Low-frequency sound propagation in the South Fiji Basin

In March 1976, a joint experiment of the Naval Underwater Systems Center, New London, and the New Zealand Defence Scientific Establishment was carried out to investigate the characteristics of low-frequency (10–400 Hz) underwater sound propagation in the South Fiji Basin north of New Zealand using explosive sound sources. The source depth was 18 m and as a result propagation nearly everywhere within the basin was dominated by bottom-reflected modes. There were 5-dB level enhancements associated with the bathymetric features forming the southern, western, and eastern boundaries of the basin and 15–20-dB enhancements associated with the slope up to the Fiji Plateau to the north. The propagation loss across the abyssal plains is predicted well, while the level enhancements associated with all the features bounding the basin and the typically 15-dB shadowing losses associated with the ridges east and west of the basin can be approximately predicted with a model (ASTRAL) which allows for horizontal changes in the environment. However, it appears that more sophisticated models may be required to accurately predict the enhancement and shadowing.