Sound Scattering from Sea Ice: Aspects Relevant to Ice-Draft Profiling by Sonar

Abstract

Accurate discrimination between thin ice and open water using sonar is an important practical concern for the calibration of ice-draft observations and for the use of ice-profiling sonar in climate-related studies of sea ice. To guide improvement of the surface-detection algorithm used by this instrument, a narrowbeam 400-kHz sonar was used to record over 1 million surface echoes from pack ice in the Beaufort Sea during the winter of 1995?96. Large fluctuations in the amplitude of successive echoes were observed for all types of targets. The probability density of fluctuations had lognormal form, indicative of multiple-scattered incoherent returns for all targets, including calm ice-free water, uniform growing first-year ice, and ridge keels. Median values of the scattering coefficient ranged from ?6.8 dB for calm water to ?30 dB for thick level ice and ridge keels. These low values indicate that the surfaces of these targets appear very smooth to narrowbeam sonar at this frequency. Despite this, the detection of the much stronger specular returns was precluded by the narrow beam of the sonar. The use of echo amplitude for identifying open water is hindered by the absence of an abrupt change in the average backscattering coefficient when ice forms at the surface and by the wide range of random fluctuations in amplitude. Some capability to identify targets can be acquired by averaging the amplitude of successive echoes. However, the spatial scale at which averages become useful is larger than the dimension of the majority of the uniform targets present in pack ice. Errors in ice detection are caused by scattering from bubble clouds in stormy ice-free conditions. These can be reduced by careful selection of sonar characteristics, but not completely eliminated. Because of the wide variation in the strength of surface targets, it is recommended that ice-profiling sonars have a dynamic range of at least 60 dB, if the scattering strength of the target is to be determined.