Method for establishing initial shear wave velocity field in well-constrained converted wave depth migration

Abstract

A method for establishing an initial shear wave velocity field for well-constrained converted wave depth migration, which adopts the following steps: First: Read the longitudinal wave and shear wave acoustic logging time difference curves; Second: Extract the smoothed curves of the longitudinal wave logging velocity and the shear wave logging velocity, and match them to obtain the percentage; Third: Interpolate and extrapolate the multi-well percentage curves to form a percentage volume; Fourth: Read the final velocity field of the grid tomography longitudinal wave, and extract the velocity curve at the well position; Fifth: Match the smoothed curve of the longitudinal wave logging velocity with the longitudinal wave velocity to obtain the percentage; Sixth: Extrapolate the longitudinal wave single-well percentage curve to the top and bottom in depth; Seventh: Interpolate and extrapolate the longitudinal wave multi-well percentage curves to form a percentage volume; Eighth: Scale to the multi-well constrained longitudinal wave velocity volume; Ninth: Combine the shear wave multi-well percentage volume and the multi-well constrained longitudinal wave velocity volume to obtain the multi-well constrained shear wave velocity volume. The present invention can directly adopt the converted wave depth migration technology; it can also carry out depth migration to completely solve the converted wave velocity problem, so that the converted wave migration is accurately positioned.

Description

Technical Field

[0001] The present invention belongs to the field of seismic exploration processing, and particularly relates to a method for establishing an initial shear wave velocity field for well-constrained converted wave depth migration. Background Art

[0002] At present, in the process of processing converted wave seismic data, the commonly used technique is prestack time migration for converted waves. The prestack time migration technique uses simplified equations for the converted wave traveltime curve. There are two or three dozen such simplified equations or formulas. Among them, the "Series of Prestack Time Migration Formulas for Converted Waves" written by Dr. Li Xiangyang is widely recognized in the field of converted wave processing. This formula starts from the time domain of converted waves and uses four parameters to perform dynamic correction on the converted wave ccp gather. The result is the prestack time migration result in the time domain. However, when there is a large area of gas clouds, since a part of the converted waves is also affected by the gas clouds, there is a phenomenon that the common phase axis in the gas cloud area generally bends downward, which is not conducive to the imaging of converted waves.

[0003] For converted wave seismic data with a large area of gas clouds, depth migration is generally performed to completely solve the converted wave velocity problem, so that the converted wave migrates to its correct position. The converted wave velocity includes two types: compressional wave velocity and shear wave velocity. However, since the compressional wave velocity can be obtained through tomographic inversion of the compressional wave velocity field, while the shear wave velocity cannot be obtained by existing methods. Therefore, an accurate converted wave velocity field cannot be obtained. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for establishing an initial shear wave velocity field for well-constrained converted wave depth migration to solve the technical problem of obtaining the shear wave layer velocity field in the depth domain required for converted wave depth migration.

[0005] To achieve the above purpose, the specific technical solution of the method for establishing an initial shear wave velocity field for well-constrained converted wave depth migration of the present invention is as follows:

[0006] A method for establishing an initial shear wave velocity field for well-constrained converted wave depth migration, comprising the following steps:

[0007] The first step: Read the compressional wave and shear wave sonic logging time difference curves.

[0008] The second step: Extract the smoothed curves of the compressional wave logging velocity and shear wave logging velocity, and calculate the percentage correspondingly.

[0009] The third step: Interpolate and extrapolate the multi-well percentage curves to form a percentage volume.

[0010] The fourth step: Read the final velocity field of grid tomography for compressional waves, and extract the velocity curves at the well positions.

[0011] Step 5: Match the smoothed curve of the longitudinal wave logging velocity with the longitudinal wave velocity to obtain the percentage;

[0012] Step 6: Extrapolate the longitudinal wave single-well percentage curve to the top and bottom in terms of depth;

[0013] Step 7: Interpolate and extrapolate the longitudinal wave multi-well percentage curve to form a percentage volume;

[0014] Step 8: Scale to the multi-well constrained longitudinal wave velocity volume;

[0015] Step 9: Combine the shear wave multi-well percentage volume with the multi-well constrained longitudinal wave velocity volume to obtain the multi-well constrained shear wave velocity volume.

