Geotechnical and environmental investigations of buried landfill by drilling or penetration testing are difficult and hazardous. Uncontrolled fill materials can be highly variable, however, landfill is typified by low density, low seismic velocity and high seismic attenuation relative to surrounding geological materials. Buried landfill presents a good target for appropriate seismic geophysical technologies which address major geotechnical issues of the lateral extent and depth of fill, the location of significant voids and the stability of the fill mass. Surface seismic technologies have the advantage of being non-intrusive and non-destructive while downhole/crosshole seismic technologies can be applied from single or widely spaced boreholes which can, at some sites, be drilled outside the fill area to avoid compromising liner or cover systems. The surface seismic retraction method is best applied to the definition of the lateral margins of the fill and waste-filled trenches. This well-known technology has recently been improved with new ray tracing and tomographic techniques which deal with complex seismic velocity variations within the fill. The application of seismic refraction is demonstrated with field examples from solid-waste landfills in and near Sydney, Australia. Full and effective use of any boreholes drilled within landfill is essential for geotechnical and environmental investigation. Downhole and crosshole seismic technologies based on improved methods of seismic tomographic imaging (STI) provide the means by which the base of fill can be accurately mapped and features within the fill defined in more detail. In San Francisco, STI from a single borehole is used to locate dense objects under emergency landfill following a house collapse. In Sydney, seismic refraction is used define the fill margins to locate trenches in landfill. At another Sydney site STI is applied to map the base of an unstable landfill located in an old sandstone quarry.

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