RU2018885C1 - Electric geosurvey method - Google Patents

Abstract

FIELD: geophysics. SUBSTANCE: method involves the steps of: placing pairs of power supply electrodes AB and receiving electrodes MN on a profile being studied, fixing electrode N in the middle of a power supply line, moving second receiving electrode M along the profile being studied between the power supply electrodes and finally determining a structure of the investigated portion from an increment in potential difference between pairs of points equidistant from electrode N and points equidistant from each other. EFFECT: enhanced accuracy. 1 tbl, 6 dwg

Description

 The invention relates to geophysics and can be used when conducting prospecting, surveying and mapping geological and engineering-geological works on the surface of the earth in areas with horizontal or inclined boundaries, including complicated vertical and steeply varying heterogeneities such as cores, dikes, contracts, faults or overthrusts using direct and alternating current.

 A known method of geoelectrical exploration, which allows to obtain information about the geological structure of the site, based on a change in the position of the supply electrodes A and B relative to a fixed receiving line [1].

 The known method allows to obtain information about the horizontally layered structure of the section, however, profile heterogeneities, for example, subvertical structures that complicate the geological structure of the section under study, due to the significant averaging effect of the installation (the research results are averaged over a large volume), are practically not detected. Further, their identification is necessary to conduct additional research using electroprofiling methods.

A known method of geoelectrical exploration [2], also providing for measurements when changing the position of the electrodes of the internal line relative to the external, but the ratio l _AB / l _MN can tend to unity.

 This method of sensing does not require a device "overlap", and therefore has greater performance. However, this method, due to the large averaging, is not intended to detect electrical heterogeneities related to the horizontally layered structure of the studied section.

 The aim of the invention is to obtain more detailed information about the geological structure of the research site.

 This is achieved by the fact that one of the electrodes of the receiving line is fixed approximately in the middle of the supply line; another, the movable electrode of the receiving line is moved between the electrodes of the supply line, the values of the apparent specific electrical resistances are calculated using increments of the potential difference between points equidistant from the supply electrodes and points located at a predetermined step distance from each other.

In FIG. 1 shows a diagram of a measuring installation, with A and B being the electrodes of the supply line, N is the stationary electrode of the receiving line, M ₁ , M ₂ , M _{1 '} and M _2' are the partial positions of the electrode M of the receiving line, G is the current generator, P - measuring device;
figure 2 shows the sounding curve ρ _to = f (r _g ) by the proposed method (solid line) and by the prototype method (dashed line);
Figures 3-5 are graphs ρ of the mid-gradient method, wherein the solid line is calculated according to measurements by the present method, the dashed lines are calculated according to predetermined measurements by the SG method, letters a, b, c correspond to curves with values MN = 20, 70 and 100 m;
in FIG. 6 - geoelectric section, built according to the measurements of the claimed method.

-

, где

- потенциал, создаваемый электродом А в точке 1;

- потенциал, создаваемый электродом В в точке 1.The total potential at point 1 on the profile (taking into account the different polarity of electrodes A and B) will be: Ψ ₁ =

-

where

- the potential created by electrode A at point 1;

is the potential created by electrode B at point 1.

-

, где

- потенциал, создаваемый электродом А в точке N;

- то же, но создаваемый электродом В в точке N.The total potential at the point where the electrode is located is:
Ψ _N =

-

where

is the potential created by electrode A at point N;

- the same, but created by electrode B at point N.

-

)- (

-

) = Ψ₁-Ψ_N (1)
Аналогично, измеренную в точке 2 разность потенциалов можно выразить следующим образом:
ΔU₂= (

-

)- (

-

) = Ψ₂-Ψ_N (2)
Рассуждая подобным образом, можно аналогично представить измеренную разность потенциалов в любой точке профиля.The measured potential difference when the electrode is at point 1 will be:
ΔU ₁ = (

-

) - (

-

) = Ψ ₁ -Ψ _N (1)
Similarly, the potential difference measured at point 2 can be expressed as follows:
ΔU ₂ = (

-

) - (

-

) = Ψ ₂ -Ψ _N (2)
Arguing in this way, one can similarly represent the measured potential difference at any point in the profile.

