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| title | chunk | source | category | tags | date_saved | instance |
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| Ocean acoustic tomography | 2/2 | https://en.wikipedia.org/wiki/Ocean_acoustic_tomography | reference | science, encyclopedia | 2026-05-05T07:35:48.279270+00:00 | kb-cron |
== Acoustic thermometry == Acoustic thermometry is an idea to observe the world's ocean basins, and the ocean climate in particular, using trans-basin acoustic transmissions. "Thermometry", rather than "tomography", has been used to indicate basin-scale or global scale measurements. Prototype measurements of temperature have been made in the North Pacific Basin and across the Arctic Basin. Starting in 1983, John Spiesberger of the Woods Hole Oceanographic Institution, and Ted Birdsall and Kurt Metzger of the University of Michigan developed the use of sound to infer information about the ocean's large-scale temperatures, and in particular to attempt the detection of global warming in the ocean. This group transmitted sounds from Oahu that were recorded at about ten receivers stationed around the rim of the Pacific Ocean over distances of 4,000 km (2,500 mi). These experiments demonstrated that changes in temperature could be measured with an accuracy of about 20 millidegrees. Spiesberger et al. did not detect global warming. Instead they discovered that other natural climatic fluctuations, such as El Nino, were responsible in part for substantial fluctuations in temperature that may have masked any slower and smaller trends that may have occurred from global warming. The Acoustic Thermometry of Ocean Climate (ATOC) program was implemented in the North Pacific Ocean, with acoustic transmissions from 1996 through fall 2006. The measurements terminated when agreed-upon environmental protocols ended. The decade-long deployment of the acoustic source showed that the observations are sustainable on even a modest budget. The transmissions have been verified to provide an accurate measurement of ocean temperature on the acoustic paths, with uncertainties that are far smaller than any other approach to ocean temperature measurement. Repeating earthquakes acting as naturally occurring acoustic sources have also been used in acoustic thermometry, which may be particularly useful for inferring temperature variability in the deep ocean which is presently poorly sampled by in-situ instruments.
== Acoustic transmissions and marine mammals ==
The ATOC project was embroiled in issues concerning the effects of acoustics on marine mammals (e.g. whales, porpoises, sea lions, etc.). 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 so loud the signals would kill 500,000 animals. The sound power employed, 250 W, was comparable those made by blue or fin whales, although those whales vocalize at much lower frequencies. The ocean carries sound so efficiently that sounds do not have to be that loud to cross ocean basins. Other factors in the controversy were the extensive history of activism where marine mammals are concerned, stemming from the ongoing whaling conflict, and the sympathy that much of the public feels toward marine mammals. As a result of this controversy, the ATOC program conducted a $6 million study of the effects of the acoustic transmissions on a variety of marine mammals. The acoustic source was mounted on the bottom about a half mile deep, hence marine mammals, which are bound to the surface, were generally further than a half mile from the source. The source level was modest, less than the sound level of large whales, and the duty cycle was 2% (i.e., the sound is on only 2% of the day). After six years of study the official, formal conclusion from this study was that the ATOC transmissions have "no biologically significant effects". Other acoustics activities in the ocean may not be so benign insofar as marine mammals are concerned. Various types of man-made sounds have been studied as potential threats to marine mammals, such as airgun shots for geophysical surveys, or transmissions by the U.S. Navy for various purposes. 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.
== Types of transmitted acoustic signals == Tomographic transmissions consist of long coded signals (e.g., "m-sequences") lasting 30 seconds or more. The frequencies employed range from 50 to 1000 Hz and source powers range from 100 to 250 W, depending on the particular goals of the measurements. With precise timing such as from GPS, travel times can be measured to a nominal accuracy of 1 millisecond. While these transmissions are audible near the source, beyond a range of several kilometers the signals are usually below ambient noise levels, requiring sophisticated spread-spectrum signal processing techniques to recover them.
== See also ==
Acoustical oceanography Ray tracing SOFAR channel SOSUS Speed of sound TOPEX/Poseidon satellite altimetry Underwater acoustics
== References ==
== Further reading == B. D. Dushaw, 2013. "Ocean Acoustic Tomography" in Encyclopedia of Remote Sensing, E. G. Njoku, Ed., Springer, Springer-Verlag Berlin Heidelberg, 2013. ISBN 978-0-387-36698-2. W. Munk, P. Worcester, and C. Wunsch (1995). Ocean Acoustic Tomography. Cambridge: Cambridge University Press. ISBN 0-521-47095-1. P. F. Worcester, 2001: "Tomography," in Encyclopedia of Ocean Sciences, J. Steele, S. Thorpe, and K. Turekian, Eds., Academic Press Ltd., 2969–2986.
== External links == [1] Oceans toolbox for Matlab by Rich Pawlowicz. Ocean Acoustics Lab (OAL) - the Woods Hole Oceanographic Institution. The North Pacific Acoustic Laboratory (NPAL) - the Scripps Institution of Oceanography, La Jolla, CA. Acoustic Thermometry of Ocean Climate - the Scripps Institution of Oceanography, La Jolla, CA. Discovery of Sound in the Sea - DOSITS is an educational website concerned with acoustics in the ocean. Sounds of acoustic signals employed for tomography - the DOSITS web page. A day in the life of a tomography mooring - University of Washington, Seattle, WA. Sounding Out the Ocean's Secrets - National Academy of Sciences. Sound Measures the Ocean's Secrets - Acoustical Society of America. The Acoustic Thermometry of Ocean Climate/Marine Mammal Research Program Cornell University Laboratory of Ornithology, Bioacoustics Research Program