Seismic refraction is commonly used to map depth to bedrock and is able to look as deep as 40m below grade. Variations in material density and acoustic velocity of the subsurface, together referred to as acoustic impedance, enable the mapping of earth layering, and an interpretation of material type. This is done by measuring refracted seismic waves with respect to the known geometry of the seismic wave path.
A seismic event is produced by an acoustic source, usually activated at surface, with resulting arrival times recorded at receiver sites (geophones) located at known distances from the source. Acoustic energy is imparted to the subsurface by means of a sledgehammer striking a metal plate. An instrument such as a seismograph is then used to measure the return of the refracted waves.
The seismic refraction method is non-invasive, easy to deploy method that provides numerical information about the spatial variability of depth to bedrock. Depth to bedrock can be of critical importance during a geotechnical investigation. Bedrock influences the stability of structures built above it. Shallow bedrock can be a benefit where solid foundation shoring is needed and minimal soil removal is required.