A design method for a shear wave observation system

Abstract

This invention belongs to the field of shear wave exploration technology and discloses a design method for a shear wave observation system. The method involves: acquiring target layer parameter information in the exploration area; determining the shear wave elements of the shear wave observation system based on the target layer parameter information; determining the shear wave receiver spacing of the shear wave observation system based on the target layer parameter information; quantitatively calculating the maximum shear wave offset based on the time-distance curves of the direct shear wave, refracted shear wave, and reflected shear wave; verifying the effectiveness of the shear wave elements in step S2, the shear wave receiver spacing in step S3, and the maximum shear wave offset in step S4 based on forward modeling of the shear wave equation and actual shear wave seismic records acquired through field experiments; and calculating the coverage number and coverage density of the shear wave observation system based on the signal-to-noise ratio of the actual shear wave data and the effective offset range of the shear wave. This invention obtains high-quality shear wave seismic data through qualitative and quantitative design of a nine-component three-dimensional shear wave observation system. This invention is applicable to the design of shear wave observation systems.

Description

Technical Field

[0001] This invention belongs to the field of shear wave exploration technology, specifically a design method for a shear wave observation system. Background Technology

[0002] With the increasing total consumption of natural gas, China's total imports of natural gas have been rising year by year. To discover more natural gas resources, reduce dependence on imports, and ensure energy security, my country has gradually increased its efforts in natural gas exploration. Sanhu is the main gas-producing area in the Qaidam Basin, where several gas-bearing structures have been discovered. Years of exploration practice and research results indicate that the target subsurface strata in this area are mainly Quaternary Q... 1+2 The loose sandstone has a low tectonic amplitude structure, with the target layer buried at a depth of 800-2000m. The gas layer is relatively shallow. Due to the influence of gas absorption and attenuation, the P-wave profile generally exhibits seismic anomalies of "low velocity, low frequency and in-phase axis pull-down", resulting in the low amplitude structure of the "gas cloud area" not being imaged.

[0003] Seismic exploration in this area began in the 1990s, with subsequent explorations including conventional 2D, high-resolution 2D, high-precision 2D, and high-density 3D. However, single-wave P-wave exploration failed to identify gas-bearing anomalies and perform low-amplitude structural imaging. In 2006 and 2009, converted-wave 2D and 3D explorations were attempted, significantly improving the quality of low-amplitude structural data in the "gas cloud region." However, converted-wave data could not fully reconstruct the low-amplitude structures of the complex "gas cloud region," and substantial breakthroughs in natural gas exploration were still not achieved. In 2017, BGP Inc. developed and manufactured three new shear-wave controlled seismic sources, the BV300S, and conducted 800km high-density wide-line 2D P-wave and S-wave acquisitions in the area. This yielded high-quality broadband S-wave data for the first time, making significant progress in delineating the boundaries of the low-amplitude structural "gas cloud region" and reconstructing the structures within it. In 2019, BGP Inc. further improved and upgraded the performance of the shear-wave controlled seismic source, manufacturing six EV56S units, and conducted nine-component P-wave and S-wave 3D acquisition experiments and feasibility studies in the area. In 2019, Qinghai Oilfield Company deployed a nine-component P- and S-wave three-dimensional full-coverage system covering an area of ​​82.08 km² in this region. 2 The project was implemented by the Qinghai Geophysical Exploration Department of BGP Inc., marking the world's first industrial application of nine components for both P-wave and S-wave. With the large-scale promotion of P-wave and S-wave exploration technologies, substantial progress has been made in S-wave acquisition technology, accumulating considerable experience. The design of S-wave observation systems is a crucial aspect of S-wave acquisition construction; however, most current observation systems are designed for P-waves and converted waves, and observation systems designed specifically for pure S-waves have not yet been reported. Summary of the Invention

[0004] The purpose of this invention is to provide a design method for a shear wave observation system to obtain high-quality shear wave seismic data.

[0005] To achieve the above objectives, the present invention employs the following technical methods:

[0006] A design method for a shear wave observation system includes the following steps:

[0007] S1. Obtain target layer parameter information in the exploration area;

[0008] S2. Determine the shear wave elements of the shear wave observation system based on the target layer parameter information of the exploration area;

[0009] S3. Determine the shear wave receiver spacing of the shear wave observation system based on the target layer parameter information of the exploration area.

[0010] S4. Quantitatively calculate the maximum offset distance of the shear wave based on the time-distance curves of the shear wave direct wave, the shear wave refracted wave, and the shear wave reflected wave.

