A Quantitative Evaluation Method for Joint Upward and Downward Wave Formation Q Absorption Compensation in VSP

Abstract

The present invention discloses a quantitative evaluation method for joint formation Q absorption compensation of VSP up and down waves. First, perform near-surface Q absorption compensation on surface seismic data, then perform Q absorption compensation for consolidated rock formations below the water table on the surface seismic data, perform non-zero offset correction on VSP well data and preprocess the zero-offset VSP well data. Then, use the discrete frequency shift method of the VSP down wave to obtain the Q value of the consolidated rock formation below the water table at the wellhead, perform Q absorption compensation for the consolidated rock formation below the water table of the VSP up wave, and then calculate the correlation coefficient between the VSP up wave seismic record and the surface seismic record. Finally, perform quantitative evaluation of the Q absorption compensation of surface seismic waves. The method of the present invention uses the correlation between the VSP up wave seismic record after joint Q compensation of VSP up and down waves and the surface seismic record with Q absorption compensation of underground media to quantitatively evaluate the reliability of Q absorption compensation of surface seismic waves, and solves the bottleneck problem of quantitative evaluation of Q absorption attenuation compensation of onshore surface seismic waves.

Description

Technical Field

[0001] The present invention belongs to the technical field of exploration geophysics, and particularly relates to a quantitative evaluation method for joint up - and - down - going wave formation Q absorption compensation in VSP. Background Technique

[0002] When seismic waves propagate in a viscoelastic inhomogeneous underground medium, due to the absorption of the formation, the energies of different frequency components will be attenuated to different degrees. In terms of the absorption degree of the formation, absorption is mainly divided into two categories: near - surface absorption and absorption in consolidated rock formations below the water table. As oil and gas exploration gradually moves towards deep - ultra - deep and other fields. For on - land surface seismic exploration, loose near - surface media absorb most of the effective seismic signals. At the same time, as the propagation distance increases, the reflected waves are absorbed and attenuated by underground consolidated rock formations. Since seismic waves with different frequencies have different propagation speeds and are affected differently by formation absorption, dispersion phenomena will occur, causing phase characteristics to be distorted, seriously reducing the resolution and imaging quality of seismic data for deep - ultra - deep target reservoirs. Therefore, solving the absorption compensation problems of the near - surface and consolidated rock formations below the water table is of great significance for petroleum seismic exploration.

[0003] During the propagation of seismic waves, the changes in amplitude, phase, and frequency are related to the absorption attenuation of the overlying formation. Formation absorption attenuation is a function of the formation quality factor Q and the propagation travel time t. Usually, in attenuation compensation research, the study of the quality factor Q is used to describe the overall effect of this absorption and attenuation. As the propagation distance increases, the attenuation of reflected waves in deep - ultra - deep layers is particularly obvious, and their amplitude, phase, and frequency have all undergone serious distortions. Without considering the influence of formation absorption effects, it will lead to a reduction in the imaging resolution and quality of deep - ultra - deep reservoirs, making it difficult to meet the requirements of reservoir prediction. Therefore, solving the formation absorption compensation problem is of great significance for petroleum seismic exploration.

[0004] The primary issue in formation Q absorption compensation is the extraction and modeling of the Q parameter of the subsurface medium. For onshore surface seismic exploration, there are two important characteristics in formation Q absorption compensation: 1. In terms of the absorption degree of the formation, absorption is mainly divided into two categories: near-surface absorption and absorption in the consolidated rock layer below the water table. The loose near-surface medium absorbs most of the effective seismic signals, representing the near-surface absorption effect. The refraction wave information above the water table is mainly used; while the seismic wave undergoes absorption attenuation in the consolidated rock layer below the water table, and the reflection wave information of the rock layer needs to be used. Therefore, it is necessary to compensate for near-surface absorption and absorption in the consolidated rock layer below the water table separately; 2. How to accurately obtain the quality factor of the formation has always been a research hotspot for geophysicists. Currently, multiple types of Q-value estimation methods based on signal theory and tomography theory have been formed in the industry. However, the absorption attenuation mechanism of the actual medium for seismic waves is very complex, and these methods all have their own limitations. In actual production, they often lead to unreasonable Q absorption compensation results, seriously affecting the reliability of the subsequent seismic data processing and interpretation results. Therefore, it is crucial to develop a quality control evaluation method for the Q absorption compensation effect of seismic waves. However, currently in actual seismic data processing, a reasonable quality control evaluation method has not been established. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a quantitative evaluation method for VSP up and down wave joint formation Q absorption compensation, effectively solving the bottleneck problem of the quantitative evaluation of onshore surface seismic wave Q absorption attenuation compensation, providing a basis for the fidelity of the seismic wave Q absorption compensation technology, and having good application prospects and technological leading roles.

