RU1840791C - Method and device for measuring impedance of earth's crust in very low frequency range of radio-waves - Google Patents

Abstract

FIELD: physics; geophysics.

SUBSTANCE: invention relates to geological prospecting and can be used for determining effective complex electrical conductivity of the Earth's crust. The method involves measuring surface impedance of the Earth's crust at different frequencies in the very low frequency range of radio-waves from which effective complex electrical conductivity and structure of the geological section are determined. The probing electromagnetic field used is radio-wave emission of a horizontal earthed electrical antenna or system of antennae, powered by highly stable sinusoidal current. The device for realising the method has sensors of horizontal electrical and magnetic components of the electromagnetic field with a preamplifier. The sensor of the horizontal electrical component is made in form of a symmetrical three-electrode earthed electric dipole with a differential preamplifier. The device also has two synchronous detectors, two low-pass filters, a phase regulator and a highly stable reference frequency generator.

EFFECT: more accurate amplitude-phase measurement of surface impedance.

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Description

The invention relates to the field of geoelectric exploration and can be used to determine the effective integrated electrical conductivity of the earth’s crust to the depth of the skin layer, to compile electrical conductivity maps of different scales, necessary for the design of power lines and communications, in antenna technology when choosing areas for placing ultra-low-frequency (ELF) emitters, when studying the propagation conditions of radio waves in the cavity "Earth - ionosphere" and in the earth's crust, to determine the geoelectric section of the upper part of the earth bark.

Known methods of geoelectric exploration using the electromagnetic field of natural [1], [2], [3] and artificial [4] sources. In the method [1] using an alternating natural electric field (PEEP) in the range of sound and infrasound frequencies (4-2000 Hz), caused mainly by lightning discharge, in relation to the average horizontal E _G and vertical E _B electric field components judge the magnitude of the slope of the front of the electromagnetic wave and the effective resistance of the upper part of the earth's crust. The disadvantage of this method of PEEP is the low accuracy of determining the modulus of the slope of the wave front and effective resistance due to the instability of the ratio (E _G / E _B ) in time. The method does not allow to obtain information about the phase of the slope of the wave front. In the method of geoelectric exploration [4], using an artificial sounding field created by the action of a powerful amplitude-modulated short-wave or medium-wave radio emission on the ionosphere, the surface impedance is measured at a modulation frequency in the range of 10-2500 Hz and the geoelectric section of the earth is judged by the frequency dependence of the surface impedance bark. The disadvantage of this method is that the electromagnetic field of a level sufficient for measurements is created in local areas measuring several hundred kilometers, and the radiation efficiency depends on the latitude of the emitting device and in middle latitudes is much less than in high latitudes (in the auroral zone).

The closest set of similar features to the proposed technical solution is the method of magnetotelluric sounding (MTZ), based on the fact that at the observation point at the air-to-ground interface, the electric and magnetic components of the Earth’s natural geomagnetic field are measured by horizontal mutually perpendicular electric and magnetic sensors [3]. Each sensor is connected through its own amplifier to an electronic recorder or digital recorder. Based on the obtained synchronous records of variations in the electric and magnetic components of the geomagnetic field and the known characteristics of the receiving paths, the surface impedance of the earth's crust is determined, the frequency dependence of which determines the structure of the geoelectric section.

A disadvantage of the known MTZ method is that the surface impedance of the earth's crust is determined in the range of geomagnetic pulsations (frequencies below 1 Hz). In this case, information is obtained only on the structure of the deepest layers of the earth's crust and upper mantle. To obtain high-quality information, variations of the quasi-sinusoidal form are used, for the accumulation of the necessary volume of which in the range of 0.001-1 Hz a considerable time is required, reaching 5-7 days. The extension of this method to the range of ultra-low-frequency and extremely low-frequency electromagnetic fields (ELF - 30-300 Hz; ELF - 3-30 Hz), where the main source of the probing magnetotelluric field is lightning discharge radiation, encounters great difficulties. The complex pulse-noise nature of a random electromagnetic field, as well as the uncertainty in space and time of the sources that create it, significantly reduce the quality of the information received - currently, when measuring surface impedance, only the surface impedance module is reliably estimated and its phase is not estimated [2 ]. The lack of information on the phase of the surface impedance leads to significant errors in the calculation of the propagation of UHF radio waves and in determining the energy characteristics of the UHF emitters, which are linear vibrator antennas with grounded ends. A disadvantage of the known MTZ method is also the use of asymmetric electric sensors for measuring the horizontal component of the electric field in the high-frequency region of the geomagnetic pulsations, in the measurement results of which significant errors are introduced by the unmeasured vertical component of the electric field E _B , which is an obstacle.

