RU2071095C1 - Method for geoelectric prospecting - Google Patents

Abstract

FIELD: geoelectric prospecting for determination of parameters of geoelectric layers forming sedimentary section. SUBSTANCE: unsteady electromagnetic field is excited by ungrounded loop or by line AB of finite length grounded at ends of each observation point. Points are positioned at unspecified distances from center of source. At the same time, derivatives from radial and vertical components of magnetic induction is recorded. Ratios $$$ are found for each particular time of formation and parameters of layers of geoelectric section are determined by their assemblage. EFFECT: improved reliability.

Description

 The invention relates to geophysical methods of exploration, in particular to the field of electromagnetic sounding, designed to determine the parameters of geoelectric layers composing a sedimentary section, and can be used in structural electrical exploration, in the search for oil and gas fields, hydrogeological studies, searches for building materials, etc. .

 In the practice of electrical exploration studies of layered media, sounding according to the method of establishing an electromagnetic field in various modifications [1,2] is well known based on the fact that the components of an unsteady electromagnetic field are associated with the parameters of the studied geoelectric section. So, with the commonly used sizes of the source and receiver and with distances between them exceeding the thickness of the layered thickness, the formation process is determined by the total longitudinal conductivity of the section. However, the determination of the parameters of individual geoelectric layers is not possible.

 There is a method [3] in which an unsteady electromagnetic field in the zone closest to the source, with sizes of the source and receiver circuits smaller than the thickness of the layered thickness, is recorded in a time range that includes both smaller and larger products of magnetic permeability and total longitudinal conductivity and power layered strata. At times less than the value of the specified product, the formation process is determined by the parameters of the upper layers, and at times greater than the specified product by the total longitudinal conductivity of the layered thickness. Using the proposed method allows you to divide a multilayer incision into separate layers according to their conductivity.

 The disadvantages of this method are, firstly, the low resolution of the isolation of formations, the conductivity of which is small compared to the total longitudinal conductivity of the section, and, secondly, the multistage determination of the conductivity at each value of the formation time and the associated increase in the influence of various distorting factors , the combined effect of which, ultimately, leads to instability of the result.

 There is also known [4] a method of geoelectrical exploration, based on measuring an unsteady electromagnetic field near a source, the distinguishing feature of which is the measurement of the horizontal component of the electric field strength, the time derivative of the vertical component of the magnetic field strength, the horizontal component of the magnetic field strength and determining the cut parameters from the ratios of the first and the second magnitude to the third.

 The need to measure the three components of the electromagnetic field and the low accuracy of the measurement of horizontal components, due to the influence of interference of natural and artificial origin, lead to an increase in the time spent on observation and reduced resolution. The described method is close to the proposed invention, the purpose of which is to increase the productivity and resolution of research.

 The goal is achieved by the fact that in the proposed method of geoelectrical exploration, an unsteady electromagnetic field is excited using an ungrounded loop, and at each observation point located in the near zone of the source, the time derivative of the radial and vertical components of the magnetic induction is measured and their parameters determine the geoelectric cut.

где М момент незаземленной петли;
r расстояние между точкой наблюдения и центром незаземленной петли;

параметр (глубина залегания "середины" проводящей пленки Прайса-Шейнмана);
h_τ эффективная глубина (глубина залегания поверхности проводящей пленки Прайса-Шейнмана);
t текущее время становления;
μ = 4π•10^-7 Гн/м магнитная проницаемость;
S_τ продольная проводимость проводящей пленки Прайса-Шейнмана, совпадающая с продольной проводимостью реального слоистого разреза.The theoretical basis of the method is as follows:
When an unsteady electromagnetic field is excited using an ungrounded loop located at a height h above the Price-Sheinman conductive film, which is currently equivalent to a real geoelectric section in terms of the magnitude of the fields measured at that moment, the time derivatives of the radial and vertical components of the magnetic induction, measured on the same height at any distance from the center of the ungrounded loop are described by the expressions

where M is the moment of an ungrounded loop;
r is the distance between the observation point and the center of the ungrounded loop;

parameter (the depth of the "middle" of the conductive Price-Scheinman film);
h _τ effective depth (the depth of the surface of the conductive Price-Scheinman film);
t current formation time;
μ = 4π • 10 ^-7 GN / m magnetic permeability;
S _{τ is the} longitudinal conductivity of the Price-Scheinman conductive film, which coincides with the longitudinal conductivity of a real layered section.

