RU2408036C1 - Focused current marine geoelectric prospecting method - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: dipole source excites an electromagnetic field inside the examined medium by transmitting thereto rectangular current pulses with pauses in between. Geometric probing is carried out along the given profile during the current pulse and probing on transient process is done during the pauses. Measurements are taken using a measuring apparatus mounted on sea bed, consisting of five electrodes: a central electrode surrounded by four equidistant electrodes lying at the corners of a square, two opposite sides of which are parallel to the axis of the profile. First potential differences between the outer electrodes and the central electrode are measured. When the dipole sources passes through different points, equipotentiality of closed lines passing through the four outer electrodes of the measurement apparatus is provided, thus excluding the horizontal component of current density in each probing point, inside the area bounded by the contour of this line. The values of the measured first potential differences are used to calculate two sets of standardised interpreted electrical parametres which are not susceptible to lateral effect of three-dimensional geological irregularities lying outside the probing point. The obtained parametres are used to find the model of the medium and time sections of this model are built from electroconductivity of elements of the medium, coefficient of induced polarisation and decay time constant of the polarisation induced potential difference.

EFFECT: possibility of complete exclusion of the horizontal component of current density at the probing point and exclusion of the lateral effect, which enables finding and high-contrast delimiting of hydrocarbon accumulation in a deep sea using a relatively simple measurement electrical circuit.

4 cl, 6 dwg

Description

The invention relates to the field of geophysical research, and more specifically to methods of marine geoelectrical exploration using controlled artificial sources of electromagnetic fields, and is intended for the search and contouring of hydrocarbon accumulations (HC) on the continental shelf based on the focusing of the electric current and separate determination and mapping of each element (horizons) of the thickness of sedimentary rocks and having anomalous values in the zones of hydrocarbon accumulation of the following t ex necessary for the task, the electrical parameters: conductivity, induced polarization and decay time constant of the induced polarization potential. Together, these three electrophysical parameters make it possible to distinguish a hydrocarbon reservoir from host rocks.

Methods of geoelectrical exploration, including marine, with controlled excitation of the medium under study by electric current are known (methods of resistance on direct and alternating current, including transient processes, based on the ABMN dipole-axial installation). But they are intended to determine only one electrophysical parameter from the three listed above, namely, the apparent electrical resistance, and not the specific desired area of the investigated space, but the entire space where the source current propagating by the law of diffusion penetrates. This is far from enough for prospecting and delineating the hydrocarbon accumulations currently proposed for geological exploration, which lie at a depth of more than 1000 m below the level of the seabed.

Note that, according to the theory and practice of geoelectrical exploration based on the ABMN dipole-axial installation, high-contrast hydrocarbon deposits (with a thickness of at least 50-100 m and with an electrical resistance exceeding the resistance of surrounding rocks in 50 or more times). But such deposits are rare and to date, most of them have already been explored.

Sounding attempts based on the ABMN dipole-axial installation with controlled excitation of current in order to search for hydrocarbon accumulations over the past 90 years have been repeatedly undertaken in various versions, but because of their low efficiency for searching for hydrocarbon accumulations, they have not been widely used. Electric dipole sensing has a high resolution at depths of up to 500 m and in practice has been successfully used to search for occurring water and ores, as well as in engineering surveys and archeology.

Based on the results of sounding, the apparent electrical resistance ρ is calculated using the universal formula based on the dipole-axis ABMN installation with controlled excitation of current:

Where

I is the measured current jump in a dipole electric source;

ΔU is the measured potential difference at the ends of the receiving electrodes MN;

K is the geometric coefficient of the sounding installation ("Electrical exploration", Handbook of geophysics. Ed. A.Tarkhov M .: Nedra, 1980, p.237 and p.422-406) [1].

With this approach, which is usually used with all traditional methods for determining electrical resistance in geoelectrical prospecting with a controlled current source, due to the current distribution according to the diffusion law, only summary information on all structural elements of the medium in which the electric field develops is obtained, since the distribution in it in the space of the measured current I of the source is not controlled by anything. And there is no information about this distribution in really existing three-dimensionally inhomogeneous media.

Among these resistance methods, marine geoelectrical exploration uses bottom geometrical sounding with low-frequency alternating current based on the ABMN dipole-axial installation, called by the Norwegian company EMGS "(SBL - sea bed logging)" (SEJohansen, HEFAmundsen, T. Rosten, S. Ellingsrud , T. Eidesmo and AH Bhuyian. Subsurface hydrocarbons detected by electromagnetic sounding. First Break, 23, March 2005, 31-36) [2]. It is also known as “controlled soutce electromagnetic sounding (CSEM)” or “offshore hydrocarbons mapping (OHM).”

The publication [2] described the bottom profiling of the SBL installation with a spacing of 6.5 km between AB and MN through one of the world's largest offshore gas fields, Troll West Gas Province (TWGP). The deposit lies at a depth of about 1070 m to 1230 m from the surface of the seabed and extends along the profile from 10.5 km to 17.5 km (average sea depth - 330 m). It is characterized by an abnormally high electrical resistance (250-1000 Ohm · m) compared with the resistance of the host rocks (1-2 Ohm · m) and a large thickness of 160 m. (Note that the vast majority of hydrocarbon deposits have an electrical resistance, the value of which compared with the resistance of marginal deposits, it varies from five to thirty times, and their thicknesses range from several meters to several tens of meters.)

Nevertheless, from the description [2] and the figures attached to it, it follows that the profiling curve by the SBL method is distorted by the influence of this geometrically large and high-resistivity field already at the sixth kilometer when the measuring installation approaches it. And the four structures to the right of it between 17.5 km and 24 km are crushed by its lateral influence, and the problem of determining the presence or absence of hydrocarbons in them is not solved. It is not necessary to speak about the separation of the boundaries of the contour of deposits, since the transition from the marginal zone to the main deposit on the resistance curve (actually measured and theoretical) stretches for more than 3 km. For such a bright hydrocarbon deposit, its geometrical and electrophysical parameters are extremely scarce information.

In addition, at shallow depths of the sea, the CSEM (OHM, SBL) method does not even provide such scanty information about hydrocarbon deposits.

Thus, the methods of resistance, including geometric sounding without focusing the electric current, are not suitable for searching and delineating the majority of hydrocarbon deposits found in the geological subsoil at depths greater than 1000 m relative to the level of the seabed, for at least three reasons: the first is recorded only one of the three electrophysical parameters of the medium under study necessary for this purpose (apparent resistance), which is far from always enough to detect accumulations of hydrocarbons in the bulk of geologists eskih species; the second is the recorded parameter (apparent resistance) for the same purpose is too coarse, because due to the lack of vertical focusing of the electric current, it records the resistance of the volume of all geological objects of the studied medium in which the electric field of the current source develops, that is, the measurement results are significantly distorted by the influence lateral geological heterogeneities; third, the parameter of induced polarization η is not recorded, which, along with electrical resistance, also has an anomalous value in the zone of hydrocarbon accumulations.

