RU2248016C1 - Geophysical electric prospecting method - Google Patents

Abstract

FIELD: geophysical electric prospecting.

SUBSTANCE: method can be used for ac geophysical electric prospecting which current is induced in ground by inductive method; method can be used for searching and prospecting objects as in non-conducting and conducting media. Low-frequency electromagnet field is induced by current running in non-grounded loop onto day surface of Earth. Phase shifts of components of magnetic induction are measured at preset height for parallel profiles relatively vertical component of magnetic induction along profile crossing epicenter of loop and being perpendicular to parallel profiles. Availability of abnormal conducting objects in Earth is determined from structure in phase shifts at the area.

EFFECT: improved precision of measurement; improved efficiency of aerial prospecting.

5 dwg

Description

The present invention relates to geoelectrical exploration on alternating current, excited in the earth by an inductive method, and can be used in the search and exploration of conductive objects in a non-conductive and conductive medium, for example in sea water. Scope of primary application: searches for ore deposits occurring at depths of up to 500 m and more.

There is a known method [1] of radio comparing and direction finding (radio kip), in which the spatial components of the electromagnetic fields of remote ultra-long-range radio stations are measured. However, this method has significant drawbacks in that it has a small depth of research and low measurement accuracy due to the use of relatively high frequencies (more than 10 kHz) and the presence of variations (from short-period to long-period) signal level over time.

There is also a known method of geoelectrical exploration [2], in which a low-frequency electromagnetic field is excited using a vertical cable, grounded by both ends in the well, and the Cartesian components of magnetic induction are measured at given heights along parallel profiles, which makes it possible to sort out magnetic field anomalies caused by deep and near-surface objects.

However, the method has significant disadvantages: 1) the presence of a vertical open hole is required; 2) when measuring the real and imaginary components of the magnetic field, landing of the aircraft near the power cable is required in order to compensate for the initial phase shifts in the measuring hardware complex; 3) the need to transmit a reference signal over the air; 4) when measuring the real and imaginary components, high current stability in an ungrounded loop is required.

The closest technical solution is the method of geoelectrical exploration [3], taken by us as a prototype method. In the prototype method, an electromagnetic field is created using a vertical cable grounded at both ends in the well and the Cartesian components of the magnetic induction are measured by parallel profiles at given heights. The main advantage of this method is that by placing a deep electrode above and below a deep abnormal object, an anomalous object lying at a depth of up to 2.8 km is more clearly marked by the measurement results.

However, the prototype method, as well as the method [2], also has significant disadvantages: 1) for measurements, the presence of a vertical or low inclined open hole of a certain depth; 2) the need to land the aircraft near a vertical cable (supply line AB) to compensate for the initial phase shifts in the measuring equipment when studying the real and imaginary components of the Cartesian components of magnetic induction, while the current phase in the grounded cable is taken as the real axis of the temporary coordinate system; 3) the need for a radio channel for transmitting a reference signal from a ground installation on board a helicopter; 4) when measuring the real and imaginary components, high current stability in an ungrounded loop is required.

The purpose of the proposed technical solution is to increase the accuracy of measurements and performance in areal studies.

This goal is achieved by the fact that in the method of geoelectrical exploration, consisting in the excitation of a low-frequency electromagnetic field by a current flowing in an ungrounded loop on the Earth’s surface, phase shifts of the components of magnetic induction at a given height are measured along parallel profiles relative to the vertical component of magnetic induction along the profile passing through the epicenter loops and perpendicular parallel profiles, determined by the structure of phase shifts in the area, the presence in the Earth of abnormal watering objects.

Figure 1 shows the structural diagram of the device with which the proposed method is implemented, figure 2 gives the flight plan of the aircraft, on which the ungrounded loop is depicted as a square with side a. Figure 3 shows a plan of isolines of the phase shift, calculated for a homogeneous half-space. Figure 4 shows a plan of the isolines of the phase angle of the vertical component of magnetic induction, measured in one of the search sections of the Middle Urals. Figure 5 shows the calculated plan of the phase angle isolines, selected with the greatest similarity to the plan of the measured phase angle in the search section.

The device (Fig. 1) contains an onboard remote control 1, including a reference signal unit 2, a three-channel phase meter 3, an information storage and processing unit 4, a navigation device 5, a sensor unit 6, a generator device 7, an ungrounded loop 8.

The proposed method is implemented as follows. On the surface of the Earth lay an ungrounded loop 8 (figure 1) of a square shape with side a. A rectangular current without a constant component with a frequency ω is passed in this loop. A generator device 7 is used as a source. A current J = J _m SignCos ω t flows in loop 8, where J _m is the amplitude of the rectangular current. Sign Cosω t is the sign function of the argument Cos ω t.

The current flowing in the loop excites an electromagnetic field, parameters (amplitude and phase), the Cartesian components of which on the day surface and in the air depend on the electrical conductivity of the rocks. Measurements in air at a height h are carried out along parallel profiles with a speed V (Fig. 1) and along a profile perpendicular to them.