[0006] The technical solution adopted by the present invention is as follows: A quantitative evaluation method for VSP up and down wave joint formation Q absorption compensation, and the specific steps are as follows:

[0007] S1. Perform near-surface Q absorption compensation on the surface seismic data;

[0008] Based on the refraction wave dominant frequency of different seismic traces, calculate the relative attenuation coefficient R at the surface receiving point position, combine the near-surface travel time t calculated by the near-surface velocity model, obtain the near-surface Q value model based on the spectral ratio method, and then realize the near-surface Q absorption compensation of the seismic data through the inverse Q filtering compensation algorithm.

[0009] S2. Perform Q absorption compensation on the consolidated rock layer below the water table for the surface seismic data;

[0010] Use the attenuation travel time tomography of the underground formation reflection wave to invert the Q value model of the consolidated rock layer below the water table, and then realize the Q absorption compensation of the consolidated rock layer below the water table for the seismic data through the inverse Q filtering compensation algorithm.

[0011] S3. Non-zero offset correction of the VSP well data;

[0012] Collect the VSP well data in the study area and conduct shot deviation analysis one by one. There is a certain distance between the shot point position and the wellhead position of the field VSP well data. Then, through non-zero deviation correction, the arrival time of the first break of the accurate zero-deviation VSP well data for each well is obtained.

[0013] S4. Preprocessing of zero-deviation VSP well data;

[0014] Preprocess the zero-deviation VSP well data obtained in step S3 to obtain the VSP downgoing wave seismic record and upgoing wave seismic record at the wellhead. Then, select the frequency-divided subtraction method based on surface consistency to suppress surface waves, and monitor the frequency domain before and after denoising and the removed surface waves.

[0015] The key steps of the preprocessing include:

[0016] (1) Pick up the arrival time of the first break of the downgoing wave;

[0017] (2) Calculate the average velocity using the arrival time information of the downgoing wave, and then obtain the layer velocity of each geophone interval;

[0018] (3) Remove the amplitude loss caused by geometric spreading of seismic waves through spherical spreading compensation, and restore the relative amplitude change relationship completely caused by underground medium factors;

[0019] (4) Use the frequency-divided subtraction method to suppress surface waves and eliminate the influence of strong ground roll wave energy on useful data;

[0020] (5) Finally, extract the high-quality upgoing wave seismic record and downgoing wave seismic record of the data at each VSP wellhead through the median filtering wavefield separation method.

[0021] S5. Use the discrete frequency shift method of VSP downgoing wave to obtain the Q value of the consolidated rock layer below the water table at the wellhead;

[0022] During the downward propagation of seismic waves, regard the downgoing wavefield received by each geophone point of the zero-deviation VSP data as the source wavefield of the next geophone point, and perform discretization processing on the existing frequency shift method to obtain the discrete frequency shift method. Then, use the zero-deviation VSP well data to inversely calculate the Q value of the consolidated rock layer below the water table at the wellhead with higher accuracy for each trace, specifically as follows:

[0023] For the down-going wave of each VSP well, calculate the dominant frequency and its variance of the down-going wave seismic records received by geophones at different depths. Using the discretized frequency shift method, calculate the layer Q value between each adjacent pair of geophones at this well point. According to the seismic interpretation horizons, conduct geological horizon division on the Q value of the consolidated rock layer below the water table at the wellhead obtained by calculation. Combining the actual formation conditions, refer to the layering of the layer velocity to conduct layering processing on the Q value curve so that it conforms to the changing trend of the layer velocity. Finally, calculate the average Q value within each geological horizon to obtain a more accurate Q value of the consolidated rock layer below the water table at the wellhead.

