A prestack time migration imaging gather stretching correction method and system

Abstract

The present invention provides a method and system for stretching correction of pre-stack time migration imaging gathers, belonging to the field of exploration seismic data processing. The method for stretching correction of pre-stack time migration imaging gathers obtains stretching factors at different offsets and different migration times according to the offset and migration velocity of the pre-stack time migration imaging gathers, then determines 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 pre-stack time migration imaging gathers. The present invention makes the stretching correction of pre-stack time migration imaging gathers more scientific, can overcome the problems of unreliable amplitude and frequency caused by artificially selecting reference traces. 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 stretching correction of pre-stack time migration imaging gathers, which processes the stretching distortion of imaging gathers at large offsets in pre-stack time migration and can be applied to seismic data processing in petroleum geophysical exploration. Background Art

[0002] The quality of pre-stack time migration gathers is directly related to the accuracy and reliability of seismic pre-stack lithology, physical property, fluid inversion, and AVO analysis results. However, the currently applied pre-stack time migration algorithms cannot overcome the waveform distortion problem of far-offset seismic waves, resulting in low-quality common reflection point (CRP) gathers generated by conventional pre-stack time migration processing. The stretching phenomenon of mid- and far-offset wavelets is widespread, and the effects of pre-stack 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 No. CN108508487A discloses a wavelet stretching correction method and device for seismic trace gathers based on multi-wavelet decomposition. First, a reference trace is selected; the peak frequencies of the reference trace and each trace to be corrected are calculated using Fourier transform; the reference trace and each trace to be corrected are decomposed to search for the optimal wavelets of the reference trace and each trace to be corrected; the optimal wavelets of each trace to be corrected are stretched and corrected using the optimal wavelet of the reference trace; the stretched and corrected optimal wavelets are reconstructed to obtain the stretched and 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 No. CN102879821A discloses a method for fine flattening of seismic event axes for prestack gathers, which includes preprocessing the collected seismic data; extracting the prestack gather for which the seismic event axes are to be finely flattened, represented by a two-dimensional array D of I rows and J columns; setting the parameters for fine flattening of seismic event axes; using 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 implementing fine flattening of seismic event axes according to the stretching sampling coordinates; Chinese Patent Publication No. CN106249292B discloses a method for optimizing common reflection point gathers. First, the discrete scanning method is used to estimate the formation dip. To make the dip estimation more accurate, the 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 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 gather is improved. Finally, a model trace is established on the stacked section of the common reflection point gather, and residual moveout correction is performed on the denoised common reflection point gather. Since there is stronger information correlation in the formation dip direction, it can effectively solve the deficiencies in traditional methods, correct the residual moveout while improving the overall signal-to-noise ratio of the gather, manifested as the gather being flattened, improving the overall quality of the gather, and being able to better preserve the effective signals and formation edges and details such as faults and fractures; Chinese Patent Publication No. CN106501859B discloses a method for flattening seismic event axes by moving integration of prestack gathers, which includes: calculating the flattening amount DT1 between adjacent traces in the original gather; applying the flattening amount DT1 to the original gather to generate a locally flattened stacked gather; using the locally flattened stacked gather to calculate the flattening amount DT2 between adjacent traces again; selecting the near-trace "stacked trace", and performing moving integration flattening on the flattening amount DT2 to flatten the entire gather.

[0004] At present, for the problem of wavelet stretching in the far-offset gather, Zhou Peng (2016) further optimized the gather 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 the method of overall relocation NMO, and 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-truncation" of prestack gathers, and used the non-surface-consistent residual static correction method to flatten the in-phase axis of the gather in the target interval. Finally, the effective incident angle range of the target interval was selected to truncate the gather with large-offset stretching distortion. The current prestack time migration imaging gather stretching correction methods all need to select an initial gather or a reference gather, and then correct the gather with large offset to this gather. This is an empirical method, 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 time migration imaging gather stretching correction method and system, to solve the problems existing in the current prestack time migration imaging gather stretching correction technology, and obtain the prestack time migration imaging gather stretching correction factor through derivation, and form a prestack time migration imaging gather stretching correction method based on this stretching factor, so that the prestack time migration imaging gather stretching correction has a quantitative and reliable theoretical basis, and provides data guarantee for the use of the post-imaging gather.