[0016] Furthermore, in the first step: Read the longitudinal wave and shear wave acoustic logging time difference curves. The formula for obtaining the percentage coefficient from the logging time difference curves is:

[0017] percent = v2 / v1 (1)

[0018] The formula for applying the percentage coefficient to the logging time difference curves is:

[0019] v2 = percent * v1 (2)

[0020] The interpolation formula for the logging time difference curves is the linear interpolation formula, that is: If A(i1, b1), B(i2, b2) are two points, then the point P(i, b) is on the straight line determined by the above two points; then (b - b1) / (i - i1) = (b2 - b1) / (i2 - i1) = the slope of the straight line, and the required result can be obtained by transformation;

[0021] The extrapolation formula for the logging time difference curves is the fitting formula, that is: Substitute the existing sample points into y = ax 2 + bx + c, and obtain a functional relationship by finding the best a, b, and c coefficients, that is: It is the extrapolation formula;

[0022] The formula for extracting the smoothed curve of the logging time difference curves is: the fitting formula, that is: Substitute the existing sample points into y = ax 2 + bx + c, and obtain a functional relationship by finding the best a, b, and c coefficients, that is: It is the formula for extracting the smoothed curve.

[0023] Furthermore, in the second step: The steps for extracting the smoothed curves of the longitudinal wave logging velocity and the shear wave logging velocity are as follows:

[0024] 1) Extract the smoothed curve on the longitudinal wave acoustic time difference curve; The extraction of the smoothed curve needs to be tested according to the required accuracy; The formula for extracting the smoothed curve is the fitting formula, that is: Substitute the existing sample points into y = ax 2 + bx + c, and obtain a functional relationship by finding the best a, b, and c coefficients, that is: It is the formula for extracting the smoothed curve;

[0025] Among them, the single-well percentage curve is extrapolated to the top and bottom in terms of depth; the extrapolation formula is a fitting formula, that is, a functional relationship obtained by substituting the existing sample points into y = ax 2 + bx + c to obtain the best abc coefficients, which is the extrapolation formula;

[0026] 2) Extract a smooth curve from the shear wave acoustic travel time curve; the extraction of the smooth curve needs to be tested according to the required accuracy in actual situations; the extraction formula of the smooth curve is a fitting formula, that is, a functional relationship obtained by substituting the existing sample points into y = ax 2 + bx + c to obtain the best abc coefficients, which is the extraction formula of the smooth curve;

[0027] Among them, the single-well percentage curve is extrapolated to the top and bottom in terms of depth; the extrapolation formula is a fitting formula, that is, a functional relationship obtained by substituting the existing sample points into y = ax 2 + bx + c to obtain the best abc coefficients, that is: it is the extrapolation formula;

[0028] 3) Calculate the percentage by matching the P-wave and S-wave;

[0029] The sample point values at the same depth of the P-wave velocity and the S-wave velocity are divided to obtain the percentage coefficient. percent = vpw / vsw, where vpw is the P-wave logging velocity, vsw is the S-wave logging velocity, and percent is the percentage coefficient.

[0030] Furthermore, in the third step: the steps of interpolating and extrapolating the multi-well percentage curve to form a percentage volume are as follows:

[0031] 1) The extrapolation formula is a fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c to obtain the best abc coefficients, which is the extrapolation formula;

[0032] 2) The interpolation formula is a linear interpolation formula, that is: if A(i1, b1), B(i2, b2) are two points, then the point P(i, b) is on the straight line determined by the above two points; then (b - b1) / (i - i1) = (b2 - b1) / (i2 - i1) = the slope of the straight line, and the required result can be obtained by transformation; the required percentage volume is denoted as percent1.

[0033] Furthermore, in the fifth step: the percentage coefficient obtained by dividing the smooth curve of the P-wave logging velocity by the sample point value at the same depth of the P-wave velocity is; percent2 = vpw / vp, where vpw is: the sample point velocity value of the smooth P-wave velocity curve of logging, vp is: the velocity curve velocity value at the well position extracted from the final P-wave velocity field of grid tomography, and percent2 is: the percentage coefficient.

[0034] Furthermore, in the sixth step: the extrapolation formula is a fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c, and obtaining a functional relationship by calculating the optimal abc coefficients, which is the extrapolation formula.

