Acoustic reflectivity of Antarctic sea ice

An Upward Looking Sonar (ULS) operating at 300 kHz has been built and deployed off the Antarctic coast during 1994 as part of the Australian climate related sea ice research program. The ULS was moored 150 m below the surface in water of 3 km depth and returned data spanning 11 months. Acoustic backscatter results from the periods when sea ice covered the ULS site suggest that the effective pressure reflection coefficient associated with several ice types is significantly lower than values reported in the literature. Published scattering data (at or near 300 kHz) is generally from fast-ice or lakes and may not be representative of typical ice types found in Antarctic waters. A modified ULS unit has therefore been developed and deployed during Australia's first mid-winter Antarctic marine science cruise, using the research vessel Aurora Australis in the period July-September 1995. The modified ULS unit has been deployed to measure acoustic reflectivity directly from naturally occurring sea ice, including floes 50-80 cm thick and rapidly growing thin ice of a few centimeters thickness. Acoustic backscatter has been measured with the modified ULS at depths in the range 30 m/spl rarr/150 m. Preliminary results are presented.