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Multichannel Analysis of Surface Waves Generated During High-resolution Seismic Reflection Profiling of a Fluvial Aquifer

¹ Department of Geosciences, Colorado State University, Fort Collins, Colo., 80523-1482
² BP America, Inc., 200 Westlake Park Blvd., Houston, Tex., 77079
³ Department of Geological Sciences, University of Alabama, Box 870338, Tuscaloosa, Ala., 35487-0339

Shear wave velocity profiles are estimated from surface wave dispersion analysis of data collected during a high-resolution P-wave reflection survey of a fluvial aquifer located in Columbus, Mississippi. The results demonstrate that useful velocity profiles of the upper 4 m of a sedimentary sequence can be imaged, even when survey design parameters and noise conditions are not optimal for surface wave surveys. The data were collected with 100 Hz geophones, a 10 oz. hammer source, and a 1 m geophone spacing with near and far offsets of 1 and 12 m, respectively. In spite of these less-than-ideal survey parameters for surface wave analysis and the presence of substantial cultural noise arising from nearby quarrying operations and runway activity, the shear velocity profiles accurately locate the boundaries between a shallow soil layer and a meandering fluvial facies in the upper part of the aquifer, and the boundary between the meandering fluvial facies and a braided fluvial facies in the lower part of the aquifer. In general, shear velocities increase with increasing average grain size. Velocities average 150 m/s in the soil. In the meandering fluvial facies, velocities increase systematically with depth from 120 to 275 m/s, and correlate with a systematic downward decrease in clay+silt fraction in this upward fining sequence. In the braided fluvial facies, velocities decrease with depth from 270 m/s to 250 m/s, and correlate with a slight downward increase in clay+silt fraction.