3D Anisotropic Depth Migration for VTI Media: Theory and Case Histories

摘要

Prestack depth migration algorithms are based on approximate models of elastic wave propagation in the subsurface. One of such approximations consists in the assumption that the earth response is isotropic. In fact, in some areas it has been observed that the depth of reflectors determined from surface seismic data are often deeper than well log depths. These misties have been correlated with the presence of VTI anisotropy due to shales. Moreover, it is a common experience that VTI anisotropy induces lateral mispositioning and defocusing in conventional isotropic depth migration. These limitations can be overcome by taking VTI anisotropy into account during migration. This paper discusses the VTI extension of a proprietary Isotropic 3D Prestack Kirchhoff Depth Migration algorithm. This extension impacts mainly the ray tracing algorithm underlying Kirchhoff migration. The anisotropic ray tracer exploits a parsimonious acoustic approximation of VTI anisotropy such that only P-wave traveltimes are computed. This approximation is reasonable for weak anisotropy since in this case mode conversions are negligible. Comparisons are shown from isotropic and anisotropic depth migrations results, for both synthetic and field data, showing the improvements gained by taking anisotropy into account.