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Ocean-Bottom Seismometer

Source: Ocean Instruments

What is an OBS and why do we use it?

Seismometers measure movement in the Earth's crust. About 90 percent of all natural earthquakes occur underwater, where great pressure and cold make measurements difficult. The ocean-bottom seismometer (OBS) was developed for this task.

Scientists use seismometer data to calculate the energy released by earthquakes, like the massive one in December 2004 that caused the Indian Ocean tsunami. By using sensitive seismometers to study small earthquakes, researchers are working to predict large earthquakes or volcanic eruptions.

Other scientists use seismometers to peer inside the Earth itself. The waves that earthquakes generate get deformed or slowed down as they pass through different materials inside the Earth. Seismometers equipped with precise clocks record the shape and speed of these waves when they arrive. After an earthquake, data from many widespread seismometers help geologists to calculate the structure of Earth’s mantle and crust.

How does it work?

Seismometers work using the principle of inertia. The seismometer body rests securely on the sea floor. Inside, a heavy mass hangs on a spring between two magnets. When the earth moves, so do the seismometer and its magnets, but the mass briefly stays where it is. As the mass oscillates through the magnetic field it produces an electrical current which the instrument measures.

The seismometer itself is a small metal cylinder; the rest of the footlocker-sized OBS consists of equipment to run the seismometer (a data logger and batteries), weight to sink it to the sea floor, a remote-controlled acoustic release and flotation to bring the instrument back to the surface.

Why are there two main types of OBS and what are they?

The ground motion caused by earthquakes can be extremely small (less than a millimeter) or large (several meters). 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.  Because of this variability, engineers have designed two basic kinds of seismometers:

Short-period OBSs record high-frequency motions (up to hundreds of times per second).  They can record small, short-period earthquakes and are also useful for studying the outer tens of kilometers of the seafloor. Technical details for two models: WHOI D2 and Scripps L-CHEAPO.

Long-period OBSs record a much broader range of motions, with frequencies of about 10 per second to once or twice a minute.  They are used for recording mid-sized earthquakes and seismic activity far from the instrument. Technical details for two models: WHOI long-deployment OBS and Scripps long-deployment OBS.

What platforms are involved?

All ocean-bottom seismometers are designed so they can be deployed and recovered from almost any research vessel. The main required piece of equipment is a winch for lifting the heavy instrument package (60 to 600 kg; 132 to 1320 pounds) into the water.

Some ocean-bottom seismometers are linked to scientists in real time through connections to a mooring (such as the Nootka Buoy) or cabled observatory similar to Martha's Vineyard Coastal Observatory.

Advantages

Very stable clocks make comparable the readings from many far-flung seismometers. (Without reliable time-stamps, data from different machines would be unusable.) Development of these clocks was a crucial advance for seismologists studying the Earth's interior.

After recovering an ocean-bottom seismometer, scientists can offload the instrument's data by plugging in a data cable. This feature saves the task of gingerly disassembling the instrument's protective casing while aboard a rolling ship.

The ability to connect a seismometer to a mooring or observatory makes the instrument's data instantly available. This is a huge advantage for geologists scrambling to respond to a major earthquake.

Limitations

Ocean-bottom seismometers are hard to install with pinpoint accuracy (usually they are lowered into place through thousands of meters of water). They can wind up sitting on a cushion of sediment rather than on bedrock. That soft layer can dampen the very tremors the instrument is trying to measure.

Short-period seismometers have short battery lives, so large numbers of them must be set out repeatedly during 30-day cruises. These instruments are designed to be small and light to make deployment and recovery easier.

Seismometers record so much data that storing it requires writing to a disk drive (up to 27 Gb), which presents another drain on battery power.

Sources

Robert S. Detrick, Jr., Vice President for Marine Facilities and Operations, Woods Hole Oceanographic Institution.

John A. Collins. Research Specialist, Geology and Geophysics, Woods Hole Oceanographic Institution.

Dorman, L. M. Seismology sensors. p. 2737-2744 in J. H. Steele, K. K. Turekian and S. A. Thorpe (eds.), Encyclopedia of Ocean Science, Academic Press, San Diego, CA. (2001)

Last updated: March 10, 2012