Theory
Seismic surveys are an extremely useful geophysical method for studying the ground conditions to a significant depth and over a large area. The method operates by emitting a percussive force to the ground, producing seismic waves which travel through the subsurface, reflecting back or refracting each time a structure of geological boundary is encountered. Thus from this, the depth and shape of underground structures can be interpreted from seismic profiles. The initial force can be generated by means of explosives, or in environmentally sensitive areas (i.e. anywhere on land) dropping a significant weight onto the ground surface using a hammer-plate or buffalo gun. Refracted waves are detected by an array of geophones which are spaced at regular intervals. The geophones record the time taken by direct waves, reflected waves and refracted waves and outputs this data to a seismograph for processing so it can be interpreted for underground structures. Analyzing the time that the soundwaves take to return provides valuable information about rock types and possible fluids in the rocks, and is similar to the use of ultrasound in medicine. The seismic refraction method uses very similar equipment to seismic reflection, typically utilizing geophones in an array, and a seismic source (shot).

Applications
Stratigraphic mapping 
Estimation of depth to bedrock   
Estimation of depth to water table
Landfill investigations   
Predicting the rippability of specific rock types
Locating sinkholes  
Geotechnical investigations 
Hydrogeological mapping 
Voiding and disturbed dissolution