GUIDED-WAVES IN NEAR-SURFACE SEISMIC SURVEYS

Much of the Earth's surface is covered by a thin blanket of unconsolid ated sediments or weathered bedrock. Boundaries within or at the base of this surficial layer (e.g. watertable, bedrock surface, transition to unweathered rock) are commonly associated with a strong seismic con trast. Layers contained between the surface and shallowest boundaries may be efficient waveguides and, as such, associated with significant source-generated seismic noise; Whereas Rayleigh waves (groundroll) ca n be described in terms of normal modes, we demonstrate that the faste r shingled guided waves can be explained in terms of leaking modes. Du e to the relatively high Poisson's ratios in the shallow subsurface, g uided waves are composed dominantly of multiply reflected P-waves. We compare our results with those obtained using a conventional acoustic waveguide approximation and discuss methods for extracting information from guided waves for geotechnical and environmental applications.