Ocean-bottom seismometer

Computing and analyzing the data yields information about the kind of source and, in case of natural seismic events, the geophysics and geology of the sea floor and the deeper crust.

The deployment of OBS along a profile will give information about the deep structure of the Earth's crust and upper mantle in offshore areas.

OBS may be equipped with a maximum of a three-component geophone in addition to a hydrophone, and thus it needs a capacity of more than 144 Mbytes, which would be the minimum for adequate MCS profiling.

The 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.

Small motions have high frequencies, so monitoring them requires measuring movement many times per second and produces huge amounts of data.

Large motions are much rarer, so instruments need to record data less frequently, to save memory space and battery power for longer deployments.

[2] It can also be practical to store the datalogger and battery in a glass Benthos sphere to be able to connect to the ship through the use of a remotely operated vehicle (ROV),[3] which is a necessary advancement to have and maintain permanent OBS deployments.

The environment of these deployments complicates standard methods that are used in analyzing the data because of the ocean on top of the seismometer, as opposed to free-air above a typical land station.

[5] These seismometers also have a decreased signal-to-noise ratio because of noise created by the movement of the oceans due to wind driven tides, particularly at periods of 7 and 14 seconds.

The Cascadia Initiative[10][8] is an offshore/onshore deployment to observe the deformation of the Juan de Fuca and Gorda plates, as well as topics ranging from megathrust earthquakes to volcanic arc structure in the Pacific Northwest.

An ocean-bottom seismometer goes over the side of R/V Oceanus in early 2001, off Barbados. It will record long-period, low-frequency seismic waves for up to a year before the ship returns to retrieve the instrument. (Photo by John Whitehead, Woods Hole Oceanographic Institution)
This shows a P wave (red) converting into an S wave (blue), with the P wave having the ability to travel through the ocean and reflect bown off of the surface into the seismometer. This creates water multiples that do not exist when the seismometer has free-air above it. [ 4 ]
This is a map of the land and ocean-bottom stations that were deployed in the Cascadia Initiative. (Photo from http://cascadia.uoregon.edu/CIET/cascade-initiative-background )