A prestack depth migration imaging gather stretching correction method and system

Abstract

The present invention provides a method and system for prestack depth migration imaging gather stretch correction, belonging to the field of exploration seismic data processing. The prestack depth migration imaging gather stretch correction method obtains stretch factors at different offsets and different depths according to the offset and depth of the Kirchhoff prestack depth migration imaging gather, then determines the stretch fixed points and demarcation points, and finally obtains the stretched and corrected seismic data section by section using the stretch factors to achieve the stretch correction of the Kirchhoff prestack depth migration imaging gather. The present invention makes the stretch correction of the Kirchhoff prestack depth migration imaging gather more reasonable, can overcome the problems of unreliable amplitude and frequency caused by artificially selecting reference traces, and the method of the present invention is scientific, easy to implement, and has high calculation efficiency.

Description

Technical Field

[0001] The present invention belongs to the field of exploration seismic data processing, and particularly relates to a method and system for prestack depth migration imaging gather stretching correction, which corrects the stretching distortion of Kirchhoff prestack depth migration imaging gathers and can be applied to the seismic data processing of petroleum geophysical exploration. Background Art

[0002] The quality of Kirchhoff prestack depth migration imaging gathers is directly related to the accuracy and reliability of seismic prestack lithology, physical properties, fluid inversion, and AVO analysis results. However, the currently applied Kirchhoff prestack depth migration imaging gather generation algorithm cannot overcome the waveform distortion problem of far-offset seismic waves, resulting in low quality of the common reflection point (CRP) gathers generated by conventional Kirchhoff prestack depth migration. The stretching phenomenon of gather sub-wave calibration is widespread, and the effects of prestack seismic inversion and AVO attribute analysis based on CRP gather data are poor, seriously affecting the quality of subsequent seismic reservoir prediction and oil and gas detection results.

[0003] Chinese Patent Publication Document CN108508487A discloses a method and device for wavelet stretching correction of seismic trace gathers. It first selects a reference trace; calculates the peak frequencies of the reference trace and each trace to be corrected using Fourier transform; decomposes the reference trace and each trace to be corrected, and searches for the optimal wavelets of the reference trace and each trace to be corrected; performs stretching correction on the optimal wavelets of each trace to be corrected using the optimal wavelet of the reference trace; reconstructs the stretched-corrected optimal wavelets to obtain a stretched-corrected seismic trace gather. The present invention completely eliminates the stretching phenomenon, makes the main frequencies of each trace basically the same, maximally restores the high-frequency components of large angles and large offset distances, and performs amplitude preservation processing, providing accurate amplitude information for subsequent amplitude-versus-offset analysis; Chinese Patent Publication Document CN102879821A discloses a method for precisely flattening seismic event axes for pre-stack trace gathers, which includes pre-processing the collected seismic data; extracting the pre-stack trace gather for which the seismic event axes are to be precisely flattened, represented by a two-dimensional array D with I rows and J columns; setting parameters for precisely flattening seismic event axes; with Nw as the window size and Nm as the window movement amount, determining whether to set a flattening seed point at the center point of a certain waveform comparison window for each window, and calculating the movement amount of the flattening seed point; calculating the stretching sampling coordinates; and performing precise flattening of seismic event axes according to the stretching sampling coordinates; Chinese Patent Publication Document CN106249292B discloses a method for optimizing processing of common reflection point trace gathers. It first estimates the formation dip using the discrete scanning method. To make the dip estimation more accurate, a conventional quadratic polynomial is used to fit the seismic data to estimate the formation dip. Then, median filtering is applied in the sliding time window in the direction of the seismic event axis of the central sample point, and the filtering result is used as the output of the final central sample point, i.e., structural median filtering. After processing, the signal-to-noise ratio of the trace gather is improved. Finally, a model trace is established on the stacked section of the common reflection point trace gather, and residual moveout correction is performed on the denoised common reflection point trace gather. Since there is stronger information correlation in the formation dip direction, it can effectively solve the deficiencies existing in traditional methods, correct the residual moveout while improving the overall signal-to-noise ratio of the trace gather, manifested as the flattening of the trace gather, improving the overall quality of the trace gather, and being able to better preserve the effective signals and formation edge and detail features such as faults and fractures; Chinese Patent Publication Document CN106501859B discloses a method for flattening seismic event axes by moving integration of pre-stack trace gathers, which includes: calculating the flattening amount DT1 between adjacent traces in the original trace gather; applying the flattening amount DT1 to the original trace gather to generate a locally flattened stacked trace gather; calculating the flattening amount DT2 between adjacent traces again using the locally flattened stacked trace gather; selecting the near-trace "stacked trace", and performing moving integration flattening on the flattening amount DT2 to flatten the entire trace gather.