[0011] S5. Based on the forward modeling of the shear wave equation and the actual shear wave seismic records collected in the field, the effectiveness of the shear wave elements in step S2, the shear wave receiver spacing in step S3, and the maximum offset of the shear wave in step S4 is verified.

[0012] S6. Calculate the coverage number and coverage density of the shear wave observation system based on the signal-to-noise ratio of the actual shear wave data and the effective offset range of the shear wave.

[0013] As a limitation: the target layer parameter information in step S1 is extracted from well logging, VSP and seismic data of the exploration area. The target layer parameter information includes the root mean square velocity of the target layer P wave, the root mean square velocity of the target layer S wave, the target layer P wave velocity, the target layer S wave velocity, the target layer P wave velocity ratio, the target layer layer velocity, the target layer dip angle, the highest frequency of the target layer reflected S wave, the highest frequency of the target layer reflected P wave, the main frequency of the target layer reflected S wave, the main frequency of the target layer reflected P wave, the target layer burial depth and the target layer S wave two-way travel time.

[0014] As a further limitation: step S2 includes the following steps:

[0015] S21. According to the spatial sampling theorem, to ensure that no spatial aliasing occurs, the longitudinal wavefront element must be smaller than half the apparent wavelength, that is: In the formula, ΔX P For P-wave elements, θ is the dip angle of the target layer, and V p-rms f is the root mean square velocity of the P-waves in the strata covering the reflector layer. max Let be the highest frequency of the P-wave reflected from the target layer; the S-wave also satisfies the spatial sampling theorem. Assuming the highest frequencies of the S-wave and P-wave reflected from the target layer are equal, therefore... In the formula, ΔX s For the transverse wave element, V s-rms The root mean square velocity of the transverse waves in the strata covering the reflector layer;

[0016] The formula for calculating the P-wave / S-wave velocity ratio is: In the formula, γ is the ratio of P-wave to S-wave velocity, and V p V represents the longitudinal wave velocity of the target layer. s Let the shear wave velocity be the target layer velocity; therefore, the formula for calculating the shear wave surface element is: In the formula, ΔX s For transverse wave elements;

[0017] S22. According to the rule of thumb, at least two sampling points should be taken for the longitudinal wavelength of each dominant frequency to obtain the formula for calculating the longitudinal and transverse resolution: In the formula, ΔX P For the longitudinal wave element, f p V is the dominant frequency of the longitudinal wave reflected from the target layer. p-int The velocity of the P-wave layer above the target layer;

[0018] Transverse waves also meet the transverse resolution requirements, therefore, In the formula, ΔX s For the transverse wave element, f s V is the dominant frequency of the transverse wave reflected from the target layer. s-int The velocity of the shear wave layer above the target layer;

[0019] Assuming the highest frequency of the reflected shear wave and the dominant frequency of the reflected longitudinal wave are equal at the target layer, therefore, The formula for calculating the P-wave / S-wave velocity ratio is: Therefore, the formula for calculating the lateral resolution of a transverse wave is:

[0020] S23. Compare the numerical values ​​of the shear wave elements obtained in step S21 and step S22, and take the minimum value as the shear wave element of the shear wave observation system.

[0021] As a further clarification: Step S3 includes the following steps:

[0022] S31. According to the Fresnel zone imaging principle, the longitudinal wave receiving line spacing is less than or equal to the radius of the first Fresnel zone, i.e., the formula is: In the formula, RLI p R is the longitudinal wave receiver line spacing. p Let λ be the radius of the first Fresnel zone of the longitudinal wave. p Let H be the wavelength of the P-wave and H be the burial depth of the target layer; the formula for calculating the travel time of the P-wave reflected from the target layer is: In the formula, t p0 The travel time of the P-wave reflected from the target layer is given by: The formula for calculating the P-wave wavelength is: therefore,

[0023]

[0024]

[0025] Transverse waves also satisfy the Fresnel zone imaging principle, therefore,

[0026]

[0027] In the formula, RLI s R is the shear wave receiver line spacing. s Let λ be the radius of the first Fresnel zone of the transverse wave. s t is the wavelength of the transverse wave. s0 The travel time of the transverse wave reflected from the target layer;

[0028] The formula for calculating the P-wave / S-wave velocity ratio is: Assuming the dominant frequencies of the reflected shear waves and the dominant frequencies of the reflected longitudinal waves are equal at the target layer, therefore, S32, Divide by get Right now

[0029] As a further clarification: Step S4 includes the following steps:

[0030] S41. The formula for the time-distance curve of the shear wave direct arrival wave is: In the formula, X is the distance from the shot point to the receiver point, and V... s-LVL T is the transverse wave velocity of the medium between the shot point and the receiver point. s-dir The reflection time of the direct wave and transverse wave;

[0031] Hyperbolic formula for transverse wave reflection:

[0032] In the formula, V sa The superposition velocity T of the reflected transverse wave s-ref The reflection time of the reflected transverse wave;

[0033] When the hyperbola of the reflected transverse wave and the hyperbola of the direct transverse wave intersect, T s-dir =T s-ref X = X dir X dir Let be the offset distance at the intersection of the transverse wave reflected hyperbola and the transverse wave direct hyperbola. From this, we can obtain:

[0034]

[0035] The maximum offset of the shear wave is less than or equal to the offset of the intersection of the reflected hyperbola and the direct hyperbola of the shear wave, that is: X maxThis represents the maximum offset of the shear wave.

[0036] S42. The formula for the time-distance curve of a transverse wave refracted is: In the formula, V s0 For the shear wave velocity of the low-deceleration layer in the medium between the shot and receiver points, V s1 T is the shear wave velocity of the refractive layer between the shot and receiver points, θ′ is the dip angle of the refractive interface, and T is the shear wave velocity of the refractive layer between the shot and receiver points. s-dir ′ represents the reflection time of the refracted transverse wave;

[0037] When the hyperbola of reflected transverse wave and the hyperbola of refracted transverse wave intersect, X = X′ dir , X′ dir Let be the offset distance at which the hyperbola of the reflected transverse wave and the time-distance curve of the refracted transverse wave intersect. From this, we can obtain:

[0038]

[0039] The maximum offset distance of the shear wave is less than or equal to the offset distance at which the hyperbolic curve of the reflected shear wave and the time-distance curve of the refracted shear wave intersect, that is: X max This represents the maximum offset of the shear wave.

[0040] S43. The formula for the time-distance curve of the transverse wave reflection is:

[0041] In the formula, T s-ref T is the reflection time of the reflected transverse wave. s0 The zero-offset reflection time of the reflected transverse wave; after polynomial expansion, we get: The formula for calculating the percentage of dynamic corrected tensile distortion is: In the formula, φ is the percentage of dynamic correction stretch distortion, and ΔT is the dynamic correction time difference; for Substitute the second approximation In the middle, we get

[0042] S44. The formula for calculating dynamic correction time difference is: When the dynamic correction speed error is ΔV sa At that time, the dynamic correction time difference is: The accuracy of the velocity spectrum in identifying the dynamic correction time quantity is D t The required relative accuracy of velocity analysis is [value missing]. get:

[0043] therefore, In the formula, λ v For relative accuracy of velocity analysis, f dom The dominant frequency of the reflected shear wave of the target layer;

[0044] S45. Compare the maximum offset distances of the shear waves obtained in steps S41, S42, S43 and S44, and take the maximum value as the maximum offset distance of the shear waves of the shear wave observation system.

[0045] As a further clarification: Step S5 includes the following steps:

[0046] S51. Using the shear wave surface element calculated in step S2, the shear wave receiver line spacing calculated in step S3, and the maximum offset of the shear wave calculated in step S4, perform forward modeling of the shear wave equation and collect actual shear wave seismic records in the field.

[0047] S52. Using spectral analysis of different surface element FK spectrum forward modeling and actual shear wave single-shot records, determine the size of the permissible surface element without spatial aliasing and verify the rationality of the shear wave surface element calculated in step S2.

[0048] S53, Utilizing velocity spectra with different offset distances and the effective offset distance X of actual large-scale shear waves. val Ratio of the actual target layer burial depth H Verify the correctness of the shear wave receiver line spacing calculated in step S3.

[0049] As a further clarification: Step S6 includes the following steps:

[0050] S61. Extract the signal-to-noise ratio of the target layer from the single-shot shear wave record, and calculate the shear wave coverage number and coverage density according to the geological task requirements, i.e. In the formula, N s D represents the number of shear wave coverages. s For the density of the gun track, SNR s-section For the desired profile signal-to-noise ratio, SNR s-shot The signal-to-noise ratio for a single shot recording in shear waves is RI, where RI is the receiver channel distance and SI is the firing point distance.

[0051] S62. Calculate the effective coverage number and effective coverage density of the shear wave based on the effective offset of the shear wave, i.e. In the formula, N s-val D represents the number of times the shear wave effectively covers the area. s-val SLI represents the effective coverage density of the shear wave, X represents the excitation line spacing, and X represents the effective coverage density of the shear wave. s-val The effective shot-receiver distance for shear waves is R, where R is the number of receiver lines.