The purpose of the invention is to increase the reliability and efficiency of measuring the complex value of the surface impedance (module and phase) in the range of ultralow (30-300 Hz) radio waves.

This goal is achieved by the fact that as a probe electromagnetic field, radio wave radiation of a horizontal grounded symmetrical linear electric antenna or a system of such antennas, powered by a highly stable sinusoidal current of given frequencies and used in the practice of special radio communications, is used [5]. The field of the proposed UHF emitter as a source for geoelectric studies is characterized by small attenuation during propagation in the Earth-ionosphere waveguide of the order of 1 dB per 1000 km at frequencies of about 100 Hz, which makes it possible to measure surface impedance from one emitter anywhere in the country. The radiation efficiency of such a grounded electric antenna is 4 orders of magnitude higher than that of an artificial ionospheric source proposed in [4], which is significantly inferior to the creation of primary electromagnetic fields for sensing over a large area [6].

Thus, it was found that the proposed method has properties that do not coincide with the properties of known solutions and the proposed method has novelty and significant differences.

The relatively low level of the electromagnetic field of the proposed emitter, comparable with the level of the natural electromagnetic field of the Earth, does not allow the use of the MTZ method because the ratio of the useful signal to noise at the input of the receiving path (U _С / U _Ш ) will be less than unity.

To implement this method of measuring surface impedance, a two-channel device is proposed for the simultaneous reception and registration of horizontal magnetic and electric components of the electromagnetic field, containing, as in any minimum set of equipment for magnetotelluric sounding (MTZ), sensors of the electric and magnetic field components, amplifiers, calibrator and electronic a recording block, in which differential preamps are additionally introduced, two synchronous detectors with no filters They each output frequencies, the phase adjuster and coherent highly stable reference oscillator.

The invention is illustrated by a block diagram of a device for measuring the surface impedance of the earth's crust, shown in figure 1. The horizontal magnetic component of the field Н _Г emitted by a horizontal grounded electric antenna is received at a distance of more than 100 km by a sensitive magnetic induction sensor 1 with a core made of a material with high magnetic permeability. Perpendicular to the horizontal electric component E _T is received grounded horizontal electric dipole 2, which is a three-electrode setup with two potential electrodes, spaced the same distance from a third central electrode connected to the neutral terminal apparatus. The symmetric circuit of the electric field sensor allows using the differential preamplifier to eliminate the influence of interference, which induces in-phase signals in each sensor arm. The sensors are oriented to the maximum field of the emitter and together with the preamplifiers 3 and 4 comprise the remote part of the device located on the ground.

The signals from the pre-amplifiers are transmitted through coaxial cables to the main part of the device, where they are fed to the first inputs of synchronous detectors 7 and 8, after amplification by strip amplifiers 5 and 6 with a passband of 50-100 Hz, reference signals from a highly stable generator 9 are fed to their second inputs : directly to one of the synchronous detectors, and to the other through the phase regulator 10. After synchronous detection, the signals pass through the low-pass filters 11, 12 with an adjustable time constant τ and register camping on a dual paper tape recorder unit 13. At the output of the LPF signals allocated of the difference frequency Δf = f _c -f _o, where f _c - the frequency of the received probing field, f _of - a reference frequency signal in the form of sinusoids distorted by noise, which has passed through filters low frequencies. By decreasing the detuning Δf to 0.003-0.001 Hz and, accordingly, reducing the passband of the low-pass filter to 0.01-0.003 Hz, by increasing τ to 30-100 seconds or more, it is possible to achieve a signal-to-noise ratio of >> 1 on the tape of the recorder. Figure 2 shows the synchronous recording of the sinusoidal signals of the difference frequency Δf on a paper tape of a two-channel recording unit. The amplitudes of the sinusoids and the phase difference between them carry information about the amplitudes and phase differences of the received field components H _G and E _G , knowing which you can calculate the modulus and phase of the surface impedance | Z | = | E _G | / | H _G |, argZ = arg E -arg H _{T T.}