In geoelectrical exploration, the influence of lateral geological heterogeneities is called the lateral effect, which, when searching for hydrocarbon accumulations at depths of more than one kilometer, affects the measurement results laterally several kilometers from the sounding point, and these clusters become almost invisible.

The distorting lateral effect is caused by the fact that the electric current does not propagate in space, for example, in the form of an acoustic beam, as in seismic exploration, but spreads according to the law of diffusion in the direction of least electrical resistance. And if electric dipole sensing at shallow depths of research has a high resolution, then with increasing depth of research, due to the scattering of the source current, it loses this resolution.

Due to the low electrical resistance of seawater, a marine DEZ can be compared to logging by AMN gradient probes in conditions of filling a well with saline drilling mud. In salt-filled wells, field geophysicists abandoned such sounding more than fifty years ago, and the electrical resistivity of rock formations is determined using lateral logging based on the principle of radial focusing (Laterolog. Doll H.G.).

The task of marine electrical exploration is to determine the presence of hydrocarbons in traditionally encountered seismic structures, which, in the presence of hydrocarbons in them, differ in electrical resistance from the rocks surrounding them no more than 20-30 times. Such structures are hundreds of times larger than structures of the TWGP type. For example, all hydrocarbon deposits in Western Siberia, in the Gulf of Ob and on the shelf of the Kara Sea, including the largest in their geometrical sizes, differ in electrical resistance from the host rocks according to logging data only within three to thirty times. And if, after conducting seismic surveys, structures such as TWGP in the Norwegian Sea do not need to be additionally checked for hydrocarbons, due to the high cost of offshore drilling, it is risky to drill most structures without checking for hydrocarbons. For example, Established UK oil and gas consultancy Hannon Westwood (see “Upstream boom likely to frustrate North Sea investment opportunities.” First Break, 25 Januar 2007, pp. 22-24) does not recommend investing in drilling the remaining undrilled averages and small seismic structures in the North Sea, since the success of drilling at present at these structures is only about 20% (one successful out of five drilled wells).

An unfavorable situation for seismic exploration has developed on the Sakhalin shelf. There, British Petroleum drilled two exploration wells in large areas of the West Schmidt block (Sakhalin-4) identified by seismic exploration according to press reports worth $ 103 million in which no hydrocarbons were found (see the Vedomosti newspaper dated 06.03.2008). It also turned out to be a dry well drilled at one of the largest geometrical dimensions of the Admiralty structure in the Barents Sea (see Obmetko V.V. et al. Prospects for the oil and gas potential of the Admiralty megaval. International Conference “Oil and Gas of the Arctic Shelf - 2008”. Murmansk. 12- November 14, 2008). This is due to the fact that hydrocarbon accumulations are not always controlled only by a structural factor.

There is a known method of marine geoelectrical exploration with focusing of electric current, which, due to maintaining the zero axial or orthogonal difference of electric potentials at the sensing measuring point located in the sea layer and determining several electrophysical parameters of the elements of the geological environment, is little affected by lateral inhomogeneities and with a fairly sufficient degree of probability allows us to the presence of hydrocarbons in the structures identified by seismic surveys and outline them (Rykhlinsky NI, Davydycheva SN, Lisin AS "Method of marine geoelectrical exploration with focusing electric current." RF patent No. 2884555 dated June 01, 2005, Bull. No. 27, 2006) [ 3].

The disadvantage of this method is that it implements an incomplete vertical focusing of the current: in the first version, only the orthogonal horizontal component of the current density is excluded at the sensing point, and in the second only the axial horizontal component of the current density is excluded.

A known method of marine geoelectrical exploration with circular focusing of electric current (EN Rykhlinskaya "Method of marine geoelectrical exploration with focusing electric current (options)." RF Patent No. 2351958 of November 12, 2007, Bull. No. 10, 2009) [4]. Although this method eliminates the horizontal component of the electric current density j _xy , it is designed to solve other equally important problems. Due to the special design of the measuring sensors, it allows you to measure only the change in the electrical parameters of the geological section along the measuring profile, which is important for accurately determining the boundaries of the contour of hydrocarbon deposits. However, it does not state that a current electric dipole can be placed on the surface of the sea. In addition, its implementation requires a relatively complex electrical circuit of the meter.

In the proposed method, with a relatively simple electrical circuit of the meter, the problem of detection and contouring with a clearly defined boundary of the contour of oil and gas deposits on the continental shelf both at large and shallow depths of the sea, and at great depths of these deposits is solved.

The technical result that allows us to solve this problem lies in the possibility of completely eliminating at the sensing point the horizontal component of the current density j _xy both orthogonal j _y and axial j _x , which completely eliminates lateral influence and for this reason makes it possible at large depths of the sea, at least four kilometers, to find and with high contrast contour hydrocarbon accumulations in the thickness of the geological environment.

This technical result is achieved by the fact that in the method of marine geoelectrical exploration, in which an electromagnetic field is excited in the thickness of the medium under study, passing through it periodic rectangular current pulses with pauses after each of them using horizontal dipole electric sources; at each sensing point during the period of the current pulse and during each pause after turning off the current, a sequence of instantaneous values of the first differences of the electric potentials of the direct field and the field of transients is measured with a constant time interval Δt, while ensuring that the resulting first differences of the electric potentials between the external measuring electrodes form interpretable parameters and, using them and the differential equation of mathematical physics for the electric field strength of a dipole source in an electrochemically polarizable conducting medium

Where

∇ × is the operator rot;

- напряженность электрического поля дипольного источника, выраженная в уравнении для случая гармонического изменения величины электрического поля по времени, V/m;

- the electric field strength of the dipole source, expressed in the equation for the case of a harmonic change in the magnitude of the electric field over time, V / m;

µ - magnetic permeability - a constant for non-magnetic environments, which include sedimentary geological rocks, and is equal to 4π · 10 ^-7 GN / m;

σ (iωσ _o ητ) is the frequency-dependent conductivity of the elements of the medium,

σ _o - the electrical conductivity of the elements of the medium without taking into account the effect of the induced polarization, S / m,