[0052] S63. Calculate the signal-to-noise ratio curves of different coverage times of actual shear wave seismic data, and determine the limit values ​​of the coverage times and coverage density of the observation system.

[0053] As a further limitation: step S63 includes the following steps:

[0054] S631. Based on the effective coverage number and effective coverage density of shear waves calculated in step S62, design a high-density wide-line observation system and conduct field tests to collect actual shear wave seismic data.

[0055] S632. Extract observation systems with different coverage times and different coverage densities, and perform overlay and migration processing on the profiles;

[0056] S633. Qualitative comparison of the signal-to-noise ratio and imaging accuracy of the target layer in shear wave profiles of observation systems with different coverage times and coverage densities;

[0057] S634. Extract the signal-to-noise ratio profiles of observation systems with different coverage times and coverage densities and plot the signal-to-noise ratio curves. Based on the rate of change of the curves, determine the limit values ​​of the coverage times and coverage densities of the shear wave observation system, and verify the rationality of the effective coverage times and effective coverage densities obtained in step S62.

[0058] The beneficial effects achieved by this invention, due to the adoption of the above-described solution, compared with the prior art, are as follows:

[0059] This invention provides a design method for a shear wave observation system, which obtains high-quality shear wave seismic data by qualitatively and quantitatively designing a nine-component three-dimensional shear wave observation system.

[0060] This invention is applicable to the design of shear wave observation systems. Detailed Implementation

[0061] The present invention will be further described below with reference to the embodiments. However, those skilled in the art should understand that the present invention is not limited to the following embodiments. Any improvements and equivalent changes made based on the specific embodiments of the present invention are within the scope of protection of the claims of the present invention.

[0062] Example: A design method for a shear wave observation system

[0063] A design method for a shear wave observation system includes the following steps:

[0064] S1. Obtain target layer parameter information in the exploration area;

[0065] Based on well logging, VSP, and seismic data from the exploration area, target layer parameter information is extracted. The target layer parameter information includes the root mean square velocity of the P-wave, the root mean square velocity of the S-wave, the P-wave velocity, the S-wave velocity, the P-wave to S-wave velocity ratio, the target layer velocity, the target layer dip angle, the highest frequency of the reflected S-wave, the highest frequency of the reflected P-wave, the dominant frequency of the reflected S-wave, the dominant frequency of the reflected P-wave, the burial depth of the target layer, and the two-way travel time of the S-wave.

[0066] S2. Determine the shear wave elements of the shear wave observation system based on the target layer parameter information of the exploration area;

[0067] S21. According to the spatial sampling theorem, to ensure that no spatial aliasing occurs, the longitudinal wavefront element must be smaller than half the apparent wavelength, that is: In the formula, ΔX P For P-wave elements, θ is the dip angle of the target layer, and V p-rms f is the root mean square velocity of the P-waves in the strata covering the reflector layer. max The highest frequency of the longitudinal wave reflected from the target layer;

[0068] The transverse wave also satisfies the spatial sampling theorem; assuming the highest frequency of the reflected transverse wave and the highest frequency of the reflected longitudinal wave at the target layer are equal, therefore, In the formula, ΔX s For the transverse wave element, V s-rms The root mean square velocity of the transverse waves in the strata covering the reflector layer;

[0069] The formula for calculating the P-wave / S-wave velocity ratio is: In the formula, γ is the ratio of P-wave to S-wave velocity, and V p V represents the longitudinal wave velocity of the target layer. s Let the shear wave velocity be the target layer velocity; therefore, the formula for calculating the shear wave surface element is: Where: ΔX s In this embodiment, the transverse wave element is used.

[0070] S22. According to the rule of thumb, at least two sampling points should be taken for the wavelength of each dominant frequency to obtain the formula for calculating the lateral resolution: Where: ΔX P For the longitudinal wave element, f p V is the dominant frequency of the longitudinal wave reflected from the target layer. p-int The longitudinal wave velocity of the layer above the target layer;

[0071] Transverse waves also meet the transverse resolution requirements, therefore, Where: ΔX s For the transverse wave element, f s V is the dominant frequency of the transverse wave reflected from the target layer. s-int The velocity of the shear wave layer above the target layer;

[0072] Assuming the dominant frequencies of the reflected shear waves and the dominant frequencies of the reflected longitudinal waves are equal at the target layer, therefore, The formula for calculating the P-wave / S-wave velocity ratio is: Therefore, the formula for calculating the lateral resolution of a transverse wave is:

[0073] S23. Compare the numerical values ​​of the shear wave elements obtained in step S21 and step S22, and take the minimum value as the shear wave element of the shear wave observation system.

