THREE-DIMENSIONAL IMAGING OF SCATTERERS ON A GRANITE OUTCROP, PANOLA MOUNTAIN, GEORGIA

TOTEVA, T.D., and LONG, L.T., Georgia Institute of Technology, Atlanta, GA, 30319, USA, WILLIAMS, L.J.,. U.S. Geological Survey, Atlanta, Georgia 30360, USA, ttoteva@eas.gatech.edu, tim.long@eas.gatech.edu, lesterw@usgs.gov.

The purpose of this study was to develop a non-invasive technique for identifying scatterers in a granite outcrop. We conducted our investigation at Panola Mountain Research Watershed (PMRW), located 25 km SE of Atlanta, GA, within the Georgia Piedmont. The area is underlain at shallow depths by fractured and unweathered crystalline rock. In the Georgia Piedmont, water resources are limited to surface reservoirs and shallow wells. This study was developed to determine if scattered waves could be used to detect and characterize fracture systems with water-supply production potential. We applied semblance analysis to a set of data. Semblance is the measure of similarity between multiple channels. The analysis consisted of two main steps. The first step was to calculate the semblance coefficient as a function of azimuth and apparent velocity. Theoretical data were used to test the accuracy and resolution of the technique. Semblance coefficient for direct surface waves had maximums at back-azimuths in excellent correlation with the back-azimuths of the source locations. The second step was an imaging algorithm. Seismic waves arriving at a given time in the coda were scattered from a point on an ellipse which size and orientation are defined by the travel time and velocity of the waves. In order to achieve maximum azimuthal coverage we used 16 geophones in a nearly circular array with an aperture of 15m. The geophones had a corner frequency of 100Hz. A weight-drop source was moved around the array at distances of 10 to 50m. The quality of the coupling between geophones and the granite outcrop proved to be a critical factor in accurately recording the seismic signal. Because the method had to be developed as non-invasive to protect the natural environment of the site, we could not drill or use devices that could damage the outcrop. Instead we used modeling clay to form a level platform and to attach the geophones to the granite. The clay alone did not guarantee good coupling. Sand bags were added to improve coupling to the granite. Frequencies from 100 to 1000Hz were recorded. Surface waves dominated the records but dispersion on the granite surface was insignificant. Our results identified an area of strong surface scatterers that correlates well with the peculiarities on the rock surface and the location of a small creek. We obtained a 3D image of the location of scatterers down to a depth of 55m.