[0035] Furthermore, in the seventh step: the steps are as follows:

[0036] 1) The interpolation formula is a linear interpolation formula, that is: if A(i1, b1) and B(i2, b2) are two points, then the point P(i, b) is on the straight line determined by these two points; then (b - b1) / (i - i1) = (b2 - b1) / (i2 - i1) = the slope of the straight line, and the required formula can be obtained by transformation;

[0037] 2) The extrapolation formula is a fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c to calculate the optimal abc coefficients to obtain a functional relationship, which is the extrapolation formula: the required percentage volume is denoted as percent2.

[0038] Furthermore, in the eighth step: the steps are as follows:

[0039] Using the above percentage volume percent2 to convert the final velocity field of the shear-wave grid tomography to the multi-well constrained shear-wave velocity volume:

[0040] The formula used is vpw = vp * percent2, where vpw is the multi-well constrained shear-wave velocity volume, vp is the final velocity field of the shear-wave grid tomography, and percent2 is the percentage coefficient.

[0041] Furthermore, in the ninth step: the formula used is: vsw = vpw * percent1, where vsw is the multi-well constrained shear-wave velocity volume, vpw is the well-constrained shear-wave velocity volume, and percent1 is the percentage volume obtained in the third step.

[0042] The method for establishing the initial shear-wave velocity field of well-constrained converted-wave depth migration of the present invention has the following advantages:

[0043] 1. For the converted-wave seismic data with a large area of gas clouds, the present invention directly adopts the converted-wave depth migration technology.

[0044] 2. The present invention uses ray tracing technology to directly track the conversion points of converted waves with the longitudinal wave velocity, and tracks to the geophones through the shear-wave velocity, that is, uses the real longitudinal wave and shear-wave velocities for tracking, calculates the travel time, and then conducts depth migration to completely solve the converted-wave velocity problem, so that the converted-wave migration is accurately positioned.

[0045] 3. The P-wave velocity of the present invention can not only be obtained through tomographic inversion of the P-wave velocity field, but also a relatively accurate initial velocity of the S-wave can be obtained.

[0046] 4. The P-wave velocity of the present invention can be further optimized through tomographic inversion of converted waves.

[0047] 5. The present invention can provide an accurate converted-wave velocity field through converted-wave depth migration. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The present invention will be further described below in conjunction with the drawings and embodiments.

[0049] Figure 1 Schematic diagram of the migration profile obtained by S-wave velocity migration for well constraint of the present invention (it is the actual graph on the screen);

[0050] Figure 2 Schematic diagram of the flow chart for obtaining S-wave velocity of the present invention;

[0051] Figure 3 Schematic diagram of the smoothed curves of P-wave and S-wave acoustic wave extraction of the present invention (it is the actual graph on the screen);

[0052] Figure 4 Schematic diagram of the final P-wave velocity field of grid tomography of the present invention (it is the actual graph on the screen);

[0053] Figure 5 Schematic diagram of the multi-well constrained S-wave velocity volume of the present invention (it is the actual graph on the screen). DETAILED DESCRIPTION OF THE INVENTION

[0054] In order to better understand the purpose, structure and function of the present invention, the following further describes in detail a method for establishing an initial S-wave velocity field of converted-wave depth migration with well constraint of the present invention in conjunction with the drawings.

[0055] As Figure 1 , Figure 2 shown, in the case of knowing the final P-wave depth-domain layer velocity field, P-wave acoustic logging curve, and S-wave acoustic logging curve of the present invention, through logging data constraint, thus, an initial S-wave initial velocity field in the depth domain required for converted-wave depth migration is obtained. By performing the following series of processes, the purpose of obtaining the S-wave layer velocity field in the depth domain required for converted-wave depth migration is achieved. The present invention adopts the following steps:

[0056] The first step: Read the P-wave and S-wave acoustic logging time difference curves;

[0057] When reading the P-wave and S-wave acoustic logging time difference curves, the formula for obtaining the percentage coefficient from the logging time difference curve is: percent = v2 / v1 (1)

[0058] The formula for applying the percentage coefficient to the log travel-time curve is as follows:

[0059] v2 = percent * v1 (2)

[0060] The interpolation formula for the log travel-time curve is the linear interpolation formula, that is: if A(i1,b1) and B(i2,b2) are two points, then the point P(i,b) is on the straight line determined by the above two points; then (b - b1) / (i - i1) = (b2 - b1) / (i2 - i1) = the slope of the straight line, and the required result can be obtained by transformation;

[0061] The extrapolation formula for the log travel-time curve is a fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c, obtaining a functional relationship by finding the optimal abc coefficients, that is: it is the extrapolation formula;

