A numerical model is developed for calculating the electromagnetic response in the ionosphere from ground-based ultra-low-frequency transmitters of finite length L. Such megatransmitters include the ZEVS transmitter with a carrier frequency of 82 Hz and the FENICS transmitter, which can generate artificial radiation at frequencies from fractions of 1 Hz to a several hundred Hz. The amplitude of radiation generated in the upper ionosphere by a grounded horizontal current suspended the high-resistive Earth’s surface is calculated. The altitude profile of the plasma parameters is reconstructed using the IRI model. The modeled amplitudes of the electromagnetic response in the nighttime ionosphere can reach 60 μV m−1 for the ZEVS transmitter (L = 60 km) powered by a current of 200 A, which is confirmed by observations on the DEMETER satellite. Calculations show that the FENICS transmitter (L = 100 km) powered by a current of 100 A can generate radiation with frequencies of 10–100 Hz and amplitudes of up to 60–70 μV m−1 in the upper nighttime ionosphere. The FENICS transmitter can be used to generate artificial Pc1 pulsations that can be detected on low-orbit satellites (e.g., CSES). Oscillations in the nighttime ionosphere at a frequency of 0.5 Hz with amplitudes >1 pT and >10 μV m−1 of the magnetic and electrical components, respectively, are generated in the FENICS transmitter by a current of >140 A.

A numerical model has been developed to calculate the electromagnetic response in the ionosphere from grounded ultra-low-frequency transmitters of finite length L. Such megatransmitters are the ZEVS installation with a carrier frequency of 82 Hz and the FENICS installation, which can generate artificial emissions at frequencies from fractions of a Hz to a few hundreds of Hz. The amplitude of radiation excited in the upper ionosphere by a grounded horizontal current suspended above a high-resistance earth’s surface has been calculated. The altitude profile of the plasma parameters was reconstructed using the IRI ionospheric model. For the ZEVS transmitter (L = 60 km) powered by a current of 200 A, the simulated amplitudes of the electromagnetic response in the nighttime ionosphere can reach ~60 μV/m, which was confirmed by observations on the DEMETER satellite. According to calculations, the FENICS facility (L = 100 km), powered by a current of 100 A, can generate radiation in the nighttime upper ionosphere with a frequency of 10—100 Hz and an amplitude of up to ~60—70 μV/m. The FENICS facility can be used to excite artificial Pc1 pulsations that could be detected on low-Earth-orbit satellites (e.g., CSES). To create pulsations in the nighttime ionosphere at a frequency of 0.5 Hz with the amplitudes of the magnetic component 1 pT and the electric component 10 μV/m, the current in the FENICS antenna is to be 100 A.

At the stages of the FENICS experiment conducted in 2014 and 2019, unique data on deep electromagnetic sounding with grounded sections of industrial power lines were obtained at distances from 180 to 940 km from the center of the supply line to the measuring installation. The electromagnetic field components were recorded by VMTU-10 measuring equipment (VEGA, St. Petersburg). The fluctuations of the current in the supply line were recorded as a time series with a sampling rate of 1 kHz. Based on synchronous time series of the field components at the observation points and the current strength in the supply vibrator, estimates of the power spectra of the autocorrelation and cross-correlation functions of the recorded values were calculated using fast Fourier transform (FFT). The resulting spectral characteristics were used to determine the amplitudes of the field components of the source and phase shifts between them, as well as to estimate the impedance tensor components. To do this, correction factors were calculated for the ratio of the apparent resistivity values for the horizontal magnetic field component to those for the impedance and electric field. A 2D interpretation is made and estimates of the resistivity allocation at depth are obtained for two sublongitudinal and sublatitudinal profiles of the Karelia–Kola region.

