SU871035A1 - Method of geoelectric prospecting - Google Patents

Description

The invention relates to methods for electrical prospecting of ore deposits with alternating current, based on the polarization of geological media. The known frequency method of phase measurements of the effects of induced polarization, based on the study of the phase frequency characteristics of the electric field created by grounded current sources. The fields of polarization and induction origin are separated on the basis of the difference in the shape of their frequency characteristics. Phase frequency characteristic tics (PFH) of VP fields have one or several extremes and flat low-frequency and high-frequency branches. The position of the extremes on the frequency axis gives information about the geological features of polarisation objects. The phase frequency characteristics of the induction fields in the region of the small parameter of the electromagnetic field (low frequencies) directly proportionally depend on the frequency. Using the difference in the slope of the phase response of the fields, induction effects are excluded from the measurement results by converting the signals measured on two or three frequencies by a special SP algorithm. The disadvantage of this method is that it is applicable only in the field of relatively large parameters of the floor. There is a known method of separation of polarization and induction effects, in which the phase frequency characteristic of the total field, measured by a grounded source fed by harmonic current 21, is measured. To increase the depth and resolution of the WWP method, the relative position of the receiving and feeding lines is changed to form sharp or obtuse angles between them. The angle between the lines is changed until the appearance of the phase frequency characteristic changes, and the presence of polarizing objects is judged by the low-frequency asymptote. The disadvantage of this method is its low resolution in the high-frequency region. This is due to the fact that the method implies the use of relatively large distances to a mezkdu feeding and receiving electrodes, at which induction effects are also large. The convergence of the electrodes in order to eliminate induction leads to a decrease in the depth of research due to an increase in the contribution along the l produced by the containing medium to the measured total phase shift. This allows the method to be applied when studying frequency characteristics in a wide range of frequencies at the stage of detailed study of the anomalies revealed in order to assess their geological nature. The closest in technical essence to the invention is a method in which an approximately potential installation is used, in which the receiving electrodes are located on opposite sides of the supply electrode, one of the receiving electrodes is located in the center of the installation, and the ratio ng is strictly fixed. This installation provides partial compensation of the primary field and the receiving line 3. However, one of the receiving electrodes is located in the center of the supply line, where the potential of the field is zero. In order for the potential difference in the receiving line to be close to zero (this determines the compensation of the field and the weakening effect of the surrounding medium, the second receiving electrode needs to be removed to infinity , which increases the signals - interference induced in the receiving line, and complicates the technology of work. In addition, the PMV / AM ratio is fixed. Fixing the parameter n does not allow the position of the receiving electrodes to be changed during measurements depending on the power supply, which does not provide a deep compensation of the primary field. Addition of amplitude phase measurements using known installations does not weaken induction effects in the AC method, the purpose of the invention is to increase the measurement resolution when emitting geological features of polarizing objects The goal is achieved by the fact that, according to the proposed method of geoelectromagnetic exploration according to the method of induced polarization during the excitation of the primary field By varying the source and measuring the amplitude-phase characteristics of the total field, when the relative position of the supply and receiving electrodes changes, the relative position of the receiving and supply lines changes until the total field phase in the phase frequency characteristic passes through the value of the set of additional amplitudes. phase measurements judge the geological features of polarizing objects. To clarify the theoretical background of the method, let us consider an idealized model of a geoelectrical section consisting of a uniform polarizing half-space with an inclusion in the form of a polarizing object that does not differ in resistivity from the enclosing medium. Place one of the supply electrodes near the object and assume that the frequency of the alternating current is so low that the electric field can be considered quasi-potential. The polarizability of the environment, we take into account through the complex resistance:: -Pu. 5u (-H where o are respectively the polarizability and resistivity of the medium; i; is the time constant of the process of polarization of the medium; and is the frequency of the field. On the surface of the half-space in the vicinity of one of the supply electrodes, the polarity of the other electrode is neglected by the expression 0 (1-1, U6n) where RelJen-wUgfT is real and imaginary components of the field of EP created by a local object, depending, in general, both on the value of R and on the frequency of the field w, E is the current in the supply line We place the receiving electrodes on the same line pitakhdim electrode on opposite sides of it at distances R and RU total voltage in the receiving line can be represented by the formula, 1l.g) Cf, - CeiUen-i b4n (-) + ReflU6n-i 3niuUen + uJr3j. The phase shift of the total voltage relative to the current is determined from the expression 1 4 21T: jpo (-5--) + ReuU6Ti RI 9. Consider the denominator of expression (2). The first term in the denominator is determined by the geometrical parameters of the installer (1 and RQ) and can be both positive and negative with an arbitrarily small number. The second term depends on both R and Rg: and the frequency of the sex. Can pick up

