A method for statistical detection of weak ultra-low-frequency (ULF) electromagnetic anomalies of lithospheric origin amid intense anthropogenic noise is proposed. The method is based on the hypothesis that the statistical distribution of ambient electromagnetic noise amplitudes in the ULF range is stationary over time, whereas an additional signal perturbs this distribution. To validate this method, we conducted a numerical experiment with superimposing a synthetic noise signal onto the magnetic data recorded over a 72-day interval from December 2022 to February 2023 at the Tashkent Geodynamic Polygon. The results demonstrate that a persistent shift in the probability distribution mode of the root-mean-square deviations toward larger values serves as a reliable indicator of an electromagnetic disturbance from a remote source. The method was tested and its applicability was demonstrated for an M6.4 earthquake.

Using data from the 2D IMAGE network and magnetic stations located in Russia, Pc5 geomagnetic pulsations with a frequency of 2.8 mHz, which occurred in the afternoon sector against the background of the magnetic storm of August 27, 2014, preceded by a 5-day period with low magnetic activity, are studied in detail. In two time intervals, at the beginning of the storm and during the period of maximum magnetic activity, instantaneous 2D distributions of Pc5 magnetic field amplitudes on Earth’s surface are plotted. It has been found that the ionospheric sources of Pc5 (vortex Hall currents) have an elliptical shape with a larger axis in the south–north direction. At the beginning of the magnetic storm, a single burst of Pc5 pulsations was detected, the center of the source of which was located at the geomagnetic latitude 67.5° (L 6.8 RE) and moved westward with a velocity of 0.7 km/s. The estimated size of this ionospheric source is 150 km in the west–east direction and 330 km in the south–north direction. During the maximum of the magnetic storm, Pc5 pulsations are produced by two ionospheric sources following each other. These ionospheric sources have a more elongated elliptical shape with axes of 250 km in the west–east direction and 670 km in the south–north direction. The centers of these sources were shifted by 4° to a more southern geomagnetic latitude 63.5° (L 5 RE) and moved westward with a velocity of 1.7 km/s. Estimates of the size of the two-time magnetic field tube in which the resonance MHD waves have been generated and its velocity in the equatorial plane of the magnetosphere are presented.

The results of experimental studies of electric field disturbances generated by sources of an ionospheric origin in the coastal zones of the White and Barents seas are presented. The data recording was performed by geophysical stations GI-MTS-1. The horizontal electric field was recorded by telluric lines 50 m long. The Ex component was set orthogonally to the coastline in the direction from the shore, and the Еy component was directed along the coastline. In the coastal strip of the sea–land contact, a zone of anomalous increase in the amplitude of ULF variations (F < 1 Hz) of the Ex component adjacent to the water’s edge was revealed. An additional narrow zone of anomalous amplification of the ULF variations of the electric field is observed at a distance of 30 m from the water’s edge. The coastal effect reaches its maximum near the water’s edge in the coastal zone and decreases with distance from the seashore.

The results of a study of the deep structure of the Earth’s polar regions based on the interpretation of gravitational anomalies, geomagnetic-field anomalies, and seismological data are presented. Based on measurements on the CHAMP and Swarm satellites, vector models of the magnetic field of the Arctic and Antarctic have been verified. To study the magnetically active zones and roots of the lithosphere observed in near-Earth space, deep sections were constructed. Density sections were obtained from gravitational anomalies, and magnetic sections were obtained from anomalies in the modulus and components of the geomagnetic field. To estimate climate risks, a joint analysis of deep sections with satellite data on variations in the ice cover of the polar regions was carried out. It showed that vertical thermofluid channels play a significant role in the process of ice melting. Distinctive features of melting centers caused by endogenous factors are revealed. The migration paths of fluid flows destroying the ice cover are visualized in sections in the form of channels with reduced magnetic and density properties. The conducted studies allows to understand the possible reasons for the localization of centers of ice and permafrost destruction. It was shown that in the permafrost zones of ancient basement blocks, thermal areas of through taliks arise under the influence of heat flows of fluid-supply channels. Melting areas can be represented by hot spots of ore clusters, within which mineral deposits are concentrated under the influence of hydrothermal flows of mineralized fluids. Studies of the deep structure of the Arctic and Antarctic are of scientific and applied importance for solving exploratory geological and geophysical problems and estimating climate risks.