[0024] S6. Q absorption compensation for the consolidated rock layer below the water table of the VSP up-going wave;

[0025] Using the Q value of the consolidated rock layer below the water table at the wellhead obtained by the discretized frequency shift method of the VSP down-going wave in step S5, perform Q absorption compensation for the consolidated rock layer below the water table on the VSP up-going wave seismic records through the inverse Q filtering compensation algorithm.

[0026] S7. Calculate the correlation coefficient between the VSP up-going wave seismic records and the surface seismic records;

[0027] Use the VSP up-going wave seismic records after joint Q absorption compensation of the up and down waves and the surface seismic records before and after Q absorption compensation of the underground medium to calculate the correlation coefficient based on waveform similarity, as follows:

[0028] Let the surface seismic record be expressed as S(t), and the VSP up-going wave seismic record be expressed as V(t), and construct the correlation coefficient formula for the two seismic records:

[0029] ρ = COV(S(t), V(t)) / (σ_S(t) * σ_V(t))

[0030] Among them, COV(S(t), V(t)) represents the covariance of S(t) and V(t); if the changing trends of S(t) and V(t) are consistent, the covariance is close to 1, and if there is no linear relationship between them, the covariance is close to zero; σ_S(t) represents the standard deviation of the surface seismic signal record S(t); σ_V(t) represents the standard deviation of the seismic record V(t) in the VSP well.

[0031] By calculating the covariance of the two seismic records and dividing it by the product of their respective standard deviations, a value between 1 and 0 is obtained to characterize the linear correlation degree between S(t) and V(t).

[0032] S8. Quantitative evaluation of Q absorption compensation for surface seismic waves;

[0033] The VSP upgoing wave seismic record after compensating for the absorption of the consolidated rock layer Q below the water table is used as the evaluation criterion for judging the Q absorption compensation effect of the surface seismic record. Then, based on the correlation coefficient calculated from the VSP upgoing wave seismic record after joint Q compensation of the up and down waves and the surface seismic records before and after compensating for the Q absorption of the underground medium, the reliability of the Q absorption compensation of the surface seismic wave is quantitatively quality controlled and evaluated. Finally, the deterministic Q absorption compensation of the surface seismic data is realized.

[0034] Among them, the evaluation criterion is specifically as follows:

[0035] When the correlation coefficient ρ value between the surface seismic record after compensating for the Q absorption of the underground medium and the VSP upgoing wave seismic record after compensating for the Q absorption of the consolidated rock layer Q below the water table is larger than that before the Q absorption compensation and approaches 1, it indicates that the Q absorption compensation of the underground medium for the surface seismic record is reasonable and effective. On the contrary, when the correlation coefficient ρ value decreases or even approaches 0, it indicates that the Q absorption compensation of the underground medium for the surface seismic record is unreasonable and has the opposite effect.

[0036] The beneficial effects of the present invention: The method of the present invention first performs near-surface Q absorption compensation on the surface seismic data, then performs Q absorption compensation on the consolidated rock layer Q below the water table of the surface seismic data, performs non-zero offset correction on the VSP well data and preprocesses the zero-offset VSP well data. Then, the Q value of the consolidated rock layer Q below the water table at the wellhead is obtained by using the VSP downgoing wave discrete frequency shift method, Q absorption compensation of the VSP upgoing wave for the consolidated rock layer Q below the water table is performed, and then the correlation coefficient between the VSP upgoing wave seismic record and the surface seismic record is calculated. Finally, the quantitative evaluation of the Q absorption compensation of the surface seismic wave is carried out. The method of the present invention uses the correlation between the VSP upgoing wave seismic record after joint Q compensation of the VSP up and down waves and the surface seismic record after compensating for the Q absorption of the underground medium to quantitatively evaluate the reliability of the Q absorption compensation of the surface seismic wave, solves the bottleneck problem of quantitative evaluation of Q absorption attenuation compensation of onshore surface seismic waves, and provides a basis for the fidelity of the seismic wave Q absorption compensation technology. Description of the Drawings

[0037] Figure 1 It is a flowchart of a method for quantitatively evaluating the joint formation Q absorption compensation of VSP up and down waves according to the present invention.

