RU2710099C1 - Geo-electric prospecting method - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method: geoelectric prospecting is based on sounding illumination of rock strata. That is ensured by using a low-frequency electromagnetic field containing one or three given fundamental frequencies and a set of their harmonics generated by the source. At that electromagnetic field is simultaneously registered in three orthogonal directions with its further spectral analysis, conversion of amplitude-frequency characteristics into apparent resistance values and physical-geological interpretation, providing possibility of obtaining information on physical properties of rocks in intervals of depth of effective penetration of electromagnetic field, corresponding to each of used set of generated frequencies. Electromagnetic field penetrating power is then controlled from the source to the observation point by comparing the observed field frequency values determined as a result of spectral analysis thereof.

EFFECT: invention relates to electromagnetic exploration of the earth's interior.

1 cl, 5 dwg

Description

Technical field

The invention relates to the use of electrometric methods to study the bowels of the earth based on low-frequency electromagnetic transmission. This method is the basis of the method of probing transmission (MW), which, in contrast to the radiation principle used in the method of radio wave transmission, uses the principle of frequency sounding, which allows obtaining information about the spatial change in the electrical resistance of rocks between the receiver and the source of the generated field.

The method of sounding scans can be used in solving geological exploration, prospecting, engineering-geological and environmental problems associated with the selection and mapping of the desired objects. Its application is most relevant in order to forecast possible negative techno-geological processes during mine development of deposits, as well as in case of difficult to access ground conditions (mountainous terrain, dense built-up areas, etc.).

State of the art

A wide range of electrical exploration methods is known for detecting geological heterogeneities of the environment, based on the use of galvanic and inductive sensing methods. These include various types of ground-based electrical and electromagnetic sounding (VEZ, ChZ, ZSB, MTZ, etc.) [10], a number of which (VEZ, EP, etc.) are also used for intra-surface research in the conditions of mine workings. However, the practical implementation of these methods is associated with the presence of a number of problems associated with the influence of various kinds of geological and technogenic interference, a wide range of electromagnetic fields generated by various kinds of existing natural, technogenic and artificially generated sources, generating a variety of interference that substantially complicates (or makes impossible) the process measurements, a decrease in the resolution of the used fields with depth, the existing incorrectness of solving inverse problems, etc., which reduce informativeness and reliability of the results.

One of the ways to increase the efficiency of the use of electrical exploration methods in these conditions is the development and use of methods of ground-underground or inter-well penetration of the earth's interior. To date, several such methods are known, in particular, radio wave (RWP) and GPR methods [4, 12], based on the use of high-frequency wave fields, which make it possible to use the radiation principle of electromagnetic field penetration.

The disadvantages of these methods include their limited information capabilities, due to the fact that the kinematic characteristics and optical manifestations, in the form of reflection and refraction of high-frequency electromagnetic radiation, are based on the use of a wave model of the electromagnetic field that includes only the dielectric constant of the medium from electromagnetic parameters. In this case, the conductive rocks cause attenuation of these fields due to a significant loss of their energy, due to its transition into the kinetic energy of charged particles of the conducting zone, significantly limiting the studied region of space and not providing the necessary control of the removal of this zone from the source [2, 4].

A known method of electromagnetic induction transmission, representing a single-level interprofile underground transmission using the equipment MCHZ-12 and a set of interpretation programs [9]. Transmission is performed at the same frequency (f = 635 Hz) along two adjacent subparallel underground profiles with the measurement of three components of the magnetic field, on the basis of which the parameter of geoelectric heterogeneity and apparent resistance are determined, characterizing the degree of heterogeneity of the translucent space with the allocation of anomalous zones.

The disadvantages of this (single-frequency) method include: 1) the lack of the use of the principle of frequency sounding, which does not allow an assessment of the removal of the desired object from each of the used observation profiles; 2) the lack of sound technology for the implementation of ground-underground and interwell penetration.

The closest in technical essence and the achieved result to the proposed method of probing transillumination of the rock thickness, from the standpoint of using an integrated electromagnetic field that carries information about the physical properties of rocks at different effective depths of its penetration, is the ground based method based on the use of industrial magnetic fields (PMF) ( RF patent No. 2568986, Kolesnikov V.P. 2014).

