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The Experiment

In January-February 1991 an acoustic source lowered from a ship near Heard Island in the southern Indian Ocean was used to transmit coded signals that were detected throughout the world's oceans. This test was the Heard Island Feasibility Test. Click on the image for a better view of the map.

The location of Heard Island was chosen for a number of reasons. First, a location in the southern Indian ocean was desirable in order for the acoustic signals to reach all the world's major ocean basins. In particular, both east and west coasts of the United States were "illuminated" by acoustic transmissions from Heard Island. Second, Heard Island is a territory of Australia, which offered logistical advantages. Third, Heard Island is south of the Antarctic Circumpolar Current where the sound channel axis is near the surface. Acoustic sources could be deployed to a shallow depth, yet still be near the sound channel axis. Finally, last but not least, the notion of this experiment from "Heard" Island appealed to the poet in Walter Munk.


Planning the experiment began long before its actual execution. Most important was securing the cooperation of the U.S. Navy whose research vessel, the M/V Cory Chouest was equipped with a unique set of acoustic transmitters able to emit powerful signals at very low frequencies. The ship also had a large center well from which the vertical array of sources was suspended. It wasn't difficult to get cooperation. The grand scale of the experiment was immediately appealing to everyone who was approached. At first the experiment depended on listening with existing U.S. Navy receiving stations in Bermuda off both coasts of North America, but once the world-wide acoustics community learned of the experiment, they volunteered to establish other listening posts. Eventually listening ststions were established by Russian, Indian, Australian, French, Soth African and Canadian scientists.

Planning proceeded efficiently towards a January 1991 experiment. Special acoustic receivers were designed by the Applied Physics Laboratory (APL) of the University of Washington under Spindel's direction for distribution to participating groups. Classes on how to use those receivers were held at APL in Seattle and at the Scripps Institution of Oceanography in San Diego. Meanwhile, Birdsall at the University of Michigan designed a special phase modulated signal, with center frequency of 57 Hz, to be used in the experiment. Metzger, Dzieciuch and Spindel traveled to Japan where the Cory Chouest was in port and boarded her for a short test cruise where the acoustic sources were deployed and test signals were transmitted.

The Marine Mammal Issue

About this time, a newspaper article about the experiment caught the eye of the Marine Mammal Commission who wondered whether the Heard Island transmissions would interfere with marine mammal activities, in particular whales. After much discussion (about what was then a poorly understood subject) a decision was made to err on the side of caution, and to require the experiment to obtain a permit to proceed. The permit specified that there be marine mammal observers on the ship, and that there also would be a marine mammal survey of the experiment site before and during the actual experiment. Accordingly, Ann Bowles of Hubbs Sea World was enlisted to direct a survey program, and a second ship, the M/V Amy Chouest was secured to carry out the survey. The permit set restrictions on whether or not transmissions were allowed depending on the proximity of whales or other endangered marine mammals. However, when the Australians learned about the U.S. permitting process, they decided that since Heard Island was their territory, they too should require a permit, and began their own process of evaluating the issues.

The M/V's Cory and Amy Chouest departed Fremantle, Australia for Heard Island on January 9, 1991, arriving on station 21 January 1991. Along the way the sea surface temperature dropped from 20°C to 4°C, falling particularly rapidly as the Antarctic Circumploar Current was crossed. The sea surface temperature shown in this figure is from a state estimate using a high-resolution ocean model ("ECCO2"). At the high southerly latitude of Heard Island, the sound channel axis is near the surface, so the acoustic source can more easily be placed within it.

The Ships Depart

Meanwhile, the window of opportunity for the experiment, when the two vessels were both available, was drawing near. Finally, still without either a U.S. or an Australian permit yet granted, but with time running out, the Amy and Cory Chouest departed Freemantle, Australia on January 9, 1991, with Munk, Forbes, Bowles and Dzieciuch on board. The hope was that the permits would be issued while enroute to Heard Island. The first acoustic transmissions were scheduled for Australia Day, January 26, 1991. The U.S. permit was granted on the 19th, but the Australian permit was still pending. The two ships arrived on station around 20 January and Amy began a pre-transmission marine mammal survey. On the 21st, the Cory lowered her acoustic sources and began drifting with the Amy, waiting for the Australian permit that would allow the transmissions to occur. It arrived at the 11th hour (or in the words written in the log, the 21st hour) 9:00 PM on January 25.

The HIFT transmissions were made from a location to the southeast of Heard Island. The ship was located over the Kerguelen Plateau, which is 1–2 km deep. The blue area to the south is Antarctica with a receiving array at Mawson Station.

Antipodal Transmissions

Immediately the Cory, which had secured permission from observers on the Amy, began a series of low level transmissions to calibrate the equipment. Full power was not scheduled until the following day, Australia Day. Three hours later the test signals were detected by Metzger at Bermuda, and Birdsall at Whidbey Island, Washington State. Thus, the signals had been detected and processed around the world on both coasts of North America (the Atlantic and Pacific sides; answering the main questions of the experiment), the day before the actual experiment was to begin!

The experiment formally began on the January 26th, for a planned 10 days. But as time went on, one after the other of the 10 deployed sources began to fail, mostly due to mechanical wear due to rough weather and overheating due to the unusually low transmission frequency for the sources. On January 31st the Amy reported "Beaufort 10 seas, and their wind guage pegged out at sixty knots". One source even broke off the array and remains on the bottom of the Indian Ocean near Heard Island. By the 30th, having made 25 transmissions so far, only three functioning acoustic sources remained. The last transmission was made on the 31st using the last remaining 2 sources. The Heard Island Feasibility Test had made a total of 35 antipodal transmissions.

The Heard Island Science Daily

During the cruise daily informal reports were faxed to shore to report the day's events, called "The Heard Island Science Daily". You may download and read through the archives of these reports here:

W. Munk, A. Bowles, A. Forbes, The Heard Island Science Daily, vols. 1–20, published by daily installment, the M/Vs Cory Chouest and Amy Chouest, Southern Ocean, 5 January 1991–1 February 1991. (5 MB PDF File), or, for slightly better viewing with a much larger file, (40 MB PDF File)

Tongue-in-Cheek "Sources-Gram" graph showing the number of surviving acoustic sources as a function of day during the experiment. The acoustic sources failed at a rapid rate (they had "a half-life of 1.2 days") as a result of violent weather and over heating from the unusually low 57-Hz transmission frequency. From The Heard Island Science Daily.
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