RU2488145C1 - Method of constructing seismic images of geologic environment - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: disclosed is a method of constructing high magnification seismic images of a geologic environment based on multifold coverage technique data. Seismic traces of the common source point obtained from seismic exploration, for each of the source-receiver pairs, when forming seismic images of the environment, are repeatedly transferred to given bin centres on the exploration area. The obtained traces are summed after repeated transfer of observed traces to a network of bin cells formed beforehand on the area. The desired original high magnification seismic image of the geologic environment is formed after summation on the entire exploration area.

EFFECT: high information value of data of seismic environment probing.

1 dwg

Description

The invention relates to the field of applied geophysics and can be used to obtain seismic images of the geological environment for exploration purposes.

The method of reflected waves (MOB) is the most effective and developed seismic survey method, which is currently used in the largest volumes in the search and detailed exploration of oil, gas and a number of other minerals on land and at sea. At present, MOB is used to determine the depth and shape of occurrence of interfaces in the context of various geological strata, to identify structural and non-structural traps of minerals, especially oil and natural gas, as well as, under favorable circumstances, to obtain data on lithology and facies composition of rocks , the condition of their formation, the nature of fluids saturating the pore space and other purposes.

The elastic waves in the MOB are excited by explosions in shallow wells or by the action of special non-explosive sources on the surface of the earth. On the surface of the earth, reflected waves from sufficiently long geological boundaries are recorded, at which the wave impedance (acoustic rigidity) of neighboring strata changes markedly. The lithological and tectonic surfaces of the sections of geological media usually correspond to such boundaries. After recording elastic waves, their kinematic (arrival times, propagation velocities, etc.) and dynamic (amplitudes, frequencies, etc.) characteristics are studied. Reflected waves are always detected against the background of interference of deep and surface origin. Therefore, to isolate them, special methods of excitation, recording and processing are used, using the differences in the kinematic and dynamic characteristics of the reflected waves and interference waves. Field observations are performed using special multiple overlap systems that provide significant redundant information, which can significantly increase the ratio of the amplitudes of the useful signals to the average level of the amplitudes of all the interference (while increasing the signal-to-noise ratio). Therefore, an increase in the frequency of observations in seismic exploration is the main direction of its development.

The main modern version of the MOB implementation is the method of the common midpoint (OST), proposed in 1950 in the USA by Maine W. The principle basis of the method is: performing field observations using complex special multiple overlap systems, sorting the observed initial paths of the common excitation point (OTV) ) in the OCT seismograms according to the principle of their belonging to a common midpoint (the middle of the “source-receiver” distance), calculation and input of special static and kinematic corrections, the subsequent summation of tra seismograms obtained from OST one stack trace for each common midpoint. The set of seismic traces thus formed for common midpoints represents the main result of the OCT method - the time section of the OCT. The principal advantages of the OCT method are that in the process of obtaining time sections of high multiplicity, both regular (multiple and converted waves) and irregular interference waves will be significantly weakened.

A method of obtaining seismic images of geological environments corresponding to the above described method of a common midpoint, adopted as one of the prototypes of the present invention. The main provisions of the method were published in 1956 in US patent No. 2.732.906 by William Mayne (Mayne W.H., 1956, Seismic Surveying. U.S. Patent. No. 2.732.906).

An analysis of the existing fundamentals of the theory of the multiple overlap method in the common depth point (MMP-MOGT) version shows that the principal feature of the method is a combination of the following assumptions: regular seismic signals recorded at any vibration receiving point are formed mainly by reflected waves that formed from local sections of flat horizontal seismic boundaries lying at different depths in the area of space under a common midpoint. The normal to these reflective elements of the boundaries intersects with the observation plane - at a common midpoint (OCT) - at a point lying on the reduction plane strictly in the middle between the points of excitation (PV) and the points of reception (PP). The ray theory of mirror reflections does not allow another trajectory. From this it follows that any regular seismic signal along the path of a common excitation point (OTV) carries information only about horizontal reflecting boundaries located under a common midpoint. This means that each registered OTV path allows during the subsequent processing (summing) process to obtain for each OST only one seismic trace t ₀ (x _OST , y _OST ), the combination of which for different common midpoints and gives us the desired seismic image of the geological environment. The assumptions made on the horizontal position of all seismic boundaries under the OCT are, of course, too rigid and simplified assumptions about the structure of real geological environments. However, due to the fundamental features of the MOGT, at the first stages of processing, seismic scanners are forced to make just such assumptions about the structure of the geological environment. Of course, in the future, in order to obtain the true position of all reflecting boundaries in the context of seismic exploration, many special processing procedures (DMO, various migrations, etc.) were developed that completely or at least partially eliminate the influence of this assumption on the final results of the method OST.

