Seismic refraction is a useful method for investigating geological structure and rock properties. The technique involves the observation of a seismic signal that has been refracted  between layers of contrasting seismic velocity. Shots are deployed using a hammer/gun/explosive source at the surface and shockwave data recorded via a linear array of geophone sensors. The travel-times of refracted signal are derived from the data and are then processed to determine depth profiles of the targeted geological boundary.