η is the coefficient of their induced polarization,

τ is the decay time constant of the potential difference caused by polarization;

solve the mathematical inverse problem by determining three electrophysical parameters inherent in each element of the medium: electrical conductivity σ _о , induced polarization η and decay time constant of the potential difference caused by polarization τ;

and build three time sections according to these parameters;

according to the invention, a measuring profile is laid at the bottom of the sea, which passes through measuring devices fixed on the bottom of the sea, each of which consists of five electrodes: a central one and four equally spaced around it along the vertices of a square, two opposite sides of which are parallel to the axis of the profile;

within each pulse-pause period, geometric sounding is carried out with the current switched on and sounding on transients during the pause after turning off the current, measuring the first electric potential differences between each of the four external electrodes of the measuring unit and the central one;

in this case, measurements in each measuring installation fixed on the measuring profile are carried out while passing a horizontal dipole source along the current profile laid parallel to the measuring and shifted relative to it in the plan in the orthogonal direction along the y axis by a distance (y = -b) and height by thickness h water layer, sending current pulses to the medium under study at all positions of the dipole source from the point with coordinates [(x = -L), (y = -b), (z = + h)] to the point with coordinates [(x = + L ), (y = -b), (z = + h)], then the dipole source deploy and transfer to another parallel current profile, laid and shifted in plan relative to the measuring one in the opposite direction along the y axis by a distance (y = + b) and height by thickness h, and continue to measure when it moves in the opposite direction from the point with coordinates [(x = + L), (y = + b), (z = + h)] to the point with coordinates [(x = -L), (y = + b), (z = + h)],

using the measured values of the first differences of electric potentials, two sources independent of the current strength and the horizontal component of the current density (j _x = 0 and j _y = o) are determined at the sensing point of the interpreted parameter - one R _z (t _o ) based on geometric sensing for all the positions of the dipole source, calculated by the formula:

R _z (t _o ) = [ΔU _M1N (I _B1A1 , t _o ) -ΔU _M3N (I _B1A1 , t _o )]:

: {[ΔU _M1N (I _B1A1 , t _o ) + ΔU _M2N (I _B1A1 , t _o ) + ΔU _M3N (I _B1A1 , t _o ) + ΔU _M4N (I _B1A1 , t _o )] + k ₁ (t _o ) · [ΔU _M1N (I _A2B2 , t _o ) + ΔU _M2N (I _A2B2 , t _o ) + ΔU _M3N (I _A2B2 , t _o ) + ΔU _M4N (I _A2B2 , t _o )] + k ₂ (t _o ) · [ΔU _M1N (I _A3B3 , t _o ) + ΔU _M2N (I _A3B3 , t _o ) + ΔU _M3N (I _A3B3 , t _o ) + ΔU _M4N (I _A3B3 , t _o )] + k ₃ (t _o ) · [ ΔU _M1N (I _B4A4 , t _o ) + ΔU _M2N (I _B4A4 , t _o ) + ΔU _M3N (I _B4A4 , t _o ) + ΔU _M4N (I _B4A4 , t _o )]},

and another R _z (t _i ) based on sensing on transients for four, selected by the iteration method, the most informative distances with the coordinates of the dipole source [(x = -a), (y = -b), (z = + h)] , [(x = + a), (y = -b), (z = + h)], [(x = + a), (y = + b), (z = + h)] and [(x = -a), (y = + b), (z = + h)] of all probed, calculated by the formula:

R _z (ti) = [ΔU _M1N (I _B1A1 , ti) -ΔU _M3N (I _B1A1 , ti)]:

: {[ΔU _M1N (I _B1A1 , ti) + ΔU _M2N (I _B1A1 , ti) + ΔU _M3N (I _B1A1 , ti) + ΔU _M4N (I _B1A1 , ti)] + k ₁ (ti) · [ΔU _M1N ( I _A2B2 , ti) + ΔU _M2N (I _A2B2 , ti) + ΔU _M3N (I _A2B2 , ti) + ΔU _M4N (I _A2B2 , ti)] + k ₂ (ti) · [ΔU _M1N (I _A3B3 , ti) + ΔU _M2N (I _A3B3 , ti) + ΔU _M3N (I _A3B3 , ti) + ΔU _M4N (I _A3B3 , ti)] + k ₃ (ti) · [ΔU _M1N (I _B4A4 , ti) + ΔU _M2N (I _B4A4 , ti) + ΔU _M3N (I _B4A4 , ti) + ΔU _M4N (I _B4A4 , ti)]},

Where

k ₁ (t _o ), k ₂ (t _o ), k ₃ (t _o ) are the focusing coefficients for geometric sensing, ensuring the equipotentiality of the closed line passing through the four external electrodes of the measuring setup, and, thus, the exception inside the region bounded the contour of this line, the horizontal component of the current density j _xy at each sensing point on all geometrical spacings during the current pulse, which are determined from the system of three equations:

,

,

,

,

,

,

k ₁ (ti), k ₂ (ti), k ₃ (ti) are the focusing coefficients for transient sensing at each sensing point in the current pause at all transient times, determined from the system of three equations and ensuring the equipotentiality of the closed line passing through four external electrodes of the measuring unit:

,

,

,

,

,

,

t ₀ is the moment in time when the current pulse is transmitted, when the electric field of the transient processes does not differ from its steady-state value corresponding to direct current;

t _i - time points at which transient signals are measured at equal time intervals Δt throughout the entire pause after turning off the current;

ΔU _M1N (I _B1A1 , to), ΔU _M2N (I _B1A1 , to), ΔU _M3N (I _B1A1 , to), ΔU _M4N (I _B1A1 , to),

ΔU _M1N (I _A2B2 , to), ΔU _M2N (I _A2B2 , to), ΔU _M3N (I _A2B2 , to), ΔU _M4N (I _A2B2 , to),

ΔU _M1N (I _A3B3 , to), ΔU _M2N (I _A3B3 , to), ΔU _M3N (I _A3B3 , to), ΔU _M4N (I _A3B3 , to),

ΔU _M1N (I _B4A4 , to), ΔU _M2N (I _B4A4 , to), ΔU _M3N (I _B4A4 , to), ΔU _M4N (I _B4A4 , to)

- instantaneous values of the first differences of electric potentials between each of the four external electrodes M ₁ M ₂ M ₃ M _{4 of the} measuring unit and the central N, measured at a time t _{about the} transmission of current in a dipole source as it passes through both current profiles through equidistant from the central electrode N four points respectively with coordinates [-x, (y = -b), (z = + h)], [+ x, (y = -b), (z = + h)], [+ x, ( y = + b), (z = + h) and [-x, (y = + b), (z = + h)];

ΔU _M1N (I _B1A1 , ti), ΔU _M2N (I _B1A1 , ti), ΔU _M3N (I _B1A1 , ti), ΔU _M4N (I _B1A1 , ti),

ΔU _M1N (I _A2B2 , ti), ΔU _M2N (I _A2B2 , ti), ΔU _M3N (I _A2B2 , ti), ΔU _M4N (I _A2B2 , ti),

ΔU _M1N (I _A3B3 , ti), ΔU _M2N (I _A3B3 , ti), ΔU _M3N (I _A3B3 , ti), ΔU _M4N (I _A3B3 , ti),

ΔU _M1N (I _B4A4 , ti), ΔU _M2N (I _B4A4 , ti), ΔU _M3N (I _B4A4 , ti), ΔU _M4N (I _B4A4 , ti)

- instantaneous values of the first differences of electric potentials between each of the four external electrodes M ₁ M ₂ M ₃ M _{4 of the} measuring unit and the central N, measured at a time t _{i of} transients in a pause of the current in the dipole source when it passes through both current profiles through equidistant from the central electrode N of the measuring setup four points, respectively, with the coordinates [(x = -a), (y = -b), (z = + h)], [(x = + a), (y = -b), (z = + h)], [(x = + a), (y = + b), (z = + h)] and [(x = -a), (y = + b), (z = + h) ].

