We create a log of intrinsic dispersion and attenuation for the Antelope Shale formation of the Buena Vista Hills field, San Joaquin Valley, California. High dispersion (or low Q) values correlate with thin sand and carbonate beds within the Antelope Shale. These beds are at least ten times as permeable as the host shale formation, so this effect provides a possible avenue for seismic prediction of permeability. The dispersion log is formed through comparison of crosswell seismic velocities (measured at approximately 1 kHz) and sonic log velocities (measured at approximately 10 kHz). In order to provide a proper basis for comparison, the sonic log must first be adjusted for field anisotropy, scaling effects, and resolution of measurement. We estimate a local shale anisotropy of about 20% based on correlations generated from published measurements of other shale fields. We apply resolution enhancement to capture the thin sand and carbonate beds, and windowed Backus averaging to match the measurement scales. A modeling study verifies the technique, and shows that beds of thickness greater than 30 cm have a measurement signature. The actual resolution is on the order of the crosswell Fresnel length, or about 7 m for the model study.
Additional publication details
Characterization of dispersion, attenuation, and anisotropy at the Buena Vista Hills field, California