The deep electromagnetic (EM) sounding of the lithosphere carried out in the frame of the FENICS-2019 experiment from 12 to 21 September 2019 in terms of experimental layout was a frequency domain sounding with the source of two grounded industrial power lines operating in the frequency range of 192.2–0.382 Hz. The measurements were mostly carried out by the research team from Geological Institute of the KSC RAS (GI KSC) within 841 km from the source. The additional measurements by stations of the Polar Geophysical Institute, Luleå University, Nizhny Novgorod Radiophysical Research Institute (NIRFI) and the Institute of Geosphere Dynamics of the Russian Academy of Sciences (IDG RAS) were carried out at distances of up to 1470 km. The results obtained in GI KSC RAS were processed and consolidated into a database of apparent resistivity and impedance phase curves, calculated for the absolute values of the total electric and magnetic fields. The principal novelty of the FENICS-2019 experiment, distinguishing it from the AMT-MT sounding, was the quantitative account for static shift at each point. The method of correction for static shift is based on measurements of the apparent resistivity from the total horizontal magnetic field of the controlled source. Magnetometer sensors are not galvanically coupled to the Earth and therefore not affected by static shift. The method for assessing the static shift was applied within the far zone; it accounts for the influence of the ionosphere and displacement currents. Geoelectric sections were constructed for four observation profiles. This work was carried out in the frame of the state assignment of the Ministry of Education and Science of the Russian Federation for GI KSC RAS (topic No. 0226-2019-0052, FMEZ-2022-0025) and supported by the Russian Foundation for Basic Research (grant No 18-05-00528).

In September 2019, on the Kola Peninsula, an experiment was carried out to generate ULF–ELF signals at night using two decommissioned industrial power lines as a horizontal emitting antenna. The line current was supplied from an external 200 kW generator. The current strength varied from 240 A at low frequencies (0.382 Hz) to 20 A at the highest (194 Hz). The paper presents the results of recording ULF signals at the Staraya Pustyn magnetic station, which is 1610 km from the power line. Signals with frequencies from 0.6 to 6.4 Hz were recorded. The signal amplitudes, normalized to the emitter current, varied in the range of 0.4–0.7 fT/A. For theoretical estimates, two models were used: (1) formulas from the theory of ELF field excitation in the Earth–ionosphere waveguide and (2) a numerical model of the ULF field in the atmosphere and ionosphere created by a linear surface current of infinite length. The numerical model is based on calculation of the system of Maxwell equations in a vertically inhomogeneous atmosphere and ionosphere, the parameters of which are calculated using the IRI model. A fundamental feature of model 2 is that it takes into account the contribution of the ionospheric waveguide propagation to the excited field at large distances at frequencies above the critical waveguide frequency of ~0.5 Hz. The dependence of the amplitude of the recorded signals from an artificial source on a frequency of 2–8 Hz has a nonmonotonic character, which may be a manifestation of the effects of waveguide propagation along the ionosphere.

