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Underwater, how far is far?

Underwater, how far is far?

As the Pacific plate dives under Alaska, creating occasional monstrous earthquakes in the process, how fast does it move? To study the Earth's plates, geophysicists need to be able to measure the speeds of these great blocks of lithosphere as they crash together or spread apart. On land, researchers can use a handful of different tools to measure how distances change from year to year. But the Pacific plate and many others lie mostly covered by water, and underwater measurement techniques are much less accurate. New scientists are developing ways to help bridge the accuracy gap.

Last March, researchers at the Scripps Institution of Oceanography in La Jolla, Calif., tested new precision transponders that sit on the seafloor, transmitting and receiving sound pulses used to measure distance. While traditional systems must allow for a few meters' error when measuring several kilometers, the new transponders chart distances to within a centimeter, says project member Marie C. McIntyre.

Next March, the group will test a second new system designed to determine the absolute position of underwater locations, allowing comparisons between points on land and in water. This system uses a cigar-shaped floating platform that reaches 100 meters below the water surface to minimize motion from waves. On top of the platform, a receiver picks up signals from Global Positioning System satellites, allowing precise determination of the platform's location. Meanwhile, seafloor transponders monitor the platform's position relative to the ocean bottom.
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Title Annotation:transponders used to measure underwater plate movement
Author:Monastersky, Richard
Publication:Science News
Date:Dec 17, 1988
Words:243
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