Ocean Acoustic Tomography is a technique used to measure temperatures and currents over large regions of the ocean. 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, one an acoustic source and one a receiver, separated by ranges of 100–5000 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 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.
Read more about Ocean Acoustic Tomography: Motivation, Multipath Arrivals and Tomography, The Integrating Property of Long-range Acoustic Measurements, Reciprocal Tomography, Applications, Acoustic Thermometry of Ocean Climate (ATOC), Acoustic Transmissions and Marine Mammals, Types of Transmitted Acoustic Signals
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—Henry David Thoreau (1817–1862)