RU2427007C2 - Method of combining three-electrode, vertical and unipolar electrical sounding - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: at an observation line, two three-electrode electrical soundings are performed using an apparatus comprising four earthing contacts lying on one line symmetrically about the observation point. The fifth earthing contact relates to virtual "infinity" and is connected to one terminal of an electric current source. Central earthing contacts are connected to a voltage measuring device. When taking measurements, outermost power earthing contacts are successively connected to the other terminal of the electric current source. Potential drop Δ U_AMN and ΔU_A'MN between receiving earthing contacts is measured. The operations are repeated for all given positions of power earthing contacts. Potential drop for vertical electrical sounding and potential drop for unipolar sounding is calculated from the measured potential drop at each observation point for given differences. The distribution of apparent electrical resistance in sections for two three-electrode and vertical sounding and distribution in section of potential drop for unipolar sounding is determined from the measured and calculated potential drops. The results determine the presence and location in the section of geological irregularities.

EFFECT: high efficiency of detecting geological irregularities in the geological environment.

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Description

The present invention relates to electrical exploration by the method of electrical resistance and can improve the efficiency of the study of the upper part of the geological section and the identification of local heterogeneities in both near-surface formations and bedrocks.

The area of primary application of the proposed method: mapping of the geological environment during engineering and geological surveys; the detection of zones of tectonically fragmented, permeable rocks; identification of conductive (ore-bearing) objects covered by loose deposits; study of the condition of soil engineering structures, etc.

A known method of vertical electrical sounding (VES), which uses four grounding located on the same line (observation profile). Two of them are reception rooms (M, N), each of which is at the same distance on different sides from the center of the installation (observation point), and connected to a measuring device. Two other grounding - supply (A, B) - are placed at the same (predetermined) distance along the observation profile of the installation center and connected to the terminals of the electric current source. With one location of the supply ground, the voltage drop ΔU _MN between the receiving ground is measured (Figure 1, a). Then the supply grounding is moved to the next specified distance from the installation center, the measurement process is repeated, etc. Based on the results of electrical sensing, the values of the apparent electrical resistance of the medium for each position of the supply ground (A, B) are calculated and the geoelectric structure of the medium is judged by the change in electrical resistance depending on the distance between the supply ground [1].

The known method has disadvantages: it is designed to study horizontally layered media, therefore, with the heterogeneous structure of the studied geological section, and especially in the near-surface part, the interpretation of the obtained experimental data is ambiguous; the presence of a non-conductive layer in the geological section makes it difficult to study the lower horizons.

A known method of electric sensing with a unipolar installation (field superposition method), in which two supply (A, A ^/ ) and two receiving (M, N) grounds are placed on the same line (observation profile) so that the ground (A) and (M ) are located on the one hand, and (A ^/ ) and (N) on the other, at a given distance from the installation center (observation point), and another supply ground (B) is put into practical infinity and is connected to one of the terminals of the electric source current. Two other supply ground (A, A ^/ ) are connected to the other terminal of the electric current source and, using a special device, the currents flowing from the supply ground (A) and (A ^/ ) are brought to a single value. The receiving earth (M, N) is connected to the measuring device. In the first position of the supply ground (A, A ^/ ), the voltage drop ΔU _MN between the receiving ground is measured (Figure 1, b). Then the ground (A) and (A ^/ ) are moved at the same (predetermined) distance from the installation center, measurements are repeated, etc. [2].

In a homogeneous and horizontally layered medium, the values of the voltage drop ΔU _MN are equal to zero. If there is heterogeneity in the geological environment that differs from the host rocks in the electrophysical properties, the alternating extrema of the voltage drop with zero crossing are fixed on the ΔU _MN plots along the observation profile [3].

The known method has the following disadvantages: firstly, it requires a very clear maintenance of the same value of the electric current flowing from the supply ground (A) and (A ^/ ) during the measurement process; secondly, measuring equipment should provide the ability to determine the sign of ΔU _MN ; thirdly, the anomalous effect is manifested both over the conductive and non-conductive inhomogeneities, and therefore it is difficult to determine the nature of the identified geoelectric object.

The aim of the proposed method is to increase the detection efficiency of geoelectric heterogeneities in the geological environment and reduce the ambiguity of the interpretation of experimental data by combining three-electrode, vertical and unipolar electrical soundings; increase in labor productivity, since it is not required for the method of unipolar sounding to maintain the current flowing from the supplying grounding at each measurement; the possibility of using any electrical equipment designed for resistance methods.

This goal is achieved by the fact that the method of two three-electrode soundings is used, in which four groundings are placed on the observation profile on the same line symmetrically to the observation point, which coincides with the center of the installation, and the fifth ground is carried into practical “infinity” and connected to one of the terminals of the electric source current, central grounding is connected to the voltage meter, when performing measurements to the other terminal of the electric current source, the edge is alternately connected s supply and ground measured between the receiving earth voltage drop ΔU _AMN and ΔU _A'MN, after the measurement at one extreme position of the feeding groundings are moved by the same predetermined distance from the center of the installation and the process repeated measurements, perform these operations for all predetermined positions grounding feeding then, from the measured voltage drops for two three-electrode soundings, at each observation point, for given distances, the voltage drop for vertical electrical soundings ΔU _BEZ and the fall of voltage for unipolar soundings ΔU _MNP, the measured and computed voltage drop is determined by the distribution of apparent resistivity in sections of two three-electrode and vertical sensing and distribution sectional voltage drop for unipolar sensing and the results of probing judged about the presence and location in the context of geoelectric heterogeneities.