This paper presents the results of a study of the informativity of the anomalies of the modulus and components of the Earth's magnetic field in near-Earth space in the altitude range from 300 to 800 km. Magnetic anomalies are calculated according to a three-dimensional component model of the Earth's magnetic field of the St. Petersburg Branch of the Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation. For comparison with the empirical data obtained by the CHAMP and Swarm satellites, the magnetic anomalies and their gradients were calculated according to the component model for altitudes of 400 and 450 km. To reveal the structural features of the lithosphere of magnetoactive zones observed in near-Earth space, deep sections were constructed based on magnetic anomalies, gravity anomalies, and seismological data. The results of the study of magnetic anomalies in near-Earth space are of scientific, practical, and applied importance for solving exploratory geological and geophysical problems and issues of spacecraft navigation.

A study of the basement of Fennoscandia on magnetic and gravity anomalies has been carried out. There are detected magnetic and density features of the structure of the greenstone belts of the Early Precambrian crust of the Baltic Shield as a result of interpretation of CHAMP and GRACE satellite data and near surface magnetic anomalies. Magnetic and density sections have been constructed through the mega blocks of Fennoscandia with the age of the Earth’s crust ranging from 3.2 to 1.6 billion years that underwent different stages of metasomatosis. There are detected magnetoactive layer in the basement of the Early Precambrian crust (“magnetite zone”) and fluid systems at depths of 12–15 and 25–35 km that are the source of the formation of rich deposits of Fennoscandia greenstone belts. Based on the concept of the distribution of fluid supply flows, there are selected several areas promising for electromagnetic monitoring that can help to detail the features of the basement conductivity of the Baltic Shield.

The possibility of detecting deep polarized objects in natural electromagnetic fields is shown experimentally on the basis of synchronous measurements of the vertical component of long-period variations in the magnetic field of the Earth. Interpretation of the data of induced polarization is performed using the method of analytical continuation of geophysical fields downwards (towards the sources).

The role of altitude models of Earth’s magnetic field for solving geophysical problems using spacecraft is considered. The results of studying the features of the deep structure of Earth’s crust in the structure of magnetic anomalies according to the data of satellite altitude measurements on the CHAMP mission are presented. The informativity of the components of the magnetic anomalies in near-Earth space calculated by the component model of Earth’s magnetic field is presented for searching for minerals in the geothermal regions of Central Europe and North America, in the zone of deposits of ferruginous quartzites of the Baltic shield, and hydrocarbon deposits in the oil and gas basins of Tatarstan, Siberia, the Barents, Mediterranean, and North seas.

The article presents the results of the study of geothermal activity zones in the south of Eastern Siberia. In order to study the deep structure of the Earth’s crust in Transbaikalia, an analysis of the anomalies of the magnetic field and gravity has been carried out. The study of deep sections through the zones of known thermal springs and mud deposits resulted in an assessment of the dependence of the thermal source temperature on the depth of the feed fluid systems. An analysis of the magnetic and density sections of the south of Eastern Siberia revealed deep fluid systems and peculiarities of the hydrothermal potential in the region. Using the models of the magnetization and density distribution, the authors have constructed location schemes for the fluid systems at different depths in the earth’s crust and revealed the ways of the thermofluid flows ascending to the surface. The results of the study showed that ascending migration of fluids with the least heat loss brings hydrothermal waters to the surface through vertical faults that cut the crystalline basement in the depth interval from 2 to 10 km. Based on the analysis of heterogeneity in the deep structure of the basement, a forecast of hydrothermal zones for the south of Eastern Siberia near promising mineral resources has been made.

The article presents the results of the study of fluid systems in the Earth’s crust within the Mamsko-Bodaibin mineragenic zone of the northern Transbaikal using magnetic field anomalies, gravity and seismic data. Two-dimensional models of the density and magnetization of schistosity zones and deep sections of gold deposits allowed us to identify the location of fluid systems and possible paths of magmatic thermofluid flows in ancient rocks. The authors have revealed peculiarities of fluid systems at different depth levels and the role of vertical ore-controlling formations of the earth crust of the Mamsko-Bodaibin zone. Diagnostic features are proposed to control the paths of fluid transport to the surface and the location of the ore-generating structures, taking into account the fluid-magmatic activity of the earth’s crust and mantle. The analysis showed that the Mamsko-Bodaibin zone is prospective in the search for new ore objects. Based on the results of the study, we propose a refined model of the ore localization of the Sukholozhskoye ore field, which includes the influence of deep factors.

