SHEAR WAVE STRUCTURE IN CHARLESTON, SOUTH CAROLINA: INITIAL COMPARISON BETWEEN SCPT, REFRACTION AND AMBIENT NOISE TECHNIQUES

JAUME, S. C., BROWN, C. M. and COOPER, S. L., Department of Geology and Environmental Geosciences, College of Charleston, Charleston, SC 29424; jaumes@cofc.edu.

During May-August 2005 we collected P refraction and ambient seismic noise data at 21 sites where SCPT results exist in the greater Charleston, SC region. S refraction data were also collected at six of these sites. We used 24 vertical and horizontal (both SH and SV configuration) 4.5 Hz geophones with station spacings of 4 to 8 meters. A wooden beam weighted down by driving a vehicle upon it and struck horizontally with a sledgehammer was used a shear wave source. Shear wave structure was determined from the slope and intercept of first arrivals in the resulting refraction data. Ambient seismic noise at the six S refraction sites was processed and interpreted following Louie (2001; see abstract by Brown et al. for more detail) and a shear wave velocity model was constructed using the Louie (2001) refraction microtremor (ReMi) technique. An initial comparison between the SCPT results, the refraction derived shear velocity model and the ReMi shear velocity model shows: a) the two surficial seismic techniques generally resolve velocities to depths greater than the available SCPT data and b) in the depth range where results overlap, the ReMi technique most often (5 of 6 sites) agrees better with the SCPT results than the S refraction technique. We also estimated V_S30 using data from the two SCPT boreholes deeper than 15 meters and compared it to V_S30 estimated from the surficial seismic techniques. In both cases the SCPT and refraction V_S30 agreed closely but the ReMi derived V_S30 was ~15% larger. We will also present comparisons between SCPT and ReMi shear velocity structures at the remaining 15 sites as they become available.