The technical result is also achieved by the fact that in the method of marine geoelectrical exploration according to the invention, current profiles for passing a horizontal dipole source are laid at a given depth of the sea or on the day surface of the sea.

The technical result is also achieved by the fact that in the method of marine geoelectrical exploration according to the invention, the distance L is six kilometers or more, the distances a and b are one kilometer or more, the immersion depth h of the dipole source is set within twenty meters above the level of the seabed to the day surface.

The technical result is also achieved by the fact that in the method of marine geoelectrical exploration according to the invention, the measuring units are placed on the measuring profile with an equal step of 200 ÷ 1000 meters.

The invention is illustrated by drawings.

Figure 1 is a block diagram of a device for implementing the proposed method.

Figure 2 shows the arrangement in the plan on the seafloor of the group of measuring installations on the measuring profile of sounding and two profiles parallel to the measuring along which the dipole source is moved in the thickness of the water layer of the sea above the seabed at a distance h or along the sea surface.

Figure 3 is a diagram of the movement of the dipole source along the first of two parallel profiles relative to the measuring one.

Figure 4 is a diagram of the movement of the dipole source along the second parallel profile relative to the measuring one.

Figure 5 shows the shape of the electric field with the focus of the electric current in the direction of the vertical coordinate Z by maintaining equal to zero the first differences of electric potentials between each pair of external measuring electrodes from the resultant action of four equidistant dipole sources from the central electrode N of the measuring device. If the differences in electric potentials between the four external electrodes M ₁ M ₂ M ₃ M ₄ are equal to zero, the currents from four current sources AB, penetrating into the square formed by these electrodes, are focused and can only propagate in the vertical direction along the Z coordinate. In addition, the vertical direction electric current, provided that it is focused, is supported on the basis of isoperimetric variation according to the Lagrange function (principle of least action) (Landau L.V., Lifshits E.M. Field Theory. 2 rear M -. L. 1948). In the plane M ₁ M ₂ M ₃ M ₄ N, the electric field potential takes the form of an ellipsoid of revolution with a minimum at point N.

Figure 6 shows the shapes of single pulses as a function of time t: a) the shape of a single rectangular pulse of current I in the circuit of the dipole source AB; b) - the shape of the pulses of the first and second differences of electric potentials.

A device for implementing the method (figure 1) contains a measuring device for measuring the first differences of electric potentials. This installation is located on the measuring profile - 1, laid along the seabed.

Here 2 - N, 3 - M ₁ , 4 - M ₂ , 5 - M ₃ , 6 - M ₄ are the electrodes of the measuring unit: external M ₁ M ₂ M ₃ M ₄ and central N (of such five-electrode measuring sensors on measuring profile 1 can be placed any number (figure 2)); 7 - a meter for measuring the first potential difference of the electric field ΔU _M1N between the external electrode M _{1 of the} measuring installation and the central N; 8 is a meter for measuring the first potential difference of the electric field ΔU _M2N between the external electrode M _{2 of the} measuring installation and the central N; 9 is a meter for measuring the first potential difference of the electric field ΔU _M3N between the external electrode M _{3 of the} measuring installation and the central N; 10 is a meter for measuring the first potential difference of the electric field ΔU _M4N between the external electrode M _{4 of the} measuring installation and the central N.

To ensure focusing of the electric field at the sensing point N, two profiles are placed in the thickness of the water layer of the sea parallel to the measuring profile 1 to the right and to the left of the sea, above the seafloor, or on the sea surface, two profiles: the first 11 and second 12, along which the horizontal current dipole source is moved consisting of two electrodes 13-A and 14-B, powered by a generator 15 of rectangular current pulses with pauses between each of the pulses.

Figure 3 shows the progress of the dipole source AB along the first profile 11, located in the thickness of the water layer of the sea above the seabed at a distance h or on the surface of the sea parallel to measuring 1 and offset from it by a distance (y = -b). Promotion of the dipole source AB along the profile 11 is carried out from its beginning [-x, (y = -b), (z = + h)] to the end [+ x, (y = -b),

(z = + h)]. Then the dipole source AB (Fig. 4) is transferred to the second profile 12, parallel to the measuring one and shifted relative to it in terms of distance (y = + b) and height by thickness h. The dipole source AB is advanced along profile 12 from the point with coordinates [+ x, (y = + b), (z = + h)] to the point with coordinates [-x, (y = + b), (z = + h )].

Such complex profiling is necessary to measure the first four differences of electric potentials between each of the four external electrodes of the measuring unit M ₁ M ₂ M ₃ M ₄ and the central N to achieve equal potentials between the external electrodes M ₁ M ₂ M ₃ M ₄ from the total action of the dipole source when passing through four points with coordinates [-x, (y = -b), (z = + h)], [+ x, (y = -b), (z = + h)], [+ x, (y = + b, (z = + h)] and [-x, (y = + b), (z = + h)]. This can be seen, for example, from the measured four first differences of electric potentials: ΔU _M1N -ΔU _M2N = ΔUM ₁ M ₂ = UM ₁ -UM ₂ = 0, ΔU _M2N - ΔU _M3N = ΔUM ₂ M ₃ = UM ₂ -UM ₃ = 0, ΔU _M3N -ΔU _M4N = ΔUM ₃ M ₄ = UM ₃ -UM ₄ = 0. It follows that UM ₁ = UM ₂ = UM ₃ = UM ₄ .

Equality of potentials between the four external electrodes of the measuring installation ensures the equipotentiality of the closed line passing through these four external electrodes, and, thereby, eliminating the horizontal component of the current density j _xy inside each sensing point at each sensing point, which ensures focusing of the current along the vertical Z coordinate inside this closed equipotential line.

6 (a) shows the shape of a single rectangular current pulse I in the circuit of the dipole source AB as a function of time t. Here T is the period (current pulse - pause).