которое при известном r позволяет итерационным способом определить величину

используя в качестве первого приближения значение

В условиях ближней зоны (r меньше m) итерационный процесс достаточно быстро сходится.For each specific time t _i

which, with the known r, allows iteratively determining the quantity

using as a first approximation the value

Under near-field conditions (r less than m), the iterative process converges quite quickly.

В предлагаемом способе повышению производительности способствует то, что измеряемые компоненты помехоустойчивы, а их измерение с технической и методической стороны наиболее просто. Разрешающая способность исследований повышается в связи с тем, что точность определения параметров разреза S_τ,h_τ не меньше точности измеряемых компонент.Next, from the expression (1) we find

In the proposed method, increasing the productivity contributes to the fact that the measured components are noise-resistant, and their measurement from the technical and methodological side is the simplest. The resolution of the studies is increased due to the fact that the accuracy of determining the parameters of the section S _τ , h _{τ is} not less than the accuracy of the measured components.

Practically the proposed method is implemented as follows:
On the studied area, the generator dipole unfolds in the form of an ungrounded loop, which is connected to the output of the generator group of the type ERS-67, UGE-50. At the moment the current is turned on or off, an unsteady electromagnetic field arises in the medium.

 At each observation point, which is located in the near zone of the source, a multi-turn receiving loop (frame) and an induction sensor are deployed, from the ends of which an EMF proportional to the time derivatives of the vertical and radial components of the magnetic induction is removed, respectively. These EMFs are simultaneously fed to the inputs of the measuring channels of the TsES-2 electrical prospecting station (TsES-3, TsES-MHD, etc.) and recorded digitally on magnetic tape.

, по которому находят параметр m из выражений (4,5) и затем, используя соотношения (6,7), вычисляют S_τ,h_τ..Registered EMFs are processed and for each specific time of formation t _i determine the ratio

by which the parameter m is found from expressions (4.5) and then, using relations (6.7), S _τ , h _τ are calculated.

 The specific dimensions of the generator dipole, the duration of the supply current, the density of the network of observation points, their minimum and maximum distances from the center of the generator dipole, the parameters of the receiving frames, measuring channels (amplification, sampling, filtering), the time range of the EMF recording are determined by the real geoelectric conditions of the studied area and geological tasks of work.

So, for example, in the conditions of the Middle Ob region, when searching for hydrocarbon deposits from experience with the modification of the ZS method [3], you can use a generator loop with an area of 1 m ² , the distance between observation points from 0.5 to 1 km with a minimum distance from the center of the generator loop, equal to 1 km and a maximum of 5 km, a receiving loop with an effective area of 250,000 m ² (25 turns, 100 m per 100 m), and a standard induction CES sensor, the duration of the supply current and the time range of the EMF registration for about 20 s with channel sampling of 0.005 from.

Claims (1)

The method of geoelectrical exploration, based on the excitation in the environment of an unsteady electromagnetic field using a source in the form of an ungrounded loop and registration at the observation points located in the near zone of the source, the vertical component of the magnetic field and the measurement results judge the cut parameters, characterized in that at each point observations simultaneously measure the time derivative of the vertical component of the magnetic induction B _z and additionally the time derivative of its radial component B _r for each specific time of formation t _i and find the ratio of the measured components, which determine the parameter

and calculate the longitudinal conductivity of the geoelectric section

and effective depth

,
where M is the moment of an ungrounded loop;
r is the distance between the observation point and the center of the ungrounded loop;
μ = 4π • 10 ^-7 GN / m magnetic permeability.