Prestack elastic RTM for VTI media using vector wavefield decomposition and vector imaging conditions

摘要

To date, studies of elastic reverse time migration (RTM) have undergone much improvement, but mainly focus on isotropic media. Because anisotropic media is widespread, it is necessary to explore the application of elastic RTM in anisotropic media. We extend the wavefield decomposition method, which is based on decoupled propagation, and three vector imaging conditions to transversely isotropic media with vertical symmetry isotropy (VTI). Two of the imaging conditions are based on the excitation amplitude (EA) and the third is based on source-normalized cross-correlation. First, the wavefield decomposition method is extended to VTI media. This is then tested in a two-layer model. The results show that this extension cannot decompose P- and S-waves perfectly in VTI media; some weak residuals remain. However, the results of a simple model test show that no obvious crosstalk is generated by these weak residuals. Finally, a Hess VTI model is adopted to test the adaptive use of this method in complex media. Many subsurface structures can be clearly recognized in the migrated result, for example, the high-velocity rock body, a fault and two low-velocity interlayers. Compared with PP images, converted PS images have many merits, such as clearer imaging of the anisotropic body, higher resolution and a wider migration aperture. We conclude that the vector decomposition method and three vector imaging conditions can be applied to prestack elastic RTM for VTI media and satisfactory results obtained.