[0074] S3. Determine the shear wave receiver spacing of the shear wave observation system based on the target layer parameter information of the exploration area.

[0075] S31. According to the Fresnel zone imaging principle, the longitudinal wave receiving line spacing is less than or equal to the radius of the first Fresnel zone, i.e., the formula is: In the formula: RLI p R is the longitudinal wave receiver line spacing. p Let λ be the radius of the first Fresnel zone of the longitudinal wave. p Let H be the wavelength of the P-wave and H be the burial depth of the target layer; the formula for calculating the travel time of the P-wave reflected from the target layer is: In the formula, t p0 The travel time of the P-wave reflected from the target layer is given by: The formula for calculating the P-wave wavelength is: therefore,

[0076]

[0077]

[0078] Transverse waves also satisfy the Fresnel zone imaging principle, therefore,

[0079]

[0080] In the formula, RLI s R is the shear wave receiver line spacing. s Let λ be the radius of the first Fresnel zone of the transverse wave. s t is the wavelength of the transverse wave. s0 The travel time of the transverse wave reflected from the target layer;

[0081] The formula for calculating the P-wave / S-wave velocity ratio is: Assuming the dominant frequencies of the reflected shear waves and the dominant frequencies of the reflected longitudinal waves are equal at the target layer, therefore, S32, Divide by get In this embodiment, λ p =100m, H=2000m, therefore That is, RLI s ≤0.7RLI p .

[0082] S4. Quantitatively calculate the maximum offset distance of the shear wave based on the time-distance curves of the shear wave direct wave, the shear wave refracted wave, and the shear wave reflected wave.

[0083] S41. The formula for the time-distance curve of the shear wave direct arrival wave is: In the formula, X is the distance from the shot point to the receiver point, and V... s-LVL T is the transverse wave velocity of the medium between the shot point and the receiver point. s-dir The reflection time of the direct wave and transverse wave;

[0084] Hyperbolic formula for transverse wave reflection:

[0085] In the formula, V sa T is the superposition velocity of the reflected transverse waves. s-ref T is the reflection time of the reflected shear wave; when the hyperbola of the reflected shear wave and the hyperbola of the direct shear wave intersect, T s-dir =T s-ref X = X dir X dir Let be the offset distance at the intersection of the transverse wave reflected hyperbola and the transverse wave direct hyperbola. From this, we can obtain:

[0086]

[0087] The maximum offset of the shear wave is less than or equal to the offset of the intersection of the reflected hyperbola and the direct hyperbola of the shear wave, that is: X max In this embodiment, V represents the maximum offset distance of the shear wave. s-LVL =400m / s,V sa =1500m / s, therefore, X dir =1106.7m, X max ≤X dir =1106.7m;

[0088] S42. The formula for the time-distance curve of a transverse wave refracted is: In the formula, V s0 For the shear wave velocity of the low-deceleration layer in the medium between the shot and receiver points, V s1 T is the shear wave velocity of the refractive layer between the shot and receiver points, θ′ is the dip angle of the refractive interface, and T is the shear wave velocity of the refractive layer between the shot and receiver points. s-dir ′ represents the reflection time of the refracted transverse wave;

[0089] When the hyperbola of reflected transverse wave and the hyperbola of refracted transverse wave intersect, T s-dir ′=T s-ref X = X′ dir , X′ dir Let be the offset distance at which the hyperbola of the reflected transverse wave and the time-distance curve of the refracted transverse wave intersect. From this, we can obtain:

[0090]

[0091] The maximum offset distance of the shear wave is less than or equal to the offset distance at which the hyperbolic curve of the reflected shear wave and the time-distance curve of the refracted shear wave intersect, that is: X max In this embodiment, V represents the maximum offset distance of the shear wave. s0 =400m / s, V s1 =600m / s, θ′=0°, therefore, X d ′ ir =3348m, X max ≤X d ′ ir =3348m;

[0092] S43. The formula for the time-distance curve of the transverse wave reflection is:

[0093] In the formula, T s-ref T is the reflection time of the reflected transverse wave. s0 The zero-offset reflection time of the reflected transverse wave; after polynomial expansion, we get: The formula for calculating the percentage of dynamic corrected tensile distortion is: In the formula, φ is the percentage of dynamic correction stretch distortion, and ΔT is the dynamic correction time difference; for Substitute the second approximation In the middle, we get In this embodiment, φ = 12.5%, therefore, X = 1414m. max ≤X=1414m;