The amplitude and phase calibration of the device channels is carried out for the channel Н _Г by the standard field method using Helmholtz rings 14, fed by current from the calibration generator 15, and by supplying a calibration signal from the same generator to the input of the channel preamplifier Е _Г. Phase shifter 10 in the calibration process establishes the common mode of the calibration sinusoids on the tape recorder. The receiving electric dipole does not introduce the phase difference in the frequency range under consideration, and the phase difference (ΔТ / Т) · 360 ° sinusoid on the recorder tape (Fig. 2) when receiving the signal will be equal to the phase difference between the measured field components Е _Г and Н _Г with uncertainty ± 180 °. This uncertainty is eliminated if we take into account that the phase of the surface impedance in the LF frequency range is in the range 0 ° - -90 °. Comparative analysis with the prototype shows that the claimed device is characterized by the presence of new units: additional differential preamplifiers, synchronous detectors with low-pass filters, a phase regulator and a coherent highly stable reference frequency generator. Thus, the claimed device meets the criteria of the invention of "novelty."

A comparison of the proposed solution with other technical solutions shows that differential preamplifiers, synchronous detectors, phase regulators and coherent reference frequency generators are widely known [7], [8]. However, when they are introduced in this connection with the other elements of the circuit into the inventive device for measuring the surface impedance of the earth's crust in the microwave frequency range, the above blocks exhibit new properties, which increases the reliability and efficiency of measuring the module and phase of the surface impedance in the microwave frequency range. This allows us to conclude that the technical solution meets the criterion of "significant differences".

The proposed method for measuring the surface impedance of the earth's crust in the microwave range of radio waves is implemented as follows. A horizontal grounded linear antenna several tens of kilometers long was powered by a highly stable (frequency instability higher than 10 ^-7 ) sinusoidal current of about 300 A. The number of operating frequencies sequentially broadcasted over the air for 1 hour at each frequency reached four (according to a previously agreed schedule). The probing radio wave radiation was received at a measurement point remote from the emitter by more than 4000 km. The levels of the received field were Н _Г = (1 ÷ 4) · 10 ^-7 A / m, Е _Г = 10 ^-8 ÷ 5 · 10 ^-7 V / m and were comparable or less than the level of the Earth’s natural electromagnetic field. The time spent on measurements at one frequency was 1 hour, at four frequencies - 4 hours. Measurements were carried out in sensing and profiling modifications. Using the proposed method allowed to determine the phase of the surface impedance of the earth's crust in the LF range and increase the measurement performance compared to the MTZ method by 5-10 times. The proposed method for measuring surface impedance can be implemented using any ELF emitter [5]. The use of the method is most effective for a system of emitters powered by a current of several hundred amperes, and at distances from 100 to 8000 km in areas with high earth crust resistance (crystalline massifs). For measurements, it is necessary to know the frequency of the emitted signal.

A device for measuring the surface impedance of the earth's crust in the microwave range of radio waves was performed according to the block diagram of figure 1. Sensor 1 is two multi-section coils connected in series with 90,000 turns of PEL-0,1 wire on a core of M 3000 NM ferrite rings with a diameter of 31 mm. The sensor is placed in an electrostatic screen. The low-noise antenna preamplifier 3 with a symmetrical input and 50 and 150 Hz rejection filters is made on field-effect transistors and has a gain of about 2000. Helmholtz calibration rings 14 have a radius of 339 mm and contain 20 turns of copper wire, the calibration current is usually 0.71 μA. The length of the receiving electric dipole 2 was 100 or 200 m. Nonpolarizing electrodes from the VP-62 station or copper electrodes were used as two potential electrodes. Differential preamplifier 4 is a low-noise amplifier with a balanced input (K _us = 10 dB, input impedance 10 MΩ / 20 pF, common-mode rejection of more than 80 dB). As an amplifier and a synchronous detector with a low-pass filter (blocks 5, 7, 10, 11 and 6, 8, 12), a Unipan 232 V homodyne nanovoltmeter or a measuring amplifier-converter UPI-1 was used, as highly stable generators 9 and 15 - G-generators 3-110, synchronized from the frequency standard Ch-1-69, as a recording unit 13 - recorder H3021 with rectangular coordinates.

