V. V. Kolobov

Deep electromagnetic sounding of the lithosphere in the eastern Baltic (fennoscandian) shield with high-power controlled sources and industrial power transmission lines (FENICS experiment)

The paper addresses the technique and the first results of a unique experiment on the deep tensor frequency electromagnetic sounding, the Fennoscandian Electrical conductivity from results of sounding with Natural and Controlled Sources (FENICS). In the experiment, Energy-1 and Energy-2 generators with power of up to 200 kW and two mutually orthogonal industrial 109- and 120-km-long power transmission lines were used. The sounding frequency range was 0.1–200 Hz. The signals were measured in the Kola-Karelian region, in Finland, on Svalbard, and in Ukraine at distances up to 2150 km from the source. The parameters of electric conductivity in the lithosphere are studied down to depths on the order of 50–70 km. A strong lateral homogeneity (the one-dimensionality) of a geoelectric section of the Earth’s crust is revealed below depths of 10–15 km. At the same time, a region with reduced transverse crustal resistivity spread over about 80 000 square kilometers is identified within the depth interval from 20 to 40 km. On the southeast the contour of the anomaly borders the zone of deepening of the Moho boundary down to 60 km in Central Finland. The results are compared with the AMT-MT sounding data and a geodynamic interpretation of the obtained information is carried out.

A set-up converter and energy transfer system of an “Energy-2” generator for electromagnetic sounding and monitoring of zones of earthquake sources

A boost converter and a system of energy transfer at a high voltage (up to 1100 V) developed for an “Energy-2” generator 200 kW in power are described. This generator is intended for solving the problems of precision monitoring of the electromagnetic depth of the Earth’s crust at seismoactive regions with an aim to search for earthquake forerunners. High voltage allows taking off the total generator’s power on transmitting antennas with high resistance (7 Ω and higher) within a wide frequency range (0.1–200 Hz) as well as on low-resistance antennas (lower than 0.7 Ω) at high frequencies (greater than 20 Hz). Modern Russian and foreign literature on the problem of developing transformers and rectifiers of the generator’s power circuit, responsible for generation of powerful low-frequency electromagnetic signals, is reviewed. Theoretical research and numerical simulation of the “Energy-2” generator’s power circuit are performed and the element base of the latter is optimized. Based on the developed documentation of all components, radiators and a system of forced ventilation have been made. The proposed generator-measuring complex has no analogs in world practice and can be used for solving a wide range of fundamental and applied problems.

The KVVN-7 multifunction digital measuring station for electromagnetic monitoring of seismoactive zones

The portable highly sensitive measuring station KVVN-7 has been designed. It enables us to perform frequency sounding with controllable sources and audio-magnetotelluric sounding in the field of natural variations in an electromagnetic field within one session. Signal recording is made by seven channels (three magnetic and four electric ones) in a broad frequency spectrum (0.1–2000 Hz) with elimination of frequencies on the edges of the set frequency range and in the odd harmonics of industrial frequency (up to the ninth harmonic). The station incorporates the system of band-stop filters and anti-alias filters at resistors having a low temperature coefficient of resistance (TCR is lower than 25 × 10−6 °C−1) and capacitors having a low temperature coefficient of capacity (TCC is lower than ±30 × 10−6 °C−1). Application of the KVVN-7 station allows the electric conductivity and fluid regime of the upper crust to be studied for both implementation of geological tasks and tasks related to electromagnetic monitoring of seismoactive zones in combination with seismic methods. The example of practical application of the KVVN-7 station in the Lovozero-Pulozero profile (Kola Peninsula) has been presented. A high effectiveness of a station has been demonstrated when study of a fluid-saturated layer with conductivity of a dilatancy-diffuse origin (“DD layer”) in the upper crust. The further perfection of the KVVN-7 measuring station is aimed at the design of a completely automated recording system through data recording to a built-in data medium (flash memory). Additionally, it is suggested to use a built-in analog-to-digital converter of high resolution (24 bit) for every channel in order to broaden the dynamical range of the station.

High-voltage power inverter of the generator “Energy-2” for electromagnetic soundings and monitoring of the earthquake source zones

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 present article is devoted to description of the successive stage of works on creation of generator “Energy-2” with capacity up to 200 kW, operating from an industrial three-phase network of 380 V voltage and frequency of 50 Hz. The schematic concept, element base and parameters of the output high-voltage power inverter of up to 1100 V voltage and 400 A current are described. In the course of designing the design documentation was developed, the drawings and circuits were made, the accessories were got, bearing structures were manufactured in skilled-industrial workshops, as well as heat-protective units radiators and ventilators, settlement-theoretical works and computer modeling was carried out. The main basic element of novelty was development of the system providing the programmed (arbitrary) form of current signals in a wide spectrum of frequencies from 0.1 to 200 Hz at power consumption of up to 200 kW. The created unit of the high-voltage inverter is designed for installation in the car body of motor vehicle ZIL-131 in the set with other units of multipurpose generator “Enerrgy-2” with the capacity of up to 200 kW. The important feature of the inverter is its multifunctionality. It permits to use the generating complex Energy-2 for monitoring of earthquake focuses in one of three modes frequency sounding, formation of the field or induced polarization. Selection of on of three monitoring modes depends on the characteristics of seismically active environment and purposes of the Earth crust sounding.

