Reception at Ascension of the Heard Island Feasibility Test transmissions

Abstract

At Ascension, about 9200 km from the source ship, all transmissions from the Heard Island Feasibility Test were recorded on at least eight hydrophones and, for many transmissions, on 11 hydrophones. The depths of most of these hydrophones are near the depth of the sound channel axis but some are considerably deeper. All are bottom mounted. Signal-to-noise ratios were found to be surprisingly high. Averaged over the continuous wave (cw) transmissions and in a 1-Hz band, signal-to-noise ratios for the axial hydrophones south of the island range from about 19 to 30 decibels (dB), adjusted to a source level of 220 dB (referenced to 1 μ Pa at 1 m). The average signal-to-noise ratio for a hydrophone at a depth approximately 0.8 km below the axis is about 16 dB, suggesting acoustic energy was not restricted to a narrow interval in depth centered about the sound channel axis as had been anticipated. The travel time of the earliest arrival was found to be about 1 h, 44 min, 17 s. A late, scattered signal was observed for at least 23 min after termination of the direct signal. An unexpected combination of phase stability and amplitude variability was observed in the received signals. Continuing analysis of the Ascension data set is likely to provide considerable information about the characteristics of acoustic signals that have propagated global distances.