The measured components of the first harmonic of the magnetic field B _x , B _y , B _{z are} carried out by the expressions:

where B _xm , In _ut , B _zm - respectively, the amplitudes of the components In _x , B _y , B _z ; φ $\genfrac{}{}{0ex}{}{\text{p}}{\text{x}}$ (x; y); φ $\genfrac{}{}{0ex}{}{\text{p}}{\text{y}}$ (x; y), φ $\genfrac{}{}{0ex}{}{\text{p}}{\text{z}}$ (x; y) - respectively, the phase shifts of the measured components of B _x , B _y , B _z relative to the current of the first harmonic in the ungrounded loop 8 (Fig.1). The output voltages from the sensors x, y, z of the sensor unit 6 (Fig. 1), proportional to B _x , B _y, and B _z, are supplied to a three-channel phase meter 3, in which the phase shift between the sensor voltages and the reference voltage is determined

U _on = U _m Cos (ω t + ω _i ),

generated by the driver of the reference signal 2, where φ _i is an unknown but constant in time within profile i, the phase angle between the current in the loop and the reference voltage, i = 0, 1, 2, 3, ..., n, where n is number of profiles. Since the transit time of the profile is small, the phase deviation of the reference voltage of block 2 relative to the current in the loop during this time is very small and can be neglected. However, the phase drift gradually accumulates during the transition from profile to profile, as well as during possible interruptions in operation. Therefore, the phase shift between the reference voltage of block 2 and the current in the loop is superimposed by its own, equal to φ _i for each profile, where i is the profile number.

Measurements begin with a zero profile (PR ₀ , FIG. 2), perpendicular to parallel profiles (PR _1,2,3, ... _{, n} , FIG. 2). The zero profile passes through the epicenter of the loop with side a (Fig. 2). As a result, in a three-channel phase meter, phase shifts relative to the reference voltage U _{on are} converted into digital codes

where k is the conversion coefficient of the phase meter, which is usually 1 or a multiple of 10. Therefore, for simplicity, we will assume k = 1, then the digital codes are determined by the following expressions:

After measuring N _x0 , N _y0 , N _z0 , the phase shifts of the magnetic induction leaving are measured along parallel profiles 1, ..., i, ..., n with respect to the reference voltage U _on generated by the reference voltage shaper 2 (Fig. 1). Then the digital phase shift codes are defined by the following expressions:

where i = 1,2,3, ..., n, φ _i is the unknown phase shift between the reference signal and the current in the loop for profile i, constant within this profile.

Digital information about the measured phase shifts of the components of the magnetic induction with respect to the reference voltage U _on goes to the information storage and processing unit 4. Synchronously with the receipt of information from the three-channel phase meter 3, data on the coordinates of the position of the aircraft in space are received at the input of block 4. In block 4, the phase angles φ $\genfrac{}{}{0ex}{}{\text{p}}{\text{x}}$ (x; y), φ $\genfrac{}{}{0ex}{}{\text{p}}{\text{y}}$ (x; y), φ $\genfrac{}{}{0ex}{}{\text{p}}{\text{z}}$ by jointly solving systems of equations (1), (2) for phase angles, taking into account that the phase angle of the vertical component in the epicenter of the loop φ $\genfrac{}{}{0ex}{}{\text{p}}{\text{z}}$ (0; 0) ≈ 2.0 °, as well as the equality of the phase angle of the vertical component relative to the current in the loop defined on the zero profile and on the corresponding ith profile for each intersection point. Thus, the phase angles φ $\genfrac{}{}{0ex}{}{\text{p}}{\text{x}}$ (x; y), φ $\genfrac{}{}{0ex}{}{\text{p}}{\text{y}}$ (x; y), φ $\genfrac{}{}{0ex}{}{\text{p}}{\text{z}}$ (x; y) on the square.

The proposed method was tested experimentally in one of the search sections of the Middle Urals (figure 4). Figure 4 shows a plan of the isolines of the phase angle of one fourth of the investigated area. In the northeastern and eastern parts of the tablet, two local anomalies of the phase angle A1 and A2 are noted. For a homogeneous half-space, the isolines of the phase angle of the vertical component of the magnetic induction are concentric circles (Fig. 3). Figure 5 shows a plan of the phase angle isolines above two 3D conductive bodies (VI and V2, shown in black rectangles) according to the results of mathematical modeling with a field structure similar to the field structure of the phase angle isolines of the vertical component of magnetic induction in Fig. 4. As a result of the selection of design isolines with the greatest similarity, two anomalous bodies with dimensions V1 = 125 × 225 × 50 m and V2 = 50 × 200 × 100 m were identified (Figs. 4, 5).

Field experiments have shown high efficiency and productivity of geophysical surveys. In the proposed method, the transmission of the reference signal by radio channel is not required, which ensures high autonomy of the measuring complex located on board the aircraft (helicopter).

The proposed method allows you to search for large ore deposits in large, undeveloped, inaccessible areas (swampy, forested), where there are no deep exploratory open-hole wells and it is impossible to land a helicopter near the current loop.

Thus, the proposed method has significant advantages compared with known methods.

Sources of information

1. Electrical exploration. Handbook of geophysics and two books. Edited by V.K. Khmelevsky and V. M. Bondarenko. Book 2. M., Nedra, 1989, pp. 46-52.

2. Astafiev P.F., Pyzhyanov Yu.B., Alfutov B.A. A report on the experimental and methodological work performed to develop the methodology for aerial exploration in the search for copper-pyrite ores within the Verkhne-Uralsky ore region. - Sverdlovsk, 1987, p. 7-60, state registration number 40-35-30 / 19a.

3. Patent RU No. 2076344 C1 (Russia). The method of geoelectrical exploration, G 01 V 3/30, 03/27/97 (prototype).

Claims (1)

The method of geoelectrical exploration, consisting in the excitation of a low-frequency electromagnetic field by a current flowing in an ungrounded loop on the Earth’s surface, characterized in that it measures the phase shifts of the components of magnetic induction at a given height along parallel profiles relative to the vertical component of magnetic induction along the profile passing through the loop epicenter and perpendicular to parallel profiles, the presence of anomalous conductive objects in the Earth is determined by the structure of phase shifts in the area s.

Images