A channel for measuring the magnetic flux density based on four integrated Hall-effect magnetic sensors (IHMSs) is described. Built-in thermal compensation and dynamic offset cancellation circuits of IHMSs do not eliminate the sensor’s magnetic sensitivity drift due to the thermomechanical packaging stresses that causes an output-voltage offset. The placement of the sensors in a thermostatically controlled cell, as well as the design and circuit solutions related to the spatial orientation of the sensors relative to the magnetic induction vector and to the IHMS output voltage processing algorithm, made it possible to minimize the instability and increase the signal-to-noise ratio of the channel’s ADC input voltage. The design and operation of the hardware–software-based PI controller that regulates the thermostatted cell temperature are considered. The developed channel is used to measure the magnetic-field flux density of the magnetic analyzer of the mass-spectrometric system for isotopic analysis of noble gases and provides the following specifications: the measurement range is ±512 mT; the bandwidth is 0–4 Hz; the noise-free code resolution of the channel is 16.3 bits; the peak-to-peak value of the equivalent input magnetic noise in the bandwidth is 0.012 mT; and the equivalent input long-term drift of measurements is no more than 0.08 mT within 48 h.

In the FENICS experiments, performed in 2007, 2009, 2014, and 2019 under the guidance of A.A. Zhamaletdinov, unique deep electromagnetic sounding data were obtained using grounded sections of industrial overhead power lines (OHPL) at distances from 180 to 840 km from the center of the supply line to the measuring installation. At the 2014 and 2019 stages, the components of the electromagnetic field and current strength in the source were recorded with VMTU-10 (VEGA LLC, St. Petersburg) measuring equipment, which allowed synchronous processing of measurement data with GPS referencing. Unlike the previous stages, in 2019, L-403 (Murmansk–Nikel) was used as a source with a sublatitudinal direction, which differs in a number of parameters from L-401, previously used for these purposes. The paper describes the main features of the generation of ELF–ULF radiation for geophysical research using industrial power lines.

Experimental investigations of the propagation of electromagnetic waves in the grounding wire were carried out at the operating 150 kV substation. The wave propagation velocities obtained during the experiment are in the range of 50–100·106 m/s. Thus, the obtained wave propagation velocity along the grounding grid of the substation is several times less than the speed of light, which can radically change the design approaches of the grounding in term of lightning protection.

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).

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.

With the correct choice of the arrester by voltage class and compliance with the calculated protective distance without taking into account the propagation velocity of the current wave on the grounding grid, overvoltages exceeding discharge or residual voltage may occur on the protected equipment, in particular the transformer. Thus, when calculating the installation of the arrester that protects the substation from incoming lightning surges from a transmission lines, it is necessary to take into account the propagation of the current wave on the grounding grid. The propagation velocity of electromagnetic waves in a 150 kV substations grounding grid was measured. The measured wave propagation velocities are in the range of 50–100⋅106 m/s. Thus, the obtained velocity of wave propagation on the grounding grid used in service is several times less than the speed of light. The measured value correlates well with similar experiments conducted for buried conductors located in soils with similar parameters and the results of mathematical modeling for a grounding grid having similar dimensions and mesh size.

A digitally controlled high-precision low-noise high-voltage source is described. A push–pull parallel resonant voltage converter with a constant switching frequency of 110 kHz is used in the source. The high-voltage source is adjusted by changing the output voltage of a linear voltage regulator that feeds the resonant converter. An algorithm for controlling the converter switches that makes it possible to implement a mode of supplying the parallel circuit of the converter from the voltage source is proposed. An automatic frequency control system ensures the temperature stability of the converter operating mode. An active ripple filter was designed. Its main feature is the application of a compensating voltage between the common bus of the source and the low-potential output of the voltage multiplier. This solution made it possible to construct the filter based on one low-voltage op-amp, which is simultaneously used as an error signal amplifier of the source control loop. As a result of applying the active filter, the output noise and ripple peak-to-peak amplitude at frequencies higher than 10 Hz does not exceed 3.3 mV. The peak-to-peak noise in the frequency range of 0.1–10 Hz is below 3.5 mV and is determined by the 1/f noise level of the used voltage reference. The developed high voltage source is used in the ion source power supply of the mass-spectrometry system for isotope analysis of noble gases and has the following specifications: the output-voltage control range is 0–5500 V with a step of 84 mV; the maximum load current is 1 mA; and the overall output voltage instability is no more than 50 ppm/h in the operating temperature range of 20–50°C.

