Small-Angle AVO Response of PS-waves in Tilted Transversely Isotropic Media

摘要

Field records for small source-receiver offsets often contain intensive reflected PS-waves, which are difficult to model without introducing anisotropy on either side of the reflector. Here, we study the small-angle reflection coefficients of the split converted PS1- and PS2-waves for a horizontal interface separating two transversely isotropic (TI) media with arbitrary orientations of the symmetry axis. The normal-incidence PS reflections vanish when both halfspaces have a horizontal symmetry plane, which happens if the symmetry axis is vertical or horizontal (i.e., if the medium is VTI or HTI). However, when the symmetry axis in either halfspace is tilted (TTI medium), the magnitude of the reflection coefficients can reach substantial values exceeding 0.1, even if the anisotropy strength is moderate. To evaluate the contributions of the orientation of the symmetry axis and the anisotropy parameters to the AVO (amplitude-variation-with-offset) response of mode conversions, we develop concise weak-contrast, weak-anisotropy approximations and compare them with exact numerical results.