Figure 6 (b) shows the shape of one of the pulses Δ ⁿ U, where n = 1 and 2. Also shows one of the values of Δ ⁿ U (t ₀ ) during the passage of current to the dipole source AB and one of the values of Δ ⁿ U (t _i ) in the current pause.

Consider the basics of the proposed method, its implementation and the new capabilities of marine geoelectrical exploration.

The proposed method of marine geoelectrical exploration allows using three profiles (figure 1) to exclude lateral influence by focusing the current. This gives a greater depth of sounding and high resolution in lateral.

It is known that an electromagnetic field in a poorly conducting physical medium propagates in time t according to the differential decaying wave equation of mathematical physics for the electric field strength resulting from the first and second Maxwell equations, including in the case of its pulse change:

Where

∇ × is the operator rot;

E - electric field strength, V / m;

µ - magnetic permeability - a constant for non-magnetic environments, which include sedimentary geological rocks, it is 4π · 10 ^-7 GN / m;

σ _about - electrical conductivity of non-polarizable medium, S / m;

ε is the dielectric constant, f / m. (V. A. Govorkov. Electric and magnetic fields. M.: Gosenergoizdat, 1960, p. 257-263) [5].

In the case of a highly conductive medium, to which sedimentary rocks belong, due to the fact that σ _{о is} numerically many times larger than ε, the second term on the right side of equation (2) is small compared to the first one, and it is discarded (L.L. Vanyan. Fundamentals of electromagnetic sounding. M: Nedra, 1965, p. 28-30) [6]. Physically, this means that bias currents in conductive media are neglected due to their smallness compared to conduction currents. Then equation (2) takes the form

In geoelectrical prospecting, this equation has an analytical solution only for one-dimensional axisymmetric media, in particular for media with horizontal unboundedly extending interfaces. At the same time, we note that in reality the geological environment is always three-dimensionally heterogeneous, since, firstly, it contains near-surface local inhomogeneities, and secondly, in general, the geological environment constantly changes its electrophysical parameters along the research profile. However, equation (3), as was said above, has been analytically solved only for one-dimensional axisymmetric media, including a medium with horizontal plane-parallel interfaces.

Therefore, the use of the solution of equation (3) in inverse problems of geoelectrical exploration for the search and contouring of oil and gas deposits is permissible only in the case when field measurements focus the electric current of a dipole source of an electromagnetic field, since in this case at point N (Fig. 5) the shape field propagation from the resulting action of four dipole sources B ₁ A ₁ , A ₂ B ₂ , A ₃ B ₃ , B ₄ A ₄ while maintaining zero the resulting first electric potential differences between each pair of external measurements of solid electrodes is almost always the same both in a three-dimensionally inhomogeneous medium and in one-dimensional with plane-parallel horizontal interfaces.

From the data of formulas (4), (5), (6) and (7) below, it follows that according to Ohm's law, the total component of the horizontal current density j _xy at the sensing point N according to the superposition principle is zero.

Thus, the current is focused at the measurement point, which eliminates the horizontal component of the current density j _xy and preserves only the vertical component of the current density j _z , which eliminates the distorting lateral effect on the sensing results and allows one to correctly solve the inverse problem in three-dimensionally inhomogeneous media using the well-known analytical solution of the one-dimensional equation (14) for layered media with plane-parallel interfaces.

To ensure the exclusion of the horizontal density component j _xy at the sensing point N with coordinates [(x = 0), (y = 0), (z = 0)], the proposed method derived the corresponding formulas of the measured electrical parameters: one R _z (t _o ) based on geometric sounding at all positions of the dipole source, calculated by the formula:

and another R _z (t _i ) based on sensing on transients for four, selected by the iteration method, the most informative distances with the coordinates of the dipole source [(x = -a), (y = -b), (z = + h)] , [(x = + a), (y = -b), (z = + h)], [(x = + a), (y = + b), (z = + h)] and [(x = -a), (y = + b), (z = + h)] of all probed, calculated by the formula:

Where

k ₁ (to), k ₂ (to), k ₃ (to) are the focusing coefficients for geometric sensing, ensuring the equipotentiality of the closed line passing through the four external electrodes of the measuring setup, and, thus, the exception inside the area limited by the contour of this line , the horizontal component of the current density j _xy at each sensing point on all geometric spacings in the period of the current pulse, which are determined from the system of three equations:

,

,

,

,

k ₁ (t _i ), k ₂ (t _i ), k ₃ (t _i ) are the focusing coefficients for transient sensing at each sensing point in the current pause at all transient times, determined from the system of three equations and ensuring the closed loop equipotentiality a line passing through the four external electrodes of the measuring unit:

,

,

,

,

t ₀ is the moment in time when the current pulse is transmitted, when the electric field of the transient processes does not differ from its steady-state value corresponding to direct current;

t _i - time points at which transient signals are measured at equal time intervals Δt throughout the entire pause after turning off the current;

ΔU _M1N (I _B1A1 , to), ΔU _M2N (I _B1A1 , to), ΔU _M3N (I _B1A1 , to), ΔU _M4N (I _B1A1 , to),

ΔU _M1N (I _A2B2 , to), ΔU _M2N (I _A2B2 , to), ΔU _M3N (I _A2B2 , to), ΔU _M4N (I _A2B2 , to),

ΔU _M1N (I _A3B3 , to), ΔU _M2N (I _A3B3 , to), ΔU _M3N (I _A3B3 , to), ΔU _M4N (I _A3B3 , to),

ΔU _M1N (I _B4A4 , to), ΔU _M2N (I _B4A4 , to), ΔU _M3N (I _B4A4 , to), ΔU _M4N (I _B4A4 , to)

- instantaneous values of the first differences of electric potentials between each of the four external electrodes M ₁ M ₂ M ₃ M _{4 of the} measuring unit and the central N, measured at a time t _{0 of} passing the current in the dipole source as it passes through both current profiles through equidistant from the central electrode N four points respectively with coordinates [-x, (y = -b), (z = + h)], [+ x, (y = -b), (z = + h)], [+ x, ( y = + b), (z = + h)] and [-x, (y = + b), (z = + h)];

ΔU _M1N (I _B1A1 , ti), ΔU _M2N (I _B1A1 , ti), ΔU _M3N (I _B1A1 , ti), ΔU _M4N (I _B1A1 , ti),

ΔU _M1N (I _A2B2 , ti), ΔU _M2N (I _A2B2 , ti), ΔU _M3N (I _A2B2 , ti), ΔU _M4N (I _A2B2 , ti),

ΔU _M1N (I _A3B3 , ti), ΔU _M2N (I _A3B3 , ti), ΔU _M3N (I _A3B3 , ti), ΔU _M4N (I _A3B3 , ti),

ΔU _M1N (I _B4A4 , ti), ΔU _M2N (I _B4A4 , ti), ΔU _M3N (I _B4A4 , ti), ΔU _M4N (I _B4A4 , ti),

- instantaneous values of the first differences of electric potentials between each of the four external electrodes M ₁ M ₂ M ₃ M _{4 of the} measuring unit and the central N, measured at a time t _{i of} transients in a pause of the current in the dipole source when it passes through both current profiles through equidistant from the central electrode N of the measuring setup four points, respectively, with the coordinates [(x = -a), (y = -b), (z = + h)], [(x = + a), (y = -b), (z = + h)], [(x = + a), (y = + b), (z = + h)] and [(x = -a), (y = + b), (z = + h) ].

