The Ocean Bottom Seismometer:
An Ocean Bottom Seismometer (OBS) is a seismometer and recording device which have been placed into a water-tight aluminum sphere. The sphere is less dense than water, so it would float without the addition of an anchor (not shown). A magnetic release is attached to the bottom of the sphere. After the capsule has been on the bottom for a period of time (ranging from ten days to two months), it releases from the anchor and floats to the surface for recovery by a ship.
Typically, an OBS is deployed by being dropped from the stern of a ship; it falls to the seafloor, and records the motion of the seafloor in response to earthquakes and other seismic sources. These devices are cheap, reliable and easy to deploy, but often do not yield good results for a variety of reasons, such as:
Early Release: The seismometer floats to the surface before the ship arrives. If this happens, the seismometer is usually lost.
Failure to Release: Sometimes a capsule becomes "welded" to the anchor. If this happens, it will not float to the surface, and so cannot be recovered.
Poor Sealing: If water gets into the interior of the capsule, it can ruin both the seismometer and the recorder.
Improper Positioning: Since the OBS falls to the bottom, there is no way of selecting a landing point. If the OBS lands in a a region where the sediments are too soft, then no record is obtained.
Despite these shortcomings, OBSs offer many unique advantages. They are widely used by earth scientists and oil exploration companies to record "shots" in remote areas and under primitive conditions; they can also be deployed on short notice following an earthquake, to record aftershocks.
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