This method is based on the use of an integrated magnetic field generated as a result of the total impact of an existing set of industrial electric power sources in a frequency range including the fundamental frequency of 50 Hz and a set of its harmonics reaching 1-2 kHz. The maximum penetration depth of such fields in the study of sedimentary rocks can reach several hundred meters, and the minimum depth begins with the first tens of meters (40-50 m). This method is selected as a prototype, due to a very limited set of existing approaches to the implementation of the proposed method (MPP).

The disadvantages of the prototype, with regard to its use for ground-underground sounding, are: 1) the presence of industrial fields generated by underground sources used in mining operations, close in frequency characteristics to the transmission electromagnetic field generated by ground sources, can introduce significant distortions ( interference) into the results of mine observations; 2) the inability to expand the range of the examined depths, due to the fixed main industrial frequency (50 or 60 Hz); 3) the possible presence of terrestrial interference waves.

The objective of the invention is the creation of a method of electromagnetic transmission of the studied rock mass (ground-underground, inter-well, etc.) with the elimination of the disadvantages of the prototype and the expansion of its functionality.

The problem is solved using the essential features indicated in the claims, such as a geoelectrical exploration method based on probing scanning of the rock mass, which is characterized in that

* use a low-frequency electromagnetic field containing one or three specified fundamental frequencies and the totality of their harmonics generated by the source,

* at the same time, the electromagnetic field is recorded simultaneously in three orthogonal directions with its subsequent spectral analysis, conversion of the amplitude-frequency characteristics into apparent resistance values and a physical and geological interpretation, providing the possibility of obtaining information about the physical properties of rocks in the depth intervals of the effective penetration of the electromagnetic field corresponding to each from the used set of generated frequencies,

* then control the penetrating ability of the electromagnetic field from the source to the observation point,

* by comparing the frequencies of the observed field, determined as a result of its spectral analysis, with the set of frequencies generated by the source of this field.

The advantages of the proposed method in comparison with the prototype:

1) the use of an integrated field formed from a set of fields for a given set of frequencies, providing the ability to eliminate a number of mine and ground industrial interference with the ability to several times increase the thickness of the translucent space (up to 1000 or more meters) compared to the prototype, the lower frequency limit of which limited to 50 Hz;

2) providing control of the penetrating ability of the electromagnetic field from the source to the observation point;

3) increased contrast of the separation of the studied rock stratum due to the approximation of the measuring setup (receiving line) to heterogeneity;

4) the ability to perform work under terrestrial conditions inaccessible for shooting (mountainous terrain, boggy, high density of built-up territory, etc.);

5) the simplification of the process and the efficiency of the work.

Moreover,

1) the use of an integrated electromagnetic field, including a given set of fundamental frequencies (from fractions to the first thousand hertz), allowing (based on the principle of frequency sounding - formula (2)) to increase the effective depth of penetration of the electromagnetic field several times;

2) the simultaneous registration at each observation point of a magnetic field for a given set of generated fundamental frequencies and their harmonics, provides information on the physical properties of the rocks for the corresponding depth interval; (determined by the set of frequencies used);

3) localization of the investigated space and control of the presence at the observation points of a translucent electromagnetic field generated by its source;

4) the known specified spatial location of the source of the integrated translucent field;

5) the physical justification for the interpretation of the results of the proposed method of electromagnetic transmission, aimed at assessing the magnitude and nature of the spatial changes in the electrical resistance of the translucent medium.

6) in contrast to radio wave transmission, the frequency principle of sounding is used instead of the radiation principle, which allows obtaining information about the change in electrical resistance in the space between the receiver and the source of the generated quasistationary electromagnetic field.

The proposed method is illustrated by graphs in FIG. 1 - FIG. 5.

The physical justification of the proposed method.

Consider the essence, physical justification and technology of the proposed method based on the theoretical foundations of electrodynamics on the example of ground-underground sounding.

The proposed method uses a technology based on the excitation of an electromagnetic field on the earth's surface using a galvanic source in the form of a grounded line AB, and registration of the magnetic field components using a three-component induction sensor in the area of mine workings.

When performing ground-underground sounding, the relative position of the supply and receiving installations, and therefore the behavior of the recorded electromagnetic field, is significantly different from traditional ground-based (or mine) sounding methods, which requires the use of special methods of interpreting the recorded field, taking into account the specifics of the studied electromagnetic field .

Consider the proposed approach to the interpretation of observation results.