Abstract —In this paper, we review the results of the deep electromagnetic soundings carried out on the Archaean blocks of the Kola Peninsula over the past 40–50 years, describe the main results of the Murman-2018 experiment, and present a critical analysis of the previous studies considering the new data. The first part of the paper addresses the results of the studies with the extremely low frequency (ELF) transmitter “Zevs” and a 40 MW MHD source “Khibiny,” the frequency soundings with a 29 kW ERS-67 car generator, and the DC resistivity soundings with vertical electrical sounding (VES) and magnetotelluric (MT) sounding setups. The review focuses on the controversial issues of the previous results for their subsequent critical analysis based on the data from the Murman-2018 experiment. The second part of the paper describes the technique, procedure, and results of the Murman-2018 experiment. The experiment included distance DC resistivity soundings (DS), controlled-source frequency soundings (Control Source AudioMagnetoTellurics, CSAMT), and audio magnetotelluric soundings (AMT) using natural variations of the Earth’s electromagnetic field. The DS and CSAMT soundings were carried out with axial and equatorial setup configurations using two mutually orthogonal current lines AB1 and AB2 with the lengths of 1.9 and 1.6 km, respectively. The key novelty of the DS measurements was the use of a linear step in changing the distance OO' between the source and receiver (2.5 and 5 km) in the range of spacings from 2.5 to 56 km. The linear step pf change of the OO' distance was used for detecting and correcting the effects of lateral and static distortions in the observation results. The DS measurements were performed along three rays directed towards West, North, and East relative to the current lines AB1 and AB2. The CSAMT measurements were performed at a distance up to 105 km from the source in combination with AMTS. Based on the results of the Murman-2018 experiment, a three-layer model of crustal structure with resistivity increasing in a gradient–stepwise manner down to a depth of 20–30 km was constructed. The resistivity in the upper layer gradually (in a gradient-wise manner) increases with depth from 10 3 Ω m on the ground to 10 4 Ω m at a depth of 1–2 km. The middle layer has a constant resistivity on the order of (1–2) × 10 4 Ω m in the depth interval from 1–2 to 10 km and is identified as a “compaction” zone. It is detected at spacings from 2–3 to 30–40 km. In this spacing interval, apparent resistivity on the ground sharply varies ​​from 5 × 10 3 to 5 × 10 4 Ω m against the average background 2 × 10 4 Ω m. The sharp swings are interpreted as the profiling effect and attributed to the influence of the fractured zones and faults intersected by the sounding path. According to the geological estimates, the faults are steeply dipping near the surface and gently dipping at depth. Their overall influence “stabilizes” “flattens” the resistivity of the middle layer at a level of 2 × 10 4 Ω m and leads to the formation of effect of intermediate conductive layer having a dilatancy-diffusion origin (DD-layer) in the depth interval from 3–5 to 7–10 km (at the base of the second layer) with a longitudinal conductivity on the order of 1 S m and resistivity within 5 × 10 3 to 10 4 Ω m. The third (bottom) layer manifests itself by a sharp stepwise increase in electrical resistivity up to 10 5 –10 6 Ω m and higher. The top surface of this layer is located at a depth of 10–15 km and is conditionally interpreted as an “impenetrability boundary” for direct current. This boundary marks the Brittle–Ductile Transition Zone (BDT) of the rocks. A critical analysis of the previous results in the light of the new data obtained in the Murman-2018 experiment is conducted in the Discussion section.

In September 2019, two out-of-service power transmission lines were used as a horizontal emitting antenna in an experiment conducted on the Kola Peninsula to generate ultra-low-frequency (ULF, 0.38–6.4 Hz) and extremely low–frequency (ELF, 9.4–194 Hz) signals during nighttime hours. The current ranged from 240 A at low frequencies (LF, 0.382 Hz) to 20 A at the highest frequencies (194 Hz). The results of the recording of ULF signals at Staraya Pustyn magnetic station, which is 1610 km away from the transmission line, are presented. The recorded signals had frequencies of 0.6–6.4 Hz and amplitudes normalized to the value of the emitter current ~0.4–0.7 fT/A. Three models were used for theoretical estimates: (1) formulas based on the theory of ELF field excitation over a conducting surface; (2) a numerical model of the ULF field in the atmosphere and ionosphere generated by a linear surface current of infinite length, and (3) a numerical model of a horizontal dipole in the multilayer Earth–atmosphere–ionosphere medium. Model 2 is based on the numerical solution of the system of Maxwell equations in the vertically inhomogeneous atmosphere and ionosphere; its fundamental feature is that it takes into account the contribution of ionospheric waveguide propagation to the excited field at large distances. Model 3 demonstrated the best agreement with the amplitudes of the recorded signals. However, contrary to the predictions of models 1 and 3, the frequency dependence of the amplitude of artificial signals in the 2–8 Hz range is non-monotonic, which may be a manifestation of the effects of waveguide propagation along the ionosphere.

The article presents a method of calculating of the lithospheric temperature dependence vs depth on the base of a-priori information of electrical resistivity change vs depth (based on the results of electromagnetic soundings), the composition of rocks at depth (based on geological assumptions) and with the use of laboratory data of electrical properties of dry rocks at high temperature. It is assumed that at a depth of more than 10–15 km, the resistivity of the lithosphere is mainly determined by the composition of the rocks and the effect of temperature. The solution of the problem is illustrated by the example of calculations performed for two types of geoelectrical section of the Fennoscandian shield - “normal” and “anomalous”. Information on geoelectrical sections was obtained from the results of electromagnetic soundings with industrial power lines (FENICS experiment). The deep geological structure of the lithosphere is assumed to be the same in “normal” and “anomalous” areas. The maximum temperature difference within these two areas was about 70 °C in the depth range of 30–60 km.

