Echothermometry: The potential role of echo sounders in ocean acoustic thermometry

The sensitivity of sound speed to temperature makes it possible to use an echo sounder as a thermometer, provided that the salinity and water depth are known with sufficient precision. Could ‘echothermometry’ – i.e., the use of an echo sounder, or a network of echo sounders, to measure temperature in this way – provide a viable alternative to long range tomography for global or regional climate monitoring? One benefit comes from the higher frequency and directional nature of echo sounder sources, resulting in lower (and therefore less harmful) sound levels in the ocean at positions away from the immediate vicinity of the source. Another is the potential for measuring regional variations in temperature. The main challenges are associated with the measurement of water depth to the necessary precision of a few centimetres. The precision requirements for travel time and water depth measurements are discussed and compared with those offered by available technology. In shallow water, where tidal changes are particularly important, laser methods can provide high resolution bathymetric data, although it is not yet clear whether the necessary precision can be obtained. Pressure sensors appear to offer the most promise in deep water.