In order to effectively analyze the influence of observation system on pre-stack imaging, the analysis and evaluation methods of observation system are studied, and new evaluation technologies of 3D seismic observation system such as offset uniformity analysis are formed, which provide new technical means for observation system analysis.
1. consistency analysis technology of bin attributes of observation system
The uniformity of bin attribute of observation system includes the uniformity of offset attribute, the uniformity of azimuth attribute and the uniformity of coverage times, in which the uniformity of offset attribute includes two meanings: first, the offset is evenly distributed from small to large, without omission; Second, the number of traces on each offset gather is as consistent as possible. Azimuth attribute uniformity means that small offset is distributed in all azimuth angles, which is beneficial to attribute analysis of all azimuth angles. Under the condition of meeting the sampling criteria of prestack migration, the observation system with uniform sampling and consistent bin attributes is suitable for prestack migration and will have the smallest migration noise and the weakest acquisition trace.
Figure 4- 1 1 shows the analysis results of offset uniformity caused by 8-line receiving observation system and different rolling distances. Each circular area in the figure represents the coverage of an offset segment with a radius of 1000 m around the imaging point in the work area. The offset range in the lower left corner is 0 ~ 100 m, and from left to right, it is 10 1 ~ 200 m and 20 1 ~ 300 m respectively. As can be seen from the figure, with the increase of the rolling distance of the wire harness, the offset distribution becomes more uneven and the offset covers "holes". As a result, the prestack migration accuracy decreases with the increase of the rolling distance of the beam.
Fig. 4- Analysis Diagram of Deviation Uniformity of Wire Harness of Line Receiving Observation System at Different Rolling Distance +0 18
2. Focusing beam analysis of observation system
* * * Focused point (CFP) theory describes prestack seismic migration with dual focusing processes (transmitting focusing and receiving focusing). After double focusing, the influence of acquisition system operator and propagation operator is removed from seismic records, and the purpose of estimating reflection coefficient is realized. For the whole work area, the final dual-focus imaging response result of point scattering is the result of accumulation and summation of all templates, that is, when the whole work area is rolled, the total resolution function of the observation system is the product of the focused source beam and the focused probe beam in the template space domain of the observation system, that is, the resolution function is
RF(x，y，z，ω)=∑BS(x，y，z，ω)BR(x，y，z，ω) (4- 1)
Among them, BS(x, y, z, ω) and BR(x, y, z, ω) are focused source beam and focused detector beam defined by double focusing theory respectively. According to the product principle, the contribution of the source and the receiving point to the resolution function can be analyzed respectively, and the factors that may cause problems can be seen, and the necessary optimization can be taken at the source or the receiving point.
The above resolution function is analyzed for scattering points at specific spatial positions. The resolution function is calculated for a number of scattering points with a certain range and density on the same plane (the distance between scattering points is generally the size of the observation surface element), so that the imaging difference between the observation system template and the scattering points rolled to different positions can be analyzed, that is, the acquisition trace or the lateral illumination uniformity can be analyzed.
3. Pre-stack migration response analysis of observation system
Figure 4- 12 shows the 8-line and 8-gun observation system, the brick wall observation system evolved from it and the 45 oblique observation system. Fig. 4- 13 shows the results of prestack migration analysis of scattering point models of three observation systems. As can be seen from the figure, the imaging effects of the three observation systems are not much different, but the details are different: in the in-line direction, the brick wall observation system has more noise than the other two observation systems, and the orthogonal observation system has the best effect; In the direction of intersection line, the noise of brick wall observation system is obviously more, and there is no obvious difference between the other two observation systems. Only at the point indicated by the arrow, the noise of the oblique observation system is weaker than that of the orthogonal observation system, but it is very subtle.
Fig. 4- 12 orthogonal 8-line 8-gun observation system (left), brick wall observation system (middle) and 45 oblique observation system derived from it (right).
Figure 4- 13 Pre-stack migration responses of scattering points of three observation systems
(The on-line part is shown above, and the cross-line part is shown below)
4. Forward simulation evaluation technology of 3D observation system.
Forward modeling is an important tool for seismic acquisition design and imaging method research. According to the specific geological conditions of the target area, a more accurate geological model is designed, and the imaging effect and the ability to solve geological problems of different observation systems are analyzed through forward simulation. Kirchhoff forward modeling technology provides a high-precision and feasible technical tool for the evaluation of three-dimensional observation system. This method can not only adapt to complex models, but also overcome the singularity of ray tracing method, and its calculation efficiency is obviously better than that of wave equation method.
