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Listening to the Ocean |
With the end of the Cold War, the U.S. Navy permitted civilian scientists to use SOSUS for basic research, giving them access to information they could not get otherwise. Scientists now could apply underwater acoustics to learning more about the geology and biology of the ocean's murky depths. In 1990, Christopher Fox and his colleagues from the Pacific Marine Environmental Laboratory were part of the military's initial evaluation of this dual civilian-military use for SOSUS. Since 1991, Fox's team, working on VENTS, the Study of Hydrothermal Venting Systems, has been using SOSUS to pinpoint the location of underwater volcanic eruptions. This has given scientists a better picture of the events that occur along mid-ocean ridges the mountain-like elevations where the ocean floor actually is being created from molten rock pushing up from below Earth's crust. (For more information on seafloor spreading, see the Beyond Discovery article "When the Earth Moves: Seafloor Spreading and Plate Tectonics.")
When Fox and his colleagues listened to the recordings of the underwater eruptions they also heard other underwater noises--including the vocalizations of baleen whales. The realization that SOSUS could be used to listen to whales also was made by Christopher Clark, a biological acoustician at Cornell University, when he first visited a SOSUS station in 1992. When Clark looked at the graphic representations of sound, scrolling 24 hours day, every day, he saw the voice patterns of blue, finback, minke, and humpback whales. He also could hear the sounds. Using a SOSUS receiver in the West Indies he could hear whales that were 1,770 kilometers (1,100 miles) away.
Whales are the biggest of Earth's creatures. The blue whale, for example, can be 100 feet long and weigh as many tons. Yet these animals also are remarkably elusive. Scientists wishing to observe blue whales firsthand must simply wait in their ships for the whales to surface. A few whales have been tracked briefly in the wild this way but not for very great distances, and much about them remains unknown. Using the SOSUS stations, scientists can track the whales in real time, positioning them on a map. Moreover, they can track not just one whale at a time, but many creatures simultaneously throughout the North Atlantic and the eastern North Pacific. They also can learn to distinguish whale calls. For example, Fox and colleagues have detected changes in the calls of finback whales during different seasons and have found that blue whales in different regions of the Pacific ocean have different calls.
One of the most intriguing mysteries about whales is how they find their way across such vast distances. Christopher Clark is interested in whether whales, like dolphins and bats, echolocate. Rather than bounce sound off objects a few yards away, however, whales send their sound pinging off geologic structures hundreds of miles away. The theory that whales use their own sound to get their bearings has been around for some time; now the data from SOSUS tracking give Clark some compelling circumstantial evidence to support it. When he superimposed the SOSUS-made track of a whale on a map of the ocean floor, it looked as though the whale was slaloming from one underwater mountain to another, with these seamounts being hundreds of miles apart. He did a similar matchup with other whales and got the same results. Clark hypothesizes that whales use sound not just to communicate, but to navigate, that is, they map the ocean acoustically to find their way around in it.
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