The information content of the method of ground-underground sounding, in fact, is determined by such factors as: 1) the need to take into account the characteristics of the formation of the electromagnetic field; 2) determination of the relationship of the components of the measured electromagnetic field in a mine environment with the electrical resistance of the overlying rock mass; 3) assessment of the effective penetration depth of the electromagnetic field.

Consider the nature of the formation of a magnetic field in the presence of a conductive object in the medium (Fig. 1).

The alternating magnetic field of the source, if there is a conducting object in the medium, is determined by the relation

- первичное поле источника в точке измерений, удаленной от него на расстояние r₀;

- поле, индуцированное проводящим телом (током индукции, образующимся в этом теле) расположенном на расстоянии r_инд от точки наблюдения.Where

- the primary field of the source at the measurement point remote from it by a distance r ₀ ;

- the field induced by the conducting body (induction current generated in this body) located at a distance r _ind from the observation point.

Obviously, the higher the conductivity of the body, the greater the magnitude of the current induced in it and, according to the Bio-Savard-Laplace law, the greater the magnitude of the magnetic field excited by it

будет снижаться над проводящим телом в силу появления в нем индукционного тока.On the basis of this, when recording on the ground, the value of the recorded field

will decrease over the conductive body due to the appearance of an induction current in it.

, может увеличиваться, поскольку расстояние от проводящего тела (источника индуцированного тока) до точки наблюдения (r_инд,2) становится значительно меньшим расстояния от наземного источника до этой точки (r_0,2) и соответственно возможна ситуация, когда

.A different picture is obtained when observed in underground (mine) conditions, where the magnitude of the observed magnetic field

, may increase, since the distance from the conducting body (the induced current source) to the observation point (r _{ind, 2} ) becomes much smaller than the distance from the ground source to this point (r _0.2 ) and, accordingly, a situation is possible when

.

Thus, there is a well-defined physical relationship between the magnitude of the magnetic field and the conductivity σ = 1 / ρ (and the electrical resistance ρ) of the medium.

Effective Depth Assessment

In the theory of variable fields, the effective depth of penetration of the field Z _{eff is} taken to be the depth at which the field decreases by a factor of e (e is the base of the natural logarithm, e = 2.7) [10].

That is, the effective depth of penetration of an electromagnetic field in a homogeneous medium is determined by its specific resistance and field frequency: the greater the resistance of the medium and the lower the frequency of the field, the greater the depth of penetration of the field into the bowels of the Earth.

и заданной величины сопротивления (ρ=10 Ом⋅м и ρ=100 Ом⋅м), приведены на Фиг. 2.Corresponding graphs of the relative value of E _{x, rel} for different values of frequencies

and a given resistance value (ρ = 10 Ohm⋅m and ρ = 100 Ohm⋅m) are shown in FIG. 2.

, наиболее контрастно отображающие чувствительность относительной напряженности поля к изменению зондирующего параметра

.To assess the rate of change of the obtained graphs E _{x, rel} (z) with depth, transform them into the graphs of the derivative

most contrastively reflecting the sensitivity of the relative field strength to a change in the probing parameter

.

Find the expression for this derivative. Based on [2], we write

;

;By replacing

;

;

and considering

we get

для разных значений электрического сопротивления однородной среды (Фиг. 3) показывают возможность оценки помимо Z_эф диапазона эффективного проявления слоя ΔZ_эф(ΔZ_эф=Z_эф±DZ), оказывающего наиболее значительное влияние на результат измерений (в данном примере этот диапазон отмечен для величины

, отличающейся от ее максимального значения на 10%).Relative value calculation results

for different values of the electrical resistance of a homogeneous medium (Fig. 3) show, in addition to Z _{eff the} range of effective manifestation of the layer, ΔZ _eff (ΔZ _eff = Z _eff ± DZ), which has the most significant effect on the measurement result (in this example, this range is marked for

, differing from its maximum value by 10%).

However, in the case of a heterogeneous, for example, horizontally layered model of the medium, the task of estimating Z _{eff is} complicated.

. В этом одна из причин неоднозначности определения глубины зондирования (Z_эф), и комплексирования гальванического и индуктивного методов зондирования.As follows from (2), Z _eff is determined by the relation

. This is one of the reasons for the ambiguity in determining the depth of sounding (Z _eff ), and the integration of galvanic and inductive methods of sounding.

