On an open patch of sea floor just north of Hawaii, oceanographers have anchored three electronic devices about 620 miles apart, forming the corners of a triangle. Each device emits 80-second bursts of a bass tone that sounds something like a cow mooing, but with a volume just short of that of a rock band.
The devices are the core of an electronic experiment that is expected to yield a detailed, never-before-seen cross-sectional view of water temperatures, densities and current flow within the boundaries of the triangle, in much the same fashion that the high-tech computed tomography scanners produce cross-sectional views of the human body for medical use.
Using sound waves instead of X-rays, the technique should lead to accurate maps of current flow throughout Earth's oceans--revealing, for instance, how the oceans move energy around Earth's surface, which could significantly improve long-term studies of climate, according to Peter F. Worcester of the Scripps Institution of Oceanography in San Diego.
Tomography, he said, is one of "the prime candidates for large-scale oceanography in the future."
Such maps could also help ship captains find currents to speed their passage and reduce fuel consumption in the same way that airplane pilots use jet stream, Worcester said. Increased knowledge of ocean currents may also benefit fisherman by making it easier to locate upwelling currents that provide nutrients for fish.
The experiment taking place 2,500 feet below the Pacific Ocean's surface also has the interest of the Navy, which hopes that it will lead to a better understanding of how sound propagates through the ocean. Such an understanding might make it easier to detect enemy submarines and avoid detection by the enemy.
"Large-scale ocean circulation is one of the fundamental problems in oceanography that we want to understand," according to Walter H. Munk of Scripps. "I know of no other technique that has the potential to make these measurements."
Scientists turned to sound for studying the ocean because light, radio waves and all other forms of electromagnetic radiation are absorbed over very short distances in water and, hence, are useless. Even now, conventional measurements of ocean temperature and current flow can be obtained only with instrumented buoys or with instrument arrays towed by ships.
"But the ocean is almost transparent to sound," said Massachusetts Institute of Technology's Carl Wunsch, who conceived the technique with Munk. "Very small sounds made deep in the ocean can be heard thousands of kilometers away. . . . If I can make a sound under water, if I can hear it, and if I can measure changes, I ought to be able to tell how the ocean is changing with time."
The key to ocean acoustic tomography is the fact that the speed of sound depends on water temperature: the higher the temperature, the faster sound travels. By measuring the time required for sound to travel a known distance between an acoustic source and a receiver, the average temperature of the water between them can be determined. The use of multiple acoustic sources and receivers allows for the calculation of a three-dimensional cross section of the ocean.
Ocean acoustic tomography will also provide additional information, such as the speed of a current.
Sound normally travels through water at a speed of a little less than one mile per second. If an ocean current--which typically has a speed of inches per second--is flowing in the same direction that the sound is traveling, the sound will get a boost from the current and reach the receiver slightly faster.
If an acoustic signal is transmitted in the opposite direction at the same time, it will be delayed slightly by the current. By measuring the very small difference in the time required for transit in each direction, the scientists can deduce the speed of the current.
One potential barrier to the ocean acoustic tomography technique is the fact that sound rarely travels in a straight line under water. It is easily deflected by hills and other physical features. Perhaps more important, regions of different water densities in the ocean act like lenses to refocus the sound waves and change their direction.
"If you have a single large bang deep in the ocean," Wunsch said, "and listen (620 miles) away, you won't hear a single bang, but a complicated sequence of bangs, arising from a whole series of different paths from source to receiver, spread out over a period of about 10 seconds."
Wunsch and Munk developed a way to separate the sound arriving from each of these paths, Worcester said. If the source and receiver are 620 miles apart, he noted, as many as 30 different paths can be detected.
The first test of ocean acoustic tomography was in 1981 when a team of scientists from six research institutions installed an array of four acoustic sources and five receivers over a 180-mile-square area of the Atlantic Ocean southwest of Bermuda and mapped the temperature of the area every three days for several weeks. Simultaneously, they used conventional techniques to map the same area.
The tomographic maps fell short of expectations, however, because the equipment was very rudimentary, Worcester recalled. "But we still did well enough to demonstrate the basic validity of the technique," he said.
The researchers also showed that they could produce a reliable map of temperatures from each day's tomography data, whereas a single conventional survey with ship-towed equipment required 18 days. They also obtained some new insights into ocean dynamics.
During the first 30 days of the experiment, Worcester said, a large eddy--a circularly flowing mass of water--remained centered in the experimental area. During the next six days, the eddy split apart and drifted out of the area.
"We had no idea before then that an eddy could break apart so fast," he said.
A second experiment two years later in the same area used a pair of devices built especially for the experiment. Each of the new acoustic transceivers incorporated both an acoustic source and a receiver. This engineering experiment proved that current flow could also be measured by ocean acoustic tomography.
Transceivers Are Costly
The acoustic transceivers operating north of Hawaii were installed in August by a team from Scripps, MIT, and the Woods Hole Oceanographic Institution in Massachusetts. The project uses only three transceivers because of their high cost, about $200,000 each.
"This is our first real experiment over such large distances," Worcester said. The site was chosen because there are few eddies in the region that might complicate data analysis and, more important, because the region has unusual wind-driven currents that extend all the way from the surface to the ocean floor.
As part of the experiment, five floating buoys with temperature sensors were released inside the triangle. These buoys contain acoustic receivers that use the signals from ocean acoustic tomography transceivers to calculate their position precisely.
At the same time, the precise height of the ocean surface will be regularly monitored by satellite. By the time the buoys and transceivers are recovered next summer, Wunsch said, "we hope to be able to integrate all the pieces of data to produce the most accurate picture ever obtained for a section of the ocean."
Next summer, scientists from Woods Hole and the French oceanographic institute, Ifremer, plan to install at least seven acoustic transceivers in the North Atlantic off the coast of New England to study the Gulf Stream.
The scientists will be particularly interested in the fate of the eddies that split off from the Gulf Stream during its northward passage. Such eddies, which may be warmer or cooler than surrounding water, are the oceanic equivalent of high- and low-pressure areas in the atmosphere, and charting them is much like producing a "weather map" of the ocean, Worcester said.
The Scripps scientists also plan to install an ocean acoustic tomography array in the Greenland Sea in the summer of 1988 to study the formation of the cold, dense undersea currents that originate in the Arctic and circle the world.