Formulas (4) and (5), regardless of the magnitude of the current in dipole AB in any three-dimensionally inhomogeneous medium at each point in space in the direction of the vertical coordinate z at the sensing point N in the plane passing through the measuring electrodes at any distance x between the dipole source AB and sounding point N for geometric sounding and throughout the time t _i when sensing on transients, ensure that the horizontal component of the current density j _{xy is} equal to zero. This always happens regardless of the fact that in the process of changing the distance x between the dipole source AB and the measuring point N or in the process of changing the time t of transients, the factors k ₁ (to), k ₂ (to), k ₃ (to) and k ₁ (ti), k ₂ (ti), k ₃ (ti) change. Due to this, when solving the inverse problem, the side effect is eliminated, i.e. the electric field in a three-dimensionally inhomogeneous medium at the sensing point N, described by formulas (4) and (5), always practically coincides with the field in a one-dimensional horizontally layered medium with unlimited interface.

This allows us to solve the inverse problem at the measuring point N for a three-dimensionally inhomogeneous medium using the well-known analytical solution for a one-dimensional medium with horizontally layered interfaces.

We also note that equation (3) is an equation of time propagation of an electromagnetic field in a non-polarizable conducting medium, which coincides with the heat conduction or diffusion equation known in mathematical physics and which is usually used in geophysics in resistance methods to study the propagation of an alternating electromagnetic field deep into the thickness of the studied geological rocks.

Moreover, it is believed that the electrical conductivity σ _{of a} given geological horizon is the main and almost the only parameter that determines its electrical properties, has its constant value for each horizon, and does not depend on the frequency of excitation of the electromagnetic field. However, geological sedimentary rocks, when excited by the alternating low-frequency electric current used in geophysics, are characterized by the polarization η caused by them. The induced polarization is a dimensionless quantity that depends on the electrochemical activity of sedimentary rocks. It is defined as the ratio of potential differences measured on a sample of the studied rock after turning off current pulses after 0.5 sec (ΔU _VP ) and before turning off (ΔU). This ratio is usually expressed as a percentage:

The induced polarization of sedimentary geological rocks has unique stability among physical parameters and practically does not depend on the composition of the rocks and their temperature. For ion-conducting (sedimentary) rocks, it depends on many factors: humidity and porosity, composition and concentration of the solution in the rock pores, pore structure and size, clay mineral content, etc. (V.A. Komarov. Electrical exploration by the polarization method. L .: Nauka, 1980, p. 392) [7]. And, most importantly, the induced polarization carries basic information about the presence of a high degree of this polarization of oil and gas deposits in the geological environment.

It has been established (WHPelton, SHWard, PGHallof, WRSill and PHNelson. Mineral discrimination and removal of inductive coupling with multi-frequency JP, Geophysics 43, 1978, p. 588-603) [8] that the electrical conductivity of sedimentary rocks not constant, but depends on the induced polarization and on the frequency of excitation of the electric field according to the proposed, in particular, KSCole and RHCole in the form of its harmonic temporal change in empirical formula:

in which this electrical conductivity depends on ω, σ _о , η and τ,

Where:

η is the induced polarization of the rocks, a dimensionless quantity, usually expressed as a percentage;

τ is the time constant determining the decay rate of the potential difference associated with the induced polarization, sec;

ω is the harmonic frequency of electrical excitation, Hz;

c is a dimensionless exponent, which, although it is not a physical parameter of rocks, σ (iωσ _о ητ) also depends _on it.

The induced polarization η at low frequencies of electric excitation, in contrast to the dielectric constant ε, is numerically not so small compared to the electrical conductivity σ _o for sedimentary geological rocks, measured, for example, at high frequency currents (ω → ∞), when, as can be seen from formula (9), the induced polarization does not occur. Therefore, the polarization caused by studying with the aim of searching and delineating the oil and gas deposits of the geoelectric parameters of sedimentary geological rocks at low-frequency alternating current cannot be neglected.

It is known (Electrical Exploration. Handbook of Geophysics. Ed. V.K. Khmelevsky and others. M: Nedra, 1989, second book, p. 99-102) [9] that for certain sedimentary geological rocks in 0.5 sec after the pulse of the exciting current is turned off, the magnitude of the potential difference caused by polarization, despite its intense decay, still retains levels whose numerical values are from 0.2% to 10% of the numerical values of the potential field differences of the direct field associated with the electrical conductivity σ _o measured as noted above , at high frequency currents, when triggered I polarization is not observed. In order to keep the formula (9) in shape, we write the thermal equation (3) for the case of a harmonic change in the magnitude of the electromagnetic field with time, bearing in mind that:

and given that:

and

Then equation (3) for a conducting non-polarizable medium, taking into account transformation (10), takes the form:

But since the electrical conductivity of sedimentary rocks is not constant, but depends on the induced polarization and on the excitation frequency according to formula (9), equation (12), taking this formula into account, acquires three defining properties of the polarized medium of the parameter -σ _о , η, τ instead of one σ _about and for the case of a harmonious change in the magnitude of the electromagnetic field in time takes the form:

and in general terms, taking into account (9) -

Replacing the frequency-independent electrical conductivity σ _о , which is present in equation (12), with the frequency-dependent σ (iω) present in equation (14), is mathematically correct and theoretically proved (A.K. Kulikov, E.A. Shemyakin. Electrical exploration by the phase method of induced polarization. M: Nedra, 1978, pp. 24-26) [10] and (J.R. White. Geo-electro-magnetism. M: Nedra, 1987, p. 61 -62) [11].

For the proposed method, the problem of detecting oil and gas deposits in the studied rock mass as a mathematical inverse problem is solved according to equation (14) as a function of the distance x between the dipole source AB and the measuring point N, as a function of time t of transient processes and, as a result, as a function, depends on the distance x and time t of the penetration depth of the electromagnetic fields of three independent from each other environmental parameters: conductivity σ _o, η induced polarization and decay time constant τ difference elec induced polarization-empirical potentials.

