Project TASMAN TWO: Low-frequency propagation measurements in the South Tasman Sea

Abstract

In February 1975, a low-frequency acoustic propagation experiment, Project TASMAN TWO, was conducted in the South Tasman Sea to the west of the South Island of New Zealand. The aim was to study low-frequency acoustic propagation in a region having considerable variations in oceanic environment. Sound signals, set to detonate at the SOFAR axis, were air dropped out to a range of 3000 km along paths chosen to sample three distinct water masses (Subtropical Water, Australasian Subantarctic Water, and Circumpolar Subantarctic Water) and to traverse a large bathymetric feature (the South Tasman Rise). In addition, convergence zone (RSR) propagation in subtropical water was investigated with the sound signals set to detonate at 18 m. The results show that the three individual water masses have significantly different attenuations at frequencies below 500 Hz. It was established that Circumpolar Subantarctic Water has an attenuation nearly five times greater than Subtropical Water. For SOFAR propagation conditions, major acoustic transmission discontinuities were not always observed at the boundaries between the water masses, but strong shadowing effects with level changes of as much as 15 dB were clearly associated with the bathymetric feature. The experimental data for both SOFAR and RSR propagation are compared with predictions made using the parabolic equation model. Although the predictions match most general trends observed in the data, poor-detailed agreement was achieved where rapid horizontal gradients in either sound speed or water depth were experienced.