High-voltage rectifier of the Energy-2 generator for electromagnetic sounding and monitoring of earthquake source zones

The high-voltage rectifier is described developed for the Energy-2 generator, with a capacity of 200 kW in which a step-up power transformer is used as a converter. The Energy-2 generator is intended for solving problems of precision deep electromagnetic monitoring of seismoactive regions of the Earth’s crust to find earthquake precursors. Theoretical investigation and numerical simulation of the high-voltage rectifier are carried out and parameters of its elemental base are optimized. All the high-voltage rectifier components, heat sink, and forced cooling system were manufactured on the basis of the developed documentation. The high-voltage rectifier was successfully tested as part of the“Energy-2 generator in August 2009 during the “FENIKS-2009” experiment.

International FENICS Experiment on the Tensor Frequency Electromagnetic Sounding of the Lithosphere in the Eastern Baltic (Fennoscandian) Shield

ISSN 1028-334X, Doklady Earth Sciences, 2009, Vol. 427A, No. 6, pp. 979–984.

The Kovdor-2015 experiment: study of the parameters of a conductive layer of dilatancy–diffusion nature (DD Layer) in the Archaean crystalline basement of the Baltic Shield

This paper addresses the Kovdor-2015 Experiment involving frequency electromagnetic soundings of the Archaean basement of the Earth’s crust in the southwestern part of the Kola Peninsula. Eleven soundings were carried out using two transmitting arrangements, 85 km apart. Each arrangement consisted of two mutually orthogonal grounded electric dipoles of 1.5 km long. The distances between the source and the receiver were 25 and 50 km. Interpretation of the results took into account the influence of displacement currents and static distortions. It is found that there is an intermediate conductive layer of the dilatancy–diffusion nature (DD layer) with a longitudinal conductivity of about one siemens at depths ranging from 1.5–2 to 5–7 km. The results are interpreted in the terms of geodynamics.

Energy-2M generating-measuring complex of directional operation for seismic monitoring and sounding of ore objects

A generating-measuring complex of directional operation, which allows investigating seismically dangerous zone of stressedly-deformed state of rock, ore zones, and fracture zones in the directions which are the most sensitive from the standpoint of the aim in view, namely along and across the strike axes of the object of interest, is developed. The complex includes the Energy-2M directional operation generator and a portable high-sensitivity KVVN-7m measuring station that allows performing frequency probing with controllable sources and sounding in the field of natural electromagnetic field variations during one cycle. The generator includes a system of independent phase and amplitude adjustment of the current at the output of the Energy-2M generator inverter along two channels. A circuit of programmed generator current supply to two mutually orthogonal antenna arrays (long grounded lines) and a system of precision control over the current parameters in each arm is developed. The generator allows generating a programmed current with circular or elliptical field polarization in the vicinity of the object under study in the specified direction in the frequency range from 1 to 2000 Hz. The Energy-2M generator is mounted on board a ZIL-131 motor vehicle and fed by two PN-72 generators 15 kW each. The KVVN-7 digital measuring station has been completely modified. Moreover, all out-of-date elements have been replaced by modern operational amplifiers characterized by extremely low internal noises, which do not exceed unities or tenths of nV/Hz0.5. The generating complex is equipped with a system for control, regulation, protection, and automation (SCRPA) based on GPS time synchronization. Thermostatting of the unit for clock frequency and reference voltage is provided. All these measures allowed reaching the accuracy of synchronization over the absolute time not worse than 2 × 10−4 s, and frequency stability of the generator signal not worse than 10−7 Hz.

Electromagnetic sounding of the Earth’s crust in the region of superdeep boreholes of Yamal-Nenets autonomous district using the fields of natural and controlled sources

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.

Portable generator for deep electromagnetic soundings and monitoring of seismically active zones with the use of industrial power transmission lines

The structure and a possible practical application of the portable generator with extremely and super low frequency (ELF-SLF) range (0.1–200 Hz) and power of 2 kW are described. The generator is designed for deep sounding of the Earth’s crust for the purposes of mineral exploration and electromagnetic monitoring of source zones of earthquakes in seismically active regions. Computational and theoretical works, as well as numerical simulation for optimizing the generator’s parameters and element base, are carried out. The main constructive elements of the generator, namely, the power factor corrector, the driver control circuit for power transistors, the inverter of the high-frequency decoupling converter, the multichannel pulse power converter, the output inverter, and etc., are consistently described. The generator’s weight does not exceed 10 kg, and, therefore, the generator may be easily delivered to a testing site using any kind of transport, including planes. The generator is powered by the single-phase network with a voltage of 220 V. The full-scale testing of the generator is performed at the territory of the Yamalo-Nenets autonomous okrug via the connection to the high-voltage transmission line Urengoi-Pangody with a length of 114 km. The maximum current in the line is 12 A. The maximum distance of reliable signal recording is 110 km.