Noise cross-correlation and passive measurements of the sound speed profile in deep water.

Ambient acoustic noise in the ocean contains extensive information about its sources and the propagation environment. Successful application of the noise observations to acoustic characterization of the environment depends on identifying the noise properties which can be reliably measured and are sensitive to variations in temperature and other physical parameters of the ocean. This paper focuses on two-point noise cross-correlation and the environmental information that can be retrieved from it without any a priori knowledge about properties and locations of the noise sources. A technique to retrieve deterministic acoustic travel times from cross-correlations of noise recorded on two vertical line arrays is described. Feasibility of environmental monitoring with ambient noise is illustrated by results of passive tomography of the water column using noise recordings of opportunity, which were obtained in the North Pacific Ocean as a by-product of a long-range sound propagation experiment. Conflicting requirements on the hydrophone separation and frequency band are analyzed that originate from the demands to improve the inversion accuracy and to decrease noise averaging time. Remaining challenges in retrieving oceanographically meaningful results from acoustic noise cross-correlations are discussed. [Work supported by ONR.]