A new portable system for measuring the grounding resistance of transmission towers equipped with overhead ground wires (OHGWs) was developed. To predict the lightning performance of power lines, electric power companies until now measure the low-frequency grounding resistance. A new technique of processing the transient impedance time-dependency obtained by injects of a fast-rising pulse current into a tower base was developed. The technique allows the estimation of element values for a simple lumped RLC equivalent tower grounding circuit, suitable for use across a wide range of soil resistivity values. The tower grounding resistance value finally calculated from the obtained resistance value of the equivalent circuit, by accounting for the parallel-connected surge impedances of OHGWs. For the measuring system, a pulse generator based on inductive energy storage was designed. Since an inductive generator's current pulse shape is not distorted due to a reflection from the remote end of the current lead, therefore, the rather short potential and current leads of 50-meter long and short ground rods could be used. The pulse has 100 ns rising time and constant amplitude within the time of measurement interval, which is limited only by the arrival time of a current wave reflected from the nearest OHGW-connected tower. In the implemented prototype, the main instrumentation requirements for field measurements, such as low weight and dimensions, autonomous power, dust and moisture protection, usability, and easy maintenance, were taken into account. The general hardware structure, operational principles, and main technical specifications of the portable measuring system prototype are discussed in the paper along with the field test results.

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.

The paper presents a low-cost system for monitoring the condition of metal-oxide surge arresters (MOSAs) in service. The developed system integrates two methods currently used for evaluating the condition of surge arresters: registration of transient overvoltages experienced by a MOSA and analysis of leakage current. A surge counter with a built-in primary leakage current sensor (SCLCS) is mounted on a MOSA to register the date, time, and value of charges carried by the discharge currents through a MOSA. The SCLCS powered by a long-life battery stores the recorded data in non-volatile memory. A measurement module connected successively to each SCLCS reads the registered data and performs a leakage current analysis, which involves the determination of the resistive component and the 3rd, 5th, 7th harmonics of the total leakage current. The voltage reference signal received from the secondary side of the potential transformer is wirelessly transferred by a transmitting unit. Six-year experimental testing of the developed prototypes installed at several distribution substations produced satisfactory results. The conducted tests confirmed the efficiency of leakage current analysis not only for detecting deterioration in the MOSA condition but also for differentiating its cause. Data obtained by an SCLCS provides useful information on lightning and switching transient overvoltages arising in a particular network segment, the efficiency of the arrester operation, and the amount of energy absorbed by the MOSA. The paper describes the hardware implementation, operational principles, and main technical specifications of the proposed system. The results of the pilot testing of the system are presented.

The methodology of the Murman-2018 experiment is considered. The technique of the study is based on the distance principle of deep sounding combined with frequency and audio magnetotelluric soundings. Two mutually orthogonal grounded transmitting lines, 1.6 km and 1.9 km in length, were powered by an Energiya-4 generator, which was used as a controllable electromagnetic field source . The main focus of the paper is the description of the self-powered 29 kW Energiya-4 generator with an output voltage up to 1200 V, developed at NERC KSC RAS. The paper discusses experience of using the generator to solve of fundamental problem of studying the nature of the deep geophysical boundaries in the Archean crust of the Murmansk block in the Murman-2018 experiment. A block diagram of the Energiya-4 generator is presented and its operating principle briefly described.

