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

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 UAMN and ΔUA'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.

1 tbl, 1 dwg

 

The present invention relates to an electrical exploration method of electrical resistivity and improves the efficiency study of the upper part of geological section and identify local heterogeneities in subsurface formations, and in the bedrock.

The field of application of the proposed method: the mapping of the geological environment for engineering-geological investigations; detection zones tectonically fragmented, permeable rocks; identifying conductive (ore) of the objects covered by unconsolidated sediments; the study of the state of ground engineering structures and other

The known method of vertical electrical sounding (VES), which uses four ground located on a single line profile observations). Two of them - foster (M, N), each of which is separated at equal distances on opposite sides of the center of a plant (point observations), and is connected to the measuring device. Two other ground - feeding (a, b) is placed on the same (given) distance along profile observations of the installation center and connected to the terminals of a source of electric current. When one accommodation supply grounding measure the voltage drop ΔUMNbetween receiving the ground (Fig 1,a). Then feeding ground displacement which are stated on the following specified distance from the center of the installation, the measurement process is repeated and so On the results of electrical sensing calculate values of apparent electrical resistance of the environment for each position of the supply grounding (a, b) and for changes in the electrical resistance depending on the distance between the supply grounding judge geoelectric structure of the environment [1].

The known method has drawbacks: it is intended to explore the horizontally layered media, so when inhomogeneous structure of the studied geological section, and especially in the subsurface part of the interpretation of the obtained experimental data is ambiguous; the presence of a non-conductive layer in the geological cross section complicates the study of the lower horizons.

There is a method of electrical sounding with unipolar installation (method fields overlay), in which two feeding (a, a/and two foster (M, N) grounding placed on the same line (the profile) so that ground (a) and (M) are on the one hand, and (A/and (N) on the other hand, at a given distance from the center of the installation (point observations), and another feeding ground (In) referred to practical infinity and is connected to one terminal of a source of electric current. The other two supply grounding (a, a/) are connected to the other terminal of the source ELEH the electrical current and with the help of a special device is the reduction to a common value of electric currents, flowing from the feeding ground (a) and (A/). Foster grounding (M, N) are connected to the measuring device. When the first position of the supply grounding (a, a/) is measured by the voltage drop ΔUMNbetween receiving the ground (Fig 1,b). Then ground (a) and (A/) move at the same predetermined distance from the center of the installation, the measurements are repeated and so on [2].

In a homogeneous and horizontally layered medium values of the voltage drop ΔUMNis equal to zero. If the geological environment heterogeneity, different electrophysical properties from the surrounding rocks, the graphs ΔUMNprofile observations are recorded alternating extremes of electrostatic voltage zero-crossing [3].

The known method has the following disadvantages: first, it requires a very precise maintaining the same value of electric current flowing from the supply ground (a) and (A/in the production process of measurement; secondly, the instrumentation must be capable of determining the sign of ΔUMN; thirdly, the anomalous effect as the above conductive and non-conductive heterogeneity, and therefore difficult to determine the nature of the detected resistivity of the object.

Goal by offering the second method is to increase the efficiency of detection of the geoelectric inhomogeneities in the geological environment and the reduction of ambiguity in the interpretation of experimental data by combining three-electrode, vertical and unipolar electrical soundings; increase productivity, as it does not require to method unipolar sensing during each measurement to maintain the same current flowing from the supply earthing; the possibility of using any electrical equipment intended for methods of resistance.

This objective is achieved in that the method is used two three-electrode soundings, in which the profile of observations placed four grounding on one line symmetrically about a point observations coincident with the center of the installation, and fifth grounding refer to practical infinity" and is connected to one terminal of a source of electric current, the Central ground is connected to measure voltage, when performing measurements to the other terminal of the source of electric current alternately connected at the supply ground and measure between receiving the grounding voltage drop ΔUAMNand ΔUA MNafter making the measurements at the same position at the feeding ground of their move at the same given distance from the center of the installation and the measurement process is repeated, perform these operations for a given provisions feeding ground, then measured the voltage drops the La two three-electrode soundings calculate at each point observations at given spacings drop voltage for vertical electrical soundings ΔU VESand the drop voltage for unipolar sensing ΔUMNPon the measured and calculated voltage drop is determined by the distribution of apparent electrical resistance in sections two and three-electrode vertical soundings, as well as the distribution in terms of the voltage drop for unipolar sensing and the results of the soundings is judged on the presence and location of the incision geoelectric inhomogeneities.

Figure 1,shown in the setup diagram for three-electrode sensing. The signal ΔUAMNmeasure when used as a feeding ground (a) and (B). The signal ΔUA MNmeasure when used as a feeding ground (A/and (B).

