Ocean Acoustic Tomography

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Ocean Acoustic Tomography is a technique used to study average temperatures over large regions of the ocean. By measuring the time it takes sound to travel between known source and receiver locations, the speed of sound can determined; the solution for sound speed from acoustic travel times is an inverse problem. Changes in the speed of sound over time can be related to small changes in the average temperature of the ocean. Because ocean acoustic tomography integrates temperature variations over a large region, small-scale turbulent and internal-wave features that usually dominate point measurements are averaged out. On ocean basin scales, this technique is also known as acoustic thermometry.

With "reciprocal tomography," which is simultaneous transmissions between pairs of acoustic transceivers, both temperature and ocean currents can be measured. The temperatures are inferred from the sum of reciprocal travel times, while the currents are inferred from the difference of reciprocal travel times.

Tomographic transmissions consist of long coded signals lasting 30 seconds or more. With precise timing, travel times can be measured to a nominal accuracy of 1 millisecond. While these transmissions are audible near the source, the signals over most of the ocean they are below ambient noise levels, requiring sophisticated spread-spectrum signal processing techniques to recover them.

The idea for acoustic tomography was originally described by Walter Munk and Carl Wunsch. Many of the tools and techniques for ocean acoustics tomography originated in Ocean Acoustics Lab at the Woods Hole Oceanographic Institution.

[edit] References

• Walter Munk, Peter Worcester, and Carl Wunsch, Ocean Acoustic Tomography, Cambridge University Press, 1995. ISBN 0-521-47095-1

[edit] External links

• [1] Ocean Acoustics Lab (OAL) at the Woods Hole Oceanographic Institution
• [2] Ocean acoustic tomography and thermometry at the Applied Physics Laboratory, University of Washington, Seattle
• [3] The North Pacific Acoustic Laboratory (NPAL) at the Scripps Institution of Oceanography, La Jolla