Acoustic thermometry

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Acoustic Thermometry of Ocean Climate (ATOC) is an idea to observe the state of the world's oceans, and the ocean climate in particular, using long-range acoustic transmissions. Prototype measurements of temperature have been made in the North Pacific Basin and across the Arctic Basin.[1]

The ATOC prototype array was an acoustic source located just north of Kauai, Hawaii, and transmissions were made to receivers of opportunity in the North Pacific Basin. The source signals were broadband (a pseudorandom m-sequence, to be precise) centered on 75 Hz with a source level of 195 dB re 1 micropascal at 1 m, or about 250 watts. Six transmissions of 20-minute duration were made on every fourth day.
The ATOC prototype array was an acoustic source located just north of Kauai, Hawaii, and transmissions were made to receivers of opportunity in the North Pacific Basin. The source signals were broadband (a pseudorandom m-sequence, to be precise) centered on 75 Hz with a source level of 195 dB re 1 micropascal at 1 m, or about 250 watts. Six transmissions of 20-minute duration were made on every fourth day.

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[edit] Motivation

The oceans are opaque to electromagnetic energy (e.g., light), but are fairly transparent to low-frequency acoustics. The speed of sound depends on the ocean temperature and, to a lesser degree, salinity. This means that if the travel times of acoustic pulses between a source and a receiver 100 to 5000 km away are measured accurately, the average temperature between source and receiver can be inferred.[2] The advantage of this approach to measuring temperature is that the technique is naturally averaging, so that the small scale fluctuations of temperature (i.e., noise) that are so prevalent in the ocean are naturally averaged. Measurements by thermometers (i.e., thermistors or Argo drifting floats) have to contend with this 1-2 °C noise, so that large numbers of instruments are required to obtain an accurate measure of average temperature. For measuring the average temperature of ocean basins, therefore, the acoustic measurement is quite cost effective.

From its beginning, the idea of observations of the ocean by acoustics, known as ocean acoustic tomography, was married to estimation of the ocean's state using modern numerical ocean models and the techniques assimilating data into numerical models. As the observational technique has matured, so too have the methods of data assimilation and the computing power required to perform those calculations.

Acoustic pulses travel great distances in the ocean because they are trapped in the SOFAR channel — an acoustic "wave guide". This means that as acoustic pulses approach the surface they are turned back towards the bottom, and as they approach the ocean bottom they are turned back towards the surface. The ocean conducts sound very efficiently, particularly sound at low frequencies, i.e., less than a few hundred hertz.
Acoustic pulses travel great distances in the ocean because they are trapped in the SOFAR channel — an acoustic "wave guide". This means that as acoustic pulses approach the surface they are turned back towards the bottom, and as they approach the ocean bottom they are turned back towards the surface. The ocean conducts sound very efficiently, particularly sound at low frequencies, i.e., less than a few hundred hertz.

[edit] Marine Mammal Issue

The implementation of ATOC began in the early 1990s, and immediately became embroiled in issues concerning the effects of acoustics on marine mammals (e.g., whales, porpoises, sea lions, etc.). [3] [4] [5] Public discussion was complicated by technical issues from a variety of disciplines (physical oceanography, acoustics, marine mammal biology, etc.) that makes understanding the effects of acoustics on marine mammals difficult for the experts, let alone the general public. Many of the issues concerning acoustics in the ocean and their effects on marine mammals were unknown. Finally, there were a variety of public misconceptions initially, such as a confusion of the definition of sound levels in air vs. sound levels in water. If a given number of decibels in water are interpreted as decibels in air, the sound level will seem to be orders of magnitude larger than it really is - at one point the ATOC sound levels were erroneously interpreted as "louder than 10,000 747 airplanes". [6]In fact, the sound levels used were comparable those made by blue or fin whales; the ocean efficiently carries sound so that sounds do not have to be that loud to cross ocean basins. Other factors in the controversy was the extensive history of activism where marine mammals are concerned, stemming from the on-going whaling conflict, and the sympathy that much of the public feels toward marine mammals. The issue became so well known that the TV show Flipper presented an episode that featured evil bad guys making undersea noises that were driving all the porpoises away!

As a result of this controversy, the ATOC program conducted a $6 million study of the effects of their acoustic transmissions on a variety of marine mammals. After six years of study the official, formal conclusion from this study was that the ATOC transmissions have "no significant biological impact".

Other acoustics activities in the ocean are not so benign insofar as marine mammals are concerned: air gun shots for geophysical surveys, and some of the louder transmissions by the U.S. Navy for various purposes are documented to present a threat to marine mammals. The actual threat depends on a variety of factors beyond noise levels: sound frequency, frequency and duration of transmissions, the nature of the acoustic signal (e.g., a sudden pulse, or coded sequence), depth of the sound source, directionality of the sound source, water depth and local topography, reverberation, etc.

In the case of the ATOC source, the source was mounted on the bottom about a half mile deep, hence marine mammals, which tend to stick to the surface, were generally further than a half mile from the source. This fact, combined with the relatively benign source level, the infrequent 2% duty cycle (the sound is on only 2% of the day), and other such factors, makes the sound transmissions benign insofar as marine mammals are concerned.

[edit] Preliminary Conclusions

Although the original ATOC program formally ended some years ago, the acoustic transmissions that began in the mid-1990s have continued through Fall 2006, when agreed-upon environmental protocols ended. The decade-long deployment of the acoustic source shows that the observations are sustainable on even a modest budget. The transmissions have been verified to provide a very accurate measurement of ocean temperature on the acoustic paths, with uncertainties that are far smaller than any other approach to ocean temperature measurement. [7]

[edit] Future Possibilities

Plans are developing for a truly global array of acoustic paths. The figure below is one possibility that is based on a proposal to the National Science Foundation's ORION program for global observatories for geophysical and oceanographic research in 2005.[8][9] As you can see from the figure, the main considerations are to keep the acoustic paths no longer than about 5000 km and to avoid significant topography (i.e., mid-ocean ridges) that might block the acoustic signals.

[edit] See also

[edit] References

  1. ^ Munk, Walter; Peter Worcester and Carl Wunsch (1995). Ocean Acoustic Tomography. Cambridge, UK: Cambridge University Press. ISBN ISBN 0-521-47095-1. 
  2. ^ The Heard Island Feasibility Test. Acoustical Society of America (1994).
  3. ^ Stephanie Siegel. "Low-frequency sonar raises whale advocates' hackles", CNN, June 30, 1999. Retrieved on 2007-10-23. 
  4. ^ Malcolm W. Browne. "Global Thermometer Imperiled by Dispute", NY Times, June 30, 1999. Retrieved on 2007-10-23. 
  5. ^ Kenneth Chang. "An Ear to Ocean Temperature", ABC News, June 24, 1999. Retrieved on 2007-10-23. 
  6. ^ Walter Munk (2006). in History of Oceanography[1], M. Jochum and R. Murtugudde, eds.: Ocean Acoustic Tomography; from a stormy start to an uncertain future. New York: Springer. 
  7. ^ The ATOC Consortium. "Ocean Climate Change: Comparison of Acoustic Tomography, Satellite Altimetry, and Modeling", Science Magazine, August 28, 1998, pp. 1327-1332. Retrieved on 2007-05-28. 
  8. ^ Ocean Research Interactive Observatory Networks (ORION).
  9. ^ Peter F. Worcester et al. (May 2005). Gyre-scale ocean heat content and dynamics: Integral constraints from acoustic remote sensing. NSF ORION, Request for Assistance: Conceptual Science Experiments.

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