-

)- (

-

) = Ψ_1′-Ψ_N (3)
Рассмотрим величину приращения разности потенциалов при перестановке электрода из точки 1 в точку 1'. Она равна разности величины Δ U₁ и Δ U_1':
Δ Δ U_1-1'= Δ U₁- Δ U_1'=Ψ₁-Ψ_1', (4) т.е. величина Δ Δ U_1-1' равна разности суммарных потенциалов в точках 1 и 1'. Эту величину можно было бы измерить, если поместить один приемный электрод в точку 1, а другой - в точку 1'.Consider the case when the movable electrode M is located at points 1 and 1 'equidistant from the electrodes A and B. The potential difference measured when the electrode M is at point 1' will be:
ΔU _{1 ′} = (

-

) - (

-

) = Ψ _{1 ′} -Ψ _N (3)
Consider the increment of the potential difference when moving the electrode from point 1 to point 1 '. It is equal to the difference between Δ U ₁ and Δ U _{1 '} :
Δ Δ U _{1-1 '} = Δ U ₁ - Δ U _1' = Ψ ₁ -Ψ _{1 '} , (4) i.e. the value Δ Δ U _{1-1 '} is equal to the difference of the total potentials at points 1 and 1'. This value could be measured if one receiving electrode was placed at point 1, and the other at point 1 '.

When placing the electrode M at points 2 and 2 ', also remote at the same distance from the electrodes A and B, making measurements and subsequent calculations similar to those given above, we obtain:
Δ Δ U _{2-2 '} = Ψ ₂ -Ψ _2' , i.e. the calculated increment of the measured potential differences is equal to the difference of the total potentials at points 2 and 2 '.

Performing further calculations for a series of points equidistant from electrodes A and B, as these points approach the center of the apparatus, a set of values Δ Δ U _{i-i '} can be obtained.

(5) (здесь К - коэффициент установки, которая была бы, если бы электроды приемной линии помещались в те точки профиля, для которых рассчитывались величины Δ Δ U), то на половине длины линии АВ в ее центральной части будем получать информацию, аналогичную информации, получаемой при съемке способом срединных градиентов.The proposed method allows, in addition to electric sounding information, to obtain more detailed information about the geological structure of the site, since if we calculate the Δ Δ U values between points located on the profile at a distance of a preselected step from the Δ Δ U values taking into account the geometric coefficient K and the current value I the supply line to calculate the value of the apparent electrical resistivity according to the formula:
ρ _k = K

(5) (here K is the coefficient of installation, which would be if the electrodes of the receiving line were placed at those profile points for which Δ Δ U values were calculated), then at half the length of the AB line in its central part we will receive information similar to information obtained when shooting the method of mid-gradients.

In fact, the value Δ Δ U _1-2 in accordance with the above notation will be: Δ Δ U _1-2 = Ψ ₁ -Ψ ₂ , and Δ Δ U _2-3 = Ψ ₂ -Ψ ₃ , etc.

But Δ Δ U _1-2 , Δ Δ U _2-3 , Δ Δ U _3-4 , ... are values similar to those that could be obtained by direct measurements if the electrodes of the receiving line were placed at points 1 and 2, 2 and 3, 3-4, etc. Obviously, the values of ρ _k calculated by formula (5) are similar to those that could be calculated from the survey of mid-range gradients.

 Thus, the implementation of the proposed method provides information similar to the previously known method, allowing to study the horizontally layered structure of the geological section of the research site, as well as to obtain information about the presence in the section of electrical inhomogeneities that are not related to the horizontally layered structure, i.e. the method provides, in comparison with known methods, more detailed information about the geology of the site.

 The proposed method is characterized in that one of the electrodes of the receiving line is stationary installed approximately in the middle of the fixed supply line AB, and the other is moved along the observation profile between the supply electrodes, and the values of the apparent resistances are calculated using increments of the potential difference between points located at a distance in advance from each other the selected step.

 The method is as follows.

On the site to be studied, observation profiles are divided, as a rule, across the supposed electrical heterogeneities of the subvertical location. On the observation profile set the supply line AB. The distance between the electrodes A and B is chosen taking into account the necessary research depth h. As a rule, the value of l _AB must satisfy the condition:
l _AB ≥ 3h.

 Approximately in the center of the AB arrangement, the electrode of the receiving line N is stationary installed. The electrode M of this line is placed on the observation profile at a point remote from the electrode A by the distance of the selected shooting step. The potential difference (Δ U) between the electrodes M and N is measured as the electrode M moves along the profile in the direction of the electrode B.