This problem as an inverse mathematical problem is solved for the proposed method by using the entire array of at least two sources of normalized electrical parameters determined by this method that are described by formulas (4) (5), (6) and (7).

It should be noted that during the sounding by the proposed method, at each measuring point of the profile along its entire length, digital information is obtained on the first differences of electric potentials with a step of discreteness at equal time intervals Δt as with each switching on of the current in the dipole source in the process of sounding (geometric sounding), and in a current pause (transient sounding).

Moreover, if for geometrical sensing, obtaining information about the response of the medium from each determining spacing of the probing installation of the current pulse is necessary, then for sensing on transient processes, information from one probing installation is sufficient, but with such a separation that carries the greatest information about the sensed geological environment (note that at large spacings, the transient signals are weak and difficult to distinguish from the background of interference, and at small spacings, sensing at transients has a low innostyu). Therefore, in solving the inverse problem, in particular, the task was also set to find the most informative separation of the sounding installation among all probed ones using the iteration method.

As a result of solving the inverse problem, taking into account iterations, we find the model of the medium that is closest in geometrical structure and electrical parameters to the studied one, build geoelectric sections of the obtained parameters σ _о , η and τ and select sections with anomalous values corresponding to the position of oil and gas deposits .

Concrete example

Figure 1 presents a block diagram of a device for implementing the proposed method. The block diagram shows a dipole source AB (13 and 14) placed in the thickness of sea water at a height h relative to the seabed or on the day surface, fed by a generator 15 of rectangular current pulses. Note that when placing the dipole source on the day surface, the design of the cable braid of the dipole source is simplified and it becomes possible to increase the electric current in the circuit of the dipole source. The placement of the dipole source on the day surface allows the study of the proposed method at shallow depths of the sea (up to 300 m). At large depths of the sea (over 300 m), due to the absorption of transient signals in the bulk of highly conductive sea water, a horizontal dipole source is immersed to a predetermined depth and its passage along two current profiles is carried out at this depth.

The device (figure 1) contains a measuring installation for measuring the differences of electric potentials. This installation is located on the measuring profile 1, laid along the seabed. Here 2 - N, 3 - M ₁ , 4 - M ₂ , 5 - M ₃ , 6 - M ₄ - electrodes of the measuring unit.

The device measures the first four electric potential differences, where: 7 is a meter for measuring the first electric field potential difference ΔU _M1N between the external electrode M _{1 of the} measuring unit and the central N; 8 is a meter for measuring the first potential difference of the electric field ΔU _M2N between the external electrode M _{2 of the} measuring installation and the central N; 9 is a meter for measuring the first potential difference of the electric field ΔU _M3N between the external electrode M _{3 of the} measuring installation and the central N; 10 is a meter for measuring the first potential difference of the electric field ΔU _M4N between the external electrode M _{4 of the} measuring installation and the central N.

The measured first potential differences of the electric field are processed using formulas (4), (5), (6) and (7) using a computer to obtain the numerical values of R _z (to) and R _z (ti) used to solve the inverse problem.

Note that, according to mathematical modeling, the distance L is six kilometers or more, the distances a and b are one kilometer or more, the immersion depth h of the dipole source is set within twenty meters above the level of the seabed to the day surface.

It should also be noted that this method can also be used for research on land, where measuring sensors and current electric dipoles are placed on the surface of the earth.

Claims (4)

1. A method of marine geoelectrical exploration, in which an electromagnetic field is excited in the thickness of the medium under study, passing through it periodic rectangular current pulses with pauses after each of them using horizontal dipole electric sources; at each sensing point during the current pulse period and during each pause after turning off the current, a sequence of instantaneous values of the first differences of the electric potentials of the direct field and the field of transients is measured with a constant time interval At, while ensuring that the resulting first differences of the electric potentials between the external measuring electrodes
form interpretable parameters and, using them and the differential equation of mathematical physics for the electric field strength of a dipole source in an electrochemically polarizable conducting medium

,
where ∇x is the operator rot;

