Since 2020, magnetotelluric/magnetovariational (MT/MV) soundings have been conducted in the triple-junction area of major segments (Fennoscandia, Sarmatia and Volga–Uralia) of the East European Platform. The study area extends to the north the SMOLENSK sounding array with inclusion of new observations from summer, 2023. Thus, the SMOLENSK array approaches from the south to the LADOGA sounding array. This article presents the results of MT/MV data invariant analysis, describes the selection of data ensembles for three-dimensional (3D) inversion, and analyzes the resulting 3D resistivity model.

The Sloboda Geodynamic Intersection is considered as an area of convergence for major segments, aulacogens, faults, and suture zones of the East European Platform.Between 2020 and 2022, researchers from the Geological Faculty of Lomonosov Moscow State University and the Geoelectromagnetic Research Center (GEMRC IPE RAS) conducted deep magnetotelluric surveys in this region. The surveys were performed along the Pushkiny Gory – Andreapol, Sebezh – Velikiye Luki – Rzhev, and Ostrov – Ostashkov profiles. Through qualitative analysis of the collected data, the dimensionality of the studied medium, the total longitudinal conductance of the sedimentary cover, and the dominant strike direction of conductive structures were assessed. Quantitative interpretation using 1D, 2D, and 3D inversions resulted in geoelectric models comprising a conductive sedimentary cover and a highly resistive basement. Within the depth range of 10–40 km, transcrustal zones of enhanced electrical conductivity were consistently identified, interpreted as the southwest extension of the Ilmen-Ladoga anomaly. These conductivity anomalies are attributed to the deep submergence of heavily tectonized and metamorphosed sedimentary rocks, initially saturated with organic and carbonate matter.

In january-february of 2022 deep magnetotelluric soundings (DMTS) were carried out along the regional (300 km) profile Pushkinskie Gory - Andreapol. The field expedition included professors, graduates and students of the Geophysical department of the Geological Faculty of Lomonosov Moscow State University, as well as scientists from the Laboratory of Magnetotelluric Soundings of the Center of Geoelectromagnetic Research IPE RAS. The article presents an analysis of the results, as well as geoelectric two-dimensional models obtained during the inversion of magnetotelluric data in the ZONDMT2D program.

The spatial distribution of the satellite lithospheric magnetic field is analyzed as a parameter associated with plume magmatism processes that occurred on the territories of large igneous magmatic provinces (LIPs). We consider the lithospheric geomagnetic field at an altitude of ~250–300 km derived from the CHAMP satellite data for three LIPs of different ages: Central European, North Atlantic, and Siberian. The results are evidence that the parameters of the lithospheric geomagnetic field are controlled by complex magmatic processes. Ancient mantle plumes are reflected in the lithospheric magnetic field as conformal positive anomalies of large magnitudes, while the lithospheric magnetic field on the territory of the young North Atlantic province is significantly reduced. Therefore, the data from geomagnetic satellite surveys add valuable information to the available geological and geophysical data and should be used holistically for studying the crustal heterogeneity of LIPs.

The spatial distribution of the field of lithospheric magnetic anomalies carries information about its sources—deep tectonic structures—and reflects the processes occurring at mantle depths. Based on the geomagnetic data measured by the CHAMP satellite at an observation altitude of 290 km, the lithospheric magnetic field parameters over the territory of Africa were calculated. A series of lithospheric magnetic anomalies of the total intensity Ta maps of various scales and degrees have been constructed. The distribution of Ta over the territories of Southern and East Africa is given. An analysis of lithospheric magnetic anomalies maps over the territories of African superplume influence showed good agreement with the existing hypothesis about the passage of the mantle superplume flow from the lower mantle to the upper mantle in the northeast direction and its further continuation under the East African rift zone. The parameters of the anomalous lithospheric magnetic field contain information about the magnetization of the lithosphere deep layers, reflecting the magnetic properties of large regional tectonic structures and the topography of the Curie surface, which is associated with the geothermal regime and tectonic setting at different levels of the lithosphere. This work shows the perspective of the geomagnetic field satellite observations using in the study of the tectonics of active zones and mapping of deep lithosphere heterogeneities in hard-to-reach areas.

The distribution of the lithospheric magnetic anomalies over the territory of the Tibetan Himalayan highlands, the Western and Eastern Himalayan syntaxes, and part of the Hindustan Peninsula, obtained from measurements on the German Earth satellite CHAMP (Challenging Minisatellite Payload) in different years of its mission, is studied. The lithospheric magnetic anomalies maps for the modulus of the full vector T-a and for the horizontal component X-a at different satellite flight levels are presented. The accordance of lithospheric magnetic anomalies maps to the regional geological and tectonic data and regional geophysics is considered. The anomalous magnetic field sign inversion over the northern part of the Indian Plate is explained as a possible effect of mantle heating, the rise of the Curie isotherm, and the loss of the initial magnetization of the earth's lower crust. A series of split-level maps of magnetic anomalies are compared with each other, and the degree of their similarity is estimated. To illustrate the expediency of separation lithospheric magnetic anomalies directly from satellite measurements and to demonstrate the difference between maps of regional lithospheric and near-surface anomalies, a T-a map was constructed based on the data from the EMAG2 database of the WDMAM model at a 4 km level. The results of the work show that the identified magnetic anomalies over the Indo-Asian collision region are adequately consistent with the existing regional geological, tectonic, and geophysical conditions.

