Surface-Wave Sensitivity To A High-Velocity Inclusion

Abstract

The shear wave velocity (Vs) is an important mechanical property for the evaluation of dynamic soil behavior. Surface waves are often used to determine Vs. It is widely recognized that the existence of a large velocity reversal in a soil profile will give rise to significant partitioning of surface wave energy to higher modes in surface-wave testing. The authors have conducted a sensitivity study to examine the effects of thickness of a high-velocity layer (HVL), depth of the layer, and degree of velocity contrast on the surface-wave dispersion curve. Both the so-called dispersion curve, representing the superposition of energy into a single curve, and the multi-mode dispersion relation are generated. Results indicate that (a) the dispersion curve is more sensitive to the depth of the HVL than to its velocity or thickness; (b) the frequency/wavelength band within which the effective dispersion curve and fundamental-mode curve diverge is diagnostic of the position of the HVL; (c) model sensitivities are frequency-dependent; and (d) for the multi-mode case, peak sensitivities are higher for the fundamental mode than the first-higher mode. Introduction The shear wave velocity (Vs) is an important mechanical property for the evaluation of dynamic soil behavior. Because they are fast, cost-saving and non-intrusive, surface wave methods are widely used to determine Vs (e.g. Foti and Butcher 2004). To generate Vs profiles from surface wave data, a process of forward modeling and inversion is followed. In collecting and processing surface wave data, two approaches can be considered (Tokimatsu et al. 1992): either isolate a dominant mode and other contributing modes or develop a single so-called “apparent” (or “effective”) dispersion curve to consider all contributing modes simultaneously. For the multi-mode solution, multi-channel data acquisition is used, followed by the frequency-slowness (f-p) method for dispersion relation extraction, as in the Multi-Channel Analysis of Surface Waves (MASW) method (Park et al. 1999). A multi-mode plane wave propagation model is used as the forward model for this situation. For the effective dispersion curve solution, the two-sensor Spectral Analysis of Surface Waves (SASW) method is used, followed by the phase-spectral method for dispersion-curve extraction (Stokoe et al. 1994). An advanced cylindrical-wave model that simulates more realistically the physical process of wave propagation from a point source, including possible reflections or refractions of body waves (Kausel and Roesset 1981; Roesset et al. 1991) is used as the forward model for this situation. In the deep alluvial basin housing Las Vegas, Nevada, and elsewhere, geotechnical engineers encounter ubiquitous heavily carbonate-cemented lenses at or near the ground surface. This material is commonly known as caliche. The carbonate-cemented soils may be formed by the evaporation of either descending surface water or ascending ground water. The calcium carbonate that was dissolved in the water is left behind in the soil. As calcium carbonate continues to be deposited over thousands of years, the cemented soil is formed (Nowatzki and Almasmoum 1998). Varying degrees of cementation can be found throughout the Las Vegas valley, with the most extensively cemented soils occurring in the western and central portion of the valley, interlayered within broad alluvial fans (Wyman et al. 1993).