A Seismic Inversion Experiment to Identify Scatterers in a Granite Outcrop

Abstract

A scattering inversion experiment was conducted on exposed Panola granite located southeast of Atlanta, Georgia, in the Panola Mountain Research Watershed. The objective was to test a near-surface seismic technique for locating subsurface discontinuities, such as fractures, that can be strong scatterers of seismic waves. Shallow water-productive fractures in the crystalline and metamorphic rocks underlying expanding suburban and urban areas, like those in central Georgia, are important because they have the potential of significantly augmenting surface freshwater supplies. Sixteen vertical [Formula: see text] geophones were placed in an array with aperture of [Formula: see text]. A weight-drop source was deployed around the array at distances of [Formula: see text]. We successfully recorded seismic waves in the frequency range of [Formula: see text] traveling through the unweathered Panola granite. The lack of overburden suppressed the interference from dispersed surface waves and enhanced coupling of the high-frequency energy source to the granite. We calculated semblance coefficients for waves propagating across the array as a function of time, back-azimuth and apparent velocity and used these to locate the source of the scattered waves within a [Formula: see text] area extending [Formula: see text] below the surface. Zones that were strong scatterers of surface waves correlated with topographic irregularities on the rock outcrop and the location of a small creek. Scattered [Formula: see text]- and [Formula: see text]-waves identified a three-dimensional distribution of scatterers which outlines the downward extension of fractures below the stream and the contact of the granite with amphibolite biotite gneiss.