In seismic refraction surveys, a very small seismic wave is generated in the ground by either a hammer or small explosive change.
This wave is refracted in the subsurface and detected by a series of sensitive geophones deployed along the ground. By measuring the travel-times of the seismic waves, the nature of and depth to subsurface features can be computed.
In seismic reflection surveying, the seismic wave is reflected from the subsurface layers and detected by the geophones on the surface.
Geotechnical Investigations (including bedrock hardness)