophysical and geological data within the region. The relation of the position of the conductors to the heat flow models and to location of the earthquakes sources indicates possible geological mechanisms, such as saline pore fluids, thin films of grain boundary graphite, or partial melting, connected with recent activation zones.

Electrical conductivity of the crust and mantle of the East European platform in the western part of Ukraine from 2D inversion

2D inversion of the magnetotelluric and magnetovariational data along 40 profiles made it possible to create overview model of electric resistivity for the territory between eastern longitudes 28° and 36° and by the northern latitudes 47.8° and 52°. Analysis of sections of resistances in the depth interval from 5 to 105 km indicate the absence of objects of low resistance (less than 100 Ohm × m — OLR), the dimensions of which would allow them to be called a layer. The geometrical parameters of the OLR indicate their locality. Along the border of the East European platform are geoelectric objects of low and high (more than 4000 Ohm × m — HO) resistances, at low depths of 20 km low-resistance objects prevail. Comparison of anomalous geoelectrical objects (OLR and HO) with fault tectonics, metallogeny, oil and gas content and modern activation shows that most of the OLR are confined to fault nodes, in which nodes and areas are often concentrated of minerals having an electronic type of conductivity (graphite, polymetallic ore and noble metals). The areas promising for oil and gas content, identified on the maps of various authors, are partially consistent with the Yavoriv, Lokachi, Ternopil and Chernivtsi zones of modern activation, and in this case are accompanied by conducting objects. It is interesting to note the consistency of geological and geophysical conclusions about the prospects of the North-West oil and gas province with the presence of the OLR. Depth and power of conducting objects allows to assume the presence of a gene-tic connection of various minerals not only with sedimentary metamorphosed rocks, but also with the presence of deep fluids. It seems that the results obtained can contribute to the development of ideas about the deep structure of the studied territory and its connection with both existing deposits of various minerals and during the spillage of new ones.

Inversion of magnetotelluric data using integral equation approach with variable sensitivity domain: Application to EarthScope MT data

We have developed a multi-level parallel magnetotelluric (MT) integral equation based inversion program which uses variable sensitivity domain. The limited sensitivity of the data, which decreases with increasing frequency, is exploited by a receiver sensitivity domain, which also varies with frequency. We assess the effect of inverting principal impedances, full impedance tensor, and full tensor jointly with magnetovariational data (tipper). We first apply this method to several models and then invert the EarthScope MT data. We recover well the prominent features in the area including resistive structure associated with the Juan de Fuca slab subducting beneath the northwestern United States, the conductive zone of partially melted material above the subducting slab at the Cascade volcanic arc, conductive features in the Great Basin and in the area of Yellowstone associated with the hot spot, and resistive areas to the east corresponding to the older and more stable cratons.

Horizontal magnetovariational responses in geothermal exploration: synthetic model studies

Horizontal magnetic inter-station transfer functions, which are less distorted by near surface inhomogeneities and more sensitive to the deep targets, have been incorporated with other magnetotelluric data sets to delineate the electrical conductivity distribution in geothermal zones. Geothermal settings composed of salty fluid saturated faults and fractures, and hydrothermal alterations show a wide variation of the bulk resistivity. This makes electromagnetic methods, particularly magnetotellurics which is able to detect the geoelectric structures of deep targets, ideal techniques for the exploration of geothermal regions. In this research, the horizontal magnetovariational data are combined with other magnetotelluric transfer functions and their capabilities for improving the electrical conductivity structure obtained from 2D inversion process are examined. To this end, several synthetic models-representative of the conductivity changes in geothermal fields or conceptual modelshave been utilized. The results from two-dimensional (2D) inversion confirm the effectiveness of horizontal magnetic tensor as a part of the transfer functions, especially for the detection of deep conductive features.

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.

The thickness of the lithospheric plate and the parameters of the asthenosphere beneath the Scandinavian Shield according to the BEAR experiment

The article summarizes the works on studying the upper mantle structure beneath the Scandinavian Shield from the magnetotelluric (MT) data of the BEAR experiment combined with the global magnetovariational (MV) data. During the past few years, we improved the technique of joint interpretation of MT and global MV data, which enabled us to draw more reliable conclusions concerning the structure of the upper mantle and outline the ways of further increasing the efficiency of the MT method in determining the thickness of the lithospheric plate and geoelectrical parameters of the asthenosphere. Our study of the upper mantle structure from the BEAR data suggests the presence of the asthenosphere at a depth of 200 to 300 km with the electric resistivity of 20–40 Ω m, which corresponds to a 1–2% melt of the mantle material. For obtaining more exact estimates, the BEAR data should be interpreted together with the MV data from the observatories located in the Scandinavian Shield. In the future, this approach will probably enable a more reliable zoning of the Scandinavian shield according to the thickness and depth of the asthenosphere.

Перерахунок магнітоваріаційних даних з довільної до геомагнітної системи координат

 The algorithm for reduction of magnetovariational data acquired in a arbitrary reference system to the geomagnetic one is described. Such a reduction is needed if, for some reason, it is impossible to set  sensors correctly for data acquisition. The algorithm is based on spherical trigonometry formulas.