- the electric field strength of the dipole source, expressed in the equation for the case of a harmonic change in the magnitude of the electric field over time, V / m;
µ - magnetic permeability - a constant value for non-magnetic environments, which include sedimentary geological rocks, and is equal to 4π · 10 ^-7 GN / m;
σ (iωσ ₀ ητ) is the frequency-dependent electrical conductivity of the elements of the medium;
σ ₀ is the electrical conductivity of the elements of the medium without taking into account the effect of the induced polarization, S / m;
η is the coefficient of their induced polarization;
τ is the decay time constant of the potential difference caused by polarization;
solve the mathematical inverse problem by determining three electrophysical parameters inherent in each element of the medium: electrical conductivity σ _о , induced polarization η and decay time constant of the potential difference caused by polarization τ;
and build three time sections according to these parameters;
characterized in that a measuring profile is laid at the bottom of the sea, which passes through measuring devices fixed on the seabed, each of which consists of five electrodes: a central electrode and four equally spaced around it along the vertices of a square, two opposite sides of which are parallel to the profile axis;
within each pulse-pause period, geometric sounding is carried out with the current switched on and sounding on transients during the pause after turning off the current, measuring the first electric potential differences between each of the four external electrodes of the measuring unit and the central one;
in this case, measurements in each measuring installation fixed on the measuring profile are carried out while passing a horizontal dipole source along the current profile laid parallel to the measuring and shifted relative to it in the plan in the orthogonal direction along the y axis by a distance (y = -b) and height by thickness h water layer, sending current pulses to the medium under study at all positions of the dipole source from the point with coordinates [(x = -L), (y = -b), (z = + h)] to the point with coordinates [(x = + L ), (y = -b), (z = + h)], then the dipole source deploy and transfer to another parallel current profile, laid and shifted in plan relative to the measuring one in the opposite direction along the y axis by a distance (y = + b) and height by thickness h, and continue to measure when it moves in the opposite direction from the point with coordinates [(x = + L), (y = + b), (z = + h)] to the point with coordinates [(x = -L), (y = + b), (z = + h)];
Using the measured values of the first electric potential differences, two sources independent of the current strength and the horizontal component of the current density (j _x = 0 and j _y = 0) are determined at the sensing point of the interpreted parameter - one R _z (t ₀ ) based on geometric sensing at all positions dipole source calculated by the formula
R _z (t ₀ ) = [ΔU _M1N (I _B1A1 , t ₀ ) -ΔU _M3N (I _B1A1 , t ₀ )]:
: {[ΔU _M1N (I _B1A1 , t ₀ ) + ΔU _M2N (I _B1A1 , t ₀ ) + ΔU _M3N (I _B1A1 , t ₀ ) + ΔU _M4N (I _B1A1 , t ₀ )] +
+ k ₁ (t ₀ ) · [ΔU _M1N (I _A2B2 , t ₀ ) + ΔU _M2N (I _A2B2 , t ₀ ) + ΔU _M3N (I _A2B2 , t ₀ ) + ΔU _M4N (I _A2B2 , t ₀ )] +
+ k ₂ (t ₀ ) · [ΔU _M1N (I _A3B3 , t ₀ ) + ΔU _M2N (I _A3B3 , t ₀ ) + ΔU _M3N (I _A3B3 , t ₀ ) + ΔU _M4N (I _A3B3 , t ₀ )] +
+ k ₃ (t ₀ ) · [ΔU _M1N (I _B4A4 , t ₀ ) + ΔU _M2N (I _B4A4 , t ₀ ) + ΔU _M3N (I _B4A4 , t ₀ ) + ΔU _M4N (I _B4A4 , t ₀ )]} ,
and another R _z (t _i ) based on sensing on transients for four, selected by the iteration method, the most informative distances with the coordinates of the dipole source [(x = -a), (y = -b), (z = + h)] , [(x = + a), (y = -b), (z = + h)], [(x = + a), (y = + b), [(z = + h)] and [( x = -a), (y = + b), (z = + h)] of all probed, calculated by the formula
R _z (ti) = [ΔU _M1N (I _B1A1 , t _i ) -ΔU _M3N (I _B1A1 , t _i )]:
: {[ΔU _M1N (I _B1A1 , t _i ) + ΔU _M2N (I _B1A1 , t _i ) + ΔU _M3N (I _B1A1 , t _i ) + ΔU _M4N (I _B1A1 , t _i )] +
+ k ₁ (t _i ) · [ΔU _M1N (I _A2B2 , t _i ) + ΔU _M2N (I _A2B2 , t _i ) + ΔU _M3N (I _A2B2 , t _i ) + ΔU _M4N (I _A2B2 , t ₀ )] +
+ k ₂ (t ₀ ) · [ΔU _M1N (I _A3B3 , t _i ) + ΔU _M2N (I _A3B3 , t _i ) + ΔU _M3N (I _A3B3 , t ₀ ) + ΔU _M4N (I _A3B3 , t _i )] +
+ k ₃ (t _i ) · [ΔU _M1N (I _B4A4 , t _i ) + ΔU _M2N (I _B4A4 , t _i ) + ΔU _M3N (I _B4A4 , t _i ) + ΔU _M4N (I _B4A4 , t _i )]} ,
Where
k _i (t ₀ ), k ₂ (t ₀ ), k ₃ (t ₀ ), are the focusing coefficients for geometric sensing, ensuring the equipotentiality of the closed line passing through the four external electrodes of the measuring setup and, thus, the exception inside the region bounded the contour of this line of the horizontal component of the current density j _xy at each sensing point on all geometrical spacings during the current pulse period, which are determined from the system of three equations:

k ₁ (ti), k ₂ (ti), k ₃ (ti), are the focusing coefficients for transient sensing at each sensing point in the current pause at all transient times, determined from the system of three equations and ensuring the equipotentiality of the closed line passing through the four external electrodes of the measuring unit:

t ₀ is the moment in time when the current pulse is transmitted, when the electric field of the transient processes does not differ from its steady-state value corresponding to direct current;
t _i - time points at which transient signals are measured at equal time intervals Δt throughout the entire pause after turning off the current;
ΔU _M1N (I _B1A1 , t ₀ ), ΔU _M2N (I _B1A1 , t ₀ ), ΔU _M3N (I _B1A1 , t ₀ ), ΔU _M4N (I _B1A1 , t ₀ ),
ΔU _M1N (I _A2B2 , t ₀ ), ΔU _M2N (I _A2B2 , t ₀ ), ΔU _M3N (I _A2B2 , t ₀ ), ΔU _M4N (I _A2B2 , t ₀ ),
ΔU _M1N (I _A3B3 , t ₀ ), ΔU _M2N (I _A3B3 , t ₀ ), ΔU _M3N (I _A3B3 , t ₀ ), ΔU _M4N (I _A3B3 , t ₀ ),
ΔU _M1N (I _B4A4 , t ₀ ), ΔU _M2N (I _B4A4 , t ₀ ), ΔU _M3N (I _B4A4 , t ₀ ), ΔU _M4N (I _B4A4 , t ₀ ),
- instantaneous values of the first differences of electric potentials between each of the four external electrodes M ₁ M ₂ M ₃ M _{4 of the} measuring unit and the central N, measured at a time t _{0 of} passing the current in the dipole source as it passes through both current profiles through equidistant from the central electrode N four points, respectively, with coordinates [-x, (y = -b), (z = + h)], [+ x, (y = -b), (z = + h)], [+ x , (y = + b), (z = + h] and [-x, (y = + b), (z = + h)];
ΔU _M1N (I _B1A1 , t _i ), ΔU _M2N (I _B1A1 , t _i ), ΔU _M3N (I _B1A1 , t _i ), ΔU _M4N (I _B1A1 , t _i ),
ΔU _M1N (I _A2B2 , t _i ), ΔU _M2N (I _A2B2 , t _i ), ΔU _M3N (I _A2B2 , t _i ), ΔU _M4N (I _A2B2 , t _i ),
ΔU _M1N (I _A3B3 , t _i ), ΔU _M2N (I _A3B3 , t _i ), ΔU _M3N (I _A3B3 , t _i ), ΔU _M4N (I _A3B3 , t _i ),
ΔU _M1N (I _B4A4 , t _i ), ΔU _M2N (I _B4A4 , t _i ), ΔU _M3N (I _B4A4 , t _i ), ΔU _M4N (I _B4A4 , t _i ),
- instantaneous values of the first differences of electric potentials between each of the four external electrodes M ₁ M ₂ M ₃ M _{4 of the} measuring unit and the central N, measured at a time t _{i of} transients in the current pause in the dipole source when it passes through both current profiles through equidistant from the central electrode N of the measuring unit is four points, respectively, with the coordinates [(x = -a), (y = -b), (z = + h)], [(x = + a), (y = -b), ( z = + h)], [(x = + a), (y = + b), (z- + h)] and [{x = -a), (y = + b), {z- + h )]. 2. The method of marine geoelectrical exploration according to claim 1, characterized in that the current profiles for passing a horizontal dipole source are laid at a given depth of the sea or on the day surface of the sea. 3. The method of marine geoelectrical exploration according to claim 1, characterized in that the distance L is six kilometers or more, the distances a and b are one kilometer or more, the immersion depth h of the dipole source is set within twenty meters above the level of the seabed to the day surface . 4. The method of marine geoelectrical exploration according to claim 1, characterized in that the measuring installation is placed on the measuring profile with an equal step of 200-1000 m

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