A high-dynamic-range counting−analog detector system based on a secondary electron multiplier (SEM) is described. The current-to-voltage converter, which is common to both channels, includes a high-speed transimpedance amplifier (TIA) and an automatic bias-correction circuit. This circuit also rejects 1/f noise of the TIA in the 0−2.3 Hz passband of the digital lowpass filter of a 24-bit analog-to-digital converter (ADC) in the electrometric channel. As a result, the noise-free code resolution of the entire channel, which is 18.5 bits, is determined only by the ADC noise. A number of circuit solutions are proposed to minimize the number of circuit components, which have made it possible to place the detector system board in the measuring head of the SEM and thereby reduce the stray capacitances that limit the response speed of the TIA. The maximum counting rate of the pulse-counting channel is 7 ×107 cps and is determined by the time resolution of the SEM. The problems of optimizing the supply voltages of the circuit and the gain of the SEM in the counting−analog mode are considered. The developed system is one of the units of the mass-spectrometry system for the isotope study of noble gases. The SEM-input-referred measurement ranges of the system are as follows: from 1 × 10–16 to 1.2 × 10−11 A at a resolution of 3 × 10−17 A or better for the electrometric channel; from 5 × 10−2 to 5 × 107 ions/s for the pulse-counting channel without counting losses and with the allowance for the SEM dark current; and from 1 × 10–20 to 1.2 × 10−11 A for the entire system. Due to the wide overlap of the measurement ranges, it is possible to perform mutual verification of data obtained in different modes.

One of electromagnetic (EM) sounding techniques for investigating the distribution of electrical conductivity with depth to determine the geological structure of the Earth's interior is a controlled-source method (CSEM). The depth of investigation and a source-receiver separation for CSEM sounding with a horizontal electric dipole source are dependent on its length and on the value of current through it. From this point of view, a-grounded at the ends overhead power transmission line (OHL) can be successfully used for deep CSEM soundings. Since, zero-sequence impedance value of an OHL in a frequency range from 0.01 to 10 Hz is small, e.g. a few ohms for more than a hundred kilometers long OHL, then a current source, i.e. a transmitter, of sufficient power is able to provide the amplitude of the current through an OHL of several hundred amperes. Therefore, the use of an OHL for deep CSEM soundings allows to increase a transmitter-receiver separation up to 1000 km and to reach a depth of investigation of 80-100 km. CSEM soundings effectiveness is influenced by a transmitter's design and characteristics. Main requirements for the design of transmitters for deep CSEM soundings by using an OHL were discussed. The transmitters of 200, 30 and 2 kW power were developed. The ability to generate an arbitrary waveform, with controlled amplitude, shape, and frequency, is a main distinctive feature of the transmitters. The transmitters were used for deep CSEM soundings with OHLs in a number of experiments that took place on the Kola Peninsula and in Western Siberia.

Development of powerful generators of the alternating current working from an industrial network in a wide spectrum of ULF-ELF frequencies range (0.1-200 Hz) is a prospective direction of researches in the field of creation of effective means for electromagnetic monitoring of earthquakes focuses. The article is devoted to the electromagnetic compatibility of a powerful ULF generator and a radiating transmission line with substation equipment. The article deals with the protection of the generator from any types of overvoltages. Calculations of the output filter of low frequencies are presented. The main methods for solving the problem of electromagnetic compatibility of the generator and the substation equipment are described.

An inexpensive system for controlling and stabilizing the magnetic-flux density of an electromagnet on the basis of a monolithic Hall sensor is described. The main functional unit of the system is a precision controllable stable-current source with digital control and a regulating unit based on a power MOSFET transistor, which operates in the saturation region. A signal from a magnetic-field sensor is used to control the magnetic-field flux density, while a negative electromagnet-current feedback stabilizes the flux density. In this method, the 1/f noise of a Hall-effect sensor does not affect stability of the electromagnet flux density, while the drift and noise characteristics of the system are determined by the parameters of the used reference-voltage source. The position of the working point of the transistor in the saturation region is stabilized in the entire range of operating currents using a drain–source voltage control circuit. The developed system is used to control the magnetic-field flux density of the mass analyzer of the MI1201IG mass spectrometer and provides the following characteristics of the magnetic-field flux density control: the operating frequency band is 0–10 Hz, the flux-density control range is 0–0.4 T with a step of 1.5 µT, and the overall drift and noise instability do not exceed ±2 × 10–6 T within 20 min in the operating temperature range of 20–50°С.