JOINT PP AND PS ANISOTROPIC AVO INVERSION USING THE EXACT ZOEPPRITZ EQUATIONS

摘要

With increasing complexity of the geological structure, the conventional amplitude variation with offset (AVO) inversion methods based on the approximate Zoeppritz equations are not suitable for the current direct hydrocarbon detection. The linearized approximations of the Zoeppritz equations work well under the conditions of weak contrast boundary and small incident angle, which not only restrict the scope of application but also reduce the accuracy of the inversion results. Furthermore, since the seismic anisotropy has been found in many regions of the Earth's subsurface, the assumption of isotropic elastic media cannot exactly describe the subsurface properties. Therefore, we develop an anisotropic AVO inversion method by replacing the approximate isotropic Zoeppritz equations with the exact isotropic Zoeppritz equations, and the Ruger's approximations are still used in the anisotropic perturbation part. In addition, we add the PS seismic data as a new constrained condition to improve the inversion results. In this study, we also obtain the analytical expressions of the derivatives of reflection coefficients with respect to unknown parameters (e.g., P- and S-wave velocities, density and Thomsen parameters: epsilon and delta). Given more parameters are required in anisotropic AVO inversion, the instability of the solution becomes a critical problem. We conduct the Marquardt method to enhance the stability during the inversion process. In this paper, we use a synthetic seismic data with ideal and noisy situations to verify the performance of the proposed AVO inversion algorithm. The final inversion results indicate that our new method works well to obtain the elastic parameters (density, P- and S-wave velocities) and Thomsen parameters.