values of fixed frequency u / g, which

a3i3po (.) 4ReuUBn 0, i.e. (4) N

C 1

T

When you turn off the frequency of the floor from

Schr to a greater or lesser side, the frequency characteristic of the phase of the total voltage, monotonically changing, passes through the value - J (due to the dependence of the CPU Rely on the frequency).

It is easy to show that in the frequency range, where K - is close, the voltage component of the voltage component created by the local object is much higher than the imaginary voltage component associated with the containing medium. For rough estimates, we accept 1x 0.02, - 1 for intervening and inducing rocks, and for a local object: yes & Taking into account expression (3) and the assumptions made, we will rewrite expression (1)

(yi |: -il1 & iyp -0101 "vyi5, p-, 0 (1-3 (plvt) From here it can be seen that the measured voltage at - is equal to the imaginary component of the voltage field of the VP from the local object with an accuracy of 0.02%. Change the real and imaginary component of the field of EP does not increase the indicated error from the frequency; therefore, the frequency response of J nuUgf is determined mainly by electrochemical processes associated with the local object under study.

In a homogeneous half-space, it is sufficient to ensure geometric symmetry in the location of the receiving electrodes with respect to the power supply, in order to obtain a phase shift of magnitude, since the lines of equal potential of the point source of current are circles. Under real conditions, the symmetry of the potentiostatic field is usually violated, which requires performing special operations aimed at determining on the ground such a position of the receiving electrodes near the supply electrode, at which the phase frequency characteristic of the total voltage goes over.

With the changes in this method, the following operations are carried out.

1. In the ground, the primary electric field is set using a source in the form of a grounded two-electrode line fed by a harmonic current, with one of the (main) electrodes located within the studied anomalies of the EAP.

2. Position the receiving electrodes on opposite sides of the main supply electrode, preferably in line with it.

3. Measure the phase frequency characteristics of the total voltage

in the receiving line when the position of the electrodes changes.

. 4. Find the position of the receiving electrodes, at which the phase frequency characteristic of the cyan voltage in the usable frequency range passes through the value-.

5. When the position of the electrodes is reached, an additional amplitude frequency response is measured.

From the totality of amplitude and phase measurements at such a position of the electrodes, when the phase frequency response goes over the value of r, the geological features of the polarizing objects are judged.

five

Operation 4 provides in the receiving line compensation for the voltage associated with the containment medium., Which allows the receiving electrodes to be brought closer to the power supply and they are influenced without any loss of fluorescence from the unloaded polarizers. j

With the found optimal placement of electrodes, it is possible to construct the frequency response of the imaginary component, for example, at a receiver line.

5 and judge from it about the geological features of the POLFUEUKVTSIKhSya objects.

Thus, it is possible to reduce the induction effects at high frequencies, to obtain the frequency response of the secondary field of EP in a wide range of frequencies and to increase the resolution of the metzd during detailed research.

FIG. 1, 2 shows an installation meter5 for performing a method of geoelectromagnetic exploration in field conditions and gives the results of measurements at its different positions.

