SU1223180A1 - Method of ground geoelectric axial sounding - Google Patents

Abstract

The method of ground-based geoelectric sounding of horizontal inhomogeneities located in a layered medium with subhorizontal interfaces. Excite the incision with a source of electric current of force I, measure at a distance g from the source in the interval Dg of the first (Dig) and second () axial differences of electric potentials and determine the calculated one based on them. ./ of the mapped parameter (apparent electrical resistance ft) using the formulas 2 Хг Д г А Д- | ЕИЛИ Yas glTr-А г I is measured except the first and second axial potential differences by the third (), and the mapped parameter (vertical current penetration A), is determined by the formula LL Y Ur Y Ur Y Ur (-V by which anomalies are judged on the position of the heterogeneity boundary. Moreover, if geometrical probing is carried out, then 7 f will be studied depending on the variation of the spacing of the sounding installation; t frequency sounding, then L is studied depending m frequency variations (tO) the source field and if sensing is carried out becoming floor, then A study depending on variations vre- Meni (t) Formation floor 6 yl i (w from O to 00..

Description

one

The invention relates to methods for conducting geophysical studies using an electric field source and is intended to map the boundaries of horizontal inhomogeneities in subhorizontal environments. boundaries, in particular for mapping the boundaries of oil and gas deposits.

The aim of the invention is to improve the accuracy of the mapping of the boundaries of the horizontal inhomogeneities in a layered medium with sub horizontal separation boundaries.

FIG. 1 shows the theoretical model of the medium; Fig. 2 is a diagram of the installation of sounding when measuring the first difference of electric potential; in FIG. 3, the same for measuring the third difference; 4 is the same for measuring the second potential difference; in FIG. 3 a pallet of sounding curves; figure 6 is an example of the results of measurements of the model of the medium.

In order to suppress the influence of the direct field on the measurement results, the following options for implementing the method are possible.

If the test medium is excited by a sinusoidal current, the receiver is separated from the source and the distance is not less than the doubled depth of the test.

object and lead during the entire time interval of the excitation of a continuous measurement of the signal at the input of the receiver.

If the medium under study is excited by alternating current pulses with pauses between each of them, then using the sync switch to the receiver input, a signal from the sensor is given only during those time periods when there is no current in the current circuit (transient signal), and from this signal the receiver only the main (first) harmonic is extracted and measured, i.e. that harmonic, which follows with a frequency equal to the frequency of the switching cycle of the current pulses. In this embodiment, the measurement result is practically independent of the distance between the receiver and the source, which allows the use of installations with small spacings, for example, equal, to the depth of the object under study.

5 0 5

about

five

five

802

As a result of the measurements, a sounding parameter L is obtained, which is a function of the measured first, second and third differences of electric potentials, as well as the separation value, r (the distance between the source and the center of the base Dg, at which electrodes are arranged, with which measure the indicated potential differences). Moreover, if geometrical sounding is carried out, then L is studied depending on the change in the spacing (g) of the probe installation; if frequency sounding is performed, then A is studied depending on the frequency variation (W) of the source of the field; if probing is carried out by becoming a field, then it is studied depending on the change in the time (t) of becoming a field.

As a reference, the medium is taken not homogeneous, but a medium with plane-parallel boundaries, in which the probing parameter under study does not depend on the sounding argument. To derive the formula by which this parameter is determined, we use Fig. 1, which shows a model of a two-layer cut with a flat horizontal interface. The first medium has the power h and the remote electrical resistance f, the second has the resistance L and is unlimited in power.

The point I, which excites the electric field in the section, is supplied from the source to the point O, which is located on the day surface. For sufficiently large ratios of the coordinate r to the power of the first layer h (r / h-) and the ratio, 1 I potential of the field in the first medium does not depend on the vertical coordinate Z, then the change in this potential between two cylindrical surfaces and, where 1-8 - -coordinates of these surfaces, equal to

.dU I

P ,, g 2ThTr

(one)

or

R

(2)

where 1 is the horizontal current flowing

through the surface d - increment, separation. From the conditions of the continuity of the current vector in a closed surface, it follows that the change in the horizontal current 1- between and Bgchvv is equal to the vertical current 1 flowing in this section through the interface into the second medium, i.e.

dit 1l

On the other hand

V, 1

 Z 2T r.dr

where A2 is some path through which current flows in the second medium.

