Shallow seismic reflection profiling over the Brevard zone, South Carolina

Abstract

Shallow reflection profiles over crystalline rocks of the Brevard zone show events that correlate with projections of mapped lithologic contacts, in spite of strong attenuation and statics effects associated with a zone of severe chemical weathering (saprolite). The profiles, which were collected in western South Carolina about 5 km from Appalachian ultra Deep Core Hole (ADCOH) regional lines 1 and 3, straddle the contact between the Brevard fault zone and the wider Brevard mylonitic shear zone to the southeast. The principal goal of these experiments was to test the feasibility of using a hammer source and 24-channel seismograph to image near‐surface brittle and ductile structures to a depth of several hundred meters. We recorded two dip lines and one strike line with a total common midpoint (CMP) coverage of 1 km. Processing focused on eliminating static shifts and separating reflections from overlapping refracted S-waves. A statics approach based on the alignment of first arrivals on common‐offset gathers effectively removed most of the statics variations. Suppressing S-waves by apparent velocity filtering of shot gathers caused considerable smearing in the stacked section because of the short receiver spreads. Moreover, for shot gathers with high levels of incoherent noise and/or residual statics, velocity filtering imposed a level of coherency on the output that was not present in the original gather. Better results were obtained by applying severe surgical mutes to isolate the most reliable portions of the velocity‐filtered shot gathers prior to CMP stacking. Final CMP stacked sections show distinct packages of moderately dipping reflections with a lateral continuity of 25 to 100 m over a depth range of 85 to 400 m. The reflections are attributed to compositional layering, variations in degree of anisotropy, near‐horizontal expansion joints, and variations in fracture density. Southeast‐dipping reflections imaged for a 450-m dip line straddling the southeast flank of the brittle Brevard fault zone correlate with projected lithologic contacts of thrust‐faulted units mapped at the surface. Reflections show poorer continuity for lines recorded over a thick sequence of more uniform lithology within the adjacent Brevard mylonite zone. Reflections with apparent dips to the northwest are consistent with images in the ADCOH regional profiles and may reflect late‐stage folding and normal faulting.