The installation contains primary 1 and

0 auxiliary 2 power electrodes current generator 3, fixed 4 and. MOBILE 5 receiving electrodes and amplitude-phase meter 6.

The measurements were performed on one of the IP anomalies. The measurement setup shown in FIG. 1, was placed on the ground so that the main supply electrode was within the VP anomaly. The receiving electrodes were placed symmetrically relative to

0 supply electrode. The phase frequency response (FFM) of the total voltage in the receiving line was measured in the frequency range 0.3-156 Hz using an equipment sample.

five

Initially, the receiving electrodes (in the proposal of medium homogeneity in resistance) were positioned symmetrically with respect to the supply electrode in position C. The nature of the phase response in this position is shown in FIG. 2.1. It differs in practical phase independence from frequency. The amplitude (iU) and imaginary component (3niu (J) of the voltage, which is explained by

Claims (3)

5 by the absence of sufficiently deep compensation of the voltages in the receiving line created by the primary field of the current source. In the future, the mobile receiving electrode occupied the position of And, III, IV, etc. a few centimeters behind each other on the ground. In positions III and IV, the phase frequency characteristic passes through - jJ-, respectively, at frequencies of 1.2 and 4, V Hz. From the point of view of implementation of this method of measurement, positions III and IV are equivalent. In one of these positions, measurements of the amplitude of the field are mandatory, since they allow, in conjunction with phase measurements, to calculate the imaginary part of the field of EP (its sign is conventionally taken positive) 3vnuU uU-SinV. In positions III and IV, the OTA frequency characteristic has an extremum, whose position on the frequency axis characterizes the geological features of object 1. As an informative parameter, the value determining the slope of the frequency characteristic can also be used (-3muU (5,)) 3muU (; E -,), where 3vnuU (SQ.) and 3miU () are the voltage and frequency E /, and 3. With further movement of the electrode towards the auxiliary feeding electrode, the frequency response of the total voltage passed into the range of H values lying between - S-and - JT, reminiscent of the shape of the curves W, p obtained in positions II and I (these FP in Fig. 2 are not shown). The data indicate .0 that when measuring the transition of the phase frequency response through the value is clearly defined in real conditions. With the positions of the receiving electrodes, which correspond to the same phase response, the frequency characteristics of the imaginary voltage component show the region of extreme values most clearly. At other electrode positions, e.g. in the position. the study of the frequency response of DvDuV) with sufficient accuracy for practice cannot be carried out. By additional measurements, it was found that the character of the graphs7,1), 3muU practically does not depend on the dimensions of the receiving line, if it varies from 5 to 20 m and on the angle between the receiving and feeding lines. For the implementation of this method, well-known technical means can be used, for example, the INFAZ VP electric exploration station (frequency range 0.3-78 Hz) or EVA-203 station (frequency range 0.03-156 Hz). Claims of the method of geoelectrical exploration according to the method of induced polarization by exciting a primary field with a grounded source and measuring the amplitude-phase frequency characteristics of the total field with changes in the relative position of the supply and receiving electrodes, in order to increase the measurement resolution when studying the geological features of polarizing objects , the relative position of the receiving and supply electrodes is changed until the total field phase at the phase part tnoj characteristic can not pass through the value -, and plurality of additional amplitude-frequency and phase measurements at the same position of the electrode is judged geological features x rizuyuschihs floor objects. Sources of information taken into account during the examination 1. Aladinsky Yu.V. et al. On one method of determining the phase-frequency characteristics of induced polarization / lime in universities, Geologists and Exploration, 1973, 7250-73. 2. Authors certificate of the USSR 324601, cl. G 01 V 3/02, 1972. 3. Frolov V.Kh. The main provisions of the approximate-potential method of electrical prospecting. Proceedings of SNIHIMSA, vol. 215, Novosibirsk, 1975, p. 7883 (prototype). | cvj N tel-4 3sh se 0 a 3 § se 1-4 .g one