Variation of P-wave reflectivity with offset and azimuth in anisotropic media

Abstract

Recent results on symmetry-plane P-wave reflection coefficients and AVO attributes in azimuthally anisotropic media (Riiger, 1995) are extended to observations at arbitrary azimuth, to variations of reflection coefficients at large incidence angles and to lower symmetry systems. The approach presented here is based on analysis of linearized reflection and transmission coefficients in azimuthally anisotropic media. The approximation of the P-wave reflection coefficient in transversely isotropic media with a horizontal axis of symmetry (HTI) shows that the AVO gradient varies as a function of the squared cosine of the azimuthal angle. This change can be inverted for the symmetry-plane directions and a combination of the shear-wave splitting parameter y and the anisotropy coefficient . Because these two parameters cause the same functional form of azimuthal change of the AVO gradient, they cannot be extracted individually. Azimuthal variations in the reflection coefficient at large angles of incidence (greater than 20°) are additionally dependent on the anisotropy coefficient and provide enough information to detect the two symmetry-plane directions. The reflection coefficient study is also extended to media of orthorhombic symmetry, which are believed to be more realistic models for fractured reservoirs. The study shows that the orthorhombic and HTI reflection coefficients are similar and that the azimuthal variation in the orthorhombic P-wave reflection response is a function of the shear-wave splitting parameter y and two anisotropy parameters describing P-wave anisotropy for near-vertical propagation in the symmetry planes.