(China Youshi University (East China) School of Earth Science and Technology, Qingdao, Shandong 266555)
Fund projects: National 863 Project (2009AA06Z206) and the key project of postgraduate innovation project of China Youshi University (East China).
Author introduction: Qin Ning, female, Ph.D. candidate, mainly engaged in migration velocity analysis, tomographic inversion and waveform inversion. E-mail: geoqin @163.com.
Abstract: With the increasing demand for oil and gas resources in the world economy, the exploration and development of carbonate reservoirs is gradually put on the research agenda. Complex surface, complex steep underground structure, extremely deep target reflector and complex reservoir are the main seismic geological problems faced by carbonate seismic exploration. These characteristics limit the accuracy of conventional treatment methods suitable for clastic rocks based on the assumption of horizontal layered media. Pre-stack depth migration imaging technology is an effective technology to improve the quality of seismic data in marine carbonate areas and improve the imaging accuracy of deep complex structures and lithology. In order to realize accurate prestack depth migration imaging in carbonate exploration area, it is necessary to study the corresponding velocity analysis method. In this paper, a tomographic velocity analysis method for imaging gathers is proposed. The angular domain * * * imaging point gathers extracted by wave equation double square root operator prestack depth migration are used as velocity analysis gathers, and the depth residuals are picked up based on automatic fitting method and converted into travel time. On the one hand, angular gathers can reflect the coupling relationship between speed and depth more accurately and reduce the interference of artifacts, so the travel time difference obtained from them is more accurate; On the other hand, the ray tracing forward modeling corresponding to this method can just decompose the complex reflection in tomography into upward and downward transmission, which simplifies the problem, improves the calculation efficiency and accuracy of sensitivity matrix and makes the velocity analysis result more accurate. The trial results of seismic geological model and actual data of marine carbonate rocks show that the velocity field in depth domain obtained by this method is accurate, the depth error of horizon interface is small, and the good prestack migration results can solve the velocity analysis problem in marine carbonate exploration area, but the prestack migration data with low signal-to-noise ratio will have a great impact on the accuracy of tomographic inversion.
Key words: marine carbonate rocks; Travel time chromatography; Velocity analysis; Angle gathering; Sensitivity matrix
Analysis of tomographic imaging velocity based on image gathers in marine carbonate exploration area
Qin Ning, Li Zhenchun, Yang Xiaodong, Zhang Kai
(School of Earth Sciences, China Shiyou University, Qingdao 266555)
With the increasing demand for oil and gas resources with the development of world economy, the exploration and development of carbonate reservoirs has become a research hotspot. The main seismic geological problems in marine carbonate exploration area are rugged near the surface, complex and steep underground structures, deep target reflection layers and complex reservoirs, which make the conventional processing methods applied to clastic rock areas powerless under the assumption of horizontal layered media. Pre-stack depth migration is an effective technique to improve the seismic data quality and imaging accuracy of complex structures in marine carbonate areas. In order to realize accurate prestack depth migration in carbonate area, we must first study the corresponding velocity analysis method. In this paper, a velocity analysis method of tomography based on image gathers is proposed. In this method, the angle domain common imaging gathers with wave equation double square root operator are used as the velocity analysis gathers, and the travel time residuals are obtained from the depth residuals by automatic fitting. On the one hand, ADCIGs can accurately reflect the coupling relationship between speed and depth, with almost no artifacts and high accuracy of travel time residual. On the other hand, in the corresponding ray tracing method, complex reflection can be decomposed into upward and downward transmission, thus simplifying the forward problem, improving the efficiency and accuracy of sensitivity matrix calculation, and making the result of velocity analysis more accurate. Through the examples of seismic geological synthetic data set and actual data set in marine carbonate area, it is shown that the inversion velocity field by this method has accurate velocity value and interface depth, and produces high-quality prestack depth migration results. This method can solve the velocity problem of marine carbonate exploration area, but the prestack seismic data with low signal-to-noise ratio will affect the accuracy of tomographic velocity analysis.
Key words: marine carbonate; Travel time tomography; Velocity analysis; Angle domain common imaging gathers; Sensitivity matrix
introduce
With the rapid development of the world economy, conventional exploration and development can no longer meet the growing demand for oil and gas, and people have turned their attention to unconventional oil and gas reservoirs. In recent years, oil and gas exploration of marine carbonate rocks has been gradually put on the research agenda. Generally speaking, complex surface, complex underground and steep structures, extremely deep target reflectors and complex reservoirs are the main seismic geological problems faced by carbonate seismic exploration. These characteristics limit the accuracy of conventional treatment methods suitable for clastic rocks based on horizontal layered assumption. Seismic pre-stack imaging technology is an effective technology to improve the quality of seismic data in marine carbonate areas and improve the imaging accuracy of deep complex structures and lithology. Pre-stack migration method is very sensitive to velocity field, and accurate velocity information is needed to obtain ideal imaging results reflecting real underground structures. Therefore, how to obtain high-precision migration velocity field reasonably and effectively has become a key problem to solve seismic exploration in marine carbonate areas.