To increase the unambiguity of estimating Z _eff (or ΔZ _eff ), additional a priori information is needed (analysis of parametric sounding using data from drilling, electric logging, a comprehensive analysis with the results of ground-based sounding, etc., in order to identify the main packs of rocks that have a noticeable manifestation in the results of the observed field )

Consider one of the approaches to implement this.

We will proceed from the following:

in the case of a homogeneous environment

and if the medium is heterogeneous, for example, horizontally layered, then

- относительное сопротивление i-го и (i+1)-го слоев.Where

- the relative resistance of the i-th and (i + 1) -th layers.

From the last relation we obtain the expression for the determination of K (μ _i ):

(по проявлению соответствующего градиента изменения поля (эффективного сопротивления, отмечаемого в относительно ненарушенной части разреза), обеспечивая этим глубинную привязку наблюденных значений кажущегося сопротивления по формуле (6).which can be estimated by comparing the depth of the roof of the marking layer Z _{eff, i} (determined from the analysis of parametric sounding performed near the well) with the value

(by the manifestation of the corresponding gradient of the field change (effective resistance noted in the relatively undisturbed part of the section), thereby providing a deep reference of the observed values of the apparent resistance according to formula (6).

Work technology

, (Т=1/ƒ); д) интерпретацию графиков зондирования

с целью получения информации о пространственном изменении электрического сопротивления пород в интервале эффективных глубин распространения магнитного поля.The technology for performing translucent electromagnetic sounding of the geological environment by the proposed method (MW) includes: a) excitation of an integrated multi-frequency field on the earth's surface (or in the well); b) registration of the components of the intensity of the integral magnetic (or electric) field at each observation point (in the mine or well) in three orthogonal directions; c) determination of the amplitude-frequency characteristics of each component of the observed magnetic field using spectral analysis; d) recalculation of the amplitude-frequency characteristics into values of apparent resistance

, (T = 1 / ƒ); e) interpretation of sounding graphs

in order to obtain information about the spatial change in the electrical resistance of rocks in the range of effective depths of magnetic field propagation.

The use of instrumentation.

The implementation of this technology was performed using a hardware-software complex, including: 1) a modernized ANCH-3M generator, which allows generating a field of specified frequencies (f = 4.88, 9.76, 19.52, 39, 78, 156 and 312 Hz) and an integral field including a certain a set of these frequencies, with a maximum current of up to I _max = 2 A; 2) PMP-2 meter based on the use of a three-component induction sensor (Kolesnikov, Diaghilev and other RF patent No. 148256) [1] and programs for recording the field of given frequencies and their harmonics (Kolesnikov, Diaghilev, certificate No. 20144611489) [5] ; 3) laptop; 4) the program of spectral analysis of variable electromagnetic fields AnalyzerH3D (Diaghilev R.A., Kolesnikov V.P. and others, certificate No. 20155617490) [6]; 5) the interpretation system of the PROBE programs (Kolesnikov et al. Certificate No. 2005610058) [7].

If necessary, other generator options can be used when upgrading them in order to form an integral field, for example, the AMS-1 equipment generator, which provides the ability to generate frequencies from 0.15 to 2500 Hz (multiples of the minimum of 0.15 Hz).

An example of testing the proposed method.

The proposed method was tested at one of the mines of the Verkhnekamsk salt field when studying the thickness of the rocks in the range from the working drift to the earth's surface. The thickness of the translucent thickness was 220 m.

The basic field survey technique using ground-based underground scanning included: 1) excitation of an electromagnetic field on the earth's surface using an ANCH-3M generator and an earthed supply line AB (AB = 1000 m); 2) registration of the three components of the magnetic field in an inductive manner using the PMP-2 hardware-software complex (Kolesnikov, Diaghilev and other RF patent No. 148256) [1] along the profile with a step of 30 m

от 0.227 до 0.026), обеспечивая этим возможность контроля основной части исследуемой толщи пород со значительным (примерно в 6-7 раз) уменьшением времени выполнения полевой съемки.Processing and spectral analysis of the integrated electromagnetic field measured in the drift using the method of low-frequency electromagnetic transmission showed a confident manifestation of each of the frequencies and their main harmonics generated by the ground source (Fig. 4). In total, 47 frequencies were allocated, in the range from 4.88 to 1562 Hz, which is very extensive initial information for the analysis of the studied rock mass. At the same time, the analysis showed that the integral field, including three operating frequencies: f = 19.52, 39.04, and 156 Hz, together with their harmonics, allowed us to select 18 frequencies in the range from 19.52 to 1404 Hz (the corresponding values

from 0.227 to 0.026), thus providing the ability to control the main part of the studied rock stratum with a significant (about 6-7 times) reduction in the time taken to perform field surveys.

