Decoupled approximation and separate extrapolation of P- and SV-waves in transversely isotropic media

Characterizing the kinematics of seismic waves in elastic vertical transversely isotropic (VTI) media involves four independent parameters. To reduce the complexity, the acoustic approximation for P-waves reduces the number of required parameters to three by setting the vertical S-wave velocity to zero. However, because only the SV-wave phase velocities parallel or perpendicular to the symmetry axis are indirectly set to zero, the acoustic approximation leads to coupled P-wave components and SV-wave artifacts. The new acoustic approximation suggests setting the vertical S-wave velocity as a phase angle-dependent variable so that the SV-wave phase velocity is zero at all phase angles. We find that manipulating this parameter is a valid approach for P-wave approximation but doing so inevitably leads to zero-or nonzero-valued spurious SVwave components. Thus, we have developed a novel approach to efficiently approximate and thoroughly separate the two wave modes in VTI media. First, the exact P-and SV-wave phase velocity expressions are rewritten by introducing an auxiliary function. After confirming the insensitivity of this function, we construct a new expression for it and obtain simplified P-and SV-wave phase velocity expressions, which are three and four parameters, respectively. This approximation process leads to the same reasonable error for both wave modes. Accuracy analysis indicates that, for the P-wave, the overall accuracy performance of our approach is comparable to that of some existing three-parameter approximations. We then derive the corresponding P-and SV-wave equations in tilted transversely isotropic (TTI) media and provide two available solutions, the hybrid finite-difference/pseudospectral scheme and the low-rank approach. Numerical examples illustrate the separability and high accuracy of the proposed P-and SV-wave simulation methods in TTI media.