At present, traveltime tomography based on ray theory is the most widely used fine velocity modeling tool in industry. Conventional traveltime tomography mainly updates the velocity field based on the traveltime obtained from shot gathers or CMP gathers, which can't distinguish the reflection in-phase axes under the condition of poor data quality, resulting in large errors and inaccurate inversion results. However, travel time tomography based on CRP gathers or CIP gathers needs to consider complex reflection problems in ray tracing forward simulation. When the initial model seriously deviates from the real model, more iterations are needed. In this paper, a tomographic velocity analysis method of * * * imaging point gathers (ADCIGS for short) in angle domain is proposed. This method can decompose the complex reflection in tomographic imaging into upward and downward transmission, and obtain the angular gathers after depth migration, which can accurately reflect the coupling relationship between velocity and depth and reduce artifact interference. Therefore, the travel time difference obtained is more accurate and reliable, and the inversion result is more accurate and can be better.
1 principle
1. 1 travel time tomography
The linear equation of seismic travel time tomography can be expressed as follows:
Proceedings of the International Conference on Unconventional Oil and Gas Exploration and Development (Qingdao)
Where: l is the sensitivity matrix, and the elements in it correspond to the ray path length of the ray in the grid; δ t is the travel time difference vector; δ s is the slowness update to be inverted, which is used to update the velocity field.
It can be seen from the formula (1) that the key to update the velocity field by using travel time tomography lies in the determination of sensitivity matrix and the calculation of travel time difference. The sensitivity matrix is obtained by efficient ray tracing forward simulation, and its matrix element aij represents the ray path length of the ith ray in the j-th grid. There are two methods to determine the travel time difference: direct method and indirect method. The direct method is to compare the travel time picked up by shot set or common point gather with the travel time of corresponding ray tracing to get the travel time difference; Indirect method is to obtain travel time difference through depth residual conversion. Then the travel time difference is back projected along the ray path to get the slowness update, so as to update the speed.
1.2 Calculation of travel time difference
In this paper, the depth residual of angular gathers is extracted by combining automatic fitting with manual control. Limited to space, I won't introduce it in detail here. On the * * * imaging point gathers in the angle domain, the depth residual of each * * * imaging point is picked up and then converted into travel time difference. Among them, the conversion relationship between depth residual and travel time difference is shown in figure 1.
Figure 1 Schematic diagram of the conversion relationship between travel time difference Δ t and depth residual Δ z
As shown in figure 1, due to the change of interface position, the ray changes (that is, the real ray becomes a new ray), and the extra path length δ L = A 1+A2, and the resulting travel time difference δ T = δ L S. According to the geometric relationship shown in figure 1, it is not difficult to obtain it.
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Substituting formula (3) into formula (2), we can get
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Then the conversion relationship between travel time difference and depth residual is:
Proceedings of the International Conference on Unconventional Oil and Gas Exploration and Development (Qingdao)
Where: Δ t is the travel time difference; δ z is the depth residual; S is the slowness value of the imaging point; α is the inclination angle of the reflective layer; β is the ray incident angle, which corresponds to the angle of imaging point gathers in the angle domain.
1.3 ray tracing and calculation of sensitivity matrix
Generally, the sensitivity matrix of travel time tomography is calculated by simple and efficient ray tracing method. The tomographic velocity analysis method proposed in this paper requires that each ray direction of ray tracing must correspond to the angle of imaging point gathers in the angle domain * * *, and the reflection can be decomposed into upward and downward transmission, without considering the complex reflection problem, which reduces the difficulty of ray tracing to some extent and improves the calculation accuracy of sensitivity matrix. Therefore, it is an important aspect that velocity tomography based on angle domain imaging point gathers is superior to velocity tomography based on other gathers.
In this paper, an accurate and effective ray tracing method-constant velocity gradient method (Gan Lan, 1985) is used to obtain the sensitivity matrix. In the process of model parameterization, rectangular grid is used to divide the velocity field. Then the angle of ray tracing is determined according to the local angle of the angular gathers and the inclination of the reflecting layer. Starting from the imaging point corresponding to the angular gathers, ray tracing is carried out according to the incident angle and fixed step size. Finally, the step size of the i-th ray in the j-th grid is accumulated, and its matrix element lij is obtained. The step size of ray tracing can be selected manually according to the accuracy requirements of sensitivity matrix, which has great flexibility and can improve the calculation efficiency.
Implementation method of 1.4 travel time tomography inversion
The realization process of velocity analysis of marine carbonate imaging gathers studied in this paper includes the following steps:
Establishment of (1) initial velocity model. Pre-stack depth migration is carried out by using the layer velocity converted from stacking velocity, and the horizon interface is obtained on the migration profile, and the initial velocity field of tomography is generated with the constraints of actual seismic and geological characteristics in Shanghai carbonate exploration area.