Аннотация.Работа посвящена предварительным результатам эксперимента FENICS (Fennoscandian Electrical conductivity from Natural and Induction Controlled Sources soundings) по изучению глубинной электропроводности литосферы Фенноскандинавского щита на территории Кольского полуострова и Карелии с использованием заземленных участков промышленных линий электропередачи.В рамках эксперимента FENICS-2019 измерения выполнялись рядом отечественных и зарубежных исследователей.Измерения с контролируемым источником выполнялись в диапазоне частот от 0.1 до 194.2 Гц, при удалении точки измерений от центра источника на расстоянии от 100 до 875 км.В качестве источника электромагнитного поля использовались заземленные участки промышленных линий электропередачи с генератором тока «Источник ЭНЧ» мощностью до 200 кВт (разработка ЦЭС КНЦ РАН, изготовитель ПГИ РАН).Измерения ГИ КНЦ РАН выполнены с помощью отечественных станций VMTU-10 (ООО «ВЕГА», г.Санкт-Петербург).Предварительные результаты представлены с использованием результатов предыдущих экспериментов серии FENICS в виде вертикальных квази-двумерных разрезов по трем субмеридиональным профилям, и набора горизонтальных квази-трехмерных разрезов для глубины 3, 10, 20 и 100 км, полученных интерполяцией результатов решения одномерной обратной задачи в точках наблюдения.

Results of study of supposed boundary of impenetrability in the crystalline earth crust are discussed in the article. The Murman-2018 experiment on distance DC sounding in combination with frequency and audio magnetotelluric sounding was undertaken to solve the problem. Methods of field work and data processing are presented in the first part of the work. The second part of the work is devoted to the interpretation method description and to presentation of results obtained. Impenetrability boundary is detected in the shape of the sharp resistivity increase from 104 to 106 Ω · m at a depth of 10–15 km. That boundary is accepted as transition zone between the brittle and ductile states of the earth crust substance (Brittle-Ductile Transition zone, BDT). To obtain the more reliable information on parameters of the impenetrability boundary it is necessary to conduct additional DC studies using remote sensing at distances up to 1000 km between transmitter and receiver.

This paper aims to study the deep electrical conductivity of the Murmansk block (Fennoscandian Shield) using the controlled source audio-frequency magnetotelluric (CSAMT) and audiomagnetotelluric (AMT) methods. One of the most important tasks in deep sounding studies implying the joint application of the AMT and CSAMT methods is to take into account the influence of surface conductors on the measurement results (static shift). To solve this problem, we develop a methodology for processing and interpreting the measurement data of the CSAMT and AMT system using the results of the deep sounding experiment conducted in the frequency range from 1 to 500 Hz performed in 2018 (Murman-2018) as an example. Measurements with the controlled source in the form of two mutually orthogonal horizontal electric dipoles are taken at distances of 12 to 105 km. The generator-measuring complex consisted of the VMTU-10 measuring station (manufactured by OOO VEGA, St. Petersburg) and the Energy-4 generator (developed at the Kola Science Center, Russian Academy of Sciences, Apatity). Using the synchronous time series of the field components at the observation points and the current in the supply dipole, the power spectra of the autocorrelation and cross-correlation functions of the recorded values are calculated based on the fast Fourier transform (FFT). The obtained spectral characteristics are used (i) to determine the amplitudes of the source field's components and the phase shifts between them, and (ii) to evaluate the components of the AMT field's impedance tensor. The values of the apparent resistivity are calculated using the amplitudes of the horizontal components of the electromagnetic field and the impedance ratios. The obtained measurement results are corrected for the static shift. For this, the correction coefficients are calculated from the ratio between the apparent resistivity values for the horizontal component of the magnetic field and the apparent resistivity values inferred from the impedance and electrical field values. The data for the cases of axial and equatorial arrays, together with the data of the AMT and remote sounding, made it possible to establish a high degree of lateral homogeneity of the deep electrical section and to expand the frequency range of the obtained curves of the apparent resistivity and impedance phase. A one-dimensional interpretation is made to obtain estimates of the distribution of the deep resistivity.