[0094] S44. The formula for calculating dynamic correction time difference is: When the dynamic correction speed error is ΔV sa At that time, the dynamic correction time difference is: The accuracy of the velocity spectrum in identifying the dynamic correction time quantity is D t The required relative accuracy of velocity analysis is [value missing]. get:

[0095] therefore, In the formula, λ v For relative accuracy of velocity analysis, f dom In this embodiment, f is the dominant frequency of the reflected shear wave of the target layer. dom =35Hz, 6% < λ v <8%, Therefore, X max ≥845m;

[0096] S45. Compare the maximum offset distances of the shear waves obtained in steps S41, S42, S43 and S44, and take the maximum value as the maximum offset distance of the shear wave observation system. Therefore, in this embodiment, the maximum offset distance of the shear wave observation system is 3348m.

[0097] S5. Based on the forward modeling of the shear wave equation and the actual shear wave seismic records collected in the field, the effectiveness of the shear wave elements in step S2, the shear wave receiver spacing in step S3, and the maximum offset of the shear wave in step S4 is verified.

[0098] S51. Using the shear wave surface element calculated in step S2, the shear wave receiver line spacing calculated in step S3, and the maximum offset of the shear wave calculated in step S4, perform forward modeling of the shear wave equation and collect actual shear wave seismic records in the field.

[0099] S52. Using spectral analysis of different surface element FK spectrum forward modeling and actual shear wave single-shot records, determine the size of the permissible surface element without spatial aliasing and verify the rationality of the shear wave surface element calculated in step S2.

[0100] S53, Utilizing velocity spectra with different offset distances and the effective offset distance X of actual large-scale shear waves. val Ratio of the actual target layer burial depth H Verify the correctness of the shear wave receiver line spacing calculated in step S3.

[0101] S6. Calculate the coverage number and coverage density of the shear wave observation system based on the signal-to-noise ratio and effective offset range of the actual shear wave data.

[0102] S61. Extract the signal-to-noise ratio of the target layer from the single-shot shear wave record, and calculate the shear wave coverage number and coverage density according to the geological task requirements, i.e. In the formula, N s D represents the number of shear wave coverages. s For the density of the gun track, SNR s-section For the desired profile signal-to-noise ratio, SNR s-shot The signal-to-noise ratio for a single shot recording in shear waves is RI, where RI is the receiver channel distance and SI is the firing point distance.

[0103] S62. Calculate the effective coverage number and effective coverage density of the shear wave based on the effective offset of the shear wave, i.e. In the formula, N s-val D represents the number of times the shear wave effectively covers the area. s-val SLI represents the effective coverage density of the shear wave, X represents the excitation line spacing, and X represents the effective coverage density of the shear wave. s-val The effective shot-receiver distance for shear waves is R, where R is the number of receiver lines.

[0104] S63. Calculate the signal-to-noise ratio curves of different coverage times of actual shear wave seismic data, and determine the limit values ​​of the coverage times and coverage density of the observation system;

[0105] S631. Based on the effective coverage number and effective coverage density of shear waves calculated in step S62, design a high-density wide-line observation system and conduct field tests to collect actual shear wave seismic data.

[0106] S632. Extract observation systems with different coverage times and different coverage densities, and perform overlay and migration processing on the profiles;

[0107] S633. Qualitatively compare the profile signal-to-noise ratio and imaging accuracy of observation systems with different coverage times and coverage densities;

[0108] S634. Extract the signal-to-noise ratio profiles of observation systems with different coverage times and coverage densities and plot the signal-to-noise ratio curves. Based on the rate of change of the curves, determine the limit values ​​of the coverage times and coverage densities of the shear wave observation system, and verify the rationality of the effective coverage times and effective coverage densities obtained in step S62.