5. Lighting analysis technology of seismic wave field
Through the concepts of effective illumination source and seismic wave energy intensity, the relationship between illumination and acquisition and structural imaging is established, and the close relationship between source illumination and acquisition response of acquisition system, especially source direction illumination, acquisition inclination response and migration imaging quality is revealed. The effective lighting design method of acquisition system based on target-controlled lighting is proposed, and the corresponding evaluation and optimization principles of acquisition system are given to guide the acquisition of target-oriented structure.
For the target in Wenxi fault zone, select the target horizon as shown in Figure 4- 14 to control the lighting, and get the energy distribution of the surface composite non-point source and the lighting wave field (Figure 4- 15). The energy of surface composite area source is concentrated in the position of shot point 1 13 ~ 187, which is mainly due to the influence of these shot point data on the design.
At present, the information contained in multi-coverage seismic data is extremely uneven for imaging. In some areas of the imaging space, there may be a lot of redundant information, while in other areas, there may be insufficient coverage, resulting in insufficient information. It is a direction of observation system research and development to analyze wave field illumination, optimize the layout of target imaging observation system and improve the imaging accuracy of geological targets.
Figure 4- 14 Setting Lighting Target Based on Geological Model
Fig. 4- 15 energy distribution of surface composite plane source (top) and illumination wave field (bottom)
(2) Optimal design method of observation system based on prestack imaging.
Based on the requirements of pre-stack imaging for seismic spatial sampling, the influence of observation system parameters and layout on pre-stack imaging is analyzed, and the design method of optimized observation system based on pre-stack imaging is formed.
1. Determine the spatial sampling of the target work area according to the pre-stack migration spatial sampling standard.
According to the geological and geophysical parameters of the target area (including the buried depth, velocity, dip angle and seismic wavelet frequency of the main target layer, etc.). ), and calculate the basic sampling parameters of the observation system according to the spatial sampling criterion of prestack migration (Equation 4-2).
Theory and practice of fine oil and gas exploration in mature exploration areas
Where Δ ρ is the sampling interval when traveling underground, that is, the distance between the center points of bins. Ts is the travel time from the shot point to the imaging point, tr is the travel time from the detection point to the imaging point, and ρs and ρr are the distances from the shot point to the detection point and the imaging point on the same horizontal projection plane, which can be expressed as:
Theory and practice of fine oil and gas exploration in mature exploration areas
Subscript * is a wildcard, indicating the coordinates S and R of the shot point or detection point, and I indicates the ground position of the underground imaging point projected onto the recording surface. For prestack migration, if the spatial sampling criterion of formula (4-2) is not satisfied and the sampling interval is too large, strong aliasing will occur during migration.
2. Automatic optimization design of observation system based on the principle of uniform sampling and consistent box attributes.
The so-called uniform sampling means that the sampling distribution of offset and azimuth within bins is uniform, while the consistency of bin attributes means that the distribution of coverage times, offset and azimuth between bins should be basically consistent. Under the condition of meeting the sampling criteria of prestack migration, the observation system with uniform sampling and consistent bin attributes is suitable for prestack migration and will have the smallest migration noise and the weakest acquisition trace. In order to make the offset distribution uniform in a single trace set, it is hoped that the offset increment (or change rate) between adjacent offset pairs in the n-covered trace set is exactly (n- 1) of the maximum offset, that is, it satisfies:
Theory and practice of fine oil and gas exploration in mature exploration areas
Where Xmax and Xmin are the maximum and minimum misalignments, respectively. The offset distribution at this time is the most ideal uniform distribution. Therefore, the following formula can be used to describe the inhomogeneity of the offset distribution in the bin:
Theory and practice of fine oil and gas exploration in mature exploration areas
Where n is the number of times of coverage and Xi is the ith offset. Generally σ2≥0. The smaller σ2 is, the more uniform the offset distribution is. In the most ideal case, σ 2 = 0.
According to this requirement, given the geological target parameters and basic sampling requirements, a series of observation systems can be automatically designed according to the principle of uniform sampling and consistency of bin attributes, and sorted according to σ2. The smaller σ2 is, the better the performance of the observation system is.
