Determination of the principal directions of azimuthal anisotropy from P-wave seismic data

Abstract

In an azimuthally anisotropic medium, the principal directions of azimuthal anisotropy are the directions along which the quasi P and the quasi S waves propagate as pure P and S modes. When the azimuthal anisotropy is induced by oriented vertical fractures, two of these principal directions correspond to the directions parallel to and perpendicular to the fractures. S-waves propagating through an azimuthally anisotropic medium are extremely sensitive to the direction of their propagation with respect to the principal directions. As a result, primary or mode converted S-wave data can be used to obtain the principal directions from seismic data. Apart from the high acquisition cost, processing and interpretation of S-wave data require a special processing technology. Moreover S-wave data quality is poor in many regions. Thus, an alternative involving only P-wave data is attractive but is limited to a few qualitative studies of the amplitude-variation-with-offset (AVO) for different azimuthal directions (Mallick and Frazer, 1991; Chang and Gardner, 1993; Allen and Peddy, 1993 ; Lefeuvre, 1994 ; Lynn et.al, 1995, Rueger and Tsvankin, 1995), and to velocity studies of selected horizons on data for different common mid point (CMP) locations (Neidell and Cook, 1986).