Claims

1. A design method for a shear wave observation system, characterized in that, Includes the following steps: S1. Obtain target layer parameter information in the exploration area; S2. Determine the shear wave elements of the shear wave observation system based on the target layer parameter information of the exploration area; S3. Determine the shear wave receiver spacing of the shear wave observation system based on the target layer parameter information of the exploration area. S4. Quantitatively calculate the maximum offset of the shear wave based on the time-distance curves of the direct shear wave, refracted shear wave, and reflected shear wave; specifically including: S41. The formula for the time-distance curve of the shear wave direct arrival wave is: In the formula, The distance from the shot point to the receiver point. The transverse wave velocity of the medium between the shot point and the receiver point. The reflection time of the direct wave transverse wave; Hyperbolic formula for transverse wave reflection: In the formula, The superposition velocity of the reflected transverse waves, H represents the reflection time of the reflected shear wave, and H represents the burial depth of the target layer. When the hyperbola of the reflected transverse wave and the hyperbola of the direct transverse wave intersect... , , Let be the offset distance at the intersection of the transverse wave reflected hyperbola and the transverse wave direct hyperbola. From this, we can obtain: ； The maximum offset of the shear wave is less than or equal to the offset of the intersection of the reflected hyperbola and the direct hyperbola of the shear wave, that is: , This represents the maximum offset of the shear wave. S42. The formula for the time-distance curve of a transverse wave refracted is: In the formula, For the shear wave velocity of the low-deceleration layer of the medium between the shot and receiver points, The transverse wave velocity of the refractive layer between the shot and receiver points. For the angle of the refractive interface, The reflection time of the refracted transverse wave; When the hyperbola of reflected transverse wave and the hyperbola of refracted transverse wave intersect... , Let be the offset distance at the intersection of the transverse wave reflection hyperbola and the transverse wave refraction hyperbola. From this, we can obtain: ； The maximum offset distance of the transverse wave is less than or equal to the offset distance at the intersection of the transverse wave reflection hyperbola and the transverse wave refraction hyperbola, that is: , This represents the maximum offset of the shear wave. S43. The formula for the time-distance curve of the transverse wave reflection is: In the formula, The reflection time of the reflected transverse wave. The zero-offset reflection time of the reflected transverse wave; after polynomial expansion, we get: ; The formula for calculating the percentage of dynamic corrected tensile distortion is: In the formula, The percentage of dynamic correction for tensile distortion. For dynamic correction of time difference; Substitute the second approximation In the middle, we get ; S44. The formula for calculating dynamic correction time difference is: When the dynamic correction speed error is At that time, the dynamic correction time difference is: ; The accuracy of the velocity spectrum in identifying dynamic correction quantities is The required relative accuracy of velocity analysis is [value missing]. ,get: , therefore, In the formula, For the relative accuracy of velocity analysis, The dominant frequency of the reflected shear wave of the target layer; S45. Compare the maximum offset distances of the shear waves obtained in steps S41, S42, S43 and S44, and take the maximum value as the maximum offset distance of the shear waves of the shear wave observation system. S5. Based on the forward modeling of the shear wave equation and the actual shear wave seismic records collected in the field, the effectiveness of the shear wave elements in step S2, the shear wave receiver spacing in step S3, and the maximum offset of the shear wave in step S4 is verified. S6. Calculate the coverage number and coverage density of the shear wave observation system based on the signal-to-noise ratio of the actual shear wave data and the effective offset range of the shear wave.

2. The design method of a shear wave observation system according to claim 1, characterized in that, The target layer parameter information in step S1 is extracted from well logging and seismic data of the exploration area. The target layer parameter information includes the root mean square velocity of the target layer P-wave, the root mean square velocity of the target layer S-wave, the target layer P-wave velocity, the target layer S-wave velocity, the target layer P-wave to S-wave velocity ratio, the target layer velocity, the target layer dip angle, the highest frequency of the target layer reflected S-wave, the highest frequency of the target layer reflected P-wave, the dominant frequency of the target layer reflected S-wave, the dominant frequency of the target layer reflected P-wave, the target layer burial depth, and the target layer S-wave two-way travel time.

3. The design method of a shear wave observation system according to claim 2, characterized in that, Step S2 includes the following steps: S21. According to the spatial sampling theorem, to ensure that no spatial aliasing occurs, the longitudinal wavefront element must be smaller than half the apparent wavelength, that is: In the formula, For P-wave elements, θ represents the dip angle of the target layer. ƒ represents the root mean square velocity of the P-waves in the strata covering the reflector layer. max The highest frequency of the longitudinal wave reflected from the target layer; The transverse wave also satisfies the spatial sampling theorem. Assuming the highest frequency of the reflected transverse wave and the highest frequency of the reflected longitudinal wave are equal at the target layer, therefore... In the formula, For transverse wave elements, The root mean square velocity of the transverse waves in the strata covering the reflector layer; The formula for calculating the P-wave / S-wave velocity ratio is: In the formula, The ratio of P-wave to S-wave velocity, For the target layer P-wave velocity, Let the shear wave velocity be the target layer velocity; therefore, the formula for calculating the shear wave surface element is: In the formula, For transverse wave elements; S22. According to the rule of thumb, at least two sampling points should be taken for the longitudinal wavelength of each dominant frequency to obtain the formula for calculating the longitudinal and transverse resolution: In the formula, For longitudinal wave elements, ƒ p The target layer reflects the dominant P-wave frequency. The velocity of the longitudinal wave layer above the target layer; Transverse waves also meet the transverse resolution requirements, therefore, In the formula, For transverse wave elements, ƒ s The dominant frequency of the transverse wave reflected from the target layer. The velocity of the shear wave layer above the target layer; Assuming the dominant frequencies of the reflected shear waves and the dominant frequencies of the reflected longitudinal waves are equal at the target layer, therefore, ; The formula for calculating the P-wave / S-wave velocity ratio is: Therefore, the formula for calculating the lateral resolution of a transverse wave is: ; S23. Compare the numerical values ​​of the shear wave elements obtained in step S21 and step S22, and take the minimum value as the shear wave element of the shear wave observation system.