The use of the principle of the formation of seismic images of the geological environment in the MOGT based on only reflected waves limits the exploration capabilities of the method in terms of obtaining high seismic images. Actually, images of high multiplicity in the MOGT can be obtained only through the use of increased multiplicity of field work, which, however, leads to a noticeable increase in the cost of such work.

As the second basic prototype, we took the ideas of the method, called the method of seismic side-scan locator (SLBO).

In 1991, Dyakonov B.P., Kuznetsov O.L., Raevsky Yu.G., Fayzullin I.S., Chirkin I.A. and Shlenkin S.I. A new method of seismic exploration was proposed - the SLBO method (RF patent No. 2008697). It is based on the use of diffracted seismic waves to detect areas with increased fracturing in the geological environment.

Scattered (diffracted) waves arising in the region of cracks are present in the recorded seismic wave field, but their energy is 1-2 orders of magnitude lower than the energy of mirror-reflected waves. The selection of such weak scattered waves and the determination of their origin in the medium is based on the principle of the theory of side-scan locators, which is realized both when performing field observations and processing seismic materials.

The technology of field observations of the SLBO includes the creation on the Earth's surface of platforms (apertures) for the reception and emission of seismic waves. To implement a side view, these apertures are located away from the study area. This arrangement makes it possible to exclude intense reflected waves from seismic records and increase the relative energy level of diffracted waves caused by the presence of cracks in the medium. To excite and record seismic waves, standard seismic survey equipment and equipment are used.

The processing of field materials of SLBO includes standard kinematic processing procedures (determining and correcting static corrections, obtaining section velocity characteristics, improving the signal-to-noise ratio, AGC, etc.), as well as special procedures based on the conjugate focusing of radiation and reception apertures for extraction scattered waves and determining their origin. The result of processing are volumetric models of fracturing of the geological environment.

In 2004, on the basis of this technology, a new RF patent was obtained for a method for seismic location of fractured areas (SLTO) (RF patent No. 2251717, 2004). The changes affected mainly the observation technique - the reception aperture and the radiation aperture are proposed to be arranged in pairs on both sides of the studied rock volume. Thus, the transmission of the medium occurs in two orthogonal directions, which makes it possible to identify zones with different orientation of the cracks. In both methods, to extract waves with a lower intensity, in comparison with regular waves and interference waves, it is necessary to carry out a large number of accumulations of the useful signal. This requires a sufficiently large number of both excitation points and reception points. The multiplicity of accumulations can reach 10,000. The accumulation procedure is as follows. The studied array is divided into cubic blocks with a side no more than half the length of the recorded wave. In the center of each block, the waves are focused from all emitters with the in-phase summation of the waves along the receivers from each radiation. The obtained energy value is attributed to each block, according to the maximum values of which the scattering region is distinguished. The main result of the work is the image of the geological environment by the integrated nature of the distribution of dispersion energy in the section. Such energy images, in which there are no mirror components of the field, are very different from the methods used in seismic exploration for representing geological media. In addition, in the described patents, the accuracy of determining the location of the identified zones of increased fracturing and their shape largely depends on the accuracy of the knowledge of the true distribution of velocities in the volume of the medium under study. The determination of speed in these methods is not provided. All this leads to significant limitations that arise when using the SLBO and SLTO methods in seismic exploration practice.

The purpose of creating the described method is to develop a technology for implementing such approaches to the formation of seismic images of the geological environment of high multiplicity and contrast according to the IMF-MOGT data, which, unlike the patents considered, allow, in our opinion, to avoid the above-mentioned contradictions and shortcomings. The goal will be achieved through a different view of the return of seismic energy, which consists in simultaneously taking into account both specular and diffracted seismic waves. Based on this approach, both the process of data recording and the process of their processing will be accordingly modified.

