Imaging VSP Data Using The CRS Stack

The seismic survey is the most effective prospecting geophysical method during exploration and development of oil/gas. The structure and the lithology of the geological body become increasingly complex now. So it must assure that the seismic section own upper resolution if we need accurately describe the targets. High signal/noise ratio is the precondition of high-resolution.Nowadays, a lot of work has been done about CRS stack of seismic data. But domestic research about CRS stack of VSP data is very rare, so this paper is about the research of CRS stack of VSP data. The main characters of CRS stack are as followed. 1) CRS, independent from the model of macroscopic velocity, which is suitable for VSP data, is decided by the parameters of five wave fields, the velocity near the surface and the receiver. 2) completely drive of data, it adopt the method of automatic searching and scanning to define the parameters of the five wave fields and complete the stacking, what's more, the process is not affected by humans. 3) After being optimized, the five parameters are used for CRS stack of VSP data.Common-Reflection-Surface (CRS) stack is the best way at present to simulate zero-offset section. Common Reflection Surface is a circle segment around an underground reflector. Its travel time response in the time domain, which is CRS stack surface, can be regraded as a combination of all Common Reflection Point (CRP) trajectores in the segment. CRS stack can focus more energy in the vicinity of the reflector, therefore can obtain a better zero offset section by stacking events with same phase.This paper illustrates the basic theory of CRS satck which is the theory of paraxial ray,according to which this paper derives the travel-time formula of CRS stack. Then, with the CRS stack formula which is suitable for any kinds of earth's surface, it also derives the travel-time formula of CRS stack for VSP data.There are five attribute parameters about the travel-time formula of CRS stack for VSP data, and the process of defining the five parameters is a five-dimension optimizing problem. This report adopts three methods to optimize the five parameters by using different algorithms, and according to the results, we can find out the best method with high quality and efficiency.In order to enforce the efficiency of optimizing the parameters of 2-dimension Walkaway-VSP CRS, we must use certain trace gather to simplify the travel-time formula of CRS (for example, common shot gather (CS) and common receiver gather (CR)). Two parameters can be defined in the common shot gather, which are exit angleβ_G of central ray and curvature of wavefront K₁, at the receive spot. And two more parameter can be defined in the CR which are the enter angleβ_s and curvature of wavefront K₂ at the shot spot. Finally, the last parameter is curvature of wavefront K₃ which can be defined in the quasi-common-offset gather.Finally, the CRS stack imaging results for the VSP data, compare with the traditional VSPCDP data stack imaging results, CRS stack imaging results better than many, especially in the case of low signal to noise ratio can demonstrate the advantages of CRS stack.。...