Method And Application Of Muti-wave Velocity Analysis Of 3D-VSP In VTI Media

Velocity analysis is a core step of the converted wave(c-wave) data process. Velocity analysis can not meet the demand of high-precision exploration based on the isotrophy theory due to the wide occcurrence of anisotropy. In this paper, velocity analysis is realized by using single square root equation in transversely isotropic media with a vertical symmetry axis(VTI), which makes the process of converted wave independent of P-wave data. This method not only simplifies the process procedure, but also elemilates the error propagation dure to the inaccuracy of P-wave. A work flow and a method of estimating the c-wave parameters are provided by theoretical analysis and numerical calculation. We sum up necessary conditions of double-scanning to require good resolution and accuracy of parameter estimation. The real data processing verifies the correctness and validity of the method and software.3D-VSP technique has many advantages compared with conventional surface seismic exploration. 3D-VSP seismic data is not as intuitive as the surface seismic data and need to be processed with such as VSPCDP transformation or moveout correction and stacking process. Then the seismic data can be interpreted by standard techniques to determine the subsurface structure and to get other informations. Velocity is one of the most important parameters in the seises processes of 3D-VSP data processing and interpretation. Typically, velocity can be determined by the first break time of the direct wave of zero offset data. But the stacking velocity may be different form the velocity aquired by this method. What's more, as for the strata which burred under the geophones, there is no direct waves so the first break picking method lost its effect. Therefore, it is necessary to study the new 3D-VSP velocity analysis method.In this paper, we study the basic theory of the VTI media and study the non-zero offset moveout equations of VSP P and P-SV converted wave in isotropic and VTI media. Then we propose an iterative method for caculating the conversion point of VSP P-SV converted wave and give a procedure to determine the s-wave velocity of the geophone location. The velocity specturn technology is introduced to acquire the non-offset vsp multi-wave velocity. On the basis, we come up with methods and steps to build isotropic and anisotropic VSP multi-wave stack velocity model and anisotropic model which is coherent with the suface seismic data. The multiwave velocity and anisotropic coefficients can obtained through the procedure. What's more, the NMO section and the stack section are consistent with surface seismic data, which can be easyly interpreted and utililize for further popurse.The theory and real data processing verifies the correctness and validity of the method and software.