Joint velocity updating for anisotropic PP and PS prestack time migration based on hyperbolic correction of nonhyperbolic moveout

摘要

The analysis of anisotropic velocity has been well researched to support multicomponent seismic migration of anisotropic media. Because of anisotropy and asymmetric ray paths, PS-wave reflection moveout is more nonhyperbolic than PP-wave reflection moveout, which makes conventional root-mean-square velocity analysis no longer applicable. Hence, additional work is needed to reform the existing seismic data processing software or to develop new velocity analysis modules. For this reason, we propose a method of joint anisotropic velocity updating for PP and PS prestack time migration based on the hyperbolic correction of nonhyperbolic moveout. Our velocity analysis method can be performed using conventional velocity analysis. According to the PP-wave moveout equation in VTI media, we derive an accurate double-square-root moveout equation for PS-waves, which is then used to generate PP and PS common-imaging-point gathers. The travel time and offset of each sample are maintained when moving the amplitude from the shot record to the common-imaging-point gather. Both moveout attributes of PP-and PS-waves are validated to be insensitive to the velocity models by synthetic data analysis. We iteratively establish the anisotropic velocity models through velocity analyses using the small-offset and large-offset gathers, respectively. Then, based on the updated anisotropic velocity models, the anisotropic reflection time of PP and PS common-imaging-point gathers is corrected to the equivalent hyperbolic time. Eventually, we apply normal moveout and stacking to the hyperbolic-corrected common-imaging-point gathers to derive migration results. In our method, the generation of both PP and PS common-imaging-point gathers is independent of the velocity models. Application to synthetic and field seismic data demonstrates that the proposed method represents a feasible, new scheme for anisotropic velocity model building during the multicomponent prestack time migration process.