3. The pre-stack imaging performance of the observation system is analyzed by focusing beam and migration uniformity.
The analysis chart of offset uniformity can directly analyze the plane distribution characteristics of offset, and the focused beam can analyze the focusing amplitude accuracy of the observation system and the traces left by the rolling of the arrangement template. The combination of the two can quickly analyze the prestack migration effect of the observation system, which is beneficial to quickly optimize the observation system and the observation system with better "basic attributes"
Focusing beam technology is used to analyze beam rolling (Figure 4- 16). The observation systems used in the figure are all 8-line receivers, but the difference is the number of shots in the beam and the rolling distance of the beam caused by them. From the analysis of focused beam, it can be seen that the rolling distance of array (beam) has great influence on prestack imaging. In the design of actual observation system, it is necessary to strengthen the analysis and demonstration of beam rolling distance and make the best choice between construction cost and good imaging ability.
Fig. 4- 16 resolution function of beam rolling prestack migration at different distances in the observation system
(a)8 lines and 4 guns, rolling 200 meters; (b)8 rows 16 guns, rolling 800 meters.
4. Analyze the pre-stack imaging effect of the observation system through the pre-stack migration response of scattering points.
Scattering point model can verify the pre-stack imaging effect of observation system, analyze the subtle differences between observation systems, and get an observation system with better imaging ability. Figure 4- 17 shows the comparison between the results of observation systems with different widths and the prestack migration response of scattering points. The observation systems used in the figure have the same parameters, except the number of receiving lines and the width (and horizontal coverage time) of the array. The migration response analysis shows that even if the acquisition parameters of the observation system in the longitudinal direction remain unchanged, the imaging in this direction will be obviously improved with the increase of the array width.
Figure 4- 17 Pre-stack time migration in-line profile of recorded data with different array sheet (wire harness) widths
(a) Four wires and eight guns with a width of 400 meters; (b) Eight lines and eight guns, 800 meters wide; (c) 16 line, 8 guns, 800 meters wide.
(3) The practice of observation system optimization design technology in high-precision seismic exploration in Dongpu sag.
In recent years, the optimal design method of observation system based on prestack imaging has played an important role in seismic exploration in Zhongyuan Oilfield. Through careful design and comprehensive evaluation, the design level of the observation system has been greatly improved, laying a solid foundation for obtaining high-quality seismic data. The main parameters of observation system design are shown in Table 4-5. The sampling density of the observation system is high, and the detection point density of 50m×50m and the excitation point density of 80m×80m make the seismic spatial sampling dense and continuous.
Table 4-5 High-precision 3D acquisition parameters in Machang area in 2005
Figure 4- 18 Analysis of High Precision 3D Seismic Observation System in Racecourse
Figure 4- 18 is the analysis result of Machang high-precision 3D seismic observation system. Figure 4- 18a is the resolution function of focused beam analysis, which shows that the observation system has good energy focusing effect and weak vertical and horizontal noise, which is beneficial to high-precision prestack imaging. Figure 4- 18b shows the analysis of transverse amplitude illumination. It can be seen that the illumination uniformity of the observation system is high, the amplitude error is within 0.5%, and there are only weak acquisition traces. Fig. 4- 18c is a wave field illumination profile, which shows that the underground target is well illuminated without obvious illumination shadow. Figure 4- 18d is the prestack migration profile recorded by forward modeling, which shows that the designed observation system can basically meet the imaging requirements of complex geological targets in this area. Through careful design and comprehensive evaluation and analysis, the design level of observation system is improved, which lays a solid foundation for obtaining high-quality seismic data.
The optimized observation system scheme is used for seismic acquisition in Machang area. From the comparison between the old and new (1990 data) profiles (Figure 4- 19), it can be seen that the wave group characteristics of the new high-precision seismic data are clear, the fault imaging is clear, and the imaging accuracy is obviously improved.
Figure 4- 19 Comparison between Racecourse High-precision 3D Earthquake and Racecourse 3D Seismic Profile
In the past, the parameter analysis of observation system based on the assumption of horizontal layered homogeneous medium and the demonstration of bin attributes were far from meeting the requirements of complex surface and complex structure, which affected the imaging effect of seismic exploration. The observation mode design technology based on pre-stack imaging effect and complex model makes the observation system design more reasonable and effective, which is of great significance to improve the imaging accuracy of pre-stack migration in complex structural areas.