The ocean is largely transparent to sound, but opaque to electromagnetic radiation. Underwater sound is therefore a powerful tool for remote sensing of the ocean interior. This technique is used in Ocean Acoustic Tomography. It is used to measure temperatures and currents over large regions of the ocean (Munk et al. 1995). On ocean basin scales, this technique is also known as acoustic thermometry. The technique relies on precisely measuring the time it takes sound signals to travel between two instruments, acoustic source and a receiver, by distance within the range of 100-5,000 km. If the locations of the instruments are known precisely, the measurement of time-of-flight can be used to infer the speed of sound, averaged over the acoustic path. Changes in the speed of sound are primarily caused by changes in the temperature of the ocean; hence the measurement of the travel times is equivalent to a measurement of temperature. A 1°C change in temperature corresponds to about
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Fig. 3.3 Present status of global drifters and Moorings as on August 2010 (closed circle-drifting buoys; square-moored buoys). (Source: JCOMMOPS)
4 m/s change in sound speed. An oceanographic experiment employing tomography typically uses several source-receiver pairs in a moored array that measures an area of ocean. Sound is widely used for remote sensing of the ocean on small scales (e.g., acoustic Doppler current profilers), but acoustical measurements have been under-exploited in regional and global ocean observations relative to in-situ instruments and electromagnetic radiation (Dushaw et al. 2009).
This technique integrates temperature variations over a large region hence the smaller scale turbulent and internal-wave features that usually dominate point measurements are averaged out and we can better determine the large-scale dynamics. For example, measurements by thermometers (i.e., moored or Argo 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. Tomographic measurements also average variability over depth as well, since the ray paths cycle throughout the water column.
Basinwide and regional tomography were accepted as part of the ocean observing system by OceanObs'99 (Koblinsky and Smith 2001; Dushaw et al. 2001). Since then, a decade of measurements of basinscale temperature using acoustic thermometry have been completed in the North Pacific Ocean. In this project acoustic sources located off central California (1996-1999) and north of Kauai (1996-1999, 2002-2006) transmitted to receivers distributed throughout the northeast and north central Pacific. The result shows that the interannual, seasonal, and shorter period variability was large; as compared to the long term decadal trends. Acoustic traveltime data have been used previously in simple data assimilation experiments, and they can now be compared to assimilation products from state-of-the-art models from the ECCO (Estimating the Circulation and Climate of the Ocean) Consortium. Not surprisingly, comparisons between measured travel times and those predicted using Ocean models, constrained by satellite altimeter and other data show significant similarities and differences. Measured acoustic travel times have uncertainties much less than the differences between two model implementations by the ECCO Consortium. The acoustic data ultimately need to be combined with upper-ocean data from Argo floats and, sea surface height data from satellite altimeters to detect changes in abyssal ocean temperature and to quantitatively determine the complementarity of the various data types (Dushaw 2003).
Apart from this, the passive acoustics can be used for a variety of purposes such as: tracking, counting and studying the behavior of vocalizing marine mammals and fish; assessing and monitoring the ecological impacts of ocean warming and acidification on marine ecosystems and biodiversity; detecting nuclear tests; detecting and quantifying tsunamis; measuring rainfall (Riser et al. 2008); measuring the properties of undersea earthquakes (e.g., de GrootHedlin 2005) and volcanoes; monitoring the sound produced by high latitude sea ice; monitoring anthropogenic activities in marine protected areas and also in commercial use. The acoustic measurements supporting these projects can be real time and provide information about local ambient noise sources such as shipping, wind, rain, as well as noise from offshore wind farms.
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