Linbin Zhang

Shear waves in acoustic anisotropic media

Acoustic transversely isotropic (TI) media are defined by artificially setting the shear‐wave velocity in the direction of symmetry axis, VS0, to zero. Contrary to conventional wisdom that equating VS0 = 0 eliminates shear waves, we demonstrate their presence and examine their properties. Specifically, we show that SV‐waves generally have finite nonzero phase and group velocities in acoustic TI media. In fact, these waves have been observed in full waveform modeling, but apparently they were not understood and labeled as numerical artifacts.Acoustic TI media are characterized by extreme, in some sense infinite strength of anisotropy. It makes the following unusual wave phenomena possible: (1) there are propagation directions, where the SV‐ray is orthogonal to the corresponding wavefront normal, (2) the SV‐wave whose ray propagates along the symmetry axis is polarized parallel to the P‐wave propagating in the same direction, (3) P‐wave singularities, that is, directions where P‐ and SV‐wave phase velocitie...

3D Fourier Finite-Difference Anisotropic Depth Migration

We present a 3D Fourier finite-difference depth migration (FFD) method for waves in transversely isotropic media with a vertical axis of symmetry (VTI). The method can accommodate a wide range of anisotropy rather than weak anisotropy. The downward-continuation operator is split into three downward-continuation operators. This method can handle the strong lateral velocity variation. A complex treatment of the propagation operator is applied to mitigate inaccuracies and instabilities due to evanescent waves. Tests show that the method improves the image quality.

Split‐step complex Padé—Fourier depth migration

SUMMARY We present a split-step complex Pade-Fourier migration method based on the one-way wave equation. The downward-continuation operator is split into two downward-continuation opera- tors: one operator is a phase-shift operator and the other operator is a finite-difference operator. A complex treatment of the propagation operator is applied to mitigate inaccuracies and in- stabilities due to evanescent waves. It produces high-quality images of complex structures with fewer numerical artefacts than those obtained using a real approximation of a square-root operator in the one-way wave equation. Tests on zero-offset data from the SEG/EAGE salt data show that the method improves the image quality at the cost of an additional 10 per cent computational time compared to the conventional Fourier finite-difference method.

An Eikonal Solver in Tilted TI media

Summary We present a finite difference (FD) scheme for the computation of first arrival traveltime on regular grid for transversely isotropic (TI) media. The variable axis of symmetry (providing for dipping TI media) TI eikonal equation is solved by a FD scheme in the celerity domain. Numerical computations illustrate the accuracy and efficiency of the proposed TI eikonal solver.

Shear waves in acoustic transversely isotropic media

Acoustic transversely isotropic (TI) media are defined by artificially setting the shear-wave velocity in the direction of symmetry axis, VS0, to zero. Contrary to conventional wisdom that equating VS0 = 0 eliminates shear waves, we demonstrate their presence and examine their properties. Specifically, we show that SV-waves generally have finite nonzero phase and group velocities in acoustic TI media. In fact, these waves have been observed in full waveform modeling but apparently were not understood and labelled as numerical artifacts.