To obtain high-quality seismic images of the geological environment in a given area according to the proposed method, it is necessary to perform seismic surveys using the multiple overlap technique. To do this, a network of straight or curved profiles is created on the ground, on which receivers of seismic vibrations with a predetermined step of their location are placed. To ensure the current level of fieldwork, they must meet the following requirements. The step between the receiving channels should not be more than 50 m. The number of vibration receiving lines should be large enough - at least 8-10 receiving lines with a distance between them of no more than 200-400 meters. The total number of oscillation receivers on the research area should also be quite large - at least 800-1000, which should provide an average density of at least 100 channels per 1 square kilometer. The reception base should provide a relatively uniform coverage of the entire research area for all points of excitement. If possible, the excitation points should evenly cover the entire research area, and their total number on the work area should correspond to an average density of at least 10 excitation points per one square kilometer of the studied area. In all cases, it is necessary, using modern technical means, to determine the actual planned and altitude coordinates of all points of excitation of oscillations and points of their reception. Field work should be carried out using conveyor technology, which significantly speeds up the time of work and reduces their cost. The implementation of field work with the specified characteristics is a guarantee of obtaining high-quality geological images of the studied part of the space by the proposed method.

Subsequent actions in the inventive method suggest that the return of seismic energy from the geological environment irradiated by a seismic source is carried out not only in the form of reflected waves, but also in the form of diffracted waves. From this assumption, it follows that each recorded seismic OTV path can be considered as the result of interference summation of the set of diffracted and reflected waves formed by extended and / or local inhomogeneities existing in the geological environment. These heterogeneities in the geological environment can be imagined either as a small-sized local reflecting element of a section of arbitrary spatial orientation, or as a point local inhomogeneity of the medium. From a formal point of view, we will call these two types of objects seismic diffractors, the most likely planned location of which can occur in the central part of the space between the source and receivers. From this assumption, it follows that each observed OTV path can contain information about the structure of the medium under a set of different points on the surface of observations in this part of space, including those not lying on the source-receiver line. Therefore, each observed trace of any seismogram of the OTV can be repeatedly counted into the set of traces t ₀ for the set of points of the observation plane. Performing such recalculations for various observed traces obtained from different PVs, it is possible for preselected points of the observation plane x, y - the set of centers of calculated bins - to obtain an array of the set of traces t ₀ , the constructive interference of which during the summation, as we can assume, will give image of real diffractors at different times t ₀ (depths z) and for all bins (pickets) of the observation plane. For the practical implementation of the proposed technology for constructing seismic images of the geological environment, it is necessary to recalculate (transform) each seismic trace from each receiver to t ₀ traces on the entire set of pickets that we previously formed — centers of calculated bins — on the observation plane. The set of traces t ₀ in these bins subsequently will form the basis of the sought-after temporary (and / or deep) seismic image of the medium along the profile line (two-dimensional profile images) or in space (three-dimensional images - seismic data cubes).

The form of the formulas for such recalculations depends on the accepted velocity model of the medium. In particular, the formulas for such a conversion using an effective velocity model of the medium have the following form:

, $$t_0\left(x_D,y_D\right)=\sqrt{t^2\left(x_D,y_D\right)-\frac{2}{V^2}\cdot \left(R_{\mathrm{one}}^2+R_2^2\right)-\frac{{\left(R_{\mathrm{one}}^2-R_2^2\right)}^2}{V^{\mathrm{four}}\cdot t^2\left(x_D,y_D\right)}}$$

,

; $$R_2=\sqrt{{\left(x_{ПП}-x_D\right)}^2+{\left(y_{ПП}-y_D\right)}^2}$$

;Where $$R_{\mathrm{one}}=\sqrt{{\left(x_D-x_{P\mathrm{AT}}\right)}^2+{\left(y_D-y_{P\mathrm{AT}}\right)}^2}$$

; $$R_2=\sqrt{{\left(x_{PP}-x_D\right)}^2+{\left(y_{PP}-y_D\right)}^2}$$

;

t (x _D , y _D ) - current time on the observed track; x _PV , y _PV ; x _PP , y _PP ; x _D , y _D are the coordinates on the observation plane, respectively, of the point of excitation, the point of reception, and the center of the bin, into which each observed trace t (x _D , y _D ) is transformed.

Figure 1 shows a diagram explaining the technology of transfer routes, and the notation used.

To obtain a dynamic time section of such an image, each value of the amplitude of the observed seismic trace at any time t (x _D , y _D ) is transferred to the corresponding recalculated time of the trace of the time section t ₀ (x _D , y _D ). As a result of such a transfer, in each settlement bin centered at the point x _D , y _{D of the} settlement network, we obtain a lot of recalculated traces t ₀ (x _D , y _D ), the number of which will determine the multiplicity of the final time section. After the recalculation step, summarize all seismic traces assigned to each given bin.

