Researches On The Amplitude-preserving Converted-wave Pre-Stack Time Migration In VTI Media

In seismic data processing, seismic imaging increasingly plays an important role as a tool. In the complex data processing with a certain period, migration imaging is the last step, the purpose of migration is to providing suitable velocity structure for explanation of clearly images of the interior of the earth for earth scientists. In recent years, converted seismic wave exploration technology has developed rapidly, converted wave migration imaging technology is becoming the main content of this technology. Due to widespread anisotropy in the earth medium, migration imaging of seismic wave in anisotropic medium is more complex than in isotropic medium. So it is necessary for the study of complex medium and complex models of converted wave migration imaging method. Prestack time imaging is to achieving the real common reflection point gathers superposition. The ratio of signal to noise has greatly improved after the migration, while amplitude-preserving migration can also be achieved, and the effect of migration imaging is outstanding. So pre-stack time migration is paied much attention. This paper studies the method used in the VTI medium and the amplitude-preserving converted-wave pre-stack time imaging based on pseudo-offset migration.For VTI media, the paper starts from the basic concepts of the migration and the basic principles of the anisotropic media. On the basis of previous research, we realize the two methods and their imaging results of getting the pseudo-offset migration(POM) of conversion wave in isotropic media.And comparing the imaging results of the two migration methods. The major research of our study is converted wave POM amplitude preserve pre-stack time migration in VTI medium, we introduces the different forms of amplitude weighting function and gives the amplitude-preserving converted wave specific weight function expression of the common shot and receive offset in time domain, use weight function directly achieve amplitude-preserving when extracting common conversion scatter point gathers mapping to compensate the amplitude, and avoid the accumulation of the same time point, and improve the operation efficiency.The innovative achievements in this paper as follows:(1) In isotropic medium, using the two methods of iteration method and limit method to solve POM to extract the common conversion scatter point gathers and migration imaging, verifying the correctness of POM converted wave pre-stack time migration using the two methods in isotropic medium using inclined model,and comparing their time consuming to find the operational efficiency of limit method higher than the common interation method.Thus,the paper extracts CCSP gathers using the limit method in isotropic medium. Marmousi model to pro forma results also show the effectiveness of the method POM prestack time amplitude preserving imaging isotropic medium. The result of Marmousi model also shows the effectiveness of the method of POM for prestack time amplitude preserving imaging in isotropic medium.(2) In VTI media, because of the complexity of the converted wave time-distance curve equation, it is difficult to deduce the formula of pseudo offset as in isotropic media, so for converted wave time-distance curve equation in VTI media, this paper gives the mathematical interation expressions of pseudo offset in VTI media,papers use the dichotomy and secant method to solve ray parameters, thereby solving the pseudo offset,Because the two methods in solving the pseudo offset do not need to calculate derivative, They simplify the steps of pseudo offset.(3) In VTI medium, We analyze and compare CCSP gathers in isotropic medium and VTI medium through a single scattering point model. The result shows the traveling time of scattering point model is in accordance with hyperbolic distribution and in line with the mapping rule of common conversion point gathers scattering,And it has the good convergence.We respectively use the Secant Method and the Dichotomy Method to solve the virtual offset with the inclination model, And comparing the calculating time-consuming of the two methods, the results show that the convergence rate of the Secant Method is faster than the Dichotomy Method.While improving computational efficiency.The final imaging results also illustrate the correctness of these two methods to extract CCSP gathers. We use two flat layer model to verify that the anisotropy parameters can affect CCSP gathers the results and the final imaging results, So anisotropy parameters can not be ignored in the actual calculation. With the inclination model,we can verify the impact of virtual offset interval and migration aperture to the imaging results, showing that it is crucial to select the appropriate virtual offset interval and interval aperture during the pre-stack time migration. Using a single layer model in VTI media,we can find that it is quite critical to build accurate velocity function.(4)In VTI media, this paper uses some single layer models including depression model, convex model, fault model and multi-layer composite model to test the accuracy of POM methods and contrasts migration imaging effect in isotropic medium and VTI media. Finally, we applied this method to the actual data, and achieved good migration imaging results, The results implies in VTI medias, the use of POM method to do the amplitude-preserving converted-wave pre-stack time migration has certain advantages.