Much of the Earth's surface is covered by a thin blanket of unconsolidated
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
contrast. 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)
can be described in terms of normal modes, we demonstrate that the
faster shingled guided waves can be explained in terms of leaking
modes. Due to the relatively high Poisson's ratios in the shallow
subsurface, guided 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.