Figure 1, in shows the installation diagram for three-electrode sensing. The ΔU _{AMN signal is} measured using (A) and (B) as the supply ground. The signal ΔU _{A'MN is} measured using (A ^/ ) and (B) as the supply ground.

The proposed method is carried out with electrical exploration equipment designed for electromagnetic research (for example, ERA, ERA-SIGN, ERA-MAX, or foreign counterparts operating on direct or alternating low-frequency current), as follows. Four groundings (A, M, N, A ^/ ) are placed on the observation profile on the same line symmetrically with respect to the observation point (O), which coincides with the center of the installation (Fig. 1, c), and the fifth ground (B) is referred to “infinity” and connected to one of the terminals of the electric current source. Grounding (M) and (N) are connected to a voltage meter. When making measurements, the supply ground (A) and (A7 ^/ ) are alternately connected to another terminal of the electric current source, and the voltage drop ΔU _AMN and AU _A'MN are measured between the receiving ground (MN), respectively. After performing measurements at one position of the supply ground (A) and (A ^/ ), they are moved to the same predetermined distance from the installation center (O) and the measurement process is repeated. These operations are repeated at all specified positions of the supply ground. Thus, two sounding curves corresponding to two three-electrode installations AMN (B → ∞) and A ^/ MN (B → ∞) [4] are obtained at one observation point.

Using the measured voltage drops for two three-electrode soundings, calculate at each point at each separation:

where ΔU _BEZ is the voltage drop for vertical electric sounding, ΔU _MNP is the voltage drop for a unipolar installation (field superposition method).

To justify the expressions (1), (2) we present the measured voltage drops in the form:

In the method of vertical electrical sensing, the voltage drop across the ground (M) and (N) when connecting the ground (A) to the positive, and (A ^/ ) to the negative terminals of the electric current source is calculated by the formula:

which, taking into account formulas (3) - (4), is reduced to the form:

In those cases when modular measurements of the voltage drop at the receiving ground (M) and (N) are carried out, formula (6) should be transformed taking into account the signs of the terms included in the expression. Since the potential value is inversely proportional to the distance to the observation point, i.e. before grounding (M) and (N), then always ΔU _AMN > 0, because U _AM > U _AN , respectively, ΔU _A'MN <0 and formula (6) using modular measurements is converted to:

In the method of superimposing fields (MNP), a unipolar installation is used so that two supply earths (A) and (A ^/ ) are connected to one terminal of the current source, for example, to positive, and the third (negative) is assigned to "infinity". In this case, the voltage drop across the ground (M) and (N) is calculated by the formula:

which, taking into account (3) - (4), is reduced to the form:

When performing modular measurements of the voltage drop at the receiving ground (M) and (N), formula (9) is transformed in the same way as formula (6), taking into account the signs of the terms in the expression:

.

.

Obviously, for a homogeneous conducting half-space ΔU _MNP = 0. In the presence of geoelectric inhomogeneities ΔU in it, the _MNP can be either a positive or a negative value, reflecting the distribution of the electrophysical parameters of the medium.

Based on the surveyed observation profile, the values of (ΔU _AMN ), (ΔU _A'MN ) and (ΔU _VES ) calculate the apparent electrical _resistance (ρ _к ) and the sections ρ _k are constructed for three-electrode and four-electrode symmetric (VES) installations, as well as the section (ΔU _MNE ). According to the section of the voltage drop obtained according to the results of measurements with a unipolar installation (ΔU _MNP ), geoelectric inhomogeneities are distinguished, and the conductivity of the inhomogeneity (increased or decreased compared to the enclosing medium) is determined by the sections of the apparent electrical resistance, the morphology of the object is refined and quantitative interpretation of the results of vertical electrical sensing.

Figure 2 shows a table of values of the potential difference obtained at one of the profile points of the combined soundings during the experimental work. The soundings were carried out with half distributions (r = AA ^/ / 2) from 1.5 to 15 meters according to the method proposed in the application. In addition, for monitoring at the same spacings, vertical electric soundings were additionally performed according to the standard method [1], in which the potential differences (ΔU _MN ) practically coincide with the potential differences obtained with the combined ΔU _VES soundings (Fig. 2, column 4 and 6).

The advantage of the proposed method is to increase the efficiency of detecting geoelectric heterogeneities, both in the upper part of the geological section, and in primary geological formations due to the combination of four methods of electric sounding, which increase the geoelectric information content of studies with high productivity.

Claims (1)

The method of combining three-electrode, vertical, and unipolar soundings uses the method of two three-electrode soundings, in which four grounding lines are placed on the observation profile symmetrically with respect to the observation point, which coincides with the center of the installation, and the fifth ground is put into practical “infinity” and connected to one of the terminals an electric current source, central grounding is connected to an electric voltage meter, when performing measurements to another terminal of an electric source eskogo alternately connect current extreme feeding and grounding between the measured receiving ground voltage drop ΔU _AMN and ΔU _A'MN, after the measurement at one extreme position of the feeding groundings are moved by the same predetermined distance from the center of the installation and the process repeated measurements, perform these operations for all preset positions of the supply ground, then, according to the measured voltage drops for two three-electrode soundings, at each given point of observation, at a given separation, the electric drop is calculated voltage distribution for vertical electric soundings ΔU _VES and voltage drop for unipolar soundings ΔU _MNP , the measured and calculated voltage drops determine the distribution of apparent electrical resistance in sections for two three-electrode and vertical soundings, as well as the distribution of voltage drop for unipolar soundings and across sounding results judge the presence and location of geological heterogeneities in the context.

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