RU2642967C2 - Method of geoelectrical prospecting - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: method is based on the use of a multielectrode installation in the form of a cable. An inversion multielectrode installation is used for measurements, a simultaneous registration of potential difference at all diversions of each measured sounding with a fixed position of AB supply line for it. In order to perform each sounding, measurements are carried out at two fixed AB lines, series-connected during measurements: paired electrodes AB are used for the first of them, and two nearest adjacent electrodes, adjoining the paired electrodes, are used as the second AB size, connected when the signal level is below acceptable. Two additional electrodes at the ends of the measuring cable are used for extreme pickets.

EFFECT: reduced time of measurements, operability of research of the rock massif, widened interval and volume of the surveyed depths.

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Description

The invention relates to multichannel geophysical exploration and is intended to solve geotechnical, mine, geotechnical, environmental problems, search for minerals and groundwater.

One of the basic methods of geoelectrical exploration is the method of vertical electrical sounding (VES). The method is based on the use of the geometric principle of sensing, which consists in increasing the effective depth of penetration of electric current with increasing distance between the receiving and supply electrodes of the measuring unit, called the unit spacing r [1]. The VES method includes several types of measuring devices. The main one is the symmetric four-electrode Schlumberger installation, which consists of two supply electrodes A and B, with which a current of force J is supplied to the ground, and two receiving electrodes M and N, between which the potential difference ΔU is measured (Fig. 2a). The installation spacing is r = AB / 2. One of the modifications of the Schlumberger installation is a three-electrode installation, obtained by removing one of the supply or receiving electrodes orthogonal to its direction by a distance of about 10 times the effective sensing depth Zeff (Zeff = 2r) [1, 6 (Section 3.3.3)] , or 5 times the separation of the installation r, at which the influence of the remote electrode on the measurement result can be neglected. The separation of the installation r in the case of a three-electrode installation is the distance between the center of the paired electrodes and the second (working) electrode. In general, the VES method has strict physical and mathematical support, which allows obtaining information about changes in the electrical properties of geological rocks for a given depth interval [2].

One of the main disadvantages of the VES method is the high complexity associated with repeated (up to 15-20 times) manual rearrangement of the electrodes when performing sensing at each of the observation points, carried out when performing probing in order to increase the depth of current penetration.

In this regard, in recent years, much attention has been paid to the creation of modifications that can significantly improve the performance of such work. One of the initial modifications, which is an analogue of the proposed method, was the method of continuous sounding (SEZ) [3], later renamed the method of 2D-electrotomography. Following this, a method of group soundings (GB) was created [4], combining a number of advantages of the SEZ method and the classical VES method.

The prototype of the proposed technical solution is the method of geoelectrical exploration for performing group sensing of the geological environment, based on the use of a multi-electrode installation in the form of a braid [4], which uses a system of paired electrodes located at a constant step along the observation profile, performing in the process of sounding successively as receiving and supply lines in the variant of a three-electrode installation, providing, in contrast to the SEZ method, the independence of setting the length of the receiving line MN о t step between the pickets, reducing, if necessary, the transient grounding of the supply line by supplying current to the ground by paired electrodes and increasing the density of observations due to the additional sounding obtained, in order to ensure the same maximum sounding depth at all pickets of the group electric sensing, use the method of oncoming three-electrode installations . Electrical sensing at each point is performed by sequentially connecting the supply electrode A with a given spacing increment step Δr = r _{j + 1} -r _j .

One of the drawbacks of this method is the increased complexity and duration of the measurement process, associated with the need for multiple (n-fold, n is the number of plant spacing) connection of the supply line source, which requires time to stabilize the generated current, which limits the possibility of using hardware systems that allow simultaneous registration of the generated electric field at all plant spacings during sounding at each picket. It can also be noted that its practical application is limited due to the use of one fixed length of the receiving line equal to the distance between the paired electrodes, which, as the installation spacing increases, can reduce the measured potential difference to a value below the permissible value (0.1-0.5 mV) [1, pr. 1 (p. 97)].

The objective of the invention is to eliminate the disadvantages of the prototype.

