Tm. Georges et al., FEATURES OF THE HEARD ISLAND SIGNALS RECEIVED AT ASCENSION, The Journal of the Acoustical Society of America, 96(4), 1994, pp. 2441-2447
The Heard Island transmissions were received 9140 km away at Ascension
Island by an irregular array of bottom-mounted hydrophones. The singl
e-hydrophone signal-to-noise ratio sometimes exceeded 30 dB in a 1-Hz
band, confirming the detectability of 57-Hz underwater sound at global
distances. The arrival-time pattern consists of a single broad pulse
about 10 s long, whose fine structure decorrelates in about 12 min, in
sharp contrast with the stable, discrete sequences observed over shor
ter, midlatitude paths. The amplitude fluctuations of both the fine ar
rival structure and the unmodulated receptions are uncorrelated betwee
n hydrophones as little as 3.4 km apart. Phase varies less than one cy
cle during a 1-h transmission after correcting for source motion, and
the rms phase difference between hydrophones is about 3 rad averaged o
ver the array. Phasor diagrams suggest that the effects of both source
motion and ocean dynamics vary over the array. The probability densit
y functions of the real and imaginary parts of a downshifted cw transm
ission are nearly Gaussian.