In marine work the energy sources employed are generally non-dynamite sources, such as compressed air or gases exploding under water or an electric discharge under water. These sources are towed at the rear of the ship at a suitable depth beneath the surface of the sea. Under favourable weather conditions surveys can proceed at sea much faster than on land because the drilling of shot-holes is eliminated and the geophone spread moves continuously along the line, and as a consequence of this, the degree of multiplicity of subsurface coverage recorded on marine surveys is generally higher than that recorded on land surveys; forty-eight fold coverage is now quite common on marine surveys. As in all geophysical surveys, accurate position fixing is an important and integral part of the operation and at sea one of the radio navigational aids, such as the Decca system, is generally used in conjunction with the Satellite Navigational system which, while not as precise as the radio systems, does overcome their restriction of confining recording to daylight hours only.