Shear Wave Velocity Profiles of Sediments Determined from Surface Wave Measurements

Shear stiffness properties of sediments can be determined from shear wave velocity, V_s, measurements and estimates of sediment density. Determining V_s profiles of sediment very near the seafloor surface (top 1 to 2 m) is difficult using conventional seismic methods. Downhole and crosshole methods, for example, often overestimate the velocities in this region due to difficulties with refracted wave arrivals. In-hole loggers have also proven ineffective in this very-near-surface region. The problem is magnified offshore by the lack of visual information and the logistical difficulty involved in applying conventional methods. The properties of very-near-surface sediments are often of interest offshore for applications such as pipeline routing, pipeline spanning, and pipeline-to-riser interactions. Surface wave methods, such as the Spectral-Analysis-of-Surface-Waves (SASW), have been shown to provide better resolution of near-surface properties than conventional seismic methods. Results are presented from SASW testing performed on the Palos Verde Shelf in approximately 50 m of water. The goal of this study was to measure the properties of very-near-surface sediments. The results demonstrate the ability to detect a thin layer of low velocity contaminated sediment at the seafloor and the presence of a soil cap placed over this layer. The shear wave velocity profiles determined from the surface wave tests are consistent with predicted profiles determined from empirical relationships using other measured geotechnical parameters. This method for measuring seafloor stiffness properties could be applied in much greater water depths with only minor modifications to the field equipment.