[0004] At present, for the problem of wavelet stretching in the migrated image gathers, Zhou Peng (2016) further optimized the gathers through spectral equalization, while maintaining the relationship between amplitude and offset, and solved the problem of frequency reduction of far-offset data caused by NMO stretching. Rupert et al. (1975) proposed a whole-block move NMO method, which eliminated the distortion of far-offset data in-phase axis through static translation and merging of in-phase axis data blocks. Xiong Xiaojun (2017) proposed an optimized processing technology for the combination of "denoising-flattening-cutting" of prestack gathers, used a non-surface-consistent residual static correction method to flatten the in-phase axis of the gathers in the target layer section, and finally, selected the effective incident angle range of the target layer section to cut the gathers with large-offset stretching distortion. Currently, all Kirchhoff prestack depth migration imaging gather stretching correction methods need to select an initial gather or a reference gather, and then correct the large-offset gathers to this gather. This is an empirical approach, lacking theoretical support, and the obtained results have low credibility. Summary of the Invention

[0005] The object of the present invention is to solve the problems existing in the above-mentioned prior art, and provide a prestack depth migration imaging gather stretching correction method and system. By deriving and obtaining the stretching correction factor of the Kirchhoff prestack depth migration imaging gather, and forming a Kirchhoff prestack depth migration imaging gather stretching correction method based on this stretching factor, the Kirchhoff prestack depth migration imaging gather stretching correction has a quantitative and reliable theoretical basis, providing data guarantee for the use of the post-migration gather.

[0006] The present invention is realized by the following technical solutions:

[0007] In the first aspect of the present invention, a prestack depth migration imaging gather stretching correction method is provided. The method obtains the stretching factors at different offsets and different depths according to the offset and depth of the Kirchhoff prestack depth migration imaging gather, then determines the stretching fixed point and the demarcation point, and finally obtains the stretched and corrected seismic data segment by segment using the stretching factor, realizing the stretching correction of the Kirchhoff prestack depth migration imaging gather.

[0008] The further improvement of the present invention lies in that the method includes:

[0009] (1) Input the Kirchhoff prestack depth migration imaging gather DR(x,z);

[0010] (2) Obtain the stretching factors at different offsets and different depths;

[0011] (3) Determine the stretching fixed point;

[0012] (4) Determine the demarcation points, and each segment is between two adjacent demarcation points;

[0013] (5) Use the stretching factor to calculate the stretched and corrected seismic data point by point and segment by segment.

[0014] A further improvement of the present invention is that the operation of step (2) includes:

[0015] Calculate the stretching factors at different depths and different offsets of the Kirchhoff prestack depth migration image gather DR(x, z) using the following formula (1):

[0016]

[0017] where R(x, z) represents the stretching factor at different depths and different offsets of the Kirchhoff prestack depth migration image gather DR(x, z), x is the offset, z is the depth, and Δz represents the step size in the depth direction.

[0018] A further improvement of the present invention is that the operation of step (3) includes:

[0019] (31) Stack the seismic data with different offsets in the Kirchhoff prestack depth migration image gather DR(x, z) together to form a corresponding stacked gather;

[0020] (32) Determine the wavelet length according to the dominant frequency of the seismic data, and uniformly segment the stacked gather using the wavelet length to obtain the stretching fixed points.

[0021] A further improvement of the present invention is that the operation of step (32) includes:

[0022] Uniformly segment the stacked gather using the wavelet length, that is, the length of each segment is equal to the wavelet length, and the part outside the integer multiple of the wavelet length is taken as one segment;

[0023] Search for the maximum value of the absolute value of the amplitude in the stacked gather within each segment, and take the grid point corresponding to the depth z of this maximum value as the stretching fixed point L i .