[0038] Figure 2 It is the original profile diagram of the VSP well data in the embodiment of the present invention.

[0039] Figure 3 It is the profile diagram of the extracted downgoing wave after preprocessing the VSP well data in the embodiment of the present invention.

[0040] Figure 4 It is the curve diagram of the layer velocity change of the VSP well data in the embodiment of the present invention.

[0041] Figure 5 This is the curve of the main frequency of the VSP well varying with depth in the embodiment of the present invention.

[0042] Figure 6 This is the curve of the formation Q value variation of the VSP well data using the discrete frequency shift method in the embodiment of the present invention.

[0043] Figure 7 This is the comparison chart of the correlation coefficients calculated from the VSP up-going wave seismic record after the combined Q compensation of the up and down waves and the surface seismic records before (left) and after (right) the Q absorption compensation of the underground medium in a certain work area in the embodiment of the present invention.

[0044] Figure 8 This is the comparison chart of the time migration profiles before (left) and after (right) the formation Q compensation based on the quantitative evaluation of the combined formation Q absorption compensation of the VSP up and down waves in a certain work area in the embodiment of the present invention. Detailed implementation manner

[0045] The method of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0046] In this embodiment, the method of the present invention is applied to the actual data processing of a certain work area in western China. The surface conditions in this area are complex. The target formation depth reaches 7000m to 8000m. The reservoir rocks are mainly dolomite. The reservoir thickness is between 8m and 25m. The lateral heterogeneity is strong. The seismic reflection characteristics are variable, showing instability in lateral continuity. It is very difficult to identify and trace the seismic reflection of the dolomite reservoir, and it is difficult to predict the reservoir laterally. Therefore, it is necessary to carry out high-precision formation Q absorption compensation processing to broaden the effective information of seismic data. There are 13 VSP wells in the area. The well construction method is as follows: the observation point distance = 10m, the distance from the first trace to the wellhead is 30m, the shot distance from the wellhead is 59.5m, the receiver depth ranges from 1350m to 2400m, and the ratio of the offset to the receiver depth is much less than one-tenth. After zero-offset correction, it can be processed as zero-offset VSP data.

[0047] As Figure 1 shown, the flow chart of a method for quantitative evaluation of combined formation Q absorption compensation of VSP up and down waves of the present invention is as follows:

[0048] S1. Perform near-surface Q absorption compensation on the surface seismic data;

[0049] Based on the main frequencies of the refracted waves of different seismic traces, the relative attenuation coefficient R at the position of the surface receiver is obtained. Combining the near-surface travel time t calculated by the near-surface velocity model, a near-surface Q value model is obtained based on the spectral ratio method, and then the near-surface Q absorption compensation of the seismic data is realized through the inverse Q filtering compensation algorithm.

[0050] S2. Perform absorption compensation for the consolidated rock formation Q below the water table on the surface seismic data;

[0051] Use the travel-time tomography of the attenuation of the reflected waves in the underground strata to invert the Q-value model of the strata below the water table, and then realize the Q absorption compensation for the consolidated rock formation (medium and deep layers) below the water table of the seismic data through the inverse Q-filtering compensation algorithm.

[0052] S3. Non-zero offset correction of VSP well data;

[0053] In this embodiment, the data of 13 VSP wells in the study area are collected, and the shot offset analysis is carried out one by one. There is a certain distance between the shot point position and the wellhead position of the field VSP well data, and then the accurate first arrival time of the zero-offset VSP well data of each well is obtained through non-zero offset correction.

