Impact of fine-scale sound-speed fluctuations on acoustic autocorrelation times in the East China Sea

Abstract

Autocorrelation times of acoustic signals propagating through shallow oceans are largely driven by sound-speed fluctuations. In August 2008, the Transverse Acoustic Variability Experiment obtained measurements in the East China Sea (65–80 m water depth) of fluctuating signals propagating 33 km from a moored source to a bottomed line array. Supporting environmental measurements were obtained by a towed conductivity-temperature-depth chain. For time periods without large nonlinear internal waves, the measured internal-wave power spectrum shows excess energy at high wavenumbers in comparison with the shallow-water internal-wave model of Levine. Likewise the associated sound-speed fluctuation spectrum exhibits high-wavenumber components in excess of a simple power law. Autocorrelation times of measured 300 Hz acoustic signals were compared with simulated times obtained using a parabolic-equation model to propagate acoustic fields through sound-speed fluctuations driven by linear internal-wave displacements. Results of the comparison are that median measured autocorrelation times (115 s) are shorter than simulations (300 s) driven by spectra with depleted high-wavenumber components. Simple frozen ocean simulations that translate the environment at 0.6 m/s produce autocorrelation times close to the data. [Work supported by the Office of Naval Research.]