Body-wave radiation patterns and AVO in transversely isotropic media

摘要

The angular dependence of reflection coefficients may be significantly distorted in the presence of elastic anisotropy. However, the influence of anisotropy on amplitude variation with offset (AVO) analysis is not limited to reflection coefficients. AVO signatures (e.g., AVO gradient) in anisotropic media are also distorted by the redistribution of energy along the wavefront of the wave traveling down to the reflector and back up to the surface. Significant anisotropy above the target horizon may be rather typical of sand-shale sequences commonly encountered in AVO analysis. Here, I examine the influence of P- and S -wave radiation patterns on AVO in the most common anisotropic model-transversely isotropic media. A concise analytic solution, obtained in the weak-anisotropy approximation, provides a convenient way to estimate the impact of the distortions of the radiation patterns on AVO results. It is shown that the shape of the P-wave radiation pattern in the range of angles most important to AVO analysis (0-40°) is primarily dependent on the difference between Thomsen parameters ε and δ. For media with ε - δ > 0 (the most common case), the P-wave amplitude may drop substantially over the first 25-40° from vertical. There is no simple correlation between the strength of velocity anisotropy and angular amplitude variations. For instance, for models with a fixed positive ε - δ the amplitude distortions are less pronounced for larger values of ε and δ. The distortions of the SV-wave radiation pattern are usually much more significant than those for the P-wave. The anisotropic directivity factor for the incident wave may be of equal or greater importance for AVO than the influence of anisotropy on the reflection coefficient. Therefore, interpretation of AVO anomalies in the presence of anisotropy requires an integrated approach that takes into account not only the reflection coefficient but also the wave propagation above the reflector.