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

[0007] In the first aspect of the present invention, a prestack time migration imaging gather stretching correction method is provided. The method obtains the stretching factors at different offsets and different migration times according to the offset and migration velocity of the prestack time 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, so as to realize the stretching correction of the prestack time migration imaging gather.

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

[0009] (1) Input the prestack time migration imaging gather TR(x,t) and the corresponding migration velocity;

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

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

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

[0013] (5) Calculate the stretched and corrected seismic data point by point and segment by segment using the stretching factor.

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

[0015] Calculate the stretching factors corresponding to different times and different offsets of the prestack time migration imaging gather TR(x,t) using the following formula:

[0016]

[0017] where R(x,t) represents the stretching factor at different times and different offsets of the prestack time migration imaging gather TR(x,t), t is the time, x is the offset, v(t) represents the migration velocity corresponding to different t moments of this imaging gather, and Δt represents the step size in the time direction.

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

[0019] (31) Stack the seismic data with different offsets in the prestack time migration imaging gather TR(x,t) together to form a stacked gather;

[0020] (32) Determine the wavelet length according to the main 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 lies in 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 time t of this maximum value as the stretching fixed point L i .

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

[0025] Taking (L i + L i+1 ) / 2 as the center, search for the minimum value of the absolute value of the amplitude in the stacked gather respectively upward and downward with a radius of 1 / 4 of the wavelet length, and take the grid point corresponding to the time t 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 stretching fixed point L in this section i unchanged in terms of its time position;

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

[0030] t' j = jΔt' j

[0031] where, Δt' j = Δt / R(kΔx, jΔt)

[0032] R(kΔx, jΔt) represents the stretching factor of R(x, t) at the regular grid point (kΔx, jΔt);

[0033] After completing the stretching 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 completing the stretching section by section onto a regular grid.

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

[0037] (7) After completing the stretching 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.

[0038] In the second aspect of the present invention, a prestack time 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 prestack time migration imaging gather TR(x, t) and the corresponding migration velocity;

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

[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 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 times and different offsets of the prestack time migration imaging gather, making the stretching correction of the prestack time migration imaging gather more scientific.

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

[0047] 3) The algorithm of the present invention is scientific, easy to implement, and has high calculation efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 is the input prestack time migration imaging gather;

[0049] Figure 2 is the contour map of the stretching factor corresponding to the prestack time migration imaging gather;

[0050] Figure 3 is the corrected prestack time migration imaging gather;

[0051] Figure 4 is the comparison before and after the correction of a single trace extracted at the maximum offset of the prestack time migration imaging gather;

[0052] Figure 5 is the block diagram of the steps of the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

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

[0054] The present invention gives a determined stretching factor according to the migration velocity and offset, as well as a stretching correction method and process for the prestack time migration imaging gather based on this stretching factor. By uniformly correcting all gathers with the stretching factor, a quantitative and reliable theoretical basis for the stretching correction of the prestack time migration imaging gather is provided, ensuring data for the subsequent use of the imaging gather. The content of the present invention includes obtaining the stretching correction factor for the prestack time migration imaging gather and forming a stretching correction method for the prestack time migration imaging gather based on this stretching factor.

[0055] The present invention first obtains the stretching factors at different offsets and different migration times according to the offset and migration velocity of the prestack time migration imaging gather, then determines the stretching fixed points according to the stacking result of the migration gather, then finds the point with the minimum absolute value of the stacking result amplitude near the middle position between adjacent stretching fixed points, and uses this point as the demarcation point. Finally, the stretched and corrected seismic data is calculated point by point and segment by segment using the stretching factor to complete the stretching correction of the prestack time migration imaging gather.

[0056] Specifically, as Figure 5 shown, the method of the present invention includes the following steps:

[0057] (1) Input the prestack time migration imaging gather TR(x,t) obtained by prestack time migration and the corresponding migration velocity. Prestack time migration is a mature technology and will not be elaborated here;

[0058] (2) Calculate the stretching factor corresponding to each point of the prestack time migration imaging gather.

[0059] The stretching factor corresponding to different times and different offsets of the prestack time migration imaging gather TR(x,t) is calculated using the following formula:

[0060]

[0061] where R(x,t) in formula (1) represents the stretching factor corresponding to different times and different offsets of the prestack time migration imaging gather TR(x,t), t is time, x is offset, v(t) represents the migration velocity corresponding to different t moments of this imaging gather, and Δt represents the step size in the time direction. Through the above formula, the stretching factor corresponding to different times and different offsets of the prestack time migration imaging gather TR(x,t) can be obtained.

