Estimation of Thomsen's anisotropic parameters is very important for accurate time-to-depth conversion and depth migration data processing. Compared with other methods, it is much easier and more reliable to estimate anisotropic parameters that are required for surface seismic depth imaging from vertical seismic profile(VSP) data, because the first arrivals of VSP data can be picked with much higher accuracy. In this study, we developed a method for estimating Thomsen's P-wave anisotropic parameters in VTI media using the first arrivals from walkaway VSP data. Model first-arrival travel times are calculated on the basis of the near-offset normal moveout correction velocity in VTI media and ray tracing using Thomsen's P-wave velocity approximation. Then, the anisotropic parameters δ and ε are determined by minimizing the difference between the calculated and observed travel times for the near and far offsets. Numerical forward modeling, using the proposed method indicates that errors between the estimated and measured anisotropic parameters are small. Using field data from an eight-azimuth walkaway VSP in Tarim Basin, we estimated the parameters δ and ε and built an anisotropic depth-velocity model for prestack depth migration processing of surface 3D seismic data. The results show improvement in imaging the carbonate reservoirs and minimizing the depth errors of the geological targets.
