Shear Wave Velocity Gradients in Near-Surface Marine Sediment

Abstract

Positive gradients in values of sediment shear wave velocity were measured from several shallow-water sites near La Spezia, Italy. Shear waves were generated and received using identical bimorph ceramic bender probes inserted into the upper 2 m of sediment. Shear wave velocity was measured directly using a pulse technique. Velocities ranged from 17 m/s near the sediment-water interface in high-porosity silty clays to over 100 m/s in lower porosity sands and silty sands. Shear wave gradients of 4 to 17 s^-1were measured in the upper 2 m of sediments at the muddy sites, and a higher gradient of 85 s^-1was measured at the hard-packed sandy site. Velocities at the muddy sites were generally less than predicted, but measurements and predictions were in agreement for sandy sites. Comparisons suggest that sediment shear modulus is controlled by more than the process of consolidation in near-surface, fine-grained sediments but is adequately modelled in sands.