Two-way wave depth migration method for VTI medium

Abstract

The invention relates to the technical field of seismic exploration, in particular to a two-way wave depth migration method for a VTI medium. Respectively forming a detector wave field and a seismic source wave field; respectively reading seismic data and a seismic source wave field of a single shot, and obtaining anisotropic parameters; fourier transform is carried out on the seismic source wave field data and the detector wave field data to obtain frequency-space domain wave field data; calculating parameters required by offset; calculating an anisotropic vertical wave number; solving a wave field of a second depth grid; according to the double-boundary condition, wave field propagation operators are solved layer by layer; eliminating evanescent waves and pseudo S waves at each depth position; migration imaging is carried out; continuation calculation is repeated until the maximum depth is calculated, and an offset result is output; by popularizing the two-way wave migration from an isotropic medium to an anisotropic medium, the application range of the two-way wave depth migration is expanded, and an underground structure can be better imaged.

Description

Technical Field

[0001] The invention relates to the technical field of seismic exploration, and in particular to a double-pass wave depth migration method for VTI media. Background Art

[0002] With the continuous development of seismic exploration, the exploration targets have gradually shifted to complex high-steep structural belts, and the underground media in these areas are generally anisotropic. The actual underground structure can be approximated as a VTI medium. At present, the wave equation depth migration methods for anisotropic media are all one-way wave equation depth migration methods. The two-way wave equation depth migration has better angle imaging capability and amplitude preservation characteristics than the one-way wave equation depth migration. The current two-way wave depth migration is only for isotropic media. For example, in the patent application with publication number CN105911587B, a two-way wave prestack depth migration method using a one-way wave operator is disclosed, and the two-way wave field depth extrapolation is realized using the one-way wave operator. That is, the two-way wave field depth extrapolation is performed on the detection point wave field and the shot point wave field, and the extended detection point wave field and the shot point wave field are imaged using the cross-correlation imaging principle or the reflection coefficient imaging principle.

[0003] Moreover, in the above-mentioned isotropic two-way wave depth migration method, the one-way wave operator used is a traditional one-way wave operator based on reference velocity approximation, which has a large error in wave field propagation and cannot better image the underground structure.

[0004] Therefore, a two-way wave depth migration method for anisotropic media is proposed. This method extends the traditional two-way wave depth migration from isotropic media to anisotropic media, expands the application range of two-way wave depth migration, improves the calculation accuracy of the propagation operator, and can better image the underground structure. Summary of the invention

[0005] The purpose of the present invention is to provide a two-way wave depth migration method for VTI media, aiming to fill the gap in the two-way wave depth migration method for anisotropic media in the prior art; at the same time, it solves the technical problem that the one-way wave operator used in the traditional two-way wave method is a one-way wave operator based on reference velocity approximation, the propagation error of the recursive wave field depth direction is large, and the underground structure cannot be better imaged.

[0006] To achieve the above object, the present invention adopts a double-pass wave depth migration method for VTI media, comprising the following steps:

[0007] Acquire geophone seismic data to form geophone wave fields;

[0008] Extract seismic wavelet information from the seismic data of the geophone to form the source wave field;

[0009] Read the seismic data and source wave field of a single shot respectively to obtain the anisotropic parameter V P , ε and δ;

[0010] Perform Fourier transform on the source wave field and the detector wave field data to obtain the frequency-space domain wave field data;

[0011] Calculate the parameters required for the migration based on the anisotropy parameters;

[0012] Calculate the anisotropic vertical wave number k z ;

[0013] Get the wave field of the second depth grid Provide boundary conditions for two-way depth extrapolation;

[0014] According to the double boundary conditions, the wave field propagation operator is obtained layer by layer Two-pass depth continuation in the frequency-space domain;

[0015] Eliminate evanescent waves and pseudo S waves at every depth;

[0016] Performing offset imaging;

[0017] Repeat the extension calculation until the maximum depth is reached and output the offset result.

