RU2332690C1 - Method of geological prospecting - Google Patents

Abstract

FIELD: geoelectrical prospecting.

SUBSTANCE: invention relates to geoelectrical prospecting by the electrical resistance method. The method uses two fixed supplying grounding circuits, the first of them being located in practical infinity, the other one along with two fixed reception grounding circuits being arranged nearby the observation profile, two additional movable grounding circuits located at equal distance from the second supplying grounding circuit. In measurements, in every position of the movable grounding circuits, the latter are connected in turns to a power source or an instrument. On connecting them to the power source, a voltage drop between the fixed reception grounding circuits is measured. On connecting them to instrument and measuring the voltage drop between them, the fixed supplying grounding circuits are connected to electric power source. The aforesaid operations are effected for all preset positions of the movable grounding circuits. Proceeding the measurement results, sections of apparent electrical resistance and voltage drop are plotted to estimate the availability of geoelectrical irregularities in the section.

EFFECT: higher efficiency of revealing geoelectrical irregularities and lower ambiguity in experimental data interpretation.

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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 section, the identification of local geoelectric heterogeneities in bedrocks.

The area of primary application of the proposed method: engineering-geological surveys; detection of tectonically fragmented, permeable rocks; identification of ore-bearing objects covered by loose sediments; study of the distribution in the geological environment of technologically polluted groundwater; study of the state of soil hydraulic structures, etc.

A known method of vertical electrical sensing (VES), which uses four grounding located on the same line (observation profile), two of which - receiving - are at the same distance from the center of the installation and connected to the terminals of the measuring device, and the other two - power placed at the same distance from the installation center and connected to the terminals of the electric current source. After performing measurements at one position of the supply ground, they move to the next specified distance from the installation center, etc. Based on the results of electrical sensing, the values of the apparent electrical resistance of the rocks for each position of the supply ground are determined and the presence of geoelectric heterogeneities is judged by the change in the electrical resistance depending on the distance between the supply ground [1].

The known method has significant drawbacks: firstly, it is designed to study horizontally layered media, therefore, with the heterogeneous structure of the upper part of the section, the presence of deep non-horizontal interfaces, the results of VES cannot be directly quantified; secondly, experimental materials are significantly distorted with uneven terrain [2].

A known method of electric dipole sensing using a supply dipole (grounding A and B) and a receiving dipole (grounding M and N), the centers of which at the beginning of measurements are placed at a given distance (separation). In the process, one of the dipoles remains stationary, and the second is moved along the observation profile with a given step, i.e. increase the separation of the sounding installation. At each separation, the apparent electrical resistance ρ _{k is} determined. Depending on the electrical resistance versus spacing, the geoelectric structure of the studied section is judged [3].

The known method has the following main disadvantages: firstly, the results of sounding with a dipole setup are significantly distorted with an inhomogeneous structure of the medium in horizontal directions (especially in near-surface formations); secondly, small inaccuracies in determining the orientation of the moving dipole lead to significant errors [3].

The closest in technical essence to the proposed one is the method of differential electrical profiling, in which the first supply ground (B) is taken to practical infinity, and the second supply ground (A) and two receiving ground (M and N) are placed on one straight line (observation profile) so that the receiving earths are located symmetrically with respect to the central supply ground (A). The three-electrode differential setup is moved along the observation profile with a given step. At each parking place, the supply ground is connected to a source of electric current, and the receiving ground to the measuring device and measure the voltage drop between the receiving ground (ΔU _MN ). When electroprofiling over a homogeneous half-space, ΔU _MN = 0, and in the presence of geoelectric inhomogeneities in the section, ΔU _MN assumes non-zero values [4].

The prototype method has a significant drawback: during electrical profiling over geoelectric heterogeneity, the anomalous ΔU _MN values are proportional to the electrical resistance of the rocks enclosing the heterogeneities. This leads to the fact that without information on the electrical resistance of the host rocks, the recorded anomaly is uncertain.

The aim of the proposed method is to increase the efficiency of detecting geoelectric heterogeneities in the geological environment and reduce the ambiguity of the interpretation of experimental data in the heterogeneous structure of the upper part of the section.

This goal is achieved by the fact that in the proposed method of geoelectrical exploration, in which they use the first supply ground, referred to practical infinity, as well as three ground connections located along the observation profile at the same distance between the extreme and central ones, of which the central one is used as the second supply grounding, and the other two are receiving grounding and are used to measure the voltage drop between them, which consists in the fact that in addition to four new grounding, use two additional grounding, located along the observation profile at the same distance from the central supply ground, and during each installation during the work, connect additional grounding either to a source of electric current and measure the voltage drop between the main fixed receiving grounding, or to a measuring device , and the main fixed supply grounding is connected to the electric current source, the voltage drop is measured voltage between the additional receiving grounding, find the dependences of the voltage drop between the additional receiving grounding, the apparent electrical resistance between the main receiving grounding at all positions of the additional supplying grounding and judging by their distribution and location in the context of geoelectric heterogeneities.

