Seismic refraction and reflection methods measure the transmission of "sound waves" through the subsurface generated using a hammer blow or explosive energy source. Individual subsurface layer depths and thickness can be estimated based on the analysis of sonic wave arrival times. Quality Geophysics uses seismic methods to obtain the depth to bedrock surfaces and the water table; to characterize rock type and degree of weathering; and to locate fractures, faults, and buried channels.