Near-surface waveguides and source-generated noise

A near-surface low-velocity layer consisting of weathered bedrock or unconsolidated or unsaturated sediments with a sharp seismic interface to the substratum is typical of many surficial environments. Within the offset range of high-resolution seismic surveys, the lower boundary of this surficial layer can generally be regarded as subhorizontal. Seismic waves are multiply reflected between the free surface and this interface. By interference, they generate horizontally propagating guided waves. In analogy to surface waves, guided waves can be described by modal theory: Rayleigh waves (i.e. ground roll) correspond to normal elastic modes, where as guided waves correspond to leaking modes (Roth et al., 1998). In a typical seismic shot gat her guided waves and ground roll occupy two cone shaped timeoffset windows bounding the optimum time-offset window, in which primary reflections can be observed (Robertsson et al., 1996). Therefore, these wavetypes are generally considered as "source-generated noise" in reflection seismology. However, ground roll and guided waves contain information about the physical and geometrical parameters of the shallow subsurface, which cannot be resolved by reflected or refracted waves. Here, we show some basic characteristics of guided waves for a variety of realistic 1D models consisting of a layer overlying a halfspace.