The immediate subsurface is often characterized by a layer of weathered
bedrock or unconsolidated sediments. Typically, this surficial layer
has a very high Poisson's ratio and an unusually strong impedance
contrast at its lower boundary, comparable, for example, to that
observed at a ?hard? seafloor. Within the offset range of high?resolution
seismic surveys, the layer can be regarded as horizontal and seismic
energy trapped within the layer generates horizontally propagating
guided?wave trains. Together with Rayleigh waves, guided waves pose
serious problems to high?resolution seismic imaging and are commonly
referred to as ?source?generated noise.? In a typical seismic shot
gather, Rayleigh and guided waves define two cone?shaped time?offset
windows that bound the optimum time?offset window in which primary
reflections can be observed.