[0018] Wherein, in the step of calculating the parameters required for the migration from the anisotropy parameters:

[0019] The key parameters in ground earthquakes can be expressed as the normal time difference velocity V nmo and horizontal velocity V h , the formula is:

[0020]

[0021] Wherein, in the step of calculating the parameters required for the migration from the anisotropy parameters:

[0022] This reduces the number of parameters to the parameter η, defined as the ellipticity, given by:

[0023]

[0024] Among them, in calculating the anisotropic vertical wave number k z In the steps:

[0025] Anisotropic vertical wave number k z The calculation formula is:

[0026]

[0027] Rewrite the anisotropic dispersion relation:

[0028]

[0029] Among them, in obtaining the wave field of the second depth grid To provide boundary conditions for two-pass depth extrapolation:

[0030] TRON The formula is:

[0031]

[0032] Where F' stands for inverse Fourier transform.

[0033] Among them, according to the double boundary conditions, the wave field propagation operator is calculated layer by layer. In the step of double-pass depth continuation in the frequency-space domain:

[0034] The extension formula is:

[0035]

[0036] Wherein, in the step of performing offset imaging:

[0037] Methods for performing migration imaging include utilizing the cross-correlation imaging principle and the reflection coefficient imaging principle.

[0038] The present invention provides a two-way wave depth migration method for VTI media, which obtains geophone seismic data to form a geophone wave field; extracts seismic wavelet information from the geophone seismic data to form a source wave field; reads the seismic data and source wave field of a single shot respectively to obtain anisotropic parameters V P , ε and δ; perform Fourier transform on the source wave field and the detector wave field data to obtain the frequency-space domain wave field data; calculate the parameters required for migration based on the anisotropic parameters; calculate the anisotropic vertical wave number k z ; Obtain the wave field of the second depth grid Provide boundary conditions for double-pass depth extrapolation; according to the double boundary conditions, find the wave field propagation operator layer by layer Perform two-way depth extension in the frequency-space domain; eliminate evanescent waves and pseudo-S waves at each depth position; perform migration imaging; repeat the extension calculation until the maximum depth is calculated, and output the migration result; expand the application range of two-way wave depth migration by extending two-way wave migration from isotropic media to anisotropic media, which can better image the underground structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0040] Figure 1 It is the P wave velocity model of the non-uniform variable velocity medium of the present invention.

[0041] Figure 2 It is a schematic diagram of the pulse response result of the present invention.

[0042] Figure 3 It is a schematic diagram of the parameters of the simple anisotropic model of the present invention.

[0043] Figure 4 It is a schematic diagram comparing the double-pass wave depth migration results of the present invention.

[0044] Figure 5 It is a schematic diagram of imaging amplitude comparison of the present invention.

[0045] Figure 6 It is a flow chart of the steps of the double-pass wave depth migration method for VTI media of the present invention. DETAILED DESCRIPTION

[0046] Here, exemplary embodiments are described in detail, and examples thereof are shown in the accompanying drawings. When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The implementations described in the following exemplary embodiments do not represent all implementations consistent with the present application.

[0047] The terms used in this application are for the purpose of describing specific embodiments only and are not intended to limit this application. The singular forms of "a", "said" and "the" used in this application and the appended claims are also intended to include plural forms unless the context clearly indicates other meanings. It should also be understood that the term "and / or" used herein refers to and includes any or all possible combinations of one or more associated listed items.

[0048] It should be understood that although the terms first, second, third, etc. may be used in the present application to describe various information, these information should not be limited to these terms. These terms are only used to distinguish the same type of information from each other. For example, without departing from the scope of the present application, the first information may also be referred to as the second information, and similarly, the second information may also be referred to as the first information. Depending on the context, the word "if" as used herein may be interpreted as "at the time of" or "when" or "in response to determining".

[0049] See also Figure 1 to Figure 6 ,in Figure 1 It is the P wave velocity model of non-uniform variable velocity medium. Figure 2 is a schematic diagram of the pulse response results. Figure 3 It is a schematic diagram of the parameters of a simple anisotropic model. Figure 4 This is a comparison diagram of the two-way wave depth migration results. Figure 5 This is a schematic diagram of imaging amplitude comparison. Figure 6 is a flow chart of the steps of the two-way depth migration method for VTI media.

[0050] The present invention provides a double-pass wave depth migration method for VTI media, comprising the following steps:

[0051] S101: Acquire geophone seismic data to form a geophone wave field;

[0052] S102: extracting seismic wavelet information from the seismic data of the geophone to form a source wave field;

[0053] S103: Read the seismic data and source wave field of a single shot respectively to obtain anisotropic parameters V P , ε and δ;

[0054] S104: Perform Fourier transform on the source wave field and the detector wave field data to obtain frequency-space domain wave field data.

