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Taking the Ocean's Temperature with Sound
| Article
# : |
20292 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
7 / 1992 |
3,315 Words |
| Author
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T.M. Georges
T.M. Georges is a physicist with the National Oceanic and
Atmospheric Administration (NOAA) Environmental Research
Laboratories in Boulder, Colorado. He participated in the
Heard Island Experiment at the Ascension Island receiving
station. |
In 1490, Leonardo da Vinci wrote in his notebook, "If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, your will hear ships at a great distance from you." If you performed the same experiment today, you would hear a chorus of natural and man-made sounds--perhaps the songs of the humpback whale, the clatter of snapping shrimp, the pops of air guns used in petroleum exploration--but mostly you would hear the droning and churning of distant ships' engines and propellers.
If you happened to be listening in some parts of the world during the last weak of January 1991, you might also have heard an extraordinary sound--a low-pitched hum, something like the noise you hear near an electric power substation. If so, you would have been a part of a global experimental to find out how far coherent sound can be sent through the world's oceans. The six-day experiment, called the Heard Island Feasibility Test, was motivated by more than scientific curiosity. It was a crucial test of a new technique for monitoring the effects of greenhouse warming on the world's oceans.
A Sound Channel In The Ocean
The scientific understanding of how a low-pitched hum could connect with greenhouse warming rests on foundations laid more than 50 years ago. In the early days of World War II, sonar operators learned that echoes from enemy submarines and the sounds from underwater explosions could sometimes be detected at much greater distances than previously thought possible. Navy scientists studying this unexpected phenomenon soon discovered that sound waves in the ocean are bent by variations in the water's temperature, density, and salinity, in the same way that light rays are refracted by lenses. Measuring temperature and salinity to great depths, they discovered the deep sound channel, a permanent wave guide for long-distance sound transmission.
Sound speed in the ocean depends on both temperature and pressure. Sound waves travel faster in near surface waters warmed by the sun, and at great depths, where pressures reach hundreds of atmospheres. In between, sound travels more slowly, reaching a minimum speed at depths ranging from 800 to 1,200 meters (2,400 to 3,600 feet) at temperate latitudes. The depth of the sound-speed minimum is called the sound channel axis.
If sound is produced near the sound channel axis, sound waves heading toward the surface are bent back toward the bottom by the
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