[0062] The formula for extracting the smoothed curve of the log travel-time curve is: a fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c, obtaining a functional relationship by finding the optimal abc coefficients, that is: it is the formula for extracting the smoothed curve;

[0063] As Figure 3 shown Figure 3 What is shown in

[0064] is: the log curve (the left side is the longitudinal wave velocity, the right side is the shear wave velocity, the black is the measured value, and the vertical black line in the middle of the left and right is the smoothed curve);

[0065] Step 2: Extract the smoothed curves of the longitudinal wave log velocity and the shear wave log velocity, and calculate the percentage correspondingly; the steps for extracting the smoothed curves of the longitudinal wave log velocity and the shear wave log velocity are as follows:

[0066] 1) Extract the smoothed curve on the longitudinal wave acoustic travel-time curve; the extraction of the smoothed curve needs to be tested according to the required accuracy; the formula for extracting the smoothed curve is a fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c to obtain a functional relationship by finding the optimal abc coefficients, that is: it is the formula for extracting the smoothed curve.

[0067] Among them, the single-well percentage curve is extrapolated to the top and bottom in terms of depth; the extrapolation formula is a fitting formula, that is, substituting the existing sample points into y = ax 2 2) Extract the smoothed curve on the shear wave acoustic travel-time curve; the extraction of the smoothed curve needs to be tested according to the required accuracy; the formula for extracting the smoothed curve is a fitting formula, that is: substituting the existing sample points into y = ax 2A functional relationship obtained by finding the optimal abc coefficients in y = ax² + bx + c, that is: it is the formula for extracting the smooth curve.

[0068] Among them, the single-well percentage curve is extrapolated to the top and bottom in terms of depth; the extrapolation formula is the fitting formula, that is, substituting the existing sample points into y = ax² 2 + bx + c to obtain a functional relationship for the optimal abc coefficients, that is: it is the extrapolation formula.

[0069] 3) Obtain the percentage by matching the P-wave and S-wave;

[0070] The sample point values at the same depth of the P-wave velocity and the S-wave velocity are divided to obtain the percentage coefficient. percent = vpw / vsw, where vpw is the P-wave logging velocity, vsw is the S-wave logging velocity, and percent is the percentage coefficient.

[0071] Step 3: Interpolate and extrapolate the multi-well percentage curve to form a percentage volume;

[0072] The steps for interpolating and extrapolating the multi-well percentage curve to form a percentage volume are as follows:

[0073] 1) The extrapolation formula is the fitting formula, that is: substituting the existing sample points into y = ax² 2 + bx + c to obtain a functional relationship for the optimal abc coefficients, that is: it is the extrapolation formula.

[0074] 2) The interpolation formula is the linear interpolation formula. That is, if A(i1, b1) and B(i2, b2) are two points, then the point P(i, b) is on the straight line determined by the above two points. Then (b - b1) / (i - i1) = (b2 - b1) / (i2 - i1) = the slope of the straight line, and the required result can be obtained by transformation. The required percentage volume is denoted as percent1.

[0075] Step 4: Read the final P-wave velocity field of the grid tomography and extract the velocity curve at the well position; as Figure 4 shown, it is a schematic display of the final P-wave velocity field of the grid tomography;

[0076] Step 5: Match the smooth curve of the P-wave logging velocity and the P-wave velocity to obtain the percentage; the percentage coefficient obtained by dividing the sample point values at the same depth of the smooth curve of the P-wave logging velocity and the P-wave velocity is: percent2 = vpw / vp, where vpw is: the sample point velocity value of the smooth P-wave logging velocity curve, vp is: the velocity curve velocity value extracted from the final P-wave velocity field of the grid tomography at the well position, and percent2 is: the percentage coefficient.

[0077] Step 6: The single-well P-wave percentage curve is extrapolated to the top and bottom in terms of depth;

[0078] Its extrapolation formula is a fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c to obtain the best abc coefficients, and the resulting functional relationship is the extrapolation formula.

[0079] Step 7: Interpolate and extrapolate the longitudinal wave multi-well percentage curve to form a percentage volume;

[0080] The steps are as follows:

[0081] 1) The interpolation formula is a linear interpolation formula, that is: if A(i1, b1) and B(i2, b2) are two points, then the point P(i, b) is on the straight line determined by the above two points. Then (b - b1) / (i - i1) = (b2 - b1) / (i2 - i1) = the slope of the straight line, and the required formula can be obtained by transformation.