Abstract—The geoelectric structure of the junction region between three largest segments of the East European Craton (EEC)–Volga–Uralia, Sarmatia, and Fennoscandia—is studied. We analyze electrical conductivity anomalies within the Orsha depression revealed by synchronous array magnetotelluric (MT) and magnetovariational (MV) crustal soundings. The study is based on the data from deep MT/MV survey acquired in 2018–2020 under the SMOLENSK experiment in the vicinity of the Orsha depression and the results of similar studies carried out since 2007 in the previous KIROVOGRAD experiment east on the slope of the Voronezh crystalline massif (VCM). The data are interpreted in the class of two-dimensional (2D) geoelectrical models constructed on the long latitudinal geotraverses combining the soundings from the SMOLENSK array in the west and KIROVOGRAD array in the east. Interpretation of MT/MV data within the Orsha depression is substantially complicated by intricate interference of deep and shallow responses. The latter are determined by current systems within the conductive sedimentary cover, ~2 km thick at the center of the depression and having an integrated longitudinal conductance of ~1000 S. Shallow quasi 3D effects obscure responses of crustal conductive features in a wide range of periods, challenging their analysis in the scope of 2D interpretation of impedance data. Important role in overcoming these obstacles is played by MV tipper data which are immune to surface distortions. To develop the strategy of joint MT/MV interpretation, we carried out experiments on inverting synthetic data calculated in a series of 2D models simulating possible geoelectric structure of the study region. The numerical results demonstrated the effectiveness of using the REduced Basis OCCam’s (REBOCC) algorithm in the two-stage successive 2D-inversion scheme which inverts at the first stage the electric (TE) mode MT/MV data alone, and at the second stage incorporates the magnetic (TM) mode impedance data. This technique reliably identifies deep conductive structures beneath thick sedimentary strata. The importance of using a priori information about the geoelectric structure of sedimentary cover when constructing the starting inversion model is demonstrated. The joint 2D inversion strategy worked out on the synthetic MT/MV data was used to interpret observations along three combined profiles of the SMOLENSK and KIROVOGRAD arrays. The interpretation refined the structure of the Kirov–Baryatino crustal anomaly on VCM western slope, revealed complex architecture of crustal anomalies beneath the Orsha depression, and allowed preliminary conclusions about the nature of conductivity anomalies and their connection with ancient and recent geodynamic processes.

—When solving direct and inverse problems of magnetotellurics, magnetic permeability throughout model medium is conventionally assumed to be constant and equal to vacuum permeability. To study the effect of anomalous magnetic properties of rocks on magnetotelluric data, a collection of one- (1D) and two-dimensional (2D) simulation models have been constructed. The equivalence principle for horizontally layered sections which takes into account magnetic permeability of layers is demonstrated. It is shown how the components of transfer operators—impedance, tipper, telluric and horizontal magnetic tensors—change depending on the permeability of a local conductive body. The results of the 2D inversion that excludes inhomogeneity in magnetic properties of the medium are presented for synthetic data. It is shown that at the values of relative magnetic permeability above 2, the inversion may markedly overestimate the depth to the top edge, the thickness, and electrical resistivity of the considered structures.

This paper presents results of multidisciplinary geochronologic, paleoseismic, and tectonophysical studies for local structures in the Sloboda tectono-dynamic intersection that is a segment of the junction between major tectonic structures in the East European Platform. These studies were conducted in the area of the Vitebsk–Surazh fault in the central part of the Sloboda intersection. This ancient long-lived discontinuity can be traced using geophysical data in the Precambrian basement. Activation during the platform phase of evolution made the fault penetrate the sedimentary deposits and form a negative flower structure showing signs of tectonic inversion. Since the Late Devonian until the beginning of the Quaternary, the Vitebsk–Surazh northeast striking fault has experienced at least two phases of tectonic activity in a setting of left lateral transpression and right lateral transtension. The Late Pleistocene to Early Holocene evolution of the Vitebsk–Surazh fault gave rise to the formation of graben-like hollows where late glacial and post-glacial lacustrine deposits were accumulated. The Gralevo Quarry shows horizons of seismites, indicators of paleoearthquakes, in the structure of these sediments. The earthquake of the highest intensity has been identified in the seismite structure of Early Holocene varved clay whose radiocarbon age is 9339 ± 36 years. The tectonophysical parameters of this seismic event are in agreement with conditions of normal-oblique deformation with a nearly east–west orientation of the tension axis and a nearly north–south orientation of the compression axis.