[0024] A further improvement of the present invention is that the operation of step (4) includes:

[0025] Taking (L i + L i+1 ) / 2 as the center and 1 / 4 of the wavelet length as the radius, search upward and downward respectively to find the minimum value of the absolute value of the amplitude in the stacked gather, and take the grid point corresponding to the depth z of this minimum value as the demarcation point F i .

[0026] A further improvement of the present invention lies in that the operation of step (5) includes:

[0027] Perform the following processing on a section between two adjacent demarcation points in sequence:

[0028] Keep the depth position of the stretching fixed points in this section unchanged;

[0029] For other grid points, calculate the depth position after stretching of this grid point by using the following formula:

[0030] z′ j = jΔz′ j

[0031] where Δz' j = Δz / R(kΔx, jΔz)

[0032] R(kΔx, jΔz) represents the stretching factor of R(x, z) at the grid point (kΔx, jΔz);

[0033] After the stretching is completed section by section, the seismic data after stretching correction is obtained.

[0034] A further improvement of the present invention lies in that the method further includes:

[0035] (6) Remap the irregular grid obtained after the stretching is completed section by section to a regular grid.

[0036] A further improvement of the present invention lies in that the method further includes:

[0037] (7) After the stretching is completed section by section, after stretching two adjacent sections, the demarcation point shared by the two adjacent sections is divided into two points, and zeros are filled between these two points.

[0038] In the second aspect of the present invention, a prestack depth migration imaging gather stretching correction system is provided. The system includes: a memory, a processor, and a computer program stored on the memory. When the computer program is run by the processor, the following steps are executed:

[0039] (1) Input the Kirchhoff prestack depth migration imaging gather DR(x, z);

[0040] (2) Obtain the stretching factors at different offsets and different depths;

[0041] (3) Determine the stretching fixed points;

[0042] (4) Determine the demarcation points, and a section is between two adjacent demarcation points;

[0043] (5) Calculate the seismic data after stretching correction section by section and point by point by using the stretching factors.

[0044] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0045] 1) The present invention provides a calculation formula for the stretching factor corresponding to different depths and different offsets of the Kirchhoff prestack depth migration image gather, making the stretching correction of the Kirchhoff prestack depth migration image gather more reasonable.

[0046] 2) The present invention provides a specific usage formula for the stretching factor and a stretching correction process for the Kirchhoff prestack depth migration image gather, which can overcome the problems of unreliable amplitude and frequency caused by artificially selecting reference traces.

[0047] 3) The method of the present invention is scientific, easy to implement, and has high calculation efficiency. Description of the Drawings

[0048] Figure 1 is the input Kirchhoff prestack depth migration image gather;

[0049] Figure 2 is the stretching factor diagram corresponding to the Kirchhoff prestack depth migration image gather;

[0050] Figure 3 is the corrected Kirchhoff prestack depth migration image gather;

[0051] Figure 4 is the comparison before and after the correction of a trace extracted at the maximum offset of the Kirchhoff prestack depth migration image gather;

[0052] Figure 5 is the step block diagram of the method of the present invention. Detailed Embodiment

[0053] The present invention will be further described in detail below with reference to the drawings:

[0054] The present invention proposes a quantitative stretching correction method for the Kirchhoff prestack depth migration image gather, and gives a determined stretching factor according to the offset and migration depth, as well as a stretching correction method and process for the Kirchhoff prestack depth migration image gather based on this stretching factor. The present invention uniformly corrects all gathers through the stretching factor, thereby providing a quantitative and reliable theoretical basis for the stretching correction of the Kirchhoff prestack depth migration image gather and providing data guarantee for the use of the post-image gather. The content of the present invention includes obtaining the stretching correction factor of the Kirchhoff prestack depth migration image gather and forming a stretching correction method for the Kirchhoff prestack depth migration image gather based on this stretching factor.

[0055] The method of the present invention first calculates the stretching factors at different offsets and different migration depths according to the offset and depth of the Kirchhoff prestack depth migration image gather, then determines the stretching fixed points according to the stacking result of the migrated gather at this position, then determines the demarcation points near the middle position between adjacent stretching fixed points, and finally calculates the stretched and corrected seismic data section by section and point by point using the stretching factors, completing the stretching correction of the Kirchhoff prestack depth migration image gather.

