A Frequency-Dependent Azimuthal AVO Inversion Method

The dispersion study recently has been well received due to a large amount of information carried about fluid. The frequency-dependent AVO inversion method has been proven to be an effective method for identifying fluid. However, existing dispersion methods, such as the dispersion methods based on Smith & Gidlow, Aki & Richards, Shuey and Hilterman approximation assume that the subsurface medium be an isotropic medium and influence of azimuth has not been considered. We first time expanded the Rüger approximation equation into the frequency domain and obtained the frequency-dependent azimuthal AVO inversion equation, in which a subsurface medium is considered as an anisotropic medium. Simultaneously, anisotropic dispersion gradient term, isotropic dispersion gradient term, fracture orientation and the dispersion term of the reflection coefficient of the P-wave at normal incidence were obtained by solving the equation. The main focus of this paper is on fluid identification for fractured media. The comparisons were made on differences of P-wave velocity dispersion obtained from frequency-dependent AVO inversion and the isotropic gradient dispersion term obtained from frequency-dependent azimuthal AVO inversion. The application shows that when the anisotropy is strong, the isotropic gradient dispersion term of frequency-dependent azimuthal AVO inversion works better for fluid identification.