Fracture Detection Using Back-Scattered Shear Waves

Kandiah Balachandran

Seismic wave tests were performed in the Clinker Mesas of Wyoming's Powder River Basin, at a test site near Tulsa, Oklahoma, and in a sabkha area in Eastern Saudi Arabia. All these tests show improved coherency of signals in common receiver sorted data in remarkable contrast to that in common shot gathers. This phenomenon is interpreted as being due to the conversion of the incident P-waves to S-waves in the vicinity of the receivers. This characteristic ability to convert wave types should also be present in subsurface layers.

In one of the tests performed near Tulsa, a striking event was seen on horizontal component seismometer records. This event was seen at a record time of 3 seconds; the shot-to-receiver offset was in the range of 240 to 720 feet. This situation corresponds to near-vertical propagation of waves; the event is tentatively interpreted to result from conversion of downtraveling P-waves to S-waves in a subsurface layer namely the Viola or the Arbuckle formation. These layers are believed to have the inhomogenous characteristics necessary to effect the conversion.

Tests to support the interpretation were carried out in southern Ontario. The Ordovician Trenton formation has enhanced porosity and permeability in fractured zones; a characteristic which may cause the conversion of waves. Seismic test profiles were run over fractured zones. The results support the concept that fractured zones may be identified by the strength of the converted shear waves. This may be a valuable and economical way to detect fractured reservoirs. The technique may also find applications in environmental and engineering problems.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California