[0056] As Figure 5 shown, the method of the present invention includes:

[0057] (1) Input the Kirchhoff prestack depth migration image gather DR(x, z): Input the Kirchhoff prestack depth migration image gather generated by Kirchhoff prestack depth migration. Kirchhoff prestack depth migration is a mature technology and will not be elaborated here.

[0058] (2) Calculate the stretching factors corresponding to the Kirchhoff prestack depth migration image gather:

[0059] Use the following formula (1) to calculate the stretching factors corresponding to different depths and different offsets of the Kirchhoff prestack depth migration image gather DR(x, z):

[0060]

[0061] Among them, R(x, z) in formula (1) represents the stretching factors corresponding to different depths and different offsets of the Kirchhoff prestack depth migration image gather DR(x, z), x is the offset, z is the depth, and Δz represents the step size in the depth direction. The stretching factors corresponding to different depths and different offsets of the Kirchhoff prestack depth migration image gather DR(x, z) can be obtained through formula (1).

[0062] (3) Determine the stretching fixed points L section by section according to the selected wavelet length:

[0063] The input Kirchhoff prestack depth migration imaging gather DR(x,z) is stacked together with seismic data at different offsets according to the existing method to form a corresponding stacked gather. Then, the wavelet length is determined based on the dominant frequency of the seismic data (it can be selected from 50 - 400 meters and can be estimated according to the following formula: 1.0 / f0*v0, where f0 is the dominant frequency and v0 is the migration velocity). The stacked gather is evenly segmented, and the length of each segment is equal to the selected wavelet length. The remainder (i.e., the part outside the integer multiple of the wavelet length) is taken as a segment. The maximum value of the absolute value of the amplitude of the stacked gather is searched within each segment, and the grid point corresponding to the depth z of this maximum value is used as the stretching fixed point L i Determine. i represents the number of the fixed point. The number of stretching fixed points is related to the number of segments of the stacked seismic data. The more segments, the more stretching fixed points

[0064] (4) Determine the demarcation point F segment by segment according to the selected wavelet length

[0065] Search for the minimum value of the absolute value of the amplitude of the stacked gather near the middle position between two adjacent stretching fixed points, and use the grid point corresponding to the depth z of this minimum value as the demarcation point

[0066] The specific method is as follows: Taking (L i +L i+1 ) / 2 as the center, where L i represents the position of the stretching fixed point, search for the minimum value of the absolute value of the amplitude of the stacked gather respectively upward and downward with a radius of 1 / 4 wavelet length, and use the grid point corresponding to the depth z of this minimum value as the demarcation point F i . i represents the numbers of the fixed point and the demarcation point

[0067] (5) Calculate the stretched and corrected seismic data point by point and segment by segment using the stretching factor

[0068] The stretching factor corrects the position of each point of the seismic data DR(x,z) in the depth z direction. The seismic data of the original prestack depth migration imaging gather DR(x,z) falls on the grid points of (kΔx, jΔz). Δx represents the offset step, k and j are grid numbers. The offset direction is not corrected. The depth step between adjacent grid points in the depth direction is Δz, and the corrected step is Δz'. Then, Δz' at different depth positions can be calculated by the following formula

[0069] Δz' j =Δz / R(kΔx, jΔz) (2)

[0070] Where R(kΔx, jΔz) represents the stretching factor of R(x, z) at the grid point (kΔx, jΔz), which is obtained by substituting kΔx and jΔz into formula (1) as x and z respectively.

[0071] According to the fixed point L determined in steps (2) and (3) i and the demarcation point F i , the input Kirchhoff prestack depth migration image gather DR(x, z) is divided into several segments in the depth direction according to the demarcation point F i , each segment contains several points, including the fixed point L i , one end point of each segment is F i , and the other end point is F i+1 . During the processing, the depth position corresponding to the fixed point L in each segment i remains unchanged.

[0072] For each segment segmented by using the demarcation point F i , the new grid step Δz′j is obtained through formula (2) by using the stretching factor corresponding to each point. Then, the seismic data of the image gather at the original regular grid z j = jΔz will be mapped to the new irregular grid z′ j = jΔz′ j , at this time, the depth position corresponding to the fixed point L in each segment i remains unchanged, that is, z' j = z j , and the depths corresponding to other points (including the two demarcation points as end points) in each segment have changed through the above calculation.

