OBSERVATION OF THE HEARD ISLAND SIGNALS NEAR THE GULF-STREAM

Transmission-loss and spatial-coherence data were obtained for acoustic signals of center frequency 57 Hz projected from the vicinity of Heard Island in the southern Indian Ocean during the 1991 Heard Island Feasibility Test. The acoustic signals were monitored nearly 17 000 km away from the source by a 1.8-wavelength-long array of hydrophones towed from warm water south of the Gulf Stream to the much colder water north of the Stream. An adiabatic normal mode model predicted acoustic transmission losses in good agreement with those measured in cold water near the Gulf Stream. In warmer water, the measured signal level was generally higher than predicted, possibly because of signal energy scattered from lower to higher modes by boundary scattering. At the most southerly measurement sites, no signal was detected, probably because of bathymetric blockage. The spatial coherence of the signal was limited only by incoherent background noise and occasional interfering signals from discrete directions.