(2) Obtaining sensitivity matrix and travel time difference. Based on the initial velocity field of tomography, the sensitivity matrix corresponding to the imaging point gathers in angle domain is obtained by ray tracing forward simulation. According to the actual situation of the data, the imaging gathers (ADCIGs) are extracted according to a certain angle range, and the depth residuals of each horizon are picked up and converted into travel time difference.
(3) Chromatographic inversion using regularization and prior information. Using the obtained travel time difference and sensitivity matrix, the inversion equation is established according to the formula (1), and the slowness is updated by adding regularization and prior information, so as to update the speed.
(4) Using the velocity analysis criterion to determine whether to iterate. According to the accuracy requirements of the flatness and speed of the in-phase axis on the imaging point gathers in the angle domain * * * (that is, whether the travel time difference is close to zero), it is decided whether to carry out the next iteration. If it is necessary to continue iteration, return to the first step to repeat this process, and if the accuracy requirements have been met, exit the loop. After the speed iterative update is completed, error analysis and sensitivity analysis are carried out. The implementation steps are shown in Figure 2.
Fig. 2 Flow chart of velocity analysis of marine carbonate imaging gathers.
2 Model and actual data trial calculation
2. 1 Trial calculation of seismic geological model
The following are the results of tomographic inversion processing of typical seismic geological models. The model covers many complex geological bodies, including high-steep thrust faults, various high-speed bodies (such as volcanic rocks) and the connection of many small fault blocks. In this model, the interval velocity field obtained by conventional velocity analysis is used as the initial velocity field of migration, 80 * * * imaging points are used in tomographic velocity analysis, and 36 * * imaging points are collected in angle domain (angle range 0 ~ 35, angle interval 1). Figure 3 shows the initial prestack depth migration profile and the established initial velocity field of tomography. Fig. 4 shows the comparison between the initial angular gathers and the angular gathers after tomography, and it can be seen that the flatness of the angular gathers after tomography is better. Fig. 5 shows the velocity field after tomographic updating and its corresponding prestack depth migration profile. Fig. 6 shows the comparison of initial chromatographic velocity, inversion velocity and real velocity at x = 60 10m. As can be seen from Figure 5(a), except for the thin layer of thrust fault on the left boundary and the extremely thin layer in the overlying strata, the structures at other positions can be obviously reversed; As can be seen from Figure 5(b), the reflection interface has basically returned to the correct position, and the imaging effect is good. The obtained chromatographic velocity field is basically consistent with the geological conditions in this area, with high accuracy, which provides a good prerequisite for subsequent processing and interpretation.
2.2 Trial Processing of Actual Data in Marine Carbonate Exploration Area
The following is the result of chromatographic inversion processing of actual data in a marine carbonate exploration area. The data speed changes greatly (3000 ~ 6500m/s), the target layer is buried deeply, and the data signal-to-noise ratio is low. In the analysis of tomographic velocity, 80 * * * imaging points are used, and 39 angles (angle range 0 ~ 38, angle interval 1) are used in the acquisition of * * * imaging points in the angle domain. Fig. 7 shows the initial prestack depth migration profile and the established tomographic initial velocity field. Fig. 8 shows a comparison between the initial angular focus and the angular focus after tomography. It can be seen that the angular gathers after tomography have better continuity and more accurate interface position. Fig. 9 shows the velocity field after tomography and its prestack depth migration profile. It can be seen that the velocity value and velocity interface in the updated velocity field are more realistic, and the interface in the prestack depth migration profile obtained from this is well regressed. However, this method is greatly influenced by the signal-to-noise ratio of data, and the accuracy of inversion results is lower than that of theoretical model, which needs to be improved and perfected in the future.
Fig. 3 Pre-stack depth migration profile (a) and tomographic initial velocity field (b) established thereby.
Fig. 4 Initial angular gathers (a) and tomographic updated angular gathers (b)
Fig. 5 velocity field (a) and prestack depth migration profile (b) after tomography update.
Fig. 6 Comparison of initial velocity, chromatographic update velocity and true velocity at x = 6010m.
Fig. 7 Pre-stack depth migration profile (a) and tomographic initial velocity field (b) established thereby.
Fig. 8 Initial gathers (a) and tomographic updated gathers (b)
Fig. 9 velocity field (a) and prestack depth migration profile (b) after tomography update.
3 Conclusion
Velocity analysis of marine carbonate imaging gathers can accurately reflect the coupling relationship between velocity and depth and reduce the interference of false images. Compared with other trace gather tomography inversion methods, the travel time difference obtained by this method is more accurate, which makes the velocity inversion more accurate. Another advantage is that in the process of ray tracing, there is no need to consider complex reflection problems, so a fast and accurate ray tracing method can be used. The results of model trial and actual data show that this method has high accuracy and fast calculation speed, and obtains good prestack migration results, which can solve the velocity analysis problem in marine carbonate exploration area well, but the prestack migration data with low signal-to-noise ratio will have a great impact on the accuracy of tomographic inversion.
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