Low-frequency acoustic fluctuations and internal gravity waves in the ocean

Acoustic phase and amplitude fluctuations measured during two experiments, one near Eleuthera Island and one near Bermuda, are presented and interpreted in terms of the interaction of sound with internal gravity waves. Data were collected with a variety of sensors including drifting ship-suspended hydrophones, sonobuoys (free-floating acoustic receivers equipped with a UHF transmitter for data communication), and moored hydrophones. The phase data were corrected for hydrophone motion by means of a bottom-moored Doppler navigation system. Oceanographic data are presented and compared with the acoustic data; these data comparisons show that internal waves are responsible for much of the acoustic fluctuations. Phase spectra obtained with moored hydrophones show a rapid spectral decay at frequencies above the highest osberved Brunt–Vaisala frequency as predicted by internal-wave–acoustic models. Spectra obtained with drifting hydrophones are compared with a recently proposed random, multipath model based on interaction of sound with internal waves. Spectra obtained for drifting hydrophones at a depth of 1500 m agree with the multipath-model prediction. Spectra from phones at shallower depths exhibit an order of magnitude more fluctuation energy than is predicted. Slowly drifting sonobuoys at a depth of 300 m produce spectra with a complex frequency dependence that can not be predicted by the random multipath model.