4. The design method of a shear wave observation system according to claim 3, characterized in that, Step S3 includes the following steps: S31. According to the Fresnel zone imaging principle, the longitudinal wave receiving line spacing is less than or equal to the radius of the first Fresnel zone, i.e., the formula is: In the formula, For the longitudinal wave receiving line spacing, The radius of the first Fresnel zone of the longitudinal wave. The wavelength of the longitudinal wave, The depth of the target layer; since the formula for calculating the travel time of the P-wave reflected from the target layer is: In the formula, The travel time of the P-wave reflected from the target layer is given by: The formula for calculating the P-wave wavelength is: ,therefore, ， ； Transverse waves also satisfy the Fresnel zone imaging principle, therefore, ， In the formula, The distance between the shear wave receiver lines. The radius of the first Fresnel zone of the transverse wave. The wavelength of the transverse wave. The travel time of the transverse wave reflected from the target layer; The formula for calculating the P-wave / S-wave velocity ratio is: Assuming the dominant frequencies of the reflected shear waves and the dominant frequencies of the reflected longitudinal waves are equal at the target layer, therefore, , ; S32, Divide by ,get ,Right now .

5. The design method of a shear wave observation system according to claim 1, characterized in that, Step S5 includes the following steps: S51. Using the shear wave surface element calculated in step S2, the shear wave receiver line spacing calculated in step S3, and the maximum offset of the shear wave calculated in step S4, perform forward modeling of the shear wave equation and collect actual shear wave seismic records in the field. S52. Using spectral analysis of different surface element FK spectrum forward modeling and actual shear wave single-shot records, determine the size of the permissible surface element without spatial aliasing and verify the rationality of the shear wave surface element calculated in step S2. S53, Utilizing different offset velocity spectra and the effective offset of actual large-scale shear waves. Burial depth of the actual target layer ratio Verify the correctness of the shear wave receiver line spacing calculated in step S3.

6. The design method of a shear wave observation system according to claim 5, characterized in that, Step S6 includes the following steps: S61. Extract the signal-to-noise ratio of the target layer from the single-shot shear wave record, and calculate the shear wave coverage number and coverage density according to the geological task requirements, i.e. , In the formula, For the number of shear wave coverages, For the density of the gun track, For the desired profile signal-to-noise ratio, The signal-to-noise ratio for single-shot recording of shear waves is RI, where RI is the receiver channel distance and SI is the firing point distance. S62. Calculate the effective coverage number and effective coverage density of the shear wave based on the effective offset of the shear wave, i.e. , In the formula, The number of times the shear wave effectively covers the area. SLI represents the effective coverage density of the shear wave and the excitation line spacing. For effective shot-receiver distance of shear waves, Number of receiving lines; S63. Calculate the signal-to-noise ratio curves of different coverage times of actual shear wave seismic data, and determine the limit values ​​of the coverage times and coverage density of the observation system.

7. The design method of a shear wave observation system according to claim 6, characterized in that, Step S63 includes the following steps: S631. Based on the effective coverage number and effective coverage density of shear waves calculated in step S62, design a high-density wide-line observation system and conduct field tests to collect actual shear wave seismic data. S632. Extract observation systems with different coverage times and coverage densities, and perform overlay and migration processing on the profiles; S633. Qualitative comparison of the signal-to-noise ratio and imaging accuracy of the target layer in shear wave profiles of observation systems with different coverage times and coverage densities; S634. Extract the signal-to-noise ratio profiles of observation systems with different coverage times and coverage densities and plot the signal-to-noise ratio curves. Based on the rate of change of the curves, determine the limit values ​​of the coverage times and coverage densities of the shear wave observation system, and verify the rationality of the effective coverage times and effective coverage densities obtained in step S62.

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