Very important in such operations is the question of the size of the region in which the coordinates of the points of recalculation x _D and y _D can be for any observed path, and its position on the surface of observations between the source and receiver. This region of the observation surface will be called the “large bin”, and its area will be denoted by SBB. The size, shape and position of the center of the large bin will depend on both the required depth of research and the distance between the source and receiver. We associate the minimum size of a large bin with the size of the first Fresnel zone on the main target research horizon, and for the convenience and simplicity of calculations, we will choose its shape in the form of a circle whose initial radius is equal to the radius of this zone:

R = R ₀ + kL,

where R ₀ is the radius of the first Fresnel zone, L is the source-receiver distance, k is the empirical coefficient that determines the effect of the size of the observation setup on the dimensions of the large bin.

The position of the center of the large bin on the plane of research relative to the coordinates of the PV, PP, and OST depends on what final goals will be set during the formation of seismic images. If the main purpose of such images is to extract mainly reflected waves from subhorizontal interfaces in the medium, then the center of the large bin should coincide with the position of the OCT for each data of the PV and PP. If the size of the large bin is very small and commensurate with the size of the bin in the MOGT method (25-50 m), then this technology for obtaining seismic images will almost coincide with the technology for obtaining images in the MOGT method. In this case, the seismic imaging method will be ideologically close to the MOGT.

If we want to see mainly diffracting objects in the medium or steeply inclined reflecting boundaries in the images, then the center of the large bin can be shifted from the OST to either PV or PP, or along the perpendicular from the source-receiver line. In the latter case, if the size of the large bin is correctly selected and its center is positioned, you can configure the reception system to register exclusively diffracted waves from fractured zones in the geological environment (a partial analogy to the proposal contained in the patent according to the SLTO method). Note that all of the above with minimal changes is transferred from 3D technology to the profile technology for constructing 2D seismic time sections.

The second important issue in the proposed technology for obtaining seismic images of the geological environment is the question of how and how many points of reference should be placed within a large bin. We will identify the network of traces we need to build a seismic image with the network of calculated bins. The centers of the calculated bins are the coordinates of the points of conversion x _D , y _D. Studies have shown that the size of the calculated bin can be chosen quite small, for example 20 * 20 m, 10 * 10 m or less. At the same time, it was found that the size of the network of calculated bins, in contrast to the classical technology of processing MOGT data, is practically independent of the parameters of the 3D field work methodology (from the distances between PP, PV, BOB, LPV, etc.). As a result, it turns out that the summation factor in the proposed technology (with the same bin sizes with MOGT-3D) can be 5-15 times (for 2D jobs) and 50-200 times (for 3D jobs) higher than in standard MOGT technology . This can be explained on the basis of the analysis of the modified multiplicity estimation formula in the MOGT-3D survey design theory:

, $$F_{3D}=SD\cdot NC\cdot S_{\mathrm{AT}\mathrm{AT}}\cdot \frac{N_{\mathrm{one}}}{N_2}\cdot \mathrm{one}{0}^{-6}$$

,

where SD is the density of sources (PV) per 1 km ^{2 of} the survey network; NC is the average number of active channels for 1 PV; S _BB is the area of large bin in m ² ; N ₁ is the number of calculated bins into which each observed trace is recalculated; it varies within: 1 <N ₁ <N ₂ ; N ₂ is the total number of calculated bins that make up the large bin.

Consider an example of calculating the estimated multiplicity of seismic surveys on an area of 200 km ² (20 × 10 km). Shooting parameters: the distance between the receiver - 50 m, between the receiver - 50 m, between the receiver lines - 300 m, between the receiver lines - 400 m, the number of active channels on the receive line - 120, the number of receive lines in the template - 10. In standard technology MOGT, with tight binding to the bin size to the step between PPs, we are forced to use a 25 × 25 m bin network. The total number of observations for such work will be 40 (Bondarev V.I., Krylatkov S.M., 2011, Seismic exploration. Textbook for universities in two volumes Ekaterinburg: Publishing House of the Ural State Mining University. In the proposed method, the size of the bin network can be arbitrarily selected. We will show the option of constructing images with bins of 25 × 25 m in size for comparing the multiplicity with the multiplicity in the CDP. We will select the big bin in the form of a square with a side of 400 m. As a result, applying the above parameters, we will get the expected seismic multiplicity of 9600. From here, we can do the conclusion that the multiplicity of the obtained seismic images will be significantly higher than in the MOGT, which leaves significant reserves for increasing the spatial resolution of seismic exploration by using a denser network of calculated ins.