[0055] In this embodiment, firstly, the geophone seismic data is acquired through seismic acquisition to form the geophone wave field; then, the seismic wavelet information is extracted from the geophone seismic data to form the source wave field; the seismic data and source wave field of a single shot are read respectively to obtain the anisotropy parameter V P , ε and δ; then perform Fourier transform on the source wave field and the detector wave field data to obtain the frequency-space domain wave field data.

[0056] S105: Calculate parameters required for migration according to the anisotropic parameters.

[0057] In this embodiment, the parameters required for migration are calculated based on the anisotropic parameters. In seismic exploration considering anisotropy, Thomson parameters can be equivalent to different measurable velocities. The key parameters estimated in ground seismic can be expressed as the normal moveout (NMO) velocity V nmo and horizontal velocity V h , the formula is:

[0058]

[0059] Although the Thomsen parameters were introduced to address weak anisotropy, they are convenient to use regardless of the magnitude of the anisotropy. When only P-wave kinematics are considered, the Thomsen parameters are not sensitive to S-waves along the symmetry axes x and y in VTI media. Therefore, the number of parameters is further reduced to a new parameter η, defined as the ellipticity, which is given by:

[0060]

[0061] S106: Calculate the anisotropic vertical wave number k z ;

[0062] In this embodiment, the anisotropic vertical wave number k is calculated z , anisotropic vertical wave number k z The calculation formula is:

[0063]

[0064] The present invention uses a generalized two-way phase shift operator based on local accurate velocity calculation to extrapolate the wave field, ensuring the accuracy of the propagation operator. At the same time, in order to facilitate calculation, the rewritten anisotropic dispersion relation is adopted:

[0065]

[0066] S107: Obtaining the wave field of the second depth grid Provide boundary conditions for two-pass depth extrapolation.

[0067] In this embodiment, the wave field of the second depth grid is obtained TRON The formula is:

[0068]

[0069] Where F' stands for inverse Fourier transform.

[0070] S108: According to the double boundary conditions, the wave field propagation operator is calculated layer by layer Two-pass depth continuation in the frequency-space domain;

[0071] In this embodiment, according to the double boundary conditions, the wave field propagation operator is obtained layer by layer. The extension formula is:

[0072]

[0073] S109: Eliminate evanescent waves and pseudo S waves at each depth position;

[0074] In this embodiment, the evanescent wave and the pseudo S wave are eliminated at each depth position; if A or B is negative, an evanescent wave will be generated, which can be achieved by setting k in the formula z If the value of is 0, it will be removed; if A<0 and B<0, a pseudo S wave is obtained, which can be eliminated by filtering the negative values ​​in A and B in the dispersion relation.

[0075] S110: Perform offset imaging.

[0076] In this embodiment, offset imaging is performed; wherein the offset imaging is performed by using the cross-correlation imaging principle and the reflection coefficient imaging principle.

[0077] S111: Repeat the extension calculation until the maximum depth is calculated, and output the offset result.

[0078] In this embodiment, the continuation calculation of steps S108 to S110 is repeated until the maximum depth is calculated, and the migration result is output.