The proposed method is carried out with electrical equipment designed for electromagnetic research (e.g., ERA, ERA-SIGN, ERA-MACH, or foreign counterparts working for DC or low frequency AC current), as follows. On the profile observations place four grounding (A,M,N And/on the same line symmetrical relative to the point of observation (O), coincident with the center of the installation (figure 1)and the fifth ground (In) refer to "infinity" and is connected to one terminal of a source of electric current. Ground (M) and (N) is connected to the meter is elektricheskogo voltage. When performing measurements to the other terminal of the source of electric current alternately connect the supply ground (a) and (A7/), measured between foster grounding (MN) voltage drop ΔUAMNand AUA MNrespectively. After performing the measurements at one position of the feeding ground (a) and (A/their move at the same given distance from the center of the installation (On) and the measurement process repeated. These operations repeat for all desired positions of the feeding ground. Thus, at one point observations receive two curves sensing, corresponding to two three-electrode units AMN (→∞) and A/MN(B→∞) [4].

On the measured voltage drops to two three-electrode soundings calculate at each point at each spacing:

where ΔUVES- drop voltage for vertical electrical soundings, ΔUMNP- drop voltage for a single installation (method fields overlay).

To justify the expressions (1), (2), we present the measured voltage drop:

In the method of the vertical electric soundings voltage drop on the ground (M) and (N) when connecting Zaza is of (A) positive, while (A/) to the negative terminal of the source of electric current is calculated by the formula:

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

In those cases, when are modular measurement of the voltage drop at the receiving ground (M) and (N), the formula (6) should be modified taking into account the signs of terms included in the expression. Since the magnitude of the potential is inversely proportional to the distance to the point of observation, i.e. to ground (M) and (N), it is always ΔUAMN>0, since UAM>UANaccordingly ΔUA MN<0 and the formula (6) using modular measurement is converted to the form:

In the method of superposition of fields (MNE) is a unipolar setting so that the two power supply ground (a) and (A/) is connected to one terminal of the current source, for example to the positive, and the third (negative) referred to "infinity". Thus the voltage drop across the grounding (M) and (N) is calculated by the formula:

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

When implementing modular measurement voltage drops at the receiving ground (M) and (N) formula (9) is converted in the same way as formula (6), taking into account the signs of terms that contain the x in the expression:

.

Obviously, for a uniform conducting half-space ΔUMNP=0. In the presence of geoelectric inhomogeneities ΔUMNPcan be both positive and negative, reflecting the features of the distribution of electrophysical parameters of the environment.

In the surveyed profile observations by the values of (ΔUAMN), (ΔUA MNand (ΔUVES) calculate values of apparent resistivity (ρtoand build incisions ρtofor three-electrode and four-electrode symmetrical (VES) installations, as well as a slit (ΔUMNP). In the section of the drop voltage obtained from the results of measurements with single-unit (ΔUMNP)allocate geoelectric inhomogeneities, and the sections of apparent resistivity determine the conductivity inhomogeneity (increased or decreased compared with the host environment), clarify the morphology of the object and perform quantitative interpretation of the results of vertical electrical sounding.

Figure 2 is a table of values of the potential difference obtained on one of the core points of the composite soundings for carrying out the experimental work. The soundings were performed with polarsnake (r=AA//2) from 1.5 to 15 metro station is proposed in the application method. In addition, to control when these separations were additionally performed vertical electrical sounding using the standard method [1], in which the potential difference (ΔUMN) practically coincide with the differences of potentials obtained when the composite soundings ΔUVES(figure 2, column 4 and 6).

The advantage of the proposed method is to increase the efficiency of detection of the geoelectric inhomogeneities, as in the upper part of geological section and indigenous geological formations due to the complexation of the four methods electrical sensing increasing resistivity informative studies at high productivity.

The method of combining three-electrode, vertical and unipolar sensing method uses two three-electrode soundings, in which the profile of observations placed four grounding on one line symmetrically about a point observations coincident with the center of the installation, and fifth grounding refer to practical infinity" and is connected to one terminal of a source of electric current, the Central ground is connected to measure voltage, when performing measurements to the other terminal of the source of electric current alternately connect the Ute at the feeding ground and measure between receiving the grounding voltage drop ΔU AMNand ΔUA MNafter making the measurements at the same position at the feeding ground of their move at the same given distance from the center of the installation and the measurement process is repeated, perform these operations for a given provisions feeding ground, then measured the voltage drops for the two three-electrode soundings calculate at each point observations at given spacings drop voltage for vertical electrical soundings ΔUVESand the drop voltage for unipolar sensing ΔUMNPon the measured and calculated voltage drop is determined by the distribution of apparent electrical resistance in sections two and three-electrode vertical soundings, as well as the distribution in terms of the voltage drop for unipolar sensing and the results of the soundings is judged on the presence and location in the context of geological heterogeneities.



 

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