Shear-Wave Reflections in Southeast Wyoming

Jack Cameron

M. A. Martin and T. L. Davis have interpreted shear-wave seismic reflection data across the Silo field of southeast Wyoming in terms of microfractures. The azimuthally polarized shear waves in the shallow section indicate "microfractures" oriented at N40°W, and time delays between split shear waves within the productive Niobrara (at about 8,000 ft depth) are believed to be indicators of the direction and intensity of fracturing in that zone.

This section, however, includes about 6,200 ft of Pierre Shale, which is well documented to be transversely isotropic. That characteristic is confirmed by an offset vertical seismic profile in the Silo field that shows strong shear-wave polarization in the vertical and horizontal planes. The coexistence of these two types of anisotropy may cause complications in shear-wave transmission, such as multiple splitting.

Synthetic and real data demonstrate that under these circumstances reflections are complex events, even when rotated into the optimum polarization direction. However, converted wave recordings, that is P waves reflected as S waves, may be much simpler and much easier to interpret.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.