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Results from a recent US-Russian experiment in the Arctic involving MIT engineers indicate that sound could be used to monitor the temperature of the ocean there.
The work is important because long-term measurements of ocean temperature could help determine if global warming is occurring. The polar oceans are of special interest in this respect because scientists believe they are particularly sensitive to climate change.
The Trans-Arctic Propagation (TAP) experiment is actually the Arctic version of a similar test in 1991 conducted off Heard Island in the southern Indian Ocean. That test has since led to a large-scale international experiment, known as ATOC for Acoustic Thermometry of Ocean Climate, that will monitor the temperature of the Pacific Ocean.
The TAP, Heard Island and ATOC experiments are all based on the technique of acoustic thermometry in which researchers measure how long it takes for sound to travel from an underwater source to a receiving site hundreds to thousands of miles away. From that data they can derive the average temperature of the water along the transmission path. Sound travels faster in warmer water, so shorter transmission times observed over a long period of time would indicate that the ocean is warming.
While the technique was proven feasible in temperate waters by the Heard Island test, researchers were not sure whether it would work in polar oceans. "The major uncertainty concerned the transmission losses and [acoustic] signal degradation introduced by the ice cap," said Arthur B. Baggeroer, Ford Professor of Engineering in the Department of Ocean Engineering and in the Department of Electrical Engineering and Computer Science. Professor Baggeroer and Associate Professor Henrik Schmidt of ocean engineering were on the TAP team.
The TAP experiment showed that the low-frequency signals involved in acoustic thermometry can indeed pass unimpeded from one site to another. Specifically, the US and Russian researchers broadcast sounds from a specially designed source to a receiver 2,610 kilometers (or about 1,618 miles) away. The signals, which were sent at regular intervals from April 15 through April 22, came through clearly. "We got a very high signal-to-noise ratio," said Professor Baggeroer.
The resulting data are "very exciting," he said, and will allow scientists "to answer questions about acoustic propagation and to design sources and receivers for longer-term experiments." TAP was a one-shot test-"we demonstrated what we needed to go forward with more permanent equipment," Professor Baggeroer said.
The TAP experiment was very much a collaborative effort between US and Russian scientists, Professor Baggeroer emphasized. For example, the equipment at the receiving site was designed by US scientists (led by Keith von der Heydt of the Woods Hole Oceanographic Institution), while the acoustic source was developed by Russian scientists.
Professor Baggeroer noted that this source had to transmit signals of a much lower frequency (20 Hz) than those transmitted from Heard Island (57 Hz) or those that will be transmitted from ATOC sources (75 Hz). The reason: higher-frequency signals have a high transmission loss in the Arctic, again because of interactions with the ice cap.
TAP researchers at the transmitting site, including the Russians who developed the acoustic source, were Dr. Peter Mikhalevsky of Science Applications International in Virginia (MIT PhD '79), Dr. Mark Slavinsky of the Institute for Applied Physics in Russia, Dr. Alexander Gavrilov of the Andreev Institute of Acoustics in Russia and Dr. Alexander Smerlov of the Institute of General Physics in Russia.
At the receiving site were Professor Baggeroer (MIT ScD '68), Professor Schmidt, Mr. von der Heydt and Mr. Edward K. Scheer of Woods Hole (MIT '69).
All of the TAP scientists are also involved in ATOC, as are Professors Carl I. Wunsch and John C. Marshall of the Department of Earth, Atmospheric and Planetary Sciences. That experiment will send regular signals from two sources to 18 receivers around the Pacific, and will run for 30 months. It includes researchers from seven nations and 11 institutions including MIT and Woods Hole.
ATOC, which is led by the Scripps Institution of Oceanography, is currently being reviewed by the National Marine Fisheries Service (NMFS). The NMFS must issue permits for ATOC's two sound sources, located off the coasts of Hawaii and California, before the experiment can begin. The permits have been delayed, however, because scientists unaffiliated with ATOC have expressed concerns that the acoustic signals could harm marine mammals.
As a result, ATOC transmissions probably won't begin until early August, "assuming we get the permits," Professor Baggeroer said.
He noted that the NMFS did not require permits for the TAP experiment because of its short duration and its location deep within the Arctic ice cap. "In that region in April," he explained, "there are basically no marine mammals around because more than 99 percent of the area is covered with ice" (so mammals can't come to the surface for air).
Currently the TAP researchers are analyzing their data, with help from several students in the MIT/Woods Hole Joint Program. The results will be presented this fall at a meeting of both the US and Russian Acoustical Societies. The TAP experiment was funded by the Office of Naval Research and the Russian Ministry of Science.
A version of this article appeared in the May 18, 1994 issue of MIT Tech Talk (Volume 38, Number 33).