Magazine article Oceanus

Ears in the Ocean: Hydrophones Reveal a Whole Lot of Previously Undetected Seafloor Shaking Going On

Magazine article Oceanus

Ears in the Ocean: Hydrophones Reveal a Whole Lot of Previously Undetected Seafloor Shaking Going On

Article excerpt

If you sought to delve into the forces that drive and shape the face of the Earth and that distinguish it from all other planets in our solar system, you would shine a spotlight on the mid-ocean ridges.

This 75,000-kilometer (45,000-mile)-long volcanic mountain chain bisects the seafloor and wraps around the entire globe. It is the site where magma continuously erupts to create new crust. As the crust spreads out on both sides of the ridges, it paves the surface of the planet and sets in motion the tectonic forces that cause continents to rip apart and collide, and oceans to open and close. This planetary extravaganza, full of fury and sound, is accompanied by a constant drumbeat of earthquakes and volcanic eruptions. But the oceans act like a great blue curtain, completely shrouding our view and muting the sound.

About 80 percent of volcanic and earthquake activity on Earth occurs on the seafloor, but it is like those proverbial trees that fall in forests when nobody's there. If we could eavesdrop on all that seismic activity, we could glean a great deal of information about the workings of our planet.

To do that, Earth scientists have employed instruments that record seismic waves generated by earthquakes and volcanoes. (See "Listening Closely to 'See' into the Earth" page 16.) But these studies have suffered from two primary limitations. The instruments have been installed in land-based networks, which cover broad areas over long time periods but can detect only large-magnitude earthquakes in the oceans. Experiments using ocean-bottom seismometers can detect seafloor earthquakes with precision, but they are best suited to monitor small areas.

But now, the ending of the Cold War has given Earth scientists an unprecedented opportunity to take advantage of a tool created to wage that war--a project launched by the U.S. Navy in the 1950s that went by the code name Jezebel.

SOFAR, so good

The roots of Jezebel actually began at Woods Hole Oceanographic Institution during World War II when two scientists, Maurice Ewing and J. Lamar Worzel, began to conduct basic research on acoustics in the ocean seeking any advantages that would help the Navy detect enemy submarines or mines, or help U.S. subs avoid detection. In a critical experiment, they detonated 1 pound of TNT under water near the Bahamas and detected the sound 2,000 miles away near West Africa.

The test confirmed Ewing's theory that low-frequency sound waves were less easily scattered or absorbed by water and could travel far without losing energy. The key, however, was the discovery of a layer of water in the ocean that acted like a pipeline to channel low-frequency sound and transmit it over vast distances. This sound pipeline, called the SOFAR (Sound Fixing and Ranging) channel, was independently discovered by Russian scientist Leonid Brekhovshkikh, working simultaneously with underwater explosions in the Sea of Japan.

The key to the SOFAR channel is that sound energy, traveling in waves, speeds up in waters where temperatures are warmer (near the surface) or where pressure is higher (at depths). But in between lies the SOFAR channel, which is bounded by water layers where sound velocities increase. The boundaries act like a ceiling and floor: When sound energy enters the channel from below, it slows down. When it hits the ceiling, it does not keep going, but rather, it is refracted back downward.

Then it hits the floor, and it is refracted back upward. In this way, sound is efficiently channeled horizontally with minimal loss of signal over thousands of kilometers.

The Navy immediately saw the value of the SOFAR channel, launching Jezebel, which later became the Sound Surveillance System, or SOSUS. It deployed a network of underwater microphones, called hydrophones, connected by under-sea cables to onshore facilities. The hydrophones were installed in locations to optimally exploit the SOFAR channel. …

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