Analysis of horizontal coherence during the transverse acoustic variability experiment.

Abstract

The transverse acoustic variability experiment took place in the northern limit of the East China Sea in 65–80 m of water. 300-Hz and 500-Hz cw acoustic signals were transmitted over distances of 33 and 20 km to a broadside HLA where they were received at 22–27 dB SNR. Fourteen estimates of transverse mutual-coherence functions show high correlation with scale lengths on the order of 1000–1700 m. A towed CTD chain provided simultaneous measurements of the sound-speed fluctuation spectrum due to internal waves in an effort to determine whether the observed shallow-water coherence is understandable within the framework of a normal-mode formulation of path-integral theory. An important property of the mutual-coherence function is its behavior at small hydrophone separations. Here the phase-structure function is expected to follow a power law. Measured logarithmic phase-structure function have slopes that vary between 0.6 and 1.1, which disagree with path-integral solutions that predict slope 2. The discrepancy between observation and data is not currently understood. [Work supported by the Office of Naval Research.]