[0062] (3) Determine the stretching fixed point L segment by segment according to the selected wavelet length

[0063] Stack the seismic data with different offsets of the input prestack time migration imaging gather TR(x,t) together according to the existing method to form a stacked gather, and then determine the wavelet length according to the main frequency of the seismic data (50 - 300 ms can be selected, and the wavelet length can be estimated specifically according to the following formula 1.0 / f0, where f0 is the main frequency). Divide the stacked gather evenly into segments, with the length of each segment equal to the selected wavelet length, and the remainder (the part outside the integer multiple of the wavelet length) as a segment. Search for the maximum value of the absolute value of the amplitude of the stacked gather within each segment, and use the grid point corresponding to the time t of this maximum value as the stretching fixed point L i Determine it. 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 time t of this minimum value as the demarcation point. Specifically as follows:

[0066] Using the stretching fixed point (L i+L i+1 ) centered around L / 2 i represents the position of the stretching fixed point. Search upward and downward respectively with a radius equal to one-fourth of the wavelet length of the seismic data to find the minimum value of the absolute value of the stacked trace amplitude, and use the grid point corresponding to the time t 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 TR(x,t) in the time t direction. The seismic data of the original prestack time migration imaging gather TR(x,t) falls on the grid points of (kΔx, jΔt). Δx represents the offset step, k and j are grid serial numbers. No correction is made in the offset direction. The time step between adjacent grid points in the time direction is Δt, and the corrected step is Δt'. It can be calculated by the following formula

[0069] Δt' j = Δt / R(kΔx, jΔt) (2)

[0070] where R(kΔx, jΔt) represents the stretching factor of R(x,t) at the regular grid point of (kΔx, jΔt)

[0071] According to the fixed point L i and the demarcation point F i determined in steps (2) and (3), the input prestack time migration imaging gather TR(x,t) can be divided into several segments in the time 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 time position corresponding to the fixed point L i in each segment remains unchanged

[0072] For each segment segmented by using the demarcation point F i , use the stretching factor corresponding to each point to obtain the new grid step Δt' through formula (2) j . Then, the seismic data of the imaging gather at the original regular grid t j = jΔt j will be mapped to the new irregular grid t' j = jΔt' j . At this time, the time position corresponding to the fixed point L i in each segment remains unchanged, that is, t' j = t j, the time positions corresponding to other points (including the two demarcation points as endpoints) in each segment have all changed through the above calculations.

[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. The mapping method can be implemented by interpolation, that is, the seismic data amplitude value of the imaging gather at the irregular position t′ j nearby is interpolated to obtain the new amplitude value at the time t j of the regular grid. Specific interpolation algorithms can include inverse distance interpolation, parabolic interpolation, etc.

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

[0075] In the above processing, only at the fixed point L i the "irregular position t′ j at" and the time position where t′ j is located on the regular grid are the same, and the time 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 time 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 prestack time migration imaging gather TR(x,t) and the corresponding migration velocity;

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

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

[0080] (4) Determine the demarcation points, and one 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 present invention is directed to the processing of imaging gathers with stretching distortion at large offsets in prestack time migration in the field of exploration seismic data processing, and can be applied to the seismic data processing in petroleum geophysical exploration. The present invention gives a determined stretching factor according to the migration velocity and offset, as well as a stretching correction method and process for prestack time migration imaging gathers based on this stretching factor. The calculation formula of the stretching factor corresponding to different times and different offsets in the prestack time migration imaging gathers proposed by the present invention makes the stretching correction of the prestack time migration imaging gathers more scientific. The present invention gives the specific use formula of the stretching factor and the stretching correction process of the prestack time migration imaging gathers, which can overcome the problems of unreliable amplitude and frequency caused by artificially selecting reference traces, provide a basis for subsequent velocity analysis and gather analysis, and has good application prospects.

[0083] The following are examples of the application of the present invention:

[0084]

Example 1

[0085] Figure 1 is the input prestack time migration imaging gather, Figure 1 where the longitudinal unit is seconds 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 is more severe.

