Computation of principal directions of azimuthal anisotropy from P-wave seismic data

Abstract

Principal directions for an azimuthally anisotropic medium are the directions along which the quasi P and quasi S waves propagate as pure P and S modes. For azimuthal anisotropy, induced by a single set of oriented vertical fractures, two of these principal directions are the directions parallel to and perpendicular to the fractures. To investigate if conventional P-wave data could be used in fracture detection, we computed synthetic data for an isotropic Taylor shale over a fractured Austin chalk model. We found that the modelled P-wave reflection amplitude variation with offset has different slopes along different directions. We also found that the reflection amplitude at fixed offsets is periodic in 2f where f is the orientation angle of the shooting direction with respect to one of the principal directions. For fracture induced anisotropy, this direction corresponds to the direction parallel to or perpendicular to the fractures. We use this periodic azimuthal dependence to obtain fracture orientation and a qualitative measure of the fracture density from the azimuthal P-wave data. We applied our technique to real P-wave data, collected over a wide source-to-receiver azimuth. Computations using our method gave an orientation of the principal direction consistent with the general fracture orientation in the area as inferred from other geological and geophysical evidence.