[0054] S4. Preprocessing of zero-offset VSP well data;

[0055] Preprocess the zero-offset VSP well data obtained in step S3. (In the work area of this embodiment, the signal-to-noise ratio of the VSP well data is relatively low, and the surface waves are very developed. Figure 2 It is the original profile of the VSP well data of a certain well) to obtain the down-going wave seismic record and the up-going wave seismic record of the VSP at the wellhead. Based on the principle of fidelity processing, this embodiment selects the frequency-divided subtraction method based on surface consistency to suppress the surface waves, and monitors the frequency domain before and after denoising and the removed surface waves to ensure that the relative amplitude relationship of the data is not changed during the processing process, and the frequency and phase characteristics are not damaged (that is, ensure that the amplitude, frequency and phase characteristics of the effective signal are not damaged during the denoising process).

[0056] The key steps of the preprocessing include:

[0057] (1) Pick up the first arrival time of the down-going wave;

[0058] (2) Calculate the average velocity using the first arrival time information of the down-going wave, and then obtain the layer velocity of each geophone interval;

[0059] (3) Remove the amplitude loss caused by geometric spreading of the seismic wave through spherical spreading compensation, and restore the relative amplitude change relationship completely caused by underground medium factors;

[0060] (4) Use the frequency-divided subtraction method to suppress the surface waves and eliminate the influence of the strong energy of the ground roll on the useful data;

[0061] (5) Finally, extract the high-quality up-going wave seismic record and down-going wave seismic record of the data at each VSP wellhead through the median filtering wavefield separation method.

[0062] Figure 3It is the downgoing wavefield profile extracted after the preprocessing of zero-offset VSP well data. Through the preprocessing of VSP well data, the interval velocity data of each well is calculated. Some VSP wells provide interval velocity data tables. By comparison, the velocity differences in the extremely shallow layers above 100 meters are relatively large, and the interval velocities provided by the VSP well interpretation results are all smaller than the values calculated by the processing. Further down, the interval velocities become closer. Since the non-zero-offset influence is eliminated in the processing and calculation process, the results are more accurate. Finally, accurate interval velocity data at each VSP well is obtained by combining the two. Figure 4 It is the final interval velocity change curve of a certain VSP well.

[0063] S5. Use the discretized frequency shift method for the downgoing wave of VSP to calculate the Q value of the consolidated rock layer below the water table at the wellhead;

[0064] Considering that during the downward propagation of seismic waves, the downgoing wavefield received by each geophone in the zero-offset VSP data can be regarded as the source wavefield of the next geophone at the same time. In this way, by making appropriate improvements (discretization processing) to the existing frequency shift method, the discretized frequency shift method is obtained, and the Q value of the consolidated rock layer below the water table at the wellhead with higher accuracy can be inversely calculated from the zero-offset VSP well data channel by channel.

[0065] Specifically, for the downgoing wave of each VSP well, calculate the main frequency and its variance of the downgoing wave seismic records received by geophones at different depths. Figure 5 It is the change curve of the main frequency with depth of a certain well. Using the discretized frequency shift method, calculate the layer Q value between each adjacent pair of geophones at this well point. According to the seismic interpretation horizons, conduct geological horizon division on the Q value of the consolidated rock layer below the water table at the wellhead calculated, and in combination with the actual formation situation, refer to the layering of the interval velocity to conduct layering processing on the Q value curve to make it conform to the interval velocity change trend. Finally, calculate the average Q value within each geological horizon, thereby obtaining the Q value of the consolidated rock layer below the water table at the wellhead with higher accuracy. Figure 6 It is the change curve of the Q value of the consolidated rock layer below the water table at the wellhead based on geological horizon constraints calculated for a certain well.

[0066] S6. Q absorption compensation for the consolidated rock layer (mid - deep layer) below the water table of the VSP upgoing wave;

[0067] Use the Q value of the consolidated rock layer below the water table at the wellhead obtained by the discretized frequency shift method for the VSP downgoing wave in step S5, and perform Q absorption compensation for the VSP upgoing wave seismic records through the inverse Q filtering compensation algorithm.

