OBSERVATION OF THE HEARD ISLAND SIGNALS NEAR THE GULF-STREAM

Transmission-loss and spatial-coherence data were obtained for acousti c signals of center frequency 57 Hz projected from the vicinity of Hea rd Island in the southern Indian Ocean during the 1991 Heard Island Fe asibility Test. The acoustic signals were monitored nearly 17 000 km a way from the source by a 1.8-wavelength-long array of hydrophones towe d from warm water south of the Gulf Stream to the much colder water no rth of the Stream. An adiabatic normal mode model predicted acoustic t ransmission 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 sca ttered 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 limit ed only by incoherent background noise and occasional interfering sign als from discrete directions.