The proposed technology for constructing seismic images of the geological environment has a number of advantages over all existing methods known to us:

- the method allows to obtain seismic images of very high multiplicity, the values of which can be two orders of magnitude higher than in the standard method of a common midpoint; at the same time, the multiplicity of seismic images for areal observations, even when using standard field technology today, can be brought up to ten thousand or more;

- to a large extent, restrictions on the use of arbitrary irregular observation networks are removed, which can significantly reduce the cost of field work and / or significantly reduce their environmental impact on the environment;

- this method of constructing geological images can be especially promising with widespread use in the field of cableless telemetry equipment and equipment;

- due to some reduction in the requirements for the multiplicity of the final seismic images, it is possible to reduce the number of used excitation points, which will entail a reduction in the cost of the total cost of field work;

- there is the possibility of a significant increase in the multiplicity of the final seismic images in areas directly adjacent to the boundaries of the work area;

- the influence of the structure of the observation network on the obtained seismic images is significantly reduced due to the weakening of the influence of "footprints" - traces of the structure of the field arrangement of sources and receivers;

- due to the appearance of additional control parameters - the shape, size and position of the center of the large bin on the observation plane, it becomes possible to configure the observation system to preferentially isolate certain seismic waves, and, therefore, elements of the geological environment;

- the resulting seismic image of the geological environment has the properties of a migrated image - sources of secondary seismic waves are in places of their actual location.

From the foregoing, it follows that the invention proposes the following new sequence of actions that provides the construction of seismic images of the geological environment of high multiplicity and contrast according to the results of standard seismic surveys using the multiple overlap method by implementing the following eight steps:

1) the implementation of field seismic surveys using multiple overlap technology, the main parameters of which must satisfy the above conditions;

2) creating for the area (or observation line) of research the network of centers of calculated bins we need, the density of which can be 2-5 times higher than the existing standard theory for designing 3D and 2D observation systems provides;

3) the choice of the shape, size and position of the center of the big bin we need relative to the common midpoints, these parameters will be used in the formation of images in all pairs of "source-receiver";

4) recalculation of each seismic OTV trace of the selected oscillation source to all centers of the calculated bins falling within the area of a specific large bin;

5) the repetition of the operation referred to in paragraph 4, for all other points of excitation;

6) calculation in each calculated bin of the number of recalculated traces that got here to determine the actual multiplicity in it;

7) summing the seismic traces accumulated in each calculation bin and assigning the resulting trace to this calculation bin;

8) the formation of a collection of such traces of a cube of seismic data (profile section), which after standard processing (filtering, adjustment, deconvolution, etc.) is the desired seismic image of the geological environment of high multiplicity and contrast.

As the final technical result of this invention, we obtain a method of forming seismic images of high magnification and contrast, which will allow us to solve a number of new and important geological problems based on the implementation of a number of the above advantages of the proposed method for constructing seismic images of the geological environment according to the multiple overlap method. The claimed method is original and has a scientific novelty. To implement the method in the production of seismic exploration does not require a significant change in their technology, thus, the method is industrially applicable.

Claims (1)

A method of constructing seismic images of the geological environment from seismic data using the multiple overlap method (MPP) based on the use of an array of samples of signal amplitudes taken at time instants that are spaced apart from each other by a constant step (sampling step) for a given recording time from a variety of geophones located at a given distance from each other in geographic space and defining a given spatial area for constructing the desired image, during which They summarize the set of seismic traces, with a common midpoint, for different pairs of sources and receivers into the final traces on each elementary calculated cell of the area (bin) to form the final image, characterized in that on a pre-formed array of points for assigning the desired result located inside the bin , according to the coordinates of the selected source and any point of vibration reception, a special area is formed with the center at the midpoint (large bin), the size and shape of which is fixed, and includes a series of dots previously formed inside the bins caught in this region, each of which is selected to transform the source all traces recorded from this excitation point; in a similar way, the traces obtained from all other points of excitation are transformed, as a result of which an accumulated array of transformed traces accumulates in each calculation bin, based on the subsequent summing of which the final traces of the desired three-dimensional seismic image of the geological environment of high multiplicity are formed in each calculation bin.