Transverse coherence in a winter ocean

Abstract

In December 2003 an ocean acoustics experiment was conducted in 75 m of water on the New Jersey Shelf in a nearly isothermal ocean. Sources broadcast 300 and 500 Hz narrowband signals at 15 dB SNR along a 20 km range to a bottomed horizontal line array oriented transverse to the acoustic propagation path. Estimates of the mutual coherence function were obtained by ensemble averaging 137<th>748 samples obtained over a period of 13 days, and compared to a canonical exponential form. It was found that the data were not properly described by an exponential. Rather, the data comprise two regimes: one with an exponential decay rate between 0.8–1.2 and coherence length of 12 wavelengths, the other with an exponential decay rate of 0.3 and coherence length of 27–166 wavelengths. The changeover between the two regimes occurs at hydrophone separations of 4–9 wavelengths. A scintillation index near one indicates that the acoustic fluctuations are dominated by phase variability. It is concluded that these data are not well-described by a simple exponential form and that work is necessary to develop an acoustic fluctuation theory appropriate for this environment. [Work supported by the Office of Naval Research.]