Multichannel analysis of underwater surface waves near Vancouver, B.C., Canada

Abstract

Surface (Scholte) waves acquired during underwater seismic surveys with hydrophone arrays are analyzed using the multichannel analysis of surface waves (MASW) method to construct shear-wave velocity (Vs) profiles for the upper 40-m of water-bottom sediments in the Fraser River delta area, near Vancouver, British Columbia, Canada. Shear wave profiles are obtained using the Rayleigh-wave inversion method (based on the multimodal dispersion curves) since theory suggests the difference in phase velocities between the two types of surface waves (i.e., the Scholte vs. the Rayleigh waves) is minor and usually falls below the uncertainty of the measurement. Vs profiles calculated from dispersion analysis are compared with measured Vs profiles available from nearby land boreholes (within a few hundred meters of the underwater sites). The comparison shows MASW values are in good agreement with the overall trend of borehole values, but lower in general by about ten percent. This shift seems to be attributable to the water-bottom sediments being softer and their density being greater (at depths < 5 m) than sediments on land. Simple multichannel processing (surgical mute) seems critical to suppress the influence of the strong broad-band channel waves trapped in the water layer before extracting the dispersion curve.