Considerations For Interpreting Surface Wave Data In Sites With Shallow Bedrock

The inversion of dispersive surface wave data has been shown to be successful in providing reliable estimated shear-wave velocities within unconsolidated materials in the near-surface. However, when this method is applied to a site consisting of unconsolidated material over hard bedrock (i.e. shearwave velocities >>800 m/sec), inverting the fundamental mode of Rayleigh waves results in shear-wave velocities within the rock that are less than half of the expected values. This phenomenon is illustrated using the multi-channel analysis of surface waves (MASW) method at a site consisting of clay residuum overlying basalt bedrock. In addition, some data processing techniques normally reserved for the spectral analysis of surface waves (SASW) method are applied to the MASW data set for comparisons. Forward modeling shows that for sites with a high impedance contrast (i.e. high velocity layer beneath low velocity layer), the fundamental-mode Rayleigh-wave dispersion curve is hardly sensitive to bedrock velocity perturbations over a practical range of wavelengths. Use of higher modes, additional geophysical data and/or a priori information of the site might be needed to converge to a more meaningful shear-wave velocity profile.