Seismic High-Resolution and Velocity Surveys at Southern Appalachian Drill Site

Robert A. Phinney, John B. Diebold, Kabir Roy Chowdhury

The proposed southern Appalachian deep drill site is an unprecedented opportunity to understand how seismic waves propagate and how the seismic reflection method can be best exploited in crystalline rocks. In addition to the regional seismic lines collected by the site selection study team, special seismic lines were run to push current technology for high-resolution imaging and expanding spread velocity studies. Twenty-four mi (40 km) of high resolution lines were run near the proposed drill site with two vibrators operating off-end to A-120 channel lines with 110-ft probe spacing and sweeps from 20 to 80 hz. Excellent signal quality is seen on unstacked field records, aided by special correlation processing to improve the signal band shape near the 60-hz noise line. Sub tantial static shifts produced by a variable saprolite layer had to be treated by interactive refraction statics editing of the field gathers. The stacked sections show in detail the multiple layering of the southern Appalachian thrust mass and permit correlation of these layers with nappes and thrusts exposed to the northwest of the proposed drill site. Three special expanding spread profiles (ESP) were run, including a strike line (northeast-southwest) with offsets out to 80 km. By using five vibrators and a high fold of vertical stack at larger offsets, it is possible to image the major reflectors continuously for the full offset range and consequently infer the velocity structure of the thrust mass and the underlying basement. A detailed look at shallow refraction velocities from the high resolution and regional lines provides a map of the velocity below the saprolite.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.