Each OBS then receives several acoustic waves for each generation of sound arriving at different times. Each one of these arrivals is called a ray path. By combining the time of the arrival, and the magnitude and phase of ray paths from the sound source at many different locations and from several OBS, a model of the earth's structure can be created. Seismic refraction work carried out at sea is faster and cheaper than a similar work carried out on land, because a non-explosion source can be towed behind the ship and fired repetitively while the ship is moving. On land, holes have to be drilled and explosives lowered into them.
Marine work uses fewer seismometers and many more shots.
In addition, the USGS OBS provide measurement capabilities in areas where land operations cannot go. The OBS instrument package and its use T he OBS consists of an aluminum sphere which contains sensors, electronics, enough alkaline batteries to last 10 days on the ocean bottom, and an acoustic release. The two sphere halves are put together with an O-ring and a metal clamp to hold the halves together. A slight vacuum is placed on the sphere to better ensure a seal. The sphere by itself floats, so an anchor is needed to sink the instrument to the bottom.