Converted-wave imaging in anisotropic media : theory and case studies

Common-conversion-point binning associated with converted-wave (C-wave) processing complicates the task of parameter estimation, especially in anisotropic media. To overcome this problem, we derive new expressions for converted-wave prestack time migration (PSTM) in anisotropic media and illustrate their applications using both 2D and 3D data examples. The converted-wave kinematic response in inhomogeneous media with vertical transverse isotropy is separated into two parts: the response in horizontally layered vertical transverse isotrophy media and the response from a point-scatterer. The former controls the stacking process and the latter controls the process of PSTM. The C-wave traveltime in horizontally layered vertical transverse isotrophy media is determined by four parameters: the C-wave stacking velocity VC2, the vertical and effective velocity ratios γ0 and γeff, and the C-wave anisotropic parameter χeff. These four parameters are referred to as the C-wave stacking velocity model. In contrast, the C-wave diffraction time from a point-scatterer is determined by five parameters: γ0, VP2, VS2, ηeff and ζeff, where ηeff and ζeff are, respectively, the P- and S-wave anisotropic parameters, and VP2 and VS2 are the corresponding stacking velocities. VP2, VS2, ηeff and ζeff are referred to as the C-wave PSTM velocity model. There is a one-to-one analytical link between the stacking velocity model and the PSTM velocity model. There is also a simple analytical link between the C-wave stacking velocities VC2 and the migration velocity VCmig, which is in turn linked to VP2 and VS2. Based on the above, we have developed an interactive processing scheme to build the stacking and PSTM velocity models and to perform 2D and 3D C-wave anisotropic PSTM. Real data applications show that the PSTM scheme substantially improves the quality of C-wave imaging compared with the dip-moveout scheme, and these improvements have been confirmed by drilling.