Measurements of temporal coherence of sound transmissions through shallow water

In this paper we report the measurements of temporal coherence of acoustic signals propagating through shallow water using data from three experiments in three different parts of the world, with sound speed standard deviation (STD) varying from 0.3 to 5m∕s near the layer depth. Temporal coherence is estimated from the autocorrelations of broadband channel impulse functions, the latter are deduced from broadband signals transmitted through the ocean during the experiments. The measurements covered three frequency bands: low frequencies below 1.2kHz, midfrequencies between 2 and 5kHz, and high frequencies between 18 and 22kHz. The source-receiver range covers 3, 5, 10, and 42km. The signal coherence-time is defined and deduced from the data. Motivated by previous theoretical work in deep water on the signal coherence-time as a function of the signal frequency, the source-receiver range, and sound speed STD, a similar but empirical analysis is applied to the measured data in shallow water. While the range dependence agrees with the theory, the data exhibit a different dependence on the signal frequency than the theoretical prediction for deep water.