In Situ Measurement of Damping Ratio Using Surface Waves

Abstract

Measurements of surface wave attenuation provide a means to determine the in situ material damping ratio profile of near-surface soils. Frequency-dependent surface wave attenuation coefficients are determined from measurements of seismic wave amplitudes at various offsets from a swept-sine source. The accuracy of the measured attenuation coefficients is improved by properly accounting for the geometric attenuation of multimode Rayleigh waves. Once the frequency-dependent attenuation coefficients are determined, the shear damping ratio profile is calculated using a constrained inversion analysis. Application of the method is illustrated at the Treasure Island National Geotechnical Experimentation Site. Values of shear damping ratio, obtained using surface wave measurements, were less than those measured using cross hole tests, possibly because the higher frequencies used in cross hole tests result in more apparent attenuation due to scattering and because fluid losses contribute to damping at higher frequencies. Damping ratios from surface wave tests agree more closely with resonant column and torsional shear test results.