RU2809938C1 - Method for vibration seismic exploration - Google Patents

Abstract

FIELD: geophysics.

SUBSTANCE: vibration seismic exploration method is claimed, which is based on the excitation and recording of vibrational seismic oscillations and includes increasing the relative intensity of single-reflected waves by suppressing multiple-reflected waves already at the stage of field work. It is proposed to first select a train of incident waves from the entire wave field through observations in a well and then use it as a reference signal for cross-correlation or deconvolution of seismic records. The use of a train of incident waves allows, with mutual correlation with it or with deconvolution using this train as a reference signal, to suppress multiple reflected waves contained in the seismic wave field. Upgoing waves isolated after cross-correlation or deconvolution will contain only single reflections, so the amplitude-phase correction of the sweep signal on these waves will be more reasonable and reliable. Incident waves are isolated from the VSP seismic record in at least each of the strata into which the overburden intersected by the borehole is divided. The reference signal is formed by averaging the records of incident waves obtained in different layers after compensating for the time of arrival of the direct wave at different receiving points. The signal corrected by the proposed method is entered as a user signal into the vibration source control systems and is used when mining the entire area of the region. To introduce the method into production, it is proposed to use existing technical means.

EFFECT: improvement in the quality of vibration seismic data, ensuring higher reliability of geological results.

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Description

The invention relates to geophysical methods for studying the geological environment and is intended mainly for searching and exploration of oil and gas fields.

In vibration seismic exploration, correlation processing using reference electrical signals is carried out either directly in the field or at a computer center. It is believed that the correlograms obtained as a result of this processing are the result of optimal filtering, since the reference signal used for correlation is, to a first approximation, considered to be a signal influencing a deep target object. However, the electrical signal acting on the electromechanical components of the vibration source can differ significantly from the signal propagating directly into the medium, and even more so from the signal affecting the target object under study. The signal generated in the vibrator-soil system undergoes subsequent changes in accordance with the following factors: characteristics of the interference group of vibrators; characteristics of high-surface resources; the filtering influence of the medium located under the upper surface area and the nonlinear frequency-dependent attenuation of seismic energy in this medium; characteristics of the recording path, including the cable and the seismic station.

The greatest difficulty is in identifying reflections against the background of intense multiple reflected waves. In standard vibroseismic exploration technologies, the control signal (sweep signal) does not contain multiple reflected waves formed by multiple generatrix boundaries located in the overburden. But when in a real environment a signal affects a target reflecting object, then the single reflection is followed, like shadows, by multiple reflected waves. After the formation of a cross-correlation function with a reference electrical signal (or after deconvolution with such a signal), multiple waves do not disappear anywhere, and it may be impossible to distinguish single reflections against their background. This is especially difficult to do in those areas where the kinematic characteristics of these interference and useful waves are close to each other (Eastern Siberia, Moscow syneclise, etc.).

There is a known method of vibration seismic exploration, based on the excitation and recording of vibrational seismic oscillations and including the correction of excited signals by reducing the relative intensity of the spectrum components for vibrations that are not of exploration interest (Kolesov S.V., Zhukov A.P., Shekhtman G.A. Method of vibration seismic exploration // RF Patent No. 2593782, published 08/10/2016).

This method proposes to additionally excite and record oscillations after the resonant frequency of at least one of the interferences to be suppressed has been determined. Suppression of interference can be achieved by excluding them from the spectrum of excited frequencies, for example by exciting oscillations using reference signals that do not contain resonant frequencies.

The disadvantage of this known method is its focus on suppressing resonant interference, rather than multiple waves, which may interfere with the selection of reflections of exploration interest. In the known method, multiple reflected waves are not attenuated in any way.

The closest technical solution to the proposed one (prototype) is a vibration seismic exploration method, including the excitation and recording of vibrations by geophones located in the near-surface zone and a multi-point VSP probe in a well within a depth interval homogeneous in elastic properties, as well as isolation from the wave field as a reference signal train of incident waves, with which they then perform mutual correlation or deconvolution of seismic records (Zhukov A.P., Korotkov I.P., Shekhtman G.A. et al. Vibration seismic prospecting method // RF Patent No. 278060 dated 01/28/2022, publ. 09/23/2022, Bulletin No. 27).

Compared to the prototype, the proposed method of vibration seismic exploration is characterized by the following significant differences:

• registration of seismic vibrations with the isolation of incident waves from them is carried out not in one homogeneous layer, but in different layers identified within the overburden, intersected by the well;

• formation of a reference signal from incident waves recorded in different layers allows taking into account changes in the field of multiple waves formed in different layers of the geological section;

• greater reliability of signals reflected from different objects contained in the overburden; thereby expanding the functionality of the method is achieved.

The purpose of the present invention is to expand the functionality of the method.

This goal is achieved by the fact that in the method of vibration seismic exploration, including the excitation and registration of vibrational seismic oscillations in the near-surface zone and inside the medium, the identification of incident waves inside the medium and the formation of a reference signal from the incident waves, the incident waves are identified at least in each of the layers, at which are divided by the overburden intersected by the well. In one embodiment of the method, incident wave trains for cross-correlation or deconvolution are formed by averaging incident wave trains identified at different depths within a vertical seismic profile. In addition, a correction for the attenuation of seismic waves and an amplitude-phase correction are introduced into the reference signal, having determined the required parameters from the incident and reflected waves recorded in the well.