“Static shift” appear as the frequency-independent influence on results of EM soundings made with natural or controlled sources. Static shift distortion displaces apparent resistivity curves usually down for several orders and appear as the main reason of false results. In this paper we propose the quantitative method of “static shift” correction. The use of the total horizontal magnetic field measurements in the field of controlled source frequency sounding is the base of this technique. The use of magnetic measurements with induction coils, free of galvanic connection with the ground, made possible to calculate quantitatively the static shift distortions. Experiment on control source sounding with spacing up to 25 and 50 km between transmitter and receiver is implemented for to study the static shift in the Kovdor area (Kola Peninsula). Apparent resistivity curves, calculated by the total electric field ( ρ E_tot) happened to be shifted in some measuring points down up to 300–500

In the present work, on the base of theoretical calculations and experimental data the transition boundary of the earth’s crust from the brittle to ductile state (BDT boundary) has been studied. The results of theoretical calculations show that the position of the BDT boundary varies widely - from 5–10 to 30–40 km depending on those or other a priori (indirect) data. The use of experimental estimates allows clarifying the position of the BDT boundary. In this case, the assumption was taken that the BDT boundary coincides with the conditional “impenetrability” boundary, or else with the boundary of a sharp increase in the electrical resistivity of rocks, establishing at the depths of 10–15 km based on the results of electromagnetic soundings on direct current. Results of the deep MHD-sounding “Khibiny” and the deep DC soundings on the Murmansky block from car generator and on the Russian platform from the Volgograd-Donbass DC power transmission line were taken as the main source of experimental information.

The article describes the experiment “Murman-2018” on remote electromagnetic sounding in combination with frequency and audio magnetotelluric soundings. The current from 29 kW power car-generator was fed into the ground using two mutually orthogonal grounded electric dipoles 1.6 and 1.9 km long. The measurements were carried out along three traces with maximum distances from the source up to 105 km in the mode of frequency sounding (in the range of 4-1000 Hz) and up to 56 km in the mode of remote sensing at direct current. The data processing was performed in the spectral mode and in the accumulation mode. The results of the experiment made it possible to quantify for the first time in the scientific literature the position of the boundary of a sharp increase in rock resistance at a depth of 10-15 km. The nature of the established boundary (the boundary of the “impenetrability” for direct current) is associated with the transition of the properties of the rocks of the Earth's crust from the fragile state in the upper crust to a plastic state at depths of 10-15 km and more. In foreign literature, this boundary is defined as the BDT-zone (brittle-ductile transition zone).

Both processes- and with a controlled source and fields of natural origin, contain information about the Earth. The data of the CSAMT research and the results of the AMT-MT measurements should complement each other. To this point of view, in the Geological Institute of RAS has developed a set of programs for data processing and interpretation of the results of joint data of the magnetotelluric research and data of the frequency electromagnetic sounding with controlled sources. Presents the results of the joint inversion of CSAMT data at distances 50 km from two mutually-orthogonal horizontal electric dipoles in the frequency range of 3.82–2185 Hz (“Kovdor-2015”) in west sector Kovdor-Jona area and AMT-MT measurements of the 2016 year in point of the disposition of the dipole sources.

The results of studying the influence of lunisolar tides on the electrical conductivity of the Earth’s crust in the territory of the Kola Peninsula are presented. Along with the results obtained by the authors, the data of other researchers are also considered. All the studies are based on the analysis of the field produced by the Zevs facility transmitting extremely low frequency (ELF) signals at 82–83 Hz. The measurements were carried out in different years at the Avva-Guba (1998), Lovozero (2009), and Imandra–Varzuga polygon (IVP) monitoring sites (2013) located 180, 90, and 160 km from the transmitter, respectively. The negative correlation between the tides and crustal electrical resistivity is revealed at all the points. This means that tidal rises of the Earth’s surface are accompanied by a decrease in resistivity and vice versa. The overview shows that the higher the resistivity of separate Earth’s crustal blocks the higher the relative amplitudes of the corresponding tidal responses that are observed.

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.