A technique for establishing the presence of underground geological strata using sound waves. There are two main techniques - reflection or refraction. In the oil and gas industry by far the most common is reflection. In a reflection survey sound waves are sent into the ground and the echoes from boundaries between rock with different properties of density and sound velocity are recorded. The sound waves recorded at the surface can then be used to create maps of the rock layers in the subsurface to reduce the risk of drilling dry holes. A reflection seismic survey may be either 2D, in which the reflections are recorded along a line or 3D, in which the lines are very closely spaced to deliver a three dimensional view of the subsurface. A refraction seismic survey is a different technology in which the sound waves are refracted along rock boundaries rather than reflected from them.