The article considers maps of the field of lithospheric magnetic anomalies-images of the deep structures of the Arctic Ocean-constructed from observations by the CHAMP satellite. The lithospheric magnetic anomalies corresponding to the most significant tectonic structures were revealed: Lomonosov Ridge, Mendeleev Ridge, Alpha Rise, and the Amundsen, Podvodnikov and Makarov basins. The distribution of the regional magnetic anomaly field, which is a complex set of positive and negative segments of various shapes and amplitudes, is discussed in light of the modern geological and geophysical views on the structure of the Arctic Ocean lithosphere. The constructed lithospheric magnetic field model indicates that in the region of conjugation with Eurasia the foot and the slope of Lomonosov Ridge are morphologically related to the continental margin, while the area of Mendeleev and Alpha elevations in central Arctic appears to be a single-block structure with a continental-type crust.

The EHS3D international project in the Eastern Tibet and NE India brought in recent 15 years a huge array of magnetotelluric (MT) and magnetovariational (MV) soundings. These soundings were performed simultaneously in clusters containing several broadband (BMT) and long-period (LMT) field instruments and local geomagnetic observatories (Xiao et al. 2010; Varentsov et al. 2010). Reliably estimated impedances, tippers and horizontal MV responses made possible the resolution of conductivity structures within the whole tectonosphere at depths reaching 250 km. The model resulted from 2D+ inversion of joint MT/MV data set along the submeridional EHS-3 profile crossing the whole Eastern Tibet outlined sedimentary basins, subhorizontal upper crustal conductors, bright crustal-mantle conducting anomalies above dipping plates, well resistive lithospheric mantle and several "asthenospheric" cells. Recently, the EHS3D array has been extended with two new profiles. The EHS-4 profile fills the gap between EHS-2 and EHS-3 geotraverses in China, while the EHS-IND profile continues Tibetan profiles for more than 500 km into India through the Brahmaputra Valley. We present a view at the extended EHS3D data set and first 1D model along the joint profile from the Indian Craton to the SE Tibetan Region.

The basic medium model considered in magnetotelluric (MT) and magnetovariational (MV) studies assumes the magnetic permeability being fixed everywhere at the free space level. However, this assumption can cause distortions in the MT sounding results near strong permanent magnetic field anomalies. We made 2D modelling of such distortions arising along the KIROVOGRAD project MT/MV sounding profiles at the western slope of the Voronezh Massif. Minor changes are seen in modelled data for TM mode components. At the same time, the MV anomalies in TE mode are decreasing with the increase of magnetic permeability within upper crustal magnetic blocks. This effect may cause the decrease (or even break) of deeper low crustal conductive anomalies just below strongly magnetic bodies.

—We consider results of magnetotelluric and magnetovariational soundings in the period range 10 < T ≪ 20 000 s in the North Vietnam area. The simple structure of magnetovariational responses is shown, which generally reflects the electrically conductive quasi-two-dimensional structure of the Earth’s crust. Impedance responses form as a superposition of responses of local complex subsurface and quasi-two-dimensional regional deep-seated structures. Separation of local and regional effects made it possible to construct a geoelectrical depth model of the regional tectonosphere, whose main elements are electrically conductive subvertical trans-lithospheric faults and high-resistivity disturbances of the conductive asthenosphere. These elements, favoring the flow of telluric currents induced in deep-seated electrically conductive systems into the sedimentary cover, form zones of abnormal apparent-resistivity curves. The apparent resistivity monotonously increases with increasing the period up to 20,000 s. We called the combination of such elements of the geoelectrical model a ultradeep fluid–fault system (UDFFS). The modeling has established the location of three orthogonally intersecting UDFFS of NE and SE strikes in the North Vietnam area. It shows that conductive (fluid-saturated) translithospheric faults extending to the base of the sedimentary sequence control the location of petroleum fields and ore deposits. A method for separating local and regional magnetotelluric effects is proposed. It permits one to determine reliably the main strikes of a regional two-dimensional structure and the configuration of the apparent-resistivity curves along them.

The results of 2D inversion of amplitude and phase effective curves from magnetotelluric soundings in the Tobol-Ishim interfluve of Western Siberia are presented. These results confirm the existence of conductive regional faults of submeridional strike and a conductive asthenospheric layer at depths of 70–80 km. The revealed deep conductive anomalies may indicate peculiarities of the oil and gas generation regimes.

A fundamentally new conclusion that zones of high seismic activity are controlled by electroconducting (fluid-saturated) fragments rather than by the entire deep fault is drawn. It is shown that the areal abundance of earthquake foci in geoelectric blocks of the Earth’s crust within Sakhalin and Vietnam containing electroconducting formations is mostly controlled by their roof occurring at a depth of ≤15 km.