[0086] Figure 2 is the contour map of the stretching factor corresponding to this prestack time migration imaging gather. It can be seen from Figure 2 that as the distance increases, the stretching factor becomes larger, and as the time increases, the stretching factor becomes smaller. The stretching factor is the largest in the upper left corner.

[0087]

Example 2

[0088] Figure 3 is the corrected prestack time migration imaging gather, Figure 3 where the longitudinal unit is seconds and the transverse unit is kilometers. It can be seen from Figure 3 that the corrected imaging gather is very flat, the wavelet morphology is consistent, and the energy is well preserved.

[0089]

Example 3

[0090] Figure 4 is the comparison before and after the correction of a trace extracted from the prestack time migration imaging gather at the maximum offset. The solid line is the original prestack time migration imaging gather, and the dashed line is the corrected prestack time migration imaging gather. It can be clearly seen from Figure 4 that the gather is significantly compressed. Figure 2 It can be seen 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 imaging gather at the maximum offset is the most severe, and correspondingly, its compression degree is the highest.

[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, based on 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 manners of the present invention. Therefore, the manners described above are only preferred and do not have a restrictive meaning.

Claims

1. A prestack time migration imaging gather stretching correction method, characterized in that: The method obtains stretching factors at different offsets and different migration times according to the offset and migration velocity of the prestack time migration imaging gather, then determines the stretching fixed points and demarcation points, and finally obtains the stretched and corrected seismic data segment by segment using the stretching factors, realizing the stretching correction of the prestack time migration imaging gather; The method includes: (1) Input the prestack time migration imaging gather TR(x,t) and the corresponding migration velocity; (2) Obtain the stretching factors at different times and different offsets; (3) Determine the stretching fixed points; (4) Determine the demarcation points, and one segment is between two adjacent demarcation points; (5) Calculate the stretched and corrected seismic data point by point and segment by segment using the stretching factors; The operation of step (2) includes: Calculate the corresponding stretching factors at different times and different offsets of the prestack time migration imaging gather TR(x,t) using the following formula: where R(x,t) represents the stretching factor at different times and different offsets of the prestack time migration imaging gather TR(x,t), t is the time, x is the offset, v(t) represents the migration velocity corresponding to different t moments of this imaging gather, and Δt represents the step length in the time direction.

2. The prestack time migration imaging gather stretching correction method according to claim 1, wherein: The operation of step (3) includes: (31) Superimpose the seismic data with different offsets in the prestack time migration imaging gather TR(x,t) to form a superimposed gather; (32) Determine the wavelet length according to the main frequency of the seismic data, and uniformly segment the superimposed gather using the wavelet length to obtain the stretching fixed points.

3. The prestack time migration imaging gather stretching correction method according to claim 2, wherein: The operation of step (32) includes: Uniformly segment the superimposed 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 time t corresponding to this maximum value as the stretching fixed point L i .

4. The prestack time migration imaging gather stretching correction method according to claim 3, characterized in that: 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 time t corresponding to this minimum value as the demarcation point F i .

5. The prestack time migration imaging gather stretching correction method according to claim 4, characterized in that: The operation of step (5) includes: Perform the following processing on one segment between two adjacent demarcation points in sequence: Keep the stretching fixed point L in this section i at the same time position; Calculate the stretched time position of this grid point for other grid points using the following formula: t' j = jΔt' j where, Δt' j = Δt / R(kΔx, jΔt) R(kΔx,jΔt) represents the stretching factor of R(x,t) at the regular grid point (kΔx,jΔt); After the stretching is completed segment by segment, the stretched and corrected seismic data is obtained.

6. The prestack time 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 segment by segment to a regular grid.

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

8. A prestack time 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 prestack time migration imaging gather TR(x,t) and the corresponding migration velocity; (2) Obtain the stretching factors at different times and different offsets; (3) Determine the stretching fixed points; (4) Determine the demarcation points, and one segment is between two adjacent demarcation points; (5) Calculate the stretched and corrected seismic data point by point and segment by segment using the stretching factors; The operation of step (2) includes: The stretching factors corresponding to different times and different offsets of the prestack time migration imaging gather TR(x,t) are calculated using the following formula: where R(x,t) represents the stretching factor at different times and different offsets of the prestack time migration imaging gather TR(x,t), t is time, x is the offset, v(t) represents the migration velocity corresponding to different t moments of this imaging gather, and Δt represents the step size in the time direction.

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