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.

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