Differentiating with respect to r (2) and substituting (3) and (4) into this result we get

-a -f-jzV.-

R. 1 2 where -r- g L is the desired parameter

medium models, consisting of two separable variables of this model (resistance and geometrical dimensions) and not depending on the coordination of sounding or on the supply current of source I.

The parameter A characterizes m verti-. Local penetration of the floor into the test medium. Since this parameter determines the measure of focusing the field vertically, we will call it the parameter of the vertical penetration of the field, which is determined by measuring d and dUi „

 Tr

 dy

l YE1 + 1 dr. and u r

Parameter A in a radially homogeneous layered medium does not change if equation (5) is repeatedly differentiated with respect to r, but any horizontal inhomogeneities (inclusions) and from the boundaries at each successive differentiation emerge much clearer than before this differentiation. Therefore, the parameter L is determined from equation (5) or after a single or multiple fold differentiation according to r of this equation depending on the required degree of accuracy of mapping of a local object INOG9, which lies in a layered medium, i.e. from the equation ()

(6)

d dr

d and. 1 dU, 2 dr - °

31804

As the practice of field studies has shown, to map the contour of an oil and gas reservoir, at depths of up to five kilometers, it is sufficient to use a single differentiation of equation (5).

Then

d, d u I dU

(u) o

ten

dr Mr r dr

(eight)

i.e.

L,

(9)

15

20

25 about

35

40 45 - jg

From formula (9), it follows that to determine L, it is necessary to measure the first, second, and third derivatives of the potential of the source field. In practice, Directly (we emphasize) is measured on the interval D g - the first potential difference & U proportional to dU, the second difference proportional to, and the third difference proportional to d U. Formation through the difference uUj - aU2, separately measured at points Gd and g , the first differences A и and d, and through the double difference lHD - 2AUz +, measured separately at the points r, rj and Gz of the first differences di, dig HAUj, sometimes practiced in electrical prospecting, is meaningless, since usually the potential differences div are explored accurate to 5%, and uh This means that the magnitude of the measurement error of the first differences is commensurate with the values of the second difference and is in order of magnitude higher than the values of the third difference.

In Fig. 2, point O is the source of the field, 9-12 are the measuring electrodes, 13-16 are the resistors, 17 is the measuring device for measuring the first difference of electric potentials AU in the interval Dg equal to the distance between the measuring ground 9 and 12.

Fig. 3 is a schematic diagram of the measurement of the third difference of electric potentials А U between the grounds, designated respectively 9, 10, 11 and 12, 17, the measuring device, and 13, 14, 15 and 16, respectively, the resistances RI, Rj, Rj and RM.

Grounds are equidistant from each other., The first ground is connected to

the third 11 through two serially connected electrical resistances R /, and Rj, the second grounding 10 is connected to the fourth 12 through two serially connected resistances Ra and Kc, and to the nodes of the joint. resistances R and Rj, Ra and Its. connect the measuring device, and the resistance values R, | , Ra R J and R h are imposed in such a way that R,.

To directly measure the second electric potential difference D and if there are four different ground connections from each other, the first ground 9 is connected to the fourth 12 through two series-connected resistances R and Rq, the second ground 10 is connected to the third 11 through two series-connected resistances R j and, and the measuring device connects the connection nodes of the resistances Rx and Rk, Rj and R3, and ensure that all four resistances are equal, i.e.

R. - R,

"3 -" -.

It is easy to show that at the terminals of the measuring device 17 (Fig. 3), if the condition R R is fulfilled, the value is proportional to the third potential difference, more precisely

 A and g -2 Alb + and .- UM. 4 4 4 -

where and, Ui, Uj and Shch - respectively, the potentials on the electrodes 9, 10, 11 and 12.