The first section of the paper describes the developed self-powered, chassis mounted mobile Energy-4 generator, which has a power of 29 kW and a maximum output voltage of up to 1200 V. The generator operates in the audio frequency range (2–2000 Hz) and is designed for electromagnetic sounding of the Earth’s upper crust in the search for minerals and monitoring of earthquake source zones in seismically active regions. The main power units of the generator are a PWM inverter and a step-up transformer. The inverter is powered by two DC generators mounted on the driveshaft of a truck, in the body of truck of which the generator is mounted. The circuit diagram and operation of the generator are considered, as well as individual design solutions that made it possible to increase the amplitude of the output voltage and, consequently, the current in grounded power lines. The second section is devoted to full-scale tests of the Energy-4 generator in the Kovdor-2015 experiment, during which multipath frequency soundings with 25 and 50 km spacings were carried out on the territory of the Enskii–Kovdor granite-gneiss complex composed of rocks of the Archaean basement of the Baltic Shield. As a result of the experiment, a ubiquitous intermediate conducting dilatancy-diffusion layer (DD layer) was found at depths from 2–3 to 5–9 km in an area of 100 × 200 km. The parameters of this layer have been investigated. The layer is considered a seismically active element of the brittle Earth’s upper crust with a thickness of 10–15 km.

The paper presents the results of studies of the resonance structures of the background electromagnetic noise spectrum obtained in a series of experiments in the autumn of 2012 in Karelia. The experiments are unique because the background noise was detected at stations spaced by a distance much less than the doubled effective waveguide height (about 50 km) in a region close to the main ionospheric trough, the structure and position of which were determined based on radiotomographic data. It is shown that the dimensions of the ionospheric local region, which affects both the generation of spectral resonance structures in particular and the propagation of electromagnetic waves in the considered range in general, depend significantly on the variation of ionospheric parameters in regions with strong horizontal inhomogeneity.

This paper reports the long-term teamwork results obtained by an expert group from the Radiophysical Research Institute (NIRFI) and VEGA Geophysics Ltd. (VEGA, St. Petersburg) in the course of designing modern induction-coil magnetic field sensors. A comparative analysis of the world’s most famous sensor models is carried out, and basic design and testing methods and features are considered. The range of low-noise induction-coil magnetometers (IMS-007, IMS-008, and IMS-009) designed by the authors for scientific and geophysical ground-based explorations is described. Research results are obtained from using the sensors for geothermal water exploration by broadband magnetotelluric sounding, investigation of the deep structure of the lithosphere by controlled source electromagnetic sounding, and monitoring of the Earth’s natural electromagnetic field.

The coherence conditions for local ionospheric sources of Pi 2 high-latitude geomagnetic pulsations in the 6–8 mHz frequency band were studied with a method for representing their component spatial distributions by vertical magnetic dipole field integrals over random walk trajectories in the ionospheric horizontal plane. High-quality representations are reached when the field intensity is proportional to a self-intersection density that has spatial and time walk trajectory discontinuities. According to anomalous diffusion theory, the trajectory of the subordinate Levy process is characterized by the stabilization of turning points grouped near pulsation local ionospheric sources. This makes it possible to simulate disturbances of the Pi 2 high-latitude pulsation field by a uniform motion along the Hall ionospheric source Levy trajectories with short-term pulsed current switching on-off. A coherent redistribution of the Pi 2 local ionospheric source intensity and several observed specific features in the source field distribution dynamics are explained by the independent processes of local field-aligned current formation and intensification-weakening of pulsations caused by the current carrier shift relative to wandering Levy turning points and a stationary observer.

