Non-Hyperbolic Term Estimation for VTI Inversion from Walkaway VSP and Surface Seismic Data

Walkaway VSP data provides information for interval anisotropic parameter estimates for transverse isotropy with vertical symmetry axis (VTI). For any depth tomographic/migration velocity analysis, we need an adequate initial depth velocity model. Dix-type approach provides us with explicit inversion formulas for velocity model estimation. To estimate these interval parameters, using Dix-type inversion approach, we need to estimate non-hyperbolic term of NMO function. In walkaway VSP data, NMO function can be obtained from first breaks, while in surface seismic it is estimated through non-hyperbolic semblance analysis. In both cases, determination of non-hyperbolic term is the most unstable procedure in interval VTI inversion. Because exact explicit formula for reflected time in a layered (even isotropic) media is unknown, we have to use some approximation. Different approximations have different accuracy in different geology. Here we investigate accuracy of different approximations on four models with weak and strong anisotropy and modest and large vertical velocity changes.