Comparisons of high-frequency acoustic fluctuations with ocean-temperature and sea-surface fluctuations in shallow water

Dynamic ocean processes produce small thermal variations that induce spatial and temporal variability in the ocean´s index of refraction and in the spatial scale along an acoustic propagation path. This paper reports measurements and analysis of thermal microstructure effects on ping-to-ping amplitude and phase variability of shallow-water direct-path acoustic propagation in the 20-200 kHz frequency range. These measurements were conducted during a joint experiment conducted by the Naval Research Laboratory and the North Atlantic Treaty Organization Supreme Allied Commander Atlantic (SACLANT) Undersea Research Centre, La Spezia, Italy, in 8 m of water off American Beach, located between Pisa and Livorno, Italy. Experimental observations are compared with predictions for isotropic and anisotropic turbulence, as well as for sea-surface swell. Measured phase and log-amplitude variances coincide with predictions and are relatively insensitive to weak water-column stability. The sea-surface swell dominates phase variances for this data and turbulence dominates log-amplitude variances. These results provide a reasonable lower limit on high-frequency ping-to-ping amplitude and on phase variability produced by benign shallow-water thermal fluctuations.