Ocean acoustic thermometry with ambient noise.

Abstract

Diffuse acoustic illumination, provided by ambient and shipping noise in the ocean, can be used as a probing signal to characterize the environment in a cost-effective and non-invasive manner. Valuable quantitative information about sound speed and, hence, temperature fields in water is contained in acoustic travel times between spatially separated receivers. The travel times can be retrieved from cross-correlations of noise recorded by the receivers. Practical applications of noise interferometry to ocean thermometry face a number of obstacles, the main challenge being the need to achieve very high accuracy of passive sound-speed measurements in a dynamic environment with a complex noise field. This paper reviews the theoretical background of wave interferometry in inhomogeneous media with non-perfectly diffuse noise from the viewpoint of ocean remote sensing. Feasibility of ocean acoustic thermometry is illustrated using noise recordings of opportunity, which were obtained in the North Pacific Ocean as a by-product of a long-range sound propagation experiment. Requirements to future dedicated passive acoustic systems for long-term observations of variations in the ocean heat content are discussed. [Work supported by ONR.]