The problem is solved using the features specified in the 1st paragraph of the claims, common with the prototype, such as the method of geoelectrical exploration for performing group soundings of the geological environment, based on the use of a multi-electrode installation in the form of a braid, and distinctive essential features, such as for measurements, are used inversion multi-electrode installation, carry out a one-time registration of the potential difference on all the spacings of each measured sounding at a fixed position for I have a supply line AB, and to perform each sounding, measurements are made with two fixed lines AB, connected in series during the measurement: for the first one they use paired electrodes AB, and adjacent to the second size AB, connected when the signal level is lower than acceptable, adjacent to the paired electrodes are the two nearest neighboring electrodes, while for the extreme pickets use two additional electrodes at the ends of the measuring braid.

According to paragraph 2 of the claims, the shooting process is carried out automatically using a channel switching switch.

The above set of features allows you to get the following technical result - reducing the time of measurements, the efficiency of the study of the rock mass, expanding the interval of the examined depths, the possibility of creating hardware systems for performing simultaneous multichannel measurements for the group sensing method.

The proposed method is illustrated by graphic materials, where in FIG. 1 illustrates the essence of the principle of reciprocity. In FIG. Figure 2 shows the use of this principle to convert a traditional (a) into an inversion (b) measuring setup. In FIG. Figure 3 shows the features of the manifestation of near-surface inhomogeneities when using traditional (a) and inversion (b) installations. In FIG. 4 shows a diagram for performing group soundings using an inverse multi-electrode measuring device with the number of spacing for each sounding n ₁ = 5; C is the area corresponding to single measurements of a 3-electrode measuring installation; D is the area of overlapping measurements with the direct and opposite three-electrode installations (AB-M and M-AB), the total value of the measured potential values in this area is adequate to the results of applying the 4-electrode installation [2 (p. 39)]. In FIG. Figure 5 shows the results of verification experimental work: sections of apparent resistance obtained by group probing using traditional (a) and inversion (b) measuring devices. In FIG. 6 is a diagram of the measurement process using the continuous sounding method of traditional (a) and inversion (b) installations. In FIG. 7 shows the results of the interpretation of materials obtained by the method of continuous sounding: sections of apparent resistance for traditional (a) and inversion (b) measuring systems.

The new method proposed in this application is based on the use of the principle of reciprocity [5]. According to the reciprocity principle, a source placed at point A causes at the point M the value of the potential of the electric field U _MA , the same as the value of potential U _AM at point A, formed when the source is placed at point M, regardless of the nature of the spatial change in the physical properties of the medium ( Fig. 1). That is, with the mutual replacement of the supply and receiving electrodes (Fig. 1), the ratio

When performing measurements with a traditional setup (Fig. 2a), the measured value of the potential difference between points M and N is determined by the formula

In this case, the recording point corresponds to the position of the receiving electrodes MN [2].

Using the reciprocity principle (1), relation (2) can be rewritten in the form:

The resulting relation (3) corresponds to the measuring setup shown in FIG. 2b.

A comparison of relations (2) and (3) shows that the reciprocity principle (1) ensures that ΔU ₁ = ΔU ₂ . That is, instead of the measuring installation (Fig. 1), you can use its inverse analogue (Fig. 2).

This leads to two important conclusions:

1. In view of the independence of the principle of reciprocity from the structure and physical state of the geological environment, the manifestation of the heterogeneity located at the recording point should be identical when measuring with conventional and inverse installations. In particular, the manifestation of the P-effect, which is usually associated with the presence of local heterogeneity near the receiving line MN (Fig. 3a) [2], in the case of an inversion installation is identified with the position of this heterogeneity near the supply line AB (Fig. 3b). That is, the nature of the P-effect is not associated with the functional purpose of the electrode (receiving-feeding), but with the ratio of the sizes of the paired electrodes and their location relative to local inhomogeneity. A similar conclusion can be obtained for the well-known C-effect [2], which displays the presence of heterogeneity near a separate supply electrode.

2. In view of the complete identity of the traditional and inversion installations, the position of the recording point is preserved, which, in the case of an inversion installation (Fig. 3b), must correspond to the position of the paired supply electrodes AB.

All this justifies the possibility of creating a new modification of group soundings.

