VSP Based Seismic Velocity Correction in VTI Media

Techniques for computation and correction of NMO velocities in VTI media are widely employed. However, there are discrepancies between the anisotropic parameter η values computed based on VSP data and those based on surface seismic data; this prevents the direct use of VSP-derived η values in surface seismic data processing. This paper discusses the causes of these discrepancies and proposes methods for compensating for them using actual full-azimuth seismic data, zero-offset VSP data, and Walkaway VSP data from an oil field in China. Our studies indicate that conventional computation of Vnmo, (based on the horizontal layered media and isotropy assumption, using the conventional NMO formula within a small offset range and relying on the principle of flattening CMP gathers), does not yield correct anisotropic parameters, In practice, surface seismic velocity must be corrected with the velocity computed using zero-offset VSP data. Surface seismic data are then corrected using VTI anisotropic parameters computed based on Walkaway VSP data and accurate seismic imaging results can be achieved.