[0068] S7. Calculate the correlation coefficient between the VSP upgoing wave seismic records and the surface seismic records;

[0069] Such as Figure 7As shown, the cross-correlation coefficient based on waveform similarity is calculated using the VSP upgoing wave seismic record after the combined Q absorption compensation of the up- and down-going waves and the surface seismic records before and after the Q absorption compensation of the underground medium. It can be seen that in this embodiment, the cross-correlation coefficient between the surface seismic record before and after the formation Q compensation and the VSP upgoing wave seismic record after the formation Q compensation increases from 65% to 83%, which is in good agreement with the VSP record and has a high degree of fidelity, indicating that the effect of the formation Q compensation of the surface seismic record is reasonable.

[0070] The specific method for calculating the cross-correlation coefficient is as follows:

[0071] Let the surface seismic record be denoted as S(t) and the VSP upgoing wave seismic record be denoted as V(t), and a cross-correlation coefficient formula for the two seismic records is constructed:

[0072] ρ = COV(S(t), V(t)) / (σ_S(t) * σ_V(t))

[0073] Among them, COV(S(t), V(t)) represents the covariance of S(t) and V(t), which is a statistic measuring the trend of change of two variables; if the trends of change of S(t) and V(t) are the same, the covariance is close to 1; if there is no linear relationship between them, the covariance is close to zero; σ_S(t) represents the standard deviation of the surface seismic signal record S(t), which is a statistic measuring the degree of dispersion of the data distribution of S(t); the larger the standard deviation, the higher the degree of dispersion of the data points relative to the average value; σ_V(t) represents the standard deviation of the seismic record V(t) in the VSP well, which is a statistic measuring the degree of dispersion of the data distribution of V(t).

[0074] By calculating the covariance of the two seismic records and dividing it by the product of their respective standard deviations, a value between 1 and 0 is obtained, which is used to characterize the linear correlation degree between S(t) and V(t).

[0075] S8. Quantitative evaluation of the Q absorption compensation of surface seismic waves;

[0076] Since the vertical seismic profile (VSP) records are obtained by exciting on the ground and receiving in the vertical direction in the well, the direct downgoing wave is less affected by noise and can more accurately reflect the filtering effect of the formation than surface seismic records. Therefore, using the VSP downgoing wave to invert the formation quality factor Q is the most reliable approach. The formation Q parameter is obtained through the attenuation relationship of the VSP downgoing wave, and then Q compensation is performed on the VSP upgoing wave. Theoretically, due to the characteristics of the two observation methods of surface seismic and VSP: surface seismic records undergo double absorption attenuation through the near-surface and consolidated rock layers below the water table, while VSP, due to being received in the well, only undergoes one absorption attenuation through the consolidated rock layers below the water table. Therefore, the correlation between the VSP upgoing wave seismic record after joint Q compensation of the VSP up and down waves and the surface seismic record with Q absorption compensation of the underground medium can be used to quantitatively evaluate the reliability of Q absorption compensation of surface seismic waves.

[0077] Since the VSP is excited on the ground and received in the well and is not affected by near-surface absorption, therefore, for the VSP upgoing wave seismic record after Q absorption compensation of the consolidated rock layers below the water table, it can be used as a criterion for judging the Q absorption compensation effect of surface seismic records. Then, based on the correlation between the VSP upgoing wave seismic record after joint Q compensation of the up and down waves and the surface seismic records before and after Q absorption compensation of the underground medium, the reliability of Q absorption compensation of surface seismic waves is quantitatively quality-controlled and evaluated, and finally, deterministic Q absorption compensation of surface seismic data is achieved.

[0078] Among them, the said criterion is specifically as follows:

[0079] When the correlation coefficient ρ value between the surface seismic record after Q absorption compensation of the underground medium and the VSP upgoing wave seismic record after Q absorption compensation of the consolidated rock layers below the water table is larger than that before Q absorption compensation and approaches 1, it indicates that the Q absorption compensation of the underground medium for the surface seismic record is reasonable and effective. On the contrary, when the correlation coefficient ρ value decreases or even approaches 0, it indicates that the Q absorption compensation of the underground medium for the surface seismic record is unreasonable and has the opposite effect.