It is also easy to show that according to the scheme (Fig. 4), a measurement is measured at the terminals of the measuring device 17, proportional to the second potential difference D U, or rather

4 and D - Ux -in -Us W ....

---." --- P;

under the condition of equality between the resistances R, Rj, Rj and RJ ((, which are denoted by numbers 13-16 (figure 4).

The parameter D is determined by geometric sounding as a function of the spacing (g) of the sounding installation.

L + E - f league g league h

(12)

,

5 0 5

0

"

0

or frequency sounding as a function of the frequency of the (co) source

 ..

(13)

or probing by becoming as a function of time (t) of becoming

rt) -1 + -) (14) 2 AU6 (t) g AUu (t) Ch

where index g is a sign of the axial location of the source and receivers (electrodes) gender.

Fig. 5 shows a double-layered palette of sounding curves calculated on the basis of the exact solution of the Laplace equation for the proposed method (solid curves) and for the vertical electrical sounding method (dashed curves). On the abscissa axis. the ratio of the probe installation size L to the power of the first layer is L / h, and the ordinate is the ratio of apparent resistance to the resistance of the first layer. In accordance with the earlier notation, L is equivalent to. The number of curves is the ratio of the resistance of the second layer to the resistance of the first layer Pr / Pi.

As can be seen from Figure 5, the constraints imposed by the equation view. (5) cease to occur at L / h 4, i.e.; with r equal to or greater than Lh. In this case, the parameter Л for all relations Pr / Pi is invariant to the installation spacing.

Zing on the basis of the parameter L by spacing, by frequency or time of formation of a field acquires a fundamentally new meaning, since, unlike sensing by means of resistances, it clearly defines the boundaries of local inclusions in layered media and assesses their depth, as well as exclude inclusions from the measurement results. occurring outside the radius of a probe installation.

Figure 6 shows an example of measurement results over a high-resistance heterogeneity in the form of a disk 18 by the proposed method with respect to parameter L (solid curve) and in a three-electrode method with apparent resistivity Yak (dashed curve). Here is the size

. 71

profiling installation L is equal to the diameter of the disk. As can be seen from Fig.6, anomalies from the edge of the disk. very useful, and its maximum value is only about 3% of the magnitude of the normal field. Anomalies of L are localized at the edge of the disk, where its amplitude increases many times over the magnitude of the normal field.

The application of the proposed method only for prospecting and exploration of oil and gas deposits opens up fundamentally new economic and technical capabilities, because, firstly, it allows drastically reducing the number of exploration and squalvesin, the cost of each of which is millions of rubles. The l-rum, using powerful sources of the electromagnetic field, such as MHD generators, allows searching and prospecting for oil and gas deposits at depths inaccessible to other geophysical methods.

Claims (1)

Formula of Invention The method of ground-based geoelectrical axial sounding, which includes the excitation of geoelectric 0 31 five 80 ,. the discharge by the source of the electric field, the measurement at a given distance from the source of the field of the first and second axial differences of electric potentials and the calculation of the mapped parameter, by the deviation of which from the normal, electrical discontinuities in the geoelectric section are distinguished, in order to increase the accuracy of the mapping the boundaries of the horizontal inhomogeneous, with those in a layered medium with sub-horizontal interfaces, the third axial difference of electric potentials is additionally measured and the mapped Lie parameter is determined by the formula 20 () where,,, Lit the first, second and third axial differences of electric potentials; t is the distance from the source of the electric field to the center of the base for measuring the differences of electric potentials; At is the base on which electric potential differences are measured. v /  / g ffT / 7 / 7f / // f / / /// f / FIG. g ///,  ////////////////////////// Srig.Z Г-017 f / / / f / / / f. UT /// 2r  at T "T // I / T ,. well .. t //////////, “/ -Y ABOUT IS //////// у ////////////// //////// TU // 7 ///////////// ////// fig a 5 FIG. 6