[0073] Furthermore, since the seismic data is on a regular grid, after obtaining the new irregular grid, in order to facilitate outputting the data to the original regular grid, the irregular grid can be remapped to the regular grid, and the mapping method can be realized by interpolation, that is, the amplitude value of the seismic data of the image gather near the irregular position z′ j is interpolated to obtain the new amplitude value at the depth of the regular grid z j , so that the regular grid is obtained. The specific interpolation algorithm can adopt inverse distance interpolation, parabolic interpolation, etc.

[0074] After the stretching of each segment is completed, since the demarcation point is also stretched, after stretching two adjacent segments, the demarcation point shared by the two adjacent segments is divided into two points, and zero filling can be used between these two points.

[0075] In the above processing, only at the "irregular position z′ i at the fixed point L j " is the same as z′j The depth positions of the points on the regular grid are the same, and the depth positions of the remaining points need to be calculated according to the specific Δz'. j to obtain.

[0076] The present invention also provides a prestack depth migration imaging gather stretching correction system, which includes: a memory, a processor, and a computer program stored on the memory. When the computer program is run by the processor, the following steps are executed:

[0077] (1) Input the Kirchhoff prestack depth migration imaging gather DR(x,z);

[0078] (2) Obtain the stretching factors at different offsets and different depths;

[0079] (3) Determine the stretching fixed points;

[0080] (4) Determine the demarcation points, and each segment is between two adjacent demarcation points;

[0081] (5) Use the stretching factors to calculate the stretched and corrected seismic data point by point and segment by segment.

[0082] The embodiments of the application of the present invention are as follows:

[0083]

Embodiment 1

[0084] Figure 1 is the input Kirchhoff prestack depth migration imaging gather, Figure 1 where the longitudinal unit is kilometers and the transverse unit is kilometers. It can be seen from Figure 1 that as the transverse distance increases, the seismic waveform becomes fatter and the stretching becomes more serious.

[0085] Figure 2 is the stretching factor diagram corresponding to the Kirchhoff prestack depth migration imaging gather of the present invention. It can be seen that as the distance increases, the stretching factor becomes larger, and as the depth increases, the stretching factor becomes smaller. The stretching factor in the upper left corner is the largest.

[0086]

Embodiment 2

[0087] Figure 3 is the Kirchhoff prestack depth migration imaging gather corrected by the present invention, Figure 3 where the longitudinal unit is kilometers and the transverse unit is kilometers. It can be seen from Figure 3 that the corrected imaging gather is very flat, the wavelet morphologies are consistent, and the energy is well preserved.

[0088]

Embodiment 3

[0089] Figure 4It is a comparison before and after the correction of a Kirchhoff prestack depth migration image gather extracted at the maximum offset. Figure 4 The solid line in Figure 2 is the original prestack depth migration image gather, and the dashed line is the prestack depth migration image gather after correction. It can be seen from Figure 4 that the stretching factor corresponding to the large offset and small time in the upper right corner is the largest. Therefore, the stretching distortion of the image gather at the maximum offset is the most serious, and correspondingly, its compression degree is the highest. It can be clearly seen from

[0090] that the gather has been significantly compressed. The present invention aims at the stretching correction problem of the Kirchhoff prestack depth migration image gather. The calculation formula of the stretching correction factor is derived methodologically and a stretching processing flow is established. The Kirchhoff prestack depth migration image gather is corrected by the stretching correction factor, so that there is a quantitative and reliable theoretical basis for the stretching correction of the Kirchhoff prestack depth migration image gather, providing data guarantee for the use of the post-image gather. It can be applied to the seismic data processing of petroleum geophysical exploration. The calculation formula of the stretching factor corresponding to different depths and different offsets of the Kirchhoff prestack depth migration image gather proposed by the present invention makes the stretching correction of the Kirchhoff prestack depth migration image gather more scientific, can overcome the problems of unreliable amplitude and frequency caused by artificially selecting reference traces, provides a basis for subsequent velocity analysis and gather analysis, and has good application prospects.

[0091] Finally, it should be noted that the above technical solution is only one implementation manner of the present invention. For those skilled in the art, on the basis of the disclosed application methods and principles of the present invention, it is very easy to make various types of improvements or deformations, not limited to the methods described in the above specific implementation manner of the present invention. Therefore, the above-described manner is only preferred and does not have a restrictive meaning.