The performed calculations showed that during differential profiling over a vertical formation, the graph of the voltage drop along the observation profile has characteristic features: a transition through zero above the center of the formation; alternating extremes in the zones of contact of the reservoir with the surrounding medium. If the reservoir has a higher electrical resistance compared to the surrounding medium, then the extrema are fixed in the contact areas. For a reservoir of reduced electrical resistance compared to the host medium, extreme values of the voltage drop are observed in the host medium, and their position depends on the distance between the second supply ground and receiving ground [5].

The drawing shows a diagram of the proposed installation. The signal ΔU _{MN is} measured when A and B are used as the supply ground. The signal ΔU _{M1N1 is} measured when A ₁ and B ₁ are used as the supply ground.

The proposed method is carried out with manufactured serial electric prospecting equipment (for example, ERA, ERA-SIGN) as follows. On the observation profile, two main receiving earths (M ₁ , N ₁ ) and a second supply ground (A) are placed, and the ground M ₁ and N ₁ are located symmetrically with respect to the supply ground A. The first supply ground (B) is put into practical infinity (must be observed condition: AB 10-15 times more than ₁ AM = AN _1). Additionally, on the observation profile symmetrically with respect to the supply ground A, there are two groundings M, A ₁ and N, B ₁ (see drawing). When performing measurements, the indicated grounding is connected in turn: the supplying grounding A ₁ and B ₁ to the stabilized electric current source, and the receiving grounding M ₁ and N ₁ to the measuring device and measure the voltage drop ΔU _M1N1 ; the first (A) and second (B) supply grounding - to the source of stabilized electric current, and the receiving grounding M and N - to the measuring device and measure the voltage drop ΔU _MN . After performing measurements at one position of the additional ground (M, A ₁ ) and (N, B ₁ ) they are moved to the same specified distance from the supply ground A and the measurement process is repeated. These operations are repeated for all specified positions of the additional grounding. _{Using the} surveyed survey profile, the values of ΔU _M1N1 are _{used to} calculate the apparent electrical _resistance (ρ _к ) and the sections ρ _to and ΔU _{MN are built} . Geoelectric inhomogeneities are distinguished according to the section of electric voltage drop, and inhomogeneities are classified according to the section of apparent electrical resistance into objects of either reduced or increased electrical resistance.

Thus, the advantage of the proposed method is to increase the efficiency of detecting inhomogeneities in the upper part of the section under various geoelectric conditions, since the final measurement result takes into account not only the spatial position of the inhomogeneities, but also the ratio of their electrical resistance and electrical resistance of the surrounding medium.

Information sources

1. Yakubovsky Yu.V. Electrical intelligence. - M .: Nedra, 1973, p. 56-57.

2. Matveev B.K. Electrical intelligence. - M .: Nedra, 1990, p.303.

3. Alpin L.M. Theory of dipole sounding. - M., L .: Gostoptekhizdat, 1950, p. 6; from. 88-89.

4. Tarkhov A.G. About electrical prospecting methods of pure anomaly. Proceedings of the USSR Academy of Sciences. Ser. Geophysical No. 8, 1957, pp. 981-982.

5. Ulitin R.V., Fedorova O.I., Kharus R.L. To the technique of geoelectric mapping in geoelectric studies // Theory and practice of geoelectric studies. Sat scientific Proceedings. 2. Yekaterinburg: Ural. RAS, 2000, p. 48-49, Fig. 4, 5.

Claims (1)

The method of geoelectrical exploration, using the first supply ground, connected to one of the terminals of the electric current source and assigned to practical infinity, as well as three earths located on the same line along the observation profile at the same distance between the extreme and central ones, of which the central ground is connected to the other terminal of the electric current source as the second supply ground, and the two remaining ones are used as receiving earths to measure the electric drop voltages between them, characterized in that in addition to the four main earths, it uses two additional earths located along the observation profile at the same distance from the main central supply ground and moved during the measurements along the observation profile at the same specified distance from the central ground The installation connects additional grounding to either the terminals of the electric current source and measures the voltage drop between the main fixed main receiving earthing, or to a recording device, and to the terminals of the electric current source connect the main fixed supply ground, measure the voltage drop between the additional receiving ground, perform these operations at all specified positions of the additional ground, find the dependence of the voltage drop between the additional receiving ground and apparent electrical resistance between the main receiving ground at all positions additional grounding and their distribution judge the presence and position in the context of geo-electric heterogeneities.