The problem of ocean acoustic tomography is to infer from precise measurements of travel time, or of other properties of acoustic propagation, the state of the ocean traversed by the sound field. The tomographic method was introduced by Munk and Wunsch (1979) in direct response to the demonstration in the 1970s that about 99% of the kinetic energy of the ocean circulation is associated with features that are only about 100 km in diameter, called the mesoscale. Measuring and understanding the behaviors of both the mesoscale and the larger-scale features associated with the general circulation present a formidable sampling task. Not only are the flow elements very compact spatially, but also they have long time scales (order 100 days). To produce statistically significant measurements of the fluid behavior, even in an area as compact as 1 Mm × 1 Mm (1 megameter = 1000 km), about 1% of an ocean basin, requires several full-time vessels or several hundred fixed moorings. One is accordingly led to the technology of sound propagation to measure the properties of the fluid between moorings.

Ocean acoustic tomography takes advantage of the facts that (i) travel time and other measurable acoustic parameters are functions of temperature, water velocity, and other parameters of oceanographic interest and can be interpreted to provide information about the intervening ocean using inverse methods, and (ii) the ocean is nearly transparent to low-frequency sound, so that signals can be transmitted over distances of many thousands of kilometers.