[0082] 2) The extrapolation formula is a fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c to obtain the best abc coefficients, and the resulting functional relationship is the extrapolation formula. The required percentage volume is denoted as: percent2.

[0083] Step 8: Scale to the multi-well constrained longitudinal wave velocity volume;

[0084] Use the above percentage volume percent2 to scale the final velocity field of the longitudinal wave grid tomography to the multi-well constrained longitudinal wave velocity volume.

[0085] The formula used is vpw = vp * percent2, where vpw is the multi-well constrained longitudinal wave velocity volume, vp is the final velocity field of the longitudinal wave grid tomography, and percent2 is the percentage coefficient.

[0086] Step 9: Combine the shear wave multi-well percentage volume and the multi-well constrained longitudinal wave velocity volume to obtain the multi-well constrained shear wave velocity volume;

[0087] The formula used is: vsw = vpw * percent1, where vsw is the multi-well constrained shear wave velocity volume, vpw is the well-constrained longitudinal wave velocity volume, and percent1 is the percentage volume obtained in the third step. As Figure 5 shown, it is a schematic display of the multi-well constrained shear wave velocity volume;

[0088] After the above steps of the present invention, the acquisition process of the converted wave depth migration well-constrained shear wave initial depth domain velocity field can be realized. It lays a depth domain shear wave velocity foundation for the converted wave depth migration grid tomography inversion of the shear wave accurate velocity. Then, by performing grid tomography inversion iteration from shallow to deep, the depth domain shear wave velocity field required for accurate converted wave depth migration can be obtained.

[0089] The above technologies not described are existing technologies, so they will not be elaborated here.

[0090] It can be understood that the present invention is described by means of some embodiments. Those skilled in the art will know that, without departing from the spirit and scope of the present invention, various changes or equivalent substitutions can be made to these features and embodiments. Additionally, under the teaching of the present invention, these features and embodiments can be modified to adapt to specific circumstances and materials without departing from the spirit and scope of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application belong to the scope protected by the present invention.

Claims

1. A method for establishing an initial shear wave velocity field for well-constrained converted wave depth migration, characterized in that, Adopt the following steps: The first step: Read the longitudinal wave and transverse wave acoustic logging time difference curves; The second step: Extract the smoothed curves of the longitudinal wave logging velocity and the transverse wave logging velocity, and calculate the percentages correspondingly; The third step: Interpolate and extrapolate the multi-well percentage curves to form a percentage volume; The fourth step: Read the final longitudinal wave velocity field of grid tomography, and extract the velocity curve at the well position; The fifth step: Calculate the percentage by matching the smoothed curve of the longitudinal wave logging velocity and the longitudinal wave velocity; The sixth step: Extrapolate the longitudinal wave single-well percentage curve to the top and bottom in depth; The seventh step: Interpolate and extrapolate the longitudinal wave multi-well percentage curves to form a percentage volume; The eighth step: Scale to the multi-well constrained longitudinal wave velocity volume; The ninth step: Combine the transverse wave multi-well percentage volume and the multi-well constrained longitudinal wave velocity volume to obtain the multi-well constrained transverse wave velocity volume; In the third step: The steps of interpolating and extrapolating the multi-well percentage curves to form a percentage volume are as follows: 1) The extrapolation formula is a fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c to obtain the best abc coefficients, and the resulting functional relationship is the extrapolation formula; 2) The interpolation formula is the linear interpolation formula, that is: if A(i1, b1) and B(i2, b2) are two points, then the point P(i, b) is on the straight line determined by the above two points; then (b - b1) / (i - i1) = (b2 - b1) / (i2 - i1) = the slope of the straight line, and the required result can be obtained by transformation; the obtained percentage volume is denoted as percent1.

2. The method for establishing the initial shear wave velocity field of well-constrained converted wave depth migration according to claim 1, wherein In the first step: When reading the longitudinal wave and transverse wave acoustic logging time difference curves, the formula for calculating the percentage coefficient from the logging time difference curves is: percent = v2 / v1 (1) The formula for applying the percentage coefficient to the logging time difference curve is: v2 = percent * v1; (2) The interpolation formula for the logging time difference curve is the linear interpolation formula, that is: if A(i1, b1) and B(i2, b2) are two points, then the point P(i, b) is on the straight line determined by the above two points; then (b - b1) / (i - i1) = (b2 - b1) / (i2 - i1) = the slope of the straight line, and the required result can be obtained by transformation; The extrapolation formula of the logging time difference curve is a fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c, obtaining a functional relationship by finding the best abc coefficients, that is: it is the extrapolation formula; The extraction smoothing curve formula for the logging time difference curve is: the fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c to obtain an optimal function relationship for the abc coefficients, that is: it is the extraction smoothing curve formula.