The conductivity structure across the Trans-European Suture Zone from magnetotelluric and magnetovariational data modeling

To achieve an electrical conductivity model of the Trans-European Suture Zone (TESZ), magnetotelluric measurements in the period range of 10–20,000 s along two almost parallel profiles with a total of 65 sites, starting from the North German–Polish Basin, crossing the TESZ and ending on the East European Craton, were conducted. Magnetovariational responses were combined with other magnetotelluric transfer functions, because for large periods they are free of near-surface galvanic distortions and thus lead to more reliable resolution of deep structures. Following the results of strike and dimensionality analysis, input data for two-dimensional inversion were created by rotating all transfer functions to the common strike direction. Models derived from different combinations of data were compared and several sensitivity analyses were carried out. The results show a highly conductive layer of sediments (saline aquifer) and the TESZ as a broad region of enhanced conductivity. It was also clarified that the lower crust and upper mantle of the East European Craton (EEC) are more resistive than the Paleozoic Platform (PP, underlying the Northeast German Basin) and this resistivity extends to the greater depths.

Joint three-dimensional inversion of magnetotelluric and magnetovariational data

The problem of quantitative three-dimensional interpretation of the magnetotelluric (MT) data ranks among the most difficult problems in electromagnetic (EM) geophysics. Our paper presents a new rigorous numerical method for MT inversion, based on the integral equations technique. An important feature of the proposed method is the calculation of the Frechet derivative with the aid of a quasi-analytical approximation with an inhomogeneous background. This approach simplifies the algorithm of inversion and requires only a single forward modeling on each iteration. We have also developed a method for a joint inversion of MT and magnetovariational (MV) data. We show in the present paper that the joint inversion of MT impedances and the Wiese-Parkinson vectors can automatically allow for the static shift in the observed data, which is caused by the geoelectric inhomogeneities contained in the near-surface layer.

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.

ГЕОДИНАМІКА

The results of long term investigations of induction vectors in the Transcarpathian trough are analyzed. The description of digital magnetovariational data processing and induction vectors calculation algorithm is given. The time relations of anomalous changes of induction vectors and local seismical regimen are analyzed. The direct relation between these phenomena has not been detected, however they are not entirely independent as well.

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.

Integrated modeling of EM response functions from Peninsular India and Bay of Bengal

Existing sets of magnetovariational data from the large numbers of sites distributed across the peninsular India and those in the Bay of Bengal are reanalysed to obtain inter-site vertical and horizontal field transfer functions. Maps of induction arrows relocate the earlier reported conductive zones beneath the Palk-Strait and a regional scale anomaly in the offshore region, immediately southwest of the southern tip of Indian Peninsula, named South India Offshore Conductivity Anomaly (SIOCA). Period dependence of the induction arrows suggests that with increasing period SIOCA tends to control the induction pattern over the entire peninsula. Presentation of the horizontal transfer functions in the form of ellipses of anomalous currents helps to characterize the period and spatial behaviour of horizontal fields at seafloor sites. Integrated thin sheet modeling of the on-land and seafloor induction features suggest that the greater part of the anomalous behaviour of the horizontal fields at seafloor sites can be attributed to the shielding effects due to seawater. The weak anisotropic behaviour of the horizontal fields at selected sites can be explained in terms of the concentration of the induced currents in the sediment filled troughs on either side of the 85°E Ridge. Several lines of geophysical evidence favour the hypothesis that SIOCA, low velocity zone, low magnetization anomaly, all centered near the southern tip of the India, are the relics of the interaction of Marion Plume outburst with Indian lithosphere.

The effect of the Hawaiian plume on the magnetic daily variation

Can electromagnetic techniques be employed to detect the presence of a plume below an ocean approximately 5 km deep? Recent magnetovariational data from the islands of Oahu and Hawaii provide compelling evidence that this question can be answered in the affirmative. A significant diminution of the vertical magnetic field is observed at Hawaii as compared with Oahu, which cannot arise from island/ocean bathymetry effects. The plume paradox is that the effect may not be explained by induction. This could imply distortion and current channelling due to mantle heterogeneities on a scale hitherto unconceded in global induction studies.

Magnetovariational soundings across the South Island of New Zealand: difference induction arrows and the Southern Alps conductor

Magnetovariational (MV) data from 14 sites on a transect of the South Island of New Zealand are presented. The induction arrows clearly show the effects of induction in the surrounding oceans. Comparison with the responses of an analogue model of the effect of the oceans on induction in the New Zealand region indicates that the induction arrows also contain information on anomalous on-land conductivity structure. The use of difference induction arrows to remove the effect of induction in the oceans from the observed responses is validated using both two- and three-dimensional numerical models pertinent to the New Zealand situation. Difference induction arrows calculated by subtracting the analogue model results from the observed data show a clear reversal, in both real and quadrature arrows, across the South Island. Two-dimensional numerical modeling of the projections of the difference arrows on to the transect shows that the response is consistent with the existence beneath the Southern Alps of an anomalously conducting region previously identified by magnetotelluric (MT) sounding. Use of the difference arrow technique is seen to be valid over a large period band because of the reduction in the coast effect response range as a result of the island situation.

An Interpretation of Magnetovariational Data in the Northern Tohoku District, Japan, Using Multi Sheet Modelling.

Magnetovariational data from thirty-seven sites in the northern Tohoku district of Japan are modeled quantitatively using a multi thin-sheet modelling program. The primary objective was to determine if current channeling of electromagnetic fields through the Tsugaru strait, to the north of the survey region, can account for the behavior of the observed large real induction vectors. The major conclusions that we draw are: (1) northward-directed induction vectors at 256 s period cannot be fully explained only by the channeling effect through the Tsugaru strait especially in the area to the east of the Quaternary volcanic front, and (2) a conductive anomaly is necessary at a depth of 10 km in the forearc region in order to account for the induction vectors at 16 s period.

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.