In FIG. 4 shows a diagram for performing group soundings using an inverse multi-electrode measuring installation with the number of plant spacing n ₁ = 5, where A, B are the supply electrodes; M, N _∞ - receiving electrodes; C is the area of single measurements with a 3-electrode measuring device); D is the region of overlapping measurements with the forward and backward three-electrode installations (the total potential value at each joint measurement gap by these installations corresponds to the potential values of the 4-electrode installation [2 (p. 39)]). As the electrode N _∞ , a receiving electrode is used, remote from the recording point by a distance approximately 5 times greater than the distance to the electrode M in the direction perpendicular to the axis of the installation.

General process conditions

The general scheme of the proposed method of group sensing using a multichannel inverse measuring installation (GIZ) is shown in FIG. 4. It displays a system of paired electrodes located evenly along the measurement line with a step Δx (Fig. 4a), which are used in the measurement process to form both supply (AB) and receiving (MN _∞ ) lines (Fig. 4b). Sensing at each picket is performed at a fixed position of the supply electrodes AB, with the measurement of the potential difference between the distance-variable receiving electrode M, in the range of a given number of spacings r _i (i = 1, n ₁ ), and a fixed electrode N _∞ . The difference r in this case is the distance from the center of the supply line AB to the receiving electrode M. As the active differences, depending on the required shooting detail, a set M is selected that includes either one of the paired electrodes or each of them (this is specified in the control program of the switch) . In this illustration, to simplify the disclosure of the essence of the survey, the use of one (the first along the way) of the paired electrodes is given.

при выполнении двух условий: AB≤2Δx и L=4Z_max.The length of the measuring switch with the number of spacing of the probe installation n ₁ and the distance between the pickets Δx (distance between the centers of the pair of electrodes) is L = 2n ₁ ⋅Δx + 2 (Δx-AB) (for an even number of spacings), or L = (2n ₁ - 1) Δx + 2 (Δx-AB) (in the case of an odd number of spacing). The distance between the paired electrodes corresponding to the length of the supply line AB is selected in accordance with the condition AB <2Δx. The maximum separation of the probe installation, equal to n ₁ ⋅Δr, provides the maximum probing depth Z≈0.5 n ₁ ⋅Δr maximum possible for this installation, which is the same at all points of the studied section. The number of plant spacing n ₁ is selected based on the required effective depth of sounding of the geological environment and the step between the pickets according to the ratio

when two conditions are met: AB≤2Δx and L = 4Z _max .

As a result of a group survey using a three-electrode setup, 2n ₁ (with an even number of spacing n ₁ ) is obtained, or 2n ₁ +1 (with an odd number of spacing) soundings.

In order to obtain information about the studied section to the maximum possible and the same for the entire set of group soundings of the studied depth, a measurement technique is used using counter-settings. In accordance with this technique, when performing soundings within the first half of the length of the streamer, a direct three-electrode installation AB-M is used, and after the last supply electrode M reaches the boundary of the measuring streamer, a reverse three-electrode installation M'-AB is used (Fig. 4b).

To maintain the magnitude of the variable signal within the permissible limits determined by the accuracy of the measuring equipment, an option is used, if necessary, to increase the signal level by increasing the size of the supply line AB, which, in the case of an inversion installation, is a functional analog of the receiving line (Fig. 2). To do this, it is planned to use two fixed ABs, connected in series during the measurement process: at the first (initial) stage, paired AB electrodes are used, and when the measured signal drops below an acceptable level, two adjacent electrodes adjacent to the paired AB electrodes are used. In this regard, for extra pickets, two additional electrodes are used at the ends of the measuring braid (electrodes 0 and 21 (Fig. 4a, b)).

The entire process of shooting the considered inverse measuring setup is performed automatically using a channel switching switch, providing the possibility of simultaneous registration of the set of values of the recorded signal ΔU at given spacings of electric sensing.

