Anisotropy signature in RTM extended images

Reverse-time migration can accurately image complex geologic structures in anisotropic media. Extended images at selected locations in the earth, i.e. at commonimage-point gathers (CIPs), carry rich information to characterize the angle-dependent illumination and to provide measurements for migration velocity analysis. However, characterizing the anisotropy influence on such extended images is a challenge. Extended CIPs are cheap to evaluate, since they sample the image at sparse locations indicated by the presence of strong reflectors. Such gathers are also sensitive to velocity error which manifests itself through moveout as a function of space and time lags. Furthermore, inaccurate anisotropy leaves a distinctive signature in CIPs, which can be used to evaluate anisotropy through techniques similar to the ones used in conventional wavefield tomography. It, specifically, admits a “V”-shape residual moveout with the slope of the “V” flanks depending on the ellipticity anisotropic parameter η regardless of the complexity of the velocity model. It reflects the fourth-order nature of the anisotropy influence on moveout, as it manifests it self in this distinct signature in extended images after handling the velocity properly in the imaging process. Synthetic and real data observations support this assertion.