The device operates in the following modes. The detuning of the reference signal f _{about the} received sounding signal f _c Δf = f _with -f _about = 0,01; 0.003; 0.001 Hz. The time constant of the low-pass filter τ = 10, 30, 100 sec. Exceeding the above upper limits leads to an increase in the measurement time, and a decrease in the below limits leads to a decrease in the signal-to-noise ratio, which ultimately degrades the quality of measurements of surface impedance and reduces the reliability of the obtained data.

In total, measurements were made at 60 observation points or 130 determinations of the modulus and phase of the surface impedance. The total number of hours of recording on the recorder's tape is 230 hours. Power was supplied from the AB-2 benzoelectric unit, the equipment was mounted in a heated GAZ-66 car body.

When carrying out the invention, the following positive effect is achieved:

- the phase of the surface impedance of the earth's crust is measured in the microwave frequency range of the radio wave;

- when calculating ELF emitting systems, the complex nature of the effective electrical conductivity of the earth's crust is taken into account;

- improves the accuracy of the measurements by eliminating the influence of the vertical component of the electric field, which is an obstacle to the measurements;

- the number of sensors and equipment used is reduced, the measurement process is simplified;

- increases the efficiency of determining the complex value of the surface impedance by reducing the measurement time and express data processing directly in the measurement process.

The effectiveness of the invention is confirmed by the results of field tests at 60 observation points, including a quasihomogeneous geoelectric section with known electrical characteristics.

The application of the invention in the search for areas with a high surface impedance of the earth's crust for the placement of VLF emitters has also shown its high technical and economic efficiency due to its high mobility when working in mountain taiga areas without the need for a road network. The proposed device was easily transported to a given observation region by car or helicopter.

Information sources

1. USSR author's certificate No. 646295, G01V 3/16, 1979.

2. Vladimirov N.P. Magnetotelluric sounding method. M .: Nauka, 1979, pp. 71-84.

3. USSR copyright certificate No. 488176, G01V 3/12, 1975 (prototype).

4. Copyright certificate of the USSR No. 987552, G01V 3/12, 1983.

5. Bernstein S.L. and others. Long-distance communications at extremely low frequencies. TIIER, 1974, vol. 62, No. 3, pp. 5-29.

6. Barr R., Stubbe P., "Polar Electrojet Anttenna" as soure of ELF radiation in the Earth-ionodhere wavequiede. Alm. Terr.Physics, 1984, v. 46, No. 4, p. 315-320.

7. Mandle M. 200 Selected Electronics Circuits. M.: Mir, 1985, p. 42, 86, 222.

8. Gonorovsky I.S. Radio engineering chains and M .: Sov. Radio 1977, p. 320.

Claims (2)

1. The method of geoelectrical exploration, which consists in measuring the surface impedance of the earth's crust at various frequencies in the ultra-low-frequency range of radio waves, which is used to judge the effective integrated electrical conductivity and structure of the geoelectric section, characterized in that, in order to increase the reliability of the method, a radio wave is used as a probe electromagnetic field radiation from a horizontal grounded electric antenna or a system of antennas powered by a highly stable sinusoidal current. 2. The device for implementing the method according to claim 1, containing sensors of horizontal electric and magnetic components of the electromagnetic field with preamps, the outputs of which are connected to the inputs of a two-channel strip amplifier, the outputs of which are connected to the inputs of a two-channel recorder, characterized in that, in order to increase the accuracy of the amplitude -phase measurements of the surface impedance at small signal-to-noise ratios and to eliminate the influence of the vertical component of the electric field on the measurement results, and e fields of industrial power lines, the sensor of the horizontal electric field component is made in the form of a symmetric three-electrode grounded electric dipole with a differential preamplifier, two synchronous detectors, two low-pass filters, a phase regulator and a highly stable reference frequency generator are additionally introduced, while the first inputs of synchronous detectors are connected to the outputs of the strip amplifiers, the inputs of which are connected to the outputs of the preamplifiers of the field sensors, the output of the reference signal generator ala connected to the second input of the synchronous detector, the outputs of the synchronous detectors are connected to the inputs of low pass filters with adjustable time constant, whose outputs are connected to two-channel recording unit.

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