The essence of the proposed method is as follows.

The relative intensity of seismic vibrations is influenced by many factors. Among them, multiple reflected waves occupy almost the first place. Suppression of these interferences, as a rule, is based on the difference in their kinematic characteristics from those for single reflections. However, the presence of high-velocity layers in the upper strata of the section leads to the fact that even for large offsets of vibration sources, the differences in apparent velocities are insignificant, and it is not possible to suppress multiple waves using classical methods. The ability to register multiple reflected incident waves within the medium allows one to improve vibroseismic technology in such a way that it becomes possible to exclude multiple waves from the signal used to optimize the control sweep signal. In the proposed method, this possibility is achieved by recording a train of incident waves inside a medium and then using this train to isolate single reflections. Registration of a train of incident waves in different layers makes it possible to take into account in the averaged signal the contribution of incident waves formed in different parts of the geological section. The possibility of achieving a positive effect directly in the process of borehole-land seismic exploration is beyond doubt. In the proposed method, the procedure for identifying useful single reflections is introduced directly into the process of improving the quality of field data in the field.

A train of incident waves inside a medium can be obtained using a multipoint VSP probe for this observation. In this case, the pre-sweep signal can be taken as used in standard vibroseismic surveys in the study area or in a broadband modification (Broadsweep technique). Observations within different layers are carried out so that the separation of waves into falling and rising waves using known methods is carried out most effectively. Observations inside the medium make it possible to determine not only the attenuation of waves from incident and ascending waves, but also the phase distortions introduced into the signal by the earth's strata. Taking these distortions into account makes it possible to calculate the parameters of the sweep signal that are optimal for the studied area of work.

The method is carried out as follows.

Within the area intended for research using the vibroseismic method, observations using the VSP method are carried out in one of the wells. In order to obtain high-quality material, the depth intervals selected for observation in the case of a cased well must be characterized by good quality cementation of the annulus. To carry out the work, you can use a hardware and methodological complex that implements adaptive vibroseismic technology. The vibration source control system GDS-II, developed by Geophysical Data Systems LLC (Zhukov et al., 2011, pp. 226-227), can be used as such a complex. The sequence of procedures in accordance with the invention is as follows:

• according to vertical seismic profiling data or by calculations using a medium model built using acoustic logging and other field geophysical methods, trains of incident waves are formed within each of the selected layers, including a direct wave and multiple reflected incident waves;

• before averaging different trains of incident waves, kinematic corrections are introduced into them, compensating for the time of arrival of the direct wave and multiple reflected incident waves at different receiving points;

• averaged trains of incident waves are used as a reference signal and cross-correlation or deconvolution of VSP records obtained in the well is carried out with such a signal;

• in VSP records, ascending waves are identified, which, after cross-correlation or deconvolution using a train of incident waves, will no longer contain multiple reflected waves;

• trains of ascending waves in the recording time interval corresponding to the arrival times of target reflections are converted into a spectrum;

• the spectrum of ascending wave pulses is used to correct the electrical control signal;

• the control electrical signal, after its correction according to the spectra of ascending single reflections, is used to excite vibroseismic vibrations to study the entire study area. The use of deconvolution, rather than correlation with a sweep signal, is preferable, since this increases the relative intensity of the high-frequency components of the spectrum to a greater extent, and, consequently, the resolution of the method.

The implementation of the proposed invention in the practice of vibration seismic exploration does not require the creation of new technical means and can be started already at the present time. Its use will make it possible to more reasonably control the excited signal, thereby increasing the reliability of the study of productive deposits. The technical result is improving the quality of vibration seismic data.

The proposed method, discussed in detail in the application to land seismic exploration, without significantly changing its essence, can be fully applied when conducting borehole seismic surveys, which are carried out in significant volumes using vibration sources.

In the vertical seismic profiling (VSP) method, the proposed method can be used in a variety of modern modifications of the method (Shekhtman, 2017).

INFORMATION SOURCES

Zhukov A.P., Kolesov S.V., Shekhtman G.A., Shneerson M.B. Seismic exploration with vibration sources. // Tver, GERS Publishing House LLC, 2011.

Kolesov S.V., Zhukov A.P., Shekhtman G.A. Method of vibration seismic exploration // RF Patent No. 2593782, publ. 08/10/2016.

Zhukov A.P., Korotkov I.P., Shekhtman G.A. and others. Method of vibration seismic exploration // RF Patent No. 278060 dated 01/28/2022, publ. 09/23/2022, Bulletin No. 27.

Shekhtman G.A. Vertical seismic profiling. - M: LLC "EAGE Geomodel", 2017. - 284 p.

Claims (3)

1. A method of vibration seismic exploration, including the excitation and recording of vibrational seismic oscillations in the near-surface zone and inside the medium, the identification of incident waves inside the medium and the formation of a reference signal from the incident waves, characterized in that, in order to expand the functionality of the method, the incident waves are isolated at least in each of the layers into which the overburden intersected by the well is divided. 2. The method according to claim 1, characterized in that the incident wave train for cross-correlation or deconvolution is formed by averaging the incident wave trains identified at different depths within the vertical seismic profile. 3. The method according to claim 1, characterized in that a correction for the attenuation of seismic waves and an amplitude-phase correction are introduced into the reference signal, having determined the required parameters from the incident and reflected waves recorded in the well.