The background electromagnetic field recorded with the ground generator turned off showed the presence of the fundamental frequency of the industrial field with f = 50 Hz and its harmonics. These frequencies, which differ from those used for surface-underground scanning, in this case cannot have a significant effect on the results of the survey.

Based on the interpretation of the observed data, a regular manifestation of a different penetrating ability of a quasistationary field through the examined rock thickness was revealed, indicating a change in its electrical conductivity associated with the degree of decompression and moisture saturation of the pore space. The nature of the change in electrical resistance made it possible to identify the presence in the studied section relative to the conductive zone in the southern part of the profile (Fig. 5), later certified by a set of experimental works. This description, graphs and examples are considered as material illustrating the invention, the essence of which and the scope of patent claims are defined in the following claims, a combination of essential features and their equivalents.

Literature

1. Hardware-software complex for geoelectrical exploration: US Pat. RF 148256 Ros. Federation: IPC7: G01V 3/02 / authors Kolesnikov V.P., Diaghilev R.A., Kolesnikov S.V .; 10/28/2014.

2. Kolesnikov V.P. Electrometry Fundamentals of the theory of variable electromagnetic fields. Perm: Publishing house of PSNIU, 2013.185 p.

3. Kolesnikov V.P., Laskina T.A. Electrical exploration in urbanized areas // Geophysics. 2014. No5. S. 33-40.

4. Petrovsky A.D. Radio wave methods in underground geophysics. 2nd ed. add.M .: CNITRI, 2001.290 s.

5. Program for the registration of industrial electromagnetic fields for geophysical surveys: certificate of official registration of computer programs No. 20144611489 Ros. Federation / authors Kolesnikov V.P., Diaghilev R.A .; 02/04/2014.

6. The program of spectral analysis of industrial electromagnetic fields for geophysical surveys: certificate of official registration of computer programs No. 20155617490 Ros. Federation / authors Diaghilev R.A., Kolesnikov V.P., Laskina T.A., Artemyev D.A .; 07/13/2015.

7. Program for processing and interpreting the results of vertical electric sounding PROBE: certificate of official registration of computer programs No. 2004611865 Ros. Federation / authors Kolesnikov V.P., Kutin V.A., Mokronosov S.V., copyright holder Kolesnikov V.P .; 01/11/2005.

8. Method for geoelectrical exploration: invention patent No. 2568986 Ros. Federation / author V. Kolesnikov; publ. 11/20/15 g.

9. Khachai O.A., Khachai O.Yu., Kononov A.V. 3-D method of electromagnetic induction transmission and a processing and interpretation system for studying the state of the water-proof stratum of kimberlite pipes // Gorny Informatsionno-analiticheskiy bulletin. M .: MGGU, 2009. No. 12. S. 230-236.

10. Electrical exploration: Handbook of geophysics: in 2 books. / ed. VK. Khmelevsky and V.M. Bondarenko. M .: Nedra, 1989.438 s.

11. Yavorsky B.M., Detlaf A.A. Handbook of Physics. M .: Nauka, 1981. 512 s.

12. Patent No. 2084930 (radio wave method) Ros. Federation / author Borisov B.V. and etc.; 22.07, 1993

Claims (1)

The method of geoelectrical exploration, based on probing scanning of the rock mass, characterized by the fact that they use a low-frequency electromagnetic field containing one or three predetermined fundamental frequencies and a combination of their harmonics generated by the source, while simultaneously registering the electromagnetic field in three orthogonal directions with its subsequent spectral analysis, recalculation of amplitude-frequency characteristics into values of apparent resistance and physical-geological interpretation, providing the possibility the ability to obtain information about the physical properties of rocks in the depth intervals of effective penetration of the electromagnetic field corresponding to each of the used set of generated frequencies, then they control the penetrating ability of the electromagnetic field from the source to the observation point by comparing the frequencies of the observed field, determined as a result of its spectral analysis, with the set of frequencies generated by the source of this field.

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