 The displacement step is determined by the estimated dimensions in terms of electrical heterogeneities of the subvertical location. When choosing a step, they are guided by the generally accepted approach in geophysics and, in particular, in electrical exploration, based on the theory of exploration networks: there should be at least three measurement points within the heterogeneity range. At the end of the measurements, increments of the measured values of Δ U are calculated when the electrode M is placed both at points equidistant from electrodes A and B, and at points within the middle half of the arrangement of the supply line AB, the distance between which is equal to a preselected step.

, где К - значение геометрического коэффициента: в первом случае - установки симметричного четырехэлектродного зондирования, а в другом случае - установки способа срединных градиентов;
I - величина силы тока в питающей линии.According to the calculated values of Δ Δ U calculate the value of ρ _to the formula: ρ _to = K ·

where K is the value of the geometric coefficient: in the first case, the installation of a symmetric four-electrode sounding, and in the other case, the installation of the method of mid-gradients;
I is the magnitude of the current in the supply line.

The dependence ρ _к = f (r _g ) (here r _g is the effective spacing of the four-electrode installation of electric sensing, with which values equal to Δ Δ U could be measured), is used to quantify the interpretation of electric sensing, the final output of which is the idea of horizontally stratified geological structure section within the sample portion, and curves along the ρ _to observations profile within the middle half of AB arrangement, calculated at a predetermined pitch, are interpreted by methods analogue chnymi methods for interpreting the data recording median gradients to obtain information about the electrical inhomogeneities subvertical and vertical laying.

 PRI me R. The study on the proposed method was carried out in the city of Stebnik, Lviv region.

 A geophysical profile with a length of 440 m in the west-east direction has been laid in the mine field of the Stebnikovsky Potash Plant (STKZ). In this case, picket 0 was placed at a distance of 50 m west of well N 217 t, picket 440 - 66 m east of well N 136 s. The geophysical profile is laid in such a way as to cross the salt erosion zone in underground mines, the width of which is approximately 30 m.

 The measurement setup diagram is shown in FIG. At picket 0, electrode A was installed, electrode B was installed on PC 440. The central measuring electrode N was placed on PC 220. The electrode M was moved along the profile in 10 m increments from PC 20 to PC 420. The line AB was supplied with a constant current equal to 400 mA from the generator ANCH-3 (stationary). Measurements are given using an AN4-3 microvoltmeter, and are also duplicated by a higher class meter (Щ-4300). At the end of the work according to the claimed method, measurements were performed with a symmetrical AMNB installation, as well as with conventional mid-gradient settings, with different lengths of the MN line, which in this case were 20, 70 and 100 m. The area in which measurements were made with a standard mid-gradient installation was limited pickets NN 110-330, as this is a line within 1/2 of AB length.

 Having installed electrode A on PC 0, and B on PC 440, electrode N on PC 220, and M on PC 20, the generator was turned on, the current value and the potential difference were recorded on the pair of measuring electrodes M and N.

 Then, the electrode M was transferred sequentially to new points with a step of 10 m, and the field excitation and potential difference measurements were repeated.

The table shows the results. The current strength at all measurement points is 400 mA. This table is the main document in the calculation of the apparent electrical resistivity (ρ _k ) for symmetric electrical sounding installations, as well as for the calculation of mid-gradient settings, similar to those generally accepted for the sizes of measuring lines with lengths of 20, 70 and 100 m. The choice of the size of the measuring line of the SG method is not is of fundamental importance, it is associated only with the task of dismembering the geoelectric section in plan; this size can fluctuate from the minimum equal to the measurement step, i.e. 10 m, and reach a reasonable maximum, which is advisable to choose as half the size of the measurement area, i.e. somewhere around 440/2/2 = 110 m. Thus, the dimensions MN = 20, 70 and 100 m selected for subsequent calculations almost cover the entire possible range from 10 to 110 m.

