关键词: 叠前弹性波逆时偏移, 波场分解, 解耦传播, VTI介质

Abstract: Since the elastic reverse time migration (RTM) is more representative to the actual situation, and the converted-wave RTM results have high resolution, the study of elastic RTM draws more and more attention in the recent years. Because multicomponent data is required by the elastic RTM method, and its wavefield contains both P-and S-waves, before imaging we need to separate the P-waves from S-waves so as to avoid the crosstalk. Combining with the vector-based excitation amplitude imaging condition, we applied the wavefield decomposition method based on the decoupled propagation to the elastic RTM and analyzed its application in the isotropic and vertically transversely isotropic (VTI) media. The results demonstrated that the method perfectly decomposed P-and S-waves in the isotropic media and preserved the vector information also. Although there were some small residuals in the separated wavefield for the VTI media, this residual did not generate obvious artifacts in the RTM results. Therefore, this wavefield decomposition method can be applied in the elastic RTM for an isotropic and VTI media. This method is realized in time-space domain. The separation of P-and S-waves can be directly separated during the wavefield propagation conveniently and efficiently. Compared with the elastic RTM using Helmholtz decomposition, this method avoids the problem of polarity reversal in the PS images. The migration results of the complex Hess VTI model showed that the high-velocity rock mass and a fault were clearly imaged, and even two low-velocity thin interlayers were well imaged. This suggests that the method is better adaptive to a complex model. The PS images can present a relative clear image of an anisotropic body. It demonstrates that the anisotropic elastic RTM can be used to image some subsurface structures better than the conventional RTM.

Key words: prestack elastic reverse time migration, wavefield decomposition, decoupled propagation, vertically transversely isotropic (VTI) media