Volume 31, Issue 5 (September 2008)
Potential Phase Unwrapping Errors Associated with SASW Measurements at Soft-Over-Stiff Sites
Results from simulations of spectral-analysis-of-surface-waves (SASW) measurements are presented for the common geotechnical condition of soft surficial soils overlying a stiffer half-space. Several recent experimental and numerical studies have demonstrated the dominance of higher-mode surface wave propagation at low frequencies for profile conditions where strong, shallow contrasts in elastic properties exist. This transition to higher-mode propagation has proven problematic for methods utilizing a fundamental-mode forward model in the inversion procedure, resulting in overprediction of the shear wave velocity (VS) profile. The objective of this study is to investigate the effectiveness of the SASW methodology, which uses an effective-velocity forward model, in dealing with the soft-over-stiff profile condition. Specifically, this study focuses on the phase unwrapping procedure used in the SASW method. Nine VS profiles were simulated in this study, representing different conditions of VS contrast and depth to the stiffer layer. The results show that in some cases the phase unwrapping procedure produces an experimental dispersion curve that is inconsistent with both the fundamental mode and the true effective-velocity dispersion curve for the profile. The resulting VS profile is substantially under-predicted. Experimental results from measurements at a soft-over-stiff site are presented that support the findings from the simulated measurements.