3. The method for establishing the initial shear wave velocity field of well-constrained converted wave depth migration according to claim 1, wherein In the second step: The steps of extracting the smoothed curves of the longitudinal wave logging velocity and the transverse wave logging velocity are as follows: 1) Extract a smooth curve from the longitudinal wave acoustic travel time curve; the extraction of the smooth curve needs to be tested according to the required accuracy in practice; the formula for extracting the smooth curve is a fitting formula, that is: substitute the existing sample points into y = ax 2 + bx + c to obtain a functional relationship of the optimal abc coefficients, that is: it is the formula for extracting the smooth curve; Among them, the single-well percentage curve is extrapolated to the top and bottom in terms of depth; the extrapolation formula is the fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c to obtain the best abc coefficients, and the resulting functional relationship is the extrapolation formula; 2) Extract a smooth curve from the shear wave acoustic travel time curve; the extraction of the smooth curve needs to be tested according to the required accuracy; the formula for extracting the smooth curve is a fitting formula, that is: substitute the existing sample points into y = ax 2 + bx + c to obtain the best abc coefficients, and the resulting functional relationship is the formula for extracting the smooth curve; Among them, the single-well percentage curve is extrapolated to the top and bottom in terms of depth; the extrapolation formula is the fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c to obtain an optimal function relationship for the abc coefficients, that is: it is the extrapolation formula; 3) Calculate the percentage by matching the longitudinal and transverse waves; Divide the sample point values of the longitudinal wave velocity and the transverse wave velocity at the same depth to calculate the percentage coefficient: percent = vpw / vsw, where vpw is the longitudinal wave logging velocity, vsw is the transverse wave logging velocity, and percent is the percentage coefficient.

4. The method for establishing the initial shear wave velocity field of well-constrained converted wave depth migration according to claim 1, wherein In the fifth step: Divide the sample point values of the smoothed curve of the longitudinal wave logging velocity and the longitudinal wave velocity at the same depth to calculate the percentage coefficient as; percent2 = vpw / vp, where vpw is: the sample point velocity value of the smoothed longitudinal wave velocity curve of logging, vp is: the velocity curve velocity value extracted from the final longitudinal wave velocity field of grid tomography at the well position, and percent2 is: the percentage coefficient.

5. The method for establishing an initial shear wave velocity field for well-constrained converted wave depth migration according to claim 1, wherein In the sixth step: the extrapolation formula is a fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c, and obtaining a functional relationship by calculating the optimal abc coefficients, which is: the extrapolation formula.

6. The method for establishing an initial shear wave velocity field for well-constrained converted wave depth migration according to claim 1, wherein In the seventh step: The steps are as follows: 1) The interpolation formula is the linear interpolation formula, that is: if A(i1, b1) and B(i2, b2) are two points, then the point P(i, b) is on the straight line determined by the above two points; then (b - b1) / (i - i1) = (b2 - b1) / (i2 - i1) = the slope of the straight line, and the required result can be obtained by transformation; 2) The extrapolation formula is a fitting formula, that is: substituting the existing sample points into y = ax 2 + bx + c to obtain the best abc coefficients, and the resulting functional relationship is the extrapolation formula: the percentage volume to be found is denoted as percent2.

7. The method for establishing the initial shear wave velocity field of well-constrained converted wave depth migration according to claim 1, wherein In the eighth step: The steps are as follows: Use the above percentage body percent2 ratio of the final P-wave grid tomography velocity field to the multi-well constrained P-wave velocity body: Use the formula vpw = vp * percent2, where vpw is the multi-well constrained P-wave velocity body, vp is the final P-wave grid tomography velocity field, and percent2 is the percentage coefficient.

8. The method for establishing an initial shear wave velocity field for well-constrained converted wave depth migration according to claim 1, wherein In the ninth step: The formula used is: vsw = vpw * percent1, where vsw is the multi-well constrained S-wave velocity body, vpw is the well-constrained P-wave velocity body, and percent1 is the percentage body obtained in the third step.

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