SHEAR WAVE VELOCITIES FROM AMBIENT SEISMIC NOISE, CHARLESTON, SOUTH CAROLINA

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

During the summer of 2005 we recorded ambient seismic noise at 21 sites in the greater Charleston, SC area to determine shear wave velocities. Data was collected using 24 vertical and horizontal (SV configuration) 4.5 Hz geophones spaced 4 to 8 meters apart. In addition to spiking the geophones into the ground surface, at some sites we also set the geophones upon bricks on sidewalks and roads. We recorded six ambient noise windows of 30 seconds at each site. Sites were picked based on locations where SCPT tests have been conducted. The ambient noise data was processed into slowness-frequency space following Louie (2001) and Rayleigh dispersion values were estimated from this data. The dispersion picks were then used to determine a shear wave velocity model at each site. We also modeled the shear wave velocity structure using a refraction P velocity structure determined as part of the same experiment. Preliminary results show that surficial sediment layers have velocities ranging from 125 to 250 m/sec. Velocities generally increase to 300-600 m/sec within 5-15 meters of the surface and up to 700 m/sec at depths of 40-60 meters or more. Some sites show low velocity layers within the upper 30 meters. We find V_S30 falls in the range 260-320 m/sec (NEHRP C-D boundary) when determining the shear wave velocity structure without a P velocity structure constraint, but all sites are NEHRP Class D (230-350 m/sec) when the P velocity structure is used as a constraint in the dispersion modeling. We will compare these results with the SCPT tests and some shear wave refraction results at the same site.