This article is devoted to describing the theory, technique, and first experimental results of a control source electromagnetic (CSEM) study of the Earth’s crust and ionosphere with the use of two mutually orthogonal industrial transmission lines 109 and 120 km in length in the frame of FENICS (Fennoscandian Electrical Conductivity from Natural and Induction Control Source Soundings) experiment. The main part of the measurements is executed on the territory of the Fennoscandian shield at distances from the first hundreds kilometers up to 856 km from the source with the purpose of the deep electromagnetic sounding of the Earth’s crust and upper mantle. According to the results of these studies clarifying the parameters of “normal” (standard) geoelectric section of the lithosphere to a depth of 60–70 km, the anisotropy parameters are evaluated and a geothermal and rheological interpretation in conjunction with the analysis of the seismic data is executed. Furthermore, to study the propagation of ELF–LLF waves (0.1–200 Hz) in an “Earth–Ionosphere” waveguide, the measurements are carried out apart from Fennoscandian shield at distances up to 5600 km from the source (in Ukraine, Spitsbergen, Poland, Kamchatka, and other areas). According to the results of these studies, the experimental estimates of the influence of the ionosphere and of the displacement currents on the propagation of ELF–ULF waves in the upper half-space at the different azimuths generation of the primary field are obtained.

Electromagnetic soundings with the fields of natural (magnetotelluric (MT), and audio magnetotelluric (AMT)) and high-power controlled sources have been carried out in the region of the SG-6 (Tyumen) and SG-7 (En-Yakhin) superdeep boreholes in the Yamal-Nenets autonomous district (YaNAD). In the controlled-source soundings, the electromagnetic field was generated by the VL Urengoi-Pangody 220-kV industrial power transmission line (PTL), which has a length of 114 km, and ultralow-frequency (ULF) Zevs radiating antenna located at a distance of 2000 km from the signal recording sites. In the soundings with the Urengoi-Pangody PTL, the Energiya-2 generator capable of supplying up to 200 kW of power and Energiya-3 portable generator with a power of 2 kW were used as the sources. These generators were designed and manufactured at the Kola Science Center of the Russian Academy of Sciences. The soundings with the Energiya-2 generator were conducted in the frequency range from 0.38 to 175 Hz. The external generator was connected to the PTL in upon the agreement with the Yamal-Nenets Enterprise of Main Electric Networks, a branch of OAO FSK ES of Western Siberia. The connection was carried out by the wire-ground scheme during the routine maintenance of PTL in the nighttime. The highest-quality signals were recorded in the region of the SG-7 (En-Yakhin) superdeep borehole, where the industrial noise is lowest. The results of the inversion of the soundings with PTL and Zevs ULF transmitter completely agree with each other and with the data of electric logging. The MT-AMT data provide additional information about the deep structure of the region in the low-frequency range (below 1Hz). It is established that the section of SG-6 and SG-7 boreholes contains conductive layers in the depth intervals from 0.15 to 0.3 km and from 1 to 1.5 km. These layers are associated with the variations in the lithological composition, porosity, and fluid saturation of the rocks. The top of the poorly conductive Permian-Triassic complex is identified at a depth of about 7 km. On the basis of the MT data in the lowest frequency band (hourly and longer periods) with the observations at the Novosibirsk observatory taken into account, the distribution of electric resistivity up to a depth of 800 km is reconstructed. This distribution can be used as additional information when calculating the temperature and rheology of the lithosphere and upper mantle in West Siberia. The results of our studies demonstrate the high potential of the complex electromagnetic soundings with natural and controlled sources in the study of deep structure of the lithosphere and tracing deep oil-and-gas-bearing horizons in the sedimentary cover of the West Siberian Platform within the Yamal-Nenets autonomous district.

The results of electromagnetic sounding of the Earth’s crust in the vicinities of the SG-6 and SG-7 superdeep boreholes (Yamal-Nenets Autonomous Okrug) are presented. The studies were conducted in the fields of natural sources (AMT-MTS) and in the field of the Zevs ULF antenna located at a distance of more than 2000 km from the receiver points. In the vicinity of the SG-7 superdeep borehole, where the small industrial noise was observed, the results of inverse problem solution are completely consistent with the electric logging data. The conducting layers have been identified at the depths of 150 m and 1.1 km. The roof of rocks having small electrical conductivity and belonging to the Permian-Triassic trappean complex has been found at the depth of about 7 km. The response of the Zevs signal (the frequency range of 44–182 Hz) has indicated the properties of the upper part of the geoelectrical section better than audiomagnetotelluric sounding for both boreholes. Based on the sounding in the vicinity of the SG-6 superdeep borehole, with the data of the Novosobirsk observatory taken into account, the distribution of resistivity down to about 800 km depth has been obtained. This distribution can serve as additional information in calculation of the temperature and rheological regime of the lithosphere and the upper mantle in the region of Western Siberia.