Ray tracing and parameter estimation for anisotropic depth imaging

The effects of anisotropy must be accounted for during seismic processing and imaging in order to obtain an accurate image of the subsurface. For Kirchhoff prestack depth migration, ray tracing in 3-D transversely isotropic media is carried out with a semi-analytic technique through a model representation based on tetra-hedra. Using a tetrahedra-based model allows flexible representation of complex structures while reducing the storage requirements compared to gridded models. This approach permits rapid and accurate ray tracing for calculating traveltimes for anisotropic prestack depth migration. The anisotropy parameters of the sub-surface need to be recovered accurately before anisotropic depth migration. Two approaches for estimating the anisotropy are discussed. The first method utilizes first-arrival traveltimes from walkaway VSP data. Slowness surfaces are computed from the traveltimes and are then inverted for the anisotropy parameters that best describe the change in slowness as a function of direction. Corrections for lateral heterogeneity are discussed along with the limitations of the method. The second method uses residual moveout in common image gathers resulting from prestack depth migration. The residual moveout is used to predict the amount of anisotropy needed to flatten seismic events across all offsets in the common image gather assuming a 1-D earth. Model-based moveout is used to rapidly validate the updated anisotropy model before proceeding with prestack depth migration.