VELOCITY ANISOTROPY IN SHALE DETERMINED FROM CROSSHOLE SEISMIC AND VERTICAL SEISMIC PROFILE DATA

Crosshole and vertical seismic profile (VSP) data made possible accurate characterization of the elastic properties, including noticeable velocity anisotropy, of a near-surface late Tertiary shale formation. Shear-wave splitting was obvious in both crosshole and VSP data. A transversely isotropic model, which incorporates a rapid increase in S-wave velocities with depth but a slow increase in P-wave velocities, closely fits the data over most of the measured interval. Elastic constants of the transversely isotropic model show spherical P- and S2-wave velocity surfaces but an ellipsoidal S1-wave surface with a ratio of major to minor axes of 1.15. The magnitude of the S-wave anisotropy is consistent with and lends credence to S-wave anisotropy magnitudes deduced less directly from data of many sedimentary basins. -from Authors