[0080] Figure 8 Figure 13 is the time migration profile of the surface seismic record beside a certain well before and after Q absorption compensation of the formation. By comparison, it can be seen that the resolution of the entire profile has been effectively improved after Q absorption compensation of the formation, and the recognition ability of the weak bright spot reflection characteristics of the deep dolomite thin reservoir has been effectively improved.

[0081] In summary, as can be seen from this embodiment, the actual application of deep - ultra - deep seismic exploration in a certain work area in western China shows that the method of the present invention improves the resolution and fidelity of deep - layer seismic imaging, restores the energy attenuation and phase distortion caused by attenuation, the weak wave - peak bright - spot reflection characteristics of dolomite reservoirs are clear, and it fits well with logging, improves the ability to identify thin reservoirs, and provides more reliable data for heterogeneous reservoir prediction. The method of the present invention uses the downgoing wave of VSP to invert the formation quality factor Q, and then compensates Q for the upgoing wave of VSP. On this basis, the correlation between the VSP upgoing - wave seismic record after joint Q compensation of VSP up - and - downgoing waves and the surface seismic record with Q absorption compensation of the underground medium is used to quantitatively evaluate the reliability of Q absorption compensation of surface seismic waves. The method of the present invention is aimed at onshore seismic exploration, effectively solves the technical bottleneck of quantitatively evaluating the absorption attenuation compensation of seismic waves during their propagation in underground media, and provides a basis for the fidelity of seismic wave Q absorption compensation technology.

[0082] Those of ordinary skill in the art will realize that the embodiments described herein are for helping readers understand the implementation methods of the present invention, and it should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those of ordinary skill in the art can make various other specific deformations and combinations that do not depart from the essence of the present invention according to these technical revelations disclosed in the present invention, and these deformations and combinations are still within the protection scope of the present invention.