=

, где АМ_i - разнос, определяемый взаимным расположением измерительного электрода M_i относительно питающего электрода А, м;
BM_i' - разнос, определяемый взаимным расположением измерительного электрода М' относительно питающего электрода В;
Δ Δ U_i - разность разностей потенциалов, зафиксированных на паре электродов М и N при симметричных положениях электрода М относительно электродов А и В, мB;
I - величина стабилизированного тока, мA, при этом i изменяется от 1 до максимального числа разносов переменной измерительной линии.Based on the data of the potential differences presented in the table, the calculations of the differences of the potential differences (Δ Δ U _{i-i '} ) for variable values of length MN were made, which in this case amounted to 400, 380, 360, 340, 320, 300, etc. up to 20 m inclusive. According to the obtained values of Δ Δ U, the calculation of the values of the apparent specific electrical resistance by the formula

=

where AM _i is the spacing determined by the relative position of the measuring electrode M _i relative to the supply electrode A, m;
BM _{i '} is the spacing determined by the relative position of the measuring electrode M' relative to the supply electrode B;
Δ Δ U _i - the difference of the potential differences recorded on a pair of electrodes M and N with symmetrical positions of the electrode M relative to the electrodes A and B, mB;
I is the value of the stabilized current, mA, while i varies from 1 to the maximum number of spacing of the variable measuring line.

The results of calculating the values of ρ _k according to the above formula are presented in FIG. 2, where the curve ρ _k as a function of the actual separation (ract.) Is shown by a solid line. In figure 2, the dotted line shows the curve ρ _{to the} symmetric electrical sounding.

Figure 2 shows that the discrepancy between the values of ρ _k calculated according to the measurement data by the known method does not exceed ± 5%, i.e. satisfy the accuracy of field observations.

=

·

.The calculation of the apparent electrical resistivity (ρ _to ) for the settings of the mid-gradient using the measuring lines MN of different lengths by the present method is also made on the basis of the measurement data presented in the table. The calculation of the ith values of ρ _{k is} made according to the formula

=

·

.

The distance between points M _i and M _{i + 1} is equal to the distance of the selected separation (M _i M _{i + 1} = MN); AM _i and BM _i are the distances between the i-th measuring electrode and the supply electrodes A and B, respectively, m;
AM _{i + 1} , BM _{i + 1} - the distance between the i + 1 measuring electrode and the supply electrodes A and B, respectively, m;
Δ Δ U _i - the difference of the potential difference recorded by the installation at points i and i + 1, respectively, mV;
I is the stabilized current, mA.

A graphical representation of the graphs ρ _{to the} mid-gradient method according to the measurement results of the present method is presented in figure 3-5 (solid curves).

Figure 3-5, the dotted line shows graphs ρ _{to the} mid-gradient method, performed according to the known method, with dimensions AB = 440 m and MB = 20 m (Fig. 3), MN = 70 m (Fig. 4) and MN = 100 m (figure 5).

 The discrepancy between the SG graphs made according to the generally accepted methodology and using the data obtained when applying the proposed method does not exceed ± 15%, at anomalous points ± 5% at points of the normal field, which corresponds to the requirements of field accuracy.

Based on a series of measurements according to the claimed method, carried out while moving the installation along the geophysical profile, a geoelectric section is constructed, which is shown in FIG. 6, in which layer-by-layer definitions of electrical parameters (layer capacities and their electrical resistivity) are performed based on ρ _k calculations for symmetric soundings with using the value Δ Δ U, and the profile inhomogeneities of steeply dipping (≥ 80 ^о ) are selected according to ρ _k calculations by analogy with the SG method using also the Δ Δ U.

 After making measurements according to the claimed method, you can get information similar to the totality of several methods of electrical exploration. This means that the claimed method in comparison with the known allows you to get more detailed information about the geological structure of the site.

 The results of the testing of the proposed method allow us to conclude that with its help it is possible to increase the productivity of electrical exploration no less than 2 times.

Claims (1)

 METHOD OF GEOELECTRIC EXPLORATION, which consists in placing pairs of receiving electrodes between pairs of supply electrodes on the observation profile, measuring the potential difference in the receiving line when changing the distance between its electrodes and calculating the apparent specific electrical resistivity of the layers, characterized in that one of the electrodes of the receiving line is fixed in the middle of the supply line and the second is moved within the entire distance between the electrodes of the supply line with a step specified by the detail of the studies, determine the increment the potential applied between the pairs of points equidistant from a fixed supply electrode, and the pairs of points located at equal distances from one another, wherein on the potential difference between the last further judged on the presence of sub-vertical inhomogeneities.

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