Claims

1. A prestack depth migration imaging gather stretching correction method, characterized in that: The method obtains stretching factors at different offsets and different depths according to the offset and depth of the Kirchhoff prestack depth migration imaging gather, then determines the stretching fixed points and demarcation points, and finally obtains the stretched and corrected seismic data section by section using the stretching factors, realizing the stretching correction of the Kirchhoff prestack depth migration imaging gather; The method includes: (1) Input the Kirchhoff prestack depth migration imaging gather DR(x, z); (2) Obtain the stretching factors at different offsets and different depths; (3) Determine the stretching fixed points; (4) Determine the demarcation points, and each section is between two adjacent demarcation points; (5) Calculate the stretched and corrected seismic data point by point and section by section using the stretching factors; The operation of step (2) includes: Calculate the stretching factors at different depths and different offsets of the Kirchhoff prestack depth migration imaging gather DR(x, z) using the following formula (1): where R(x, z) represents the stretching factors at different depths and different offsets of the Kirchhoff prestack depth migration imaging gather DR(x, z), x is the offset, z is the depth, and Δz represents the step size in the depth direction.

2. The prestack depth migration imaging gather stretching correction method according to claim 1, wherein: The operation of step (3) includes: (31) Stack the seismic data with different offsets in the Kirchhoff prestack depth migration imaging gather DR(x, z) together to form a corresponding stacked gather; (32) Determine the wavelet length according to the dominant frequency of the seismic data, and uniformly segment the stacked gather using the wavelet length to obtain the stretching fixed points.

3. The prestack depth migration imaging gather stretching correction method according to claim 2, characterized in that: The operation of step (32) includes: Uniformly segment the stacked gather using the wavelet length, that is, the length of each segment is equal to the wavelet length, and the part outside the integer multiple of the wavelet length is taken as one segment; Search for the maximum value of the absolute value of the amplitude in the stacked gather within each segment, and use the grid point corresponding to the depth z corresponding to this maximum value as the stretching fixed point L i .

4. The prestack depth migration imaging gather stretching correction method according to claim 3, wherein: The operation of step (4) includes: Centered on (L i + L i+1 ) / 2, search upward and downward respectively with a radius of 1 / 4 of the wavelet length to find the minimum value of the absolute value of the amplitude in the stacked trace gather, and use the grid point corresponding to the depth z corresponding to this minimum value as the demarcation point F i .

5. The prestack depth migration imaging gather stretching correction method according to claim 4, characterized in that: The operation of step (5) includes: Perform the following processing on each section between two adjacent demarcation points in turn: Keep the depth position of the stretching fixed points in this section unchanged; For other grid points, calculate the stretched depth position of this grid point using the following formula: z'j = jΔz'j where Δz'j = Δz / R(kΔx, jΔz) R(kΔx, jΔz) represents the stretching factor of R(x, z) at the grid point (kΔx, jΔz); After the stretching is completed section by section, the stretched and corrected seismic data is obtained.

6. The prestack depth migration imaging gather stretching correction method according to claim 5, wherein: The method further includes: (6) Remap the irregular grid obtained after the stretching is completed section by section to a regular grid.

7. The prestack depth migration imaging gather stretching correction method according to claim 6, wherein: The method further includes: (7) After the stretching is completed section by section, after stretching two adjacent sections, the demarcation point shared by the two adjacent sections is divided into two points, and zero is filled between these two points.

8. A prestack depth migration imaging gather stretching correction system, characterized in that: The system includes: a memory, a processor, and a computer program stored on the memory. When the computer program is run by the processor, the following steps are executed: (1) Input the Kirchhoff prestack depth migration imaging gather DR(x, z); (2) Obtain the stretching factors at different offsets and different depths; (3) Determine the stretching fixed points; (4) Determine the demarcation points, and each section is between two adjacent demarcation points; (5) Calculate the stretched and corrected seismic data point by point and segment by segment using the stretching factor; The operations in step (2) include: Calculate the stretching factors at different depths and different offsets of the Kirchhoff prestack depth migration image gather DR(x, z) using the following formula (1): where R(x, z) represents the stretching factor at different depths and different offsets of the Kirchhoff prestack depth migration image gather DR(x, z), x is the offset, z is the depth, and Δz represents the step size in the depth direction.

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