Direct estimation of shear-wave velocity profiles from surface wave investigation of geotechnical sites

Surface wave methods have been developed as a commonly used non-invasive tool for shear-wave velocity (V_s) profiling of near-surface soil and rock formations. Inversion is a key step to back-calculate the V_s profile from dispersion curves; however, the non-linear and ill-posed nature of the problem sometimes yields non-unique V_s profiles. To overcome these challenges, a new method was developed to circumvent inversion and directly estimate V_s profiles from the fundamental mode dispersion trend for sites with V_s gradually increasing with depth. This method is developed based on the relationship between the phase velocity of surface waves at a given frequency with the time-averaged shear-wave velocity within a depth of one-half wavelength. Three field sites and three simulated sites covering shallow, medium and deep locations with gradually increasing V_s with depth are employed to verify the feasibility of the proposed methodology. Results indicate that the method can efficiently give a reasonable and realistic estimation of V_s profiles. The goal of this method is to provide an alternative to obtain V_s profiles with high certainty and low computational cost for surface wave testing, but not necessarily to find a ‘better’ profile than inversion or to locate each layer interface.