) содержат информацию, соответствующую как области одинарных измерений 3-электродной измерительной установкой (область С), так и области двойных измерений, полученных прямой и встречной трехэлектродными установками (область D). Суммарное значение измеренных значений поля, во втором случае, соответствует результатам измерений 4-электродной установкой. Совокупность этих параметров и их сравнительный анализ дают возможность заверки качества получаемых материалов и расширения информационных возможностей метода за счет оценки степени латеральной неоднородности изучаемой среды и выделения локальных неоднородностей [1 (с. 101)].The resulting values of the electric field (potential difference

) contain information corresponding to both the region of single measurements by a 3-electrode measuring device (region C) and the region of double measurements obtained by direct and counter three-electrode installations (region D). The total value of the measured field values, in the second case, corresponds to the measurement results of the 4-electrode installation. The combination of these parameters and their comparative analysis make it possible to verify the quality of the materials obtained and expand the information capabilities of the method by assessing the degree of lateral heterogeneity of the medium under study and highlighting local heterogeneities [1 (p. 101)].

The advantages of the proposed method compared to the prototype include:

1) increasing the efficiency of shooting due to the possibility of simultaneous recording of the measured signal using multichannel equipment on all the spacings of the sounding performed at a fixed position of the supply line AB, compared with the prototype, which requires the serial connection of the supply electrodes on each of the spans, and associated with this increased ( n ₁ -1 times, where n ₁ is the number of plant spacing) time spent on stabilization of the current supplied to the ground;

2) the ability to maintain the value of the recorded signal within acceptable limits by increasing the size of the AV inverse installation, providing an increase in the sounding depth;

3) the ability to perform sensing using paired electrodes as supplying AV and receiving MN lines used in the sensing method with a dipole measuring setup [1, book. 1 (p. 96-97)].

It should be noted that the use of the proposed version of the inverse measuring installation expands the functional and economic capabilities of sounding, while maintaining the basic previously justified advantages of its prototype [4].

In addition, the proposed method allows to increase the efficiency of the application of a number of analogues of this method, due to the following factors: a) the possibility of obtaining and step-by-step control (visual or numerical) in the process of shooting by the SEZ method of each of the obtained sounding schedules, allowing for rapid identification and elimination reasons for a possible failure of the measured values (Fig. 6a, b); c) there is no need to create and use special programs for converting observed data obtained by the method of continuous sounding into graphs of sounding curves (VES); d) the possibility of interpreting materials obtained by the SEZ method directly using the PROBE program system [2].

The proposed method of group sensing with a multi-electrode inversion installation allows you to quickly and with the lowest labor costs carry out the following types of studies: a) study of an array of rocks lying in the depth interval from the earth's surface to the first hundreds of meters; b) the implementation of sounding of the geological environment in a confined space (mine work); c) performing advanced reconnaissance ahead of the bottomhole space; d) conducting highly accurate monitoring observations within the profile section corresponding to the length of the streamer of the measuring installation, etc.

For processing and interpretation of research results, the "PROBE" program system can be used (RF certificate No. 2005610058).

The method was tested in solving hydrogeological problems in one of the sections of the Perm region.

LITERATURE

1. Electrical Exploration: A Handbook of Geophysics. In 2 kn. / Ed. VK. Khmelevsky and V.M. Bondarenko. Prince I, M .: Nedra, 1989, p. 95-110, 174-177.

2. Kolesnikov V.P. Fundamentals of the interpretation of electrical sounding. - M .: Scientific World, 2007 .-- 248 p.

3. Bobachev A.A., Modin I.N., Pervago E.V., Shevnin V.A. Multi-electrode electrical sensing in conditions of horizontally heterogeneous media. Exploration Geophysics. - M., 1996. 50 p.

4. The method of geoelectrical exploration. Patent for invention No. 2545309, February 24, 2015 (prototype).

5. Tikhonov A.N., Samarsky A.A. Equations of mathematical physics. - M .: Nauka, 1977 .-- 736 p.

6. RSN 64-87. Electrical exploration in construction.

Claims (1)

The method of geoelectrical exploration for performing group soundings of the geological environment based on the use of a multielectrode installation in the form of a braid, characterized in that the measurements use an inverse multielectrode installation, carry out a simultaneous recording of the potential difference on all the spacings of each sensing being measured at a fixed position for the supply line AB for it, and for each sounding is measured with two fixed lines AB connected in series to Measurement process: for the first of them, paired AB electrodes are used, and as the second size of the AB, connected when the signal level is lower than acceptable, the two nearest adjacent electrodes adjacent to the paired electrodes, while for the extra pickets, two additional electrodes are used at the ends of the measuring switch.

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