Comparisons of high-frequency acoustic and shallow-water ocean temperature fluctuations

Abstract

Dynamic ocean processes produce spatial and temporal fluctuations in the ocean’s index of refraction along the propagation path that cause acoustic propagation fluctuations. This paper reports a second set of measurements designed to determine environmental limits on ping-to-ping amplitude and phase variability of direct path acoustic fluctuations in very shallow water. In September 1997, the Naval Research Laboratory in conjunction with the NATO SACLANT Undersea Research Centre conducted this series of experiments, in shallow water off American Beach, located between Pisa and Livorno, Italy. Established equations for the variance of the acoustic amplitude and phase as a function of the index of refraction variance and the ocean characteristic length scale under conditions of isotropic turbulence are used to predict acoustic fluctuations for comparison with measurements. In these results, measured phase fluctuations are more consistent with sea swell predictions than isotropic turbulence predictions. Phase fluctuations are relatively insensitive to water column stability and anomalous phase fluctuations correlate with wind direction. Amplitude fluctuations are consistent with isotropic turbulence predictions and anomalous amplitude fluctuations correlate with changing stability. [Work supported by ONR.]