[0079] In this application, if Figure 6 As shown, the seismic data of the geophone is obtained to form the geophone wave field; the seismic wavelet information is extracted from the seismic data of the geophone to form the source wave field; the seismic data and source wave field of a single shot are read respectively to obtain the anisotropy parameter V P , ε and δ; perform Fourier transform on the source wave field and the detector wave field data to obtain the frequency-space domain wave field data; calculate the parameters required for migration based on the anisotropic parameters; calculate the anisotropic vertical wave number k z ; Obtain the wave field of the second depth grid Provide boundary conditions for double-pass depth extrapolation; according to the double boundary conditions, find the wave field propagation operator layer by layer Perform two-way depth extension in the frequency-space domain; eliminate evanescent waves and pseudo-S waves at each depth position; perform migration imaging; repeat the extension calculation until the maximum depth is calculated, and output the migration result; by extending the two-way wave migration from isotropic media to anisotropic media, the application scope of two-way wave depth migration is expanded, which can better image the underground structure. Figure 1 to Figure 5 As shown, Figure 1 It is the P wave velocity model of the non-uniform variable velocity medium of the present invention. Figure 2 is a schematic diagram of the pulse response result of the present invention; wherein, Figure 2 (a) ε = 0 and δ = 0; Figure 2 (b) ε = 0.5 and δ = 0 (pseudo-S waves are not filtered out); Figure 2 (c) ε = 0.5 and δ = 0.5; Figure 2 (d) ε = 0.5 and δ = 0 (spurious S waves are filtered out). Figure 3 is a schematic diagram of the parameters of a simple anisotropic model of the present invention, wherein Figure 3(a)V P , Figure 3 (b) ε, Figure 3 (c)δ. Figure 4 is a schematic diagram comparing the results of the double-pass wave depth migration of the present invention, wherein Figure 4 (a) is the result of two-way wave depth migration in VTI medium. Figure 4 (b) is the isotropic two-way wave depth migration result. Figure 4 (c) is the imaging result of single-pass wave depth migration in VTI medium. It can be seen that the anisotropic time is not taken into account, and the imaging result has obvious errors and noise. Figure 5 is a schematic diagram of imaging amplitude comparison of the present invention, specifically, Figure 5 for Figure 4 Comparison of imaging amplitudes extracted at x=1700m. It can be seen that the dual-pass wave VTI medium depth migration method proposed in the present invention has better amplitude preservation than the traditional single-pass wave VTI medium depth migration method.

[0080] Those skilled in the art will readily appreciate other embodiments of the present application after considering the specification and practicing the contents disclosed herein. The present application is intended to cover any variations, uses or adaptations of the present application, which follow the general principles of the present application and include common knowledge or customary technical means in the art that are not disclosed in the present application.

[0081] It should be understood that the present application is not limited to the exact construction that has been described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof.

Claims

1. A two-way wave depth migration method for VTI media, characterized in that: The steps include: Acquire geophone seismic data to form geophone wave fields; Extract seismic wavelet information from the seismic data of the geophone to form the source wave field; Read the seismic data and source wave field of a single shot respectively to obtain the anisotropic parameter V P , ε and δ; Perform Fourier transform on the source wave field and the detector wave field data to obtain the frequency-space domain wave field data; Calculate the parameters required for the migration based on the anisotropy parameters; Calculate the anisotropic vertical wave number k z ; Get the wave field of the second depth grid Provide boundary conditions for two-way depth extrapolation; According to the double boundary conditions, the wave field propagation operator is obtained layer by layer Two-pass depth continuation in the frequency-space domain; Eliminate evanescent waves and pseudo S waves at every depth; Performing offset imaging; Repeat the extension calculation until the maximum depth is reached and output the offset result.

2. The double-pass wave depth migration method for VTI media according to claim 1, characterized in that: In the step of calculating the parameters needed for the offset from the anisotropy parameters: The key parameters in ground earthquakes can be expressed as the normal time difference velocity V nmo and horizontal velocity V h , the formula is:

3. The double-pass wave depth migration method for VTI media according to claim 2, characterized in that: In the step of calculating the parameters needed for the offset from the anisotropy parameters: This reduces the number of parameters to the parameter η, defined as the ellipticity, given by:

4. The double-pass wave depth migration method for VTI media according to claim 3, characterized in that: In calculating the anisotropic vertical wave number k z In the steps: Anisotropic vertical wave number k z The calculation formula is: Rewrite the anisotropic dispersion relation:

5. The double-pass wave depth migration method for VTI media according to claim 4, characterized in that: In obtaining the wave field of the second depth grid To provide boundary conditions for two-pass depth extrapolation: TRON The formula is: Where F' stands for inverse Fourier transform.

6. The double-pass wave depth migration method for VTI media according to claim 1, characterized in that: According to the double boundary conditions, the wave field propagation operator is obtained layer by layer. In the step of double-pass depth continuation in the frequency-space domain: The extension formula is:

7. The double-pass wave depth migration method for VTI media according to claim 1, characterized in that: In the steps of performing offset imaging: Methods for performing migration imaging include utilizing the cross-correlation imaging principle and the reflection coefficient imaging principle.

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