Main Perth Thermometry Science People Engineering The Experiment Epilogue
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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 Heard Island Feasibility Test (HIFT) was an experiment conducted in 1991 to test the ability of man-made acoustic signals to travel throughout the world's oceans. The experiment was used to test engineering of acoustic sources and receivers for transglobal ranges, to study the nature of acoustic signals recorded at great distances from their source, and as a preliminary test of the tool of long-range acoustics for the purposes of measuring global oceanic climate change.
  This web page documents the development of the HIFT idea, the science questions that motivated the experiment, the engineering and organization required to achieve the global test, and the conclusions and questions resulting from the experiment.
  In 1960 the sounds from a sequence explosive charge detonated off Perth, Australia were detected at Bermuda in the North Atlantic, about 19,820 km away. In the late 1980's, a question arose concerning the travel times that were recorded and the acoustic path that was required to give that travel time. (The top banner of this page shows the Perth to Bermuda acoustic paths.)
  Munk, O'Reilly, and Reid (1988) concluded that global acoustic transmissions are very sensitive to oceanographic conditions. These considerations led to the notion that long-range acoustic transmissions might be a sensitive measure of the ocean's climate.
  The Perth test and the analysis of the properties of acoustics to account for the observations raised the interesting notion of using the characteristics of sound at very long ranges in the ocean to measure small changes in the average ocean temperature. Such measurements address the question of the warming of the oceans in response to climate change. In contrast to the normal turbulent variability of the ocean, changes in the ocean's average temperature as a result of climate change are expected to be tiny - about 0.005°C per year at 1000 m depth is a nominal expectation. Measuring such small changes conventionally is difficult, while the acoustical approach appears to be ideally suited for the measurement.
  Walter Munk therefore conjectured that long-range acoustics could provide a direct way to measure oceanic climate change. The Heard Island Feasibility Test was designed to test the ability of man-made sounds to be detected and deciphered at a location on the other side of the world.
  The basic goals of HIFT were quite simple: Can controlled, man-made acoustic signals travel antipodal distances over the world's oceans? If those signals are detected, can resolvable information be extracted from them such that accurate travel times can be obtained for the measurement of temperature? Prior to HIFT, the various estimations of the attenuation of acoustic energy were widely varying, so that the detection of the transmissions at great distances was not at all assured.
  The HIFT experiment was organized by a collaboration between the Scripps Institution of Oceanography (Walter Munk), the Applied Physics Laboratory, University of Washington (Bob Spindel), the Massachusetts Institute of Technology (Art Baggeroer) and the University of Michigan (Ted Birdsall). This group enlisted the help of a larger international collaboration of researchers willing to deploy acoustic receivers at widely varying and key locations throughout the worlds oceans. The HIFT collaboration was enthusiastic and multinational. Small, portable hydrophone systems (acoustic receivers) were developed and sent to several key locations, while other collaborators used sophisticated ship-towed hydrophone arrays.
  As a result of concerns that arose about the effect the acoustic transmissions may have on the marine life around Heard Island, whales in particular, the HIFT collaboration grew to encompass a contingent of marine biologists who were to observe and monitor marine life during the experiment. The biological component of HIFT was conducted on board the M/V Amy Chouest, a sister ship to the M/V Cory Chouest.
  Engineering efforts for the HIFT experiment consisted of a number of components. The acoustic sources are specially designed for high power and low frequency. The ship used to conduct the experiment had to be robust enough to withstand the extreme weather of the Southern Ocean. A variety of hydrophone arrays, the acoustic receivers, were used to study the acoustical signals. Finally, extensive research on signal processing was conducted, both to craft an optimal collection of signals to transmit from the source and to optimally process the received signals to glean the most information possible from them.
  The Heard Island experiment occurred in January-February 1991, with the M/V's Cory Choust and Amy Chouest departing from Perth Australia on 9 January. The plan was to begin the transmissions of the experiment on Australia Day, 26 January, although the ships departed port prior to obtaining the required permits. Permits were required as a consequence of the marine mammal controversy that the experiment engendered. The permits were obtained at the last minute, but the weather did not cooperate...
  While HIFT demonstrated that global acoustic transmissions were possible, its five day duration and complicated acoustic signals precluded any use of the data for determining anything of the ocean's climate state. In addition, work with numerical ocean models showed that multi-basin measurements were less desireable than basin-scale measurements, which are more suited to the expected patterns of climate change. As a follow on of HIFT and other experiments, a decade-long experiment called the Acoustic Thermometry of Ocean Climate (ATOC) was conducted from 1996–2006 in the eastern North Pacific. This experiment ran into similar problems with marine mammal concerns, but after considerable negotiation permits were obtained, the experiment proceeded, and a decade's worth of acoustic data were obtained giving basin-wide average measurements of the seasonal and interannual temperature variations.
Main Perth Thermometry Science People Engineering The Experiment Epilogue
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