Claims

1. A quantitative evaluation method for joint formation Q absorption compensation of VSP up- and down-going waves, the specific steps are as follows: S1. Perform near-surface Q absorption compensation on surface seismic data; Based on the dominant frequencies of refracted waves of different seismic traces, calculate the relative attenuation coefficient R at the position of the surface receiving point. Combine the near-surface travel time t calculated by the near-surface velocity model, and obtain the near-surface Q value model based on the spectral ratio method. Then, through the inverse Q filtering compensation algorithm, realize the near-surface Q absorption compensation of seismic data; S2. Perform Q absorption compensation on consolidated rock formations below the water table for surface seismic data; Use the attenuation travel time tomography of underground formation reflected waves to invert the Q value model of consolidated rock formations below the water table, and then through the inverse Q filtering compensation algorithm, realize the Q absorption compensation of seismic data for consolidated rock formations below the water table; S3. Non-zero offset correction of VSP well data; Collect VSP well data in the study area, conduct shot offset analysis one by one. There is a certain distance between the shot point position and the wellhead position of the field VSP well data. Then, through non-zero offset correction, obtain the arrival time of the first break of the accurate zero-offset VSP well data for each well; S4. Preprocessing of zero-offset VSP well data; Perform preprocessing on the zero-offset VSP well data obtained in step S3 to obtain the down-going wave seismic record and up-going wave seismic record of the VSP at the wellhead. Then, select the frequency-divided subtraction method based on surface consistency to suppress surface waves, and monitor the frequency domain before and after denoising and the removed surface waves; The key steps of the preprocessing include: (1) Pick up the arrival time of the first break of the down-going wave; (2) Use the arrival time information of the down-going wave to calculate the average velocity and then obtain the layer velocity of each geophone interval; (3) Remove the amplitude loss caused by geometric spreading of seismic waves through spherical spreading compensation, and restore the relative amplitude change relationship completely caused by underground medium factors; (4) Use the frequency-divided subtraction method to suppress surface waves and eliminate the influence of strong ground roll wave energy on useful data; (5) Finally, extract high-quality up-going wave seismic records and down-going wave seismic records of the data at each VSP wellhead through the median filtering wavefield separation method; S5. Use the discrete frequency shift method of VSP down-going waves to obtain the Q value of consolidated rock formations below the water table at the wellhead; During the downward propagation of seismic waves, regard the down-going wave field received by each geophone of the zero-offset VSP data as the source wave field of the next geophone, and perform discrete processing on the existing frequency shift method to obtain the discrete frequency shift method. Then, use the zero-offset VSP well data to inversely calculate the Q value of consolidated rock formations below the water table at the wellhead with higher accuracy, specifically as follows: For the down-going wave of each VSP well, calculate the dominant frequency and its variance of the down-going wave seismic record received by geophones at different depths. Use the discrete frequency shift method to calculate the layer Q value between each adjacent geophone of this well point. According to the seismic interpretation horizon, conduct geological horizon division on the calculated Q value of consolidated rock formations below the water table at the wellhead. Combine the actual formation situation, and refer to the layering situation of layer velocity to conduct layering processing on the Q value curve to make it conform to the layer velocity change trend. Finally, calculate the average Q value within each geological horizon to obtain the Q value of consolidated rock formations below the water table at the wellhead with higher accuracy; S6. Compensation for absorption by the consolidated rock formation Q below the water table of the VSP up-going wave; Using the Q value of the consolidated rock formation below the water table at the wellhead obtained by the discrete frequency shift method of the VSP down-going wave in step S5, perform compensation for absorption by the consolidated rock formation Q below the water table on the VSP up-going wave seismic record through the inverse Q filtering compensation algorithm; S7. Calculate the correlation coefficient between the VSP up-going wave seismic record and the surface seismic record; Use the VSP up-going wave seismic record after joint Q absorption compensation of the up- and down-going waves and the surface seismic records before and after Q absorption compensation of the underground medium to calculate the correlation coefficient based on waveform similarity, specifically as follows: Let the surface seismic record be denoted as S(t), and the VSP up-going wave seismic record be denoted as V(t), and construct the correlation coefficient formula for the two seismic records: ρ = COV(S(t), V(t)) / (σ_S(t) * σ_V(t)) where COV(S(t), V(t)) represents the covariance of S(t) and V(t); if the change trends of S(t) and V(t) are consistent, the covariance is close to 1, and if there is no linear relationship between them, the covariance is close to zero; σ_S(t) represents the standard deviation of the surface seismic signal record S(t); σ_V(t) represents the standard deviation of the VSP borehole seismic record V(t); By calculating the covariance of the two seismic records and dividing it by the product of their respective standard deviations, a value between 1 and 0 is obtained to characterize the degree of linear correlation between S(t) and V(t); S8. Quantitative evaluation of Q absorption compensation of surface seismic waves; Use the VSP up-going wave seismic record after Q absorption compensation of the consolidated rock formation below the water table as the evaluation criterion for the Q absorption compensation effect of the surface seismic record, and then calculate the correlation coefficient based on the VSP up-going wave seismic record after joint Q compensation of the up- and down-going waves and the surface seismic records before and after Q absorption compensation of the underground medium to quantitatively quality control and evaluate the reliability of Q absorption compensation of surface seismic waves, and finally achieve deterministic Q absorption compensation of surface seismic data; wherein, the evaluation criterion is specifically as follows: When the correlation coefficient ρ value between the surface seismic record after Q absorption compensation of the underground medium and the VSP up-going wave seismic record after Q absorption compensation of the consolidated rock formation below the water table increases compared to the surface seismic record before Q absorption compensation and approaches 1, it indicates that the Q absorption compensation of the underground medium for the surface seismic record is reasonable and effective. On the contrary, when the correlation coefficient ρ value decreases or even approaches 0, it indicates that the Q absorption compensation of the underground medium for the surface seismic record is unreasonable and has the opposite effect.

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