Velocity anisotropy in shale determined from crosshole seismic and vertical seismic profile data

Abstract 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. In crosshole data, two orthogonally polarized shear (S) waves arrived 19 ms apart over a horizontal travel path of 246 ft (75 m). Vertically traveling S waves of the VSP separated about 10 ms in the uppermost 300 ft (90 m) but remained at nearly constant separation below that level. 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 S 2 -wave velocity surfaces but an ellipsoidal S 1 -wave surface with a ratio of major to minor axes of 1.15. The magnitude of this S-wave anisotropy is consistent with and lends credence to S-wave anisotropy magnitudes deduced less directly from data of many sedimentary basins.