Use of Rayleigh Modes in Interpretation of SASW Test

The Spectral-Analysis-of-Surface-Waves (SASW) method is a seismic method for measuring in situ elastic moduli of layered systems, like soils or pavements. The inversion process associated with SASW can be an ambiguous task in certain cases, because a field dispersion curve needs to be matched with an unknown path through a family of theoretically defined curves for an assumed profile. A study of the influence of soil stratification on participation of higher Rayleigh modes in surface wave propagation included the evaluation of theoretical dispersion curves, modal shapes and rates of energy transmission in the horizontal direction. Wave propagation due to oscillations of a circular plate on the surface of layered systems was modelled and the resulting simulated dispersion curves compared with theoretical ones. The results indicate that higher Rayleigh modes can dominate wave propagation for all but a narrow range of low frequencies for profiles where softer layers are trapped between stiffer layers. Potential ways of identification of such situations are discussed and improvements of the inversion process suggested