Processing Pp and Ps Wave in Multicomponent Sea-Floor Data for Azimuthal Anisotropy : Theory and Overview

Assuming fracture-induced azimuthal anisotropy, we review the theory and develop processing methods for recovering the fracture orientation and density from multicomponent sea-floor data. The azimuthal variations in PP amplitude, normal move-out velocity, and interval move-out show elliptical variations in an azimuthally anisotropic medium. This can be used to determine the fracture strike of the medium and has been verified from real data. However, the P-wave effects only occur with multi-azimuths, and are often complicated by other factors. This limits the application of P-wave analysis to some extent. Analysis of PS waves may thus prove to be beneficial. For near vertical propagating PS waves, the polarization and time delay of the shear-waves provide a direct measurement of the fracture orientation and intensity. For a 2D acquisition where the survey line is along the receiver cable, an optimum method is proposed for determining the fracture strike from the polarization azimuth of the fast shear-wave. The method uses rotation analysis and assumes that the fast and slow shear-waves have similar waveforms. For a 3D cross geometry where the survey line is perpendicular to the receiver cable, two deterministic methods are proposed. The first one is based on the polarity change and amplitude dimming in the azimuthal gathers of the transverse-geophone component. The second one involves a rotation of orthogonal pairs of source-receiver azimuthal gathers. The determined polarization azimuth can then be used to separate the fast and slow shear waves in the inline-shooting gathers for time-delay estimation