Quantifying suspended frazil ice using multi-frequency underwater acoustic devices

Abstract

An intensive frazil ice field sampling campaign was undertaken at the Port of Quebec on the St. Lawrence River from February-March 2009. Two underwater acoustic instruments set at different frequencies of 420 and 1228.8kHz were used to detect frazil ice in the water column. In this paper, frequency inversion methods are presented and subsequently applied to the observations to estimate frazil ice characteristics and concentration. Using inversion methods, most of the detected ice crystals had estimated radii of 0.06-0.18mm. This range compares favourably to the estimated value of 0.20mm obtained by analysing the Rouse number related to the vertical distribution of the frazil crystals. The results were in contrast to a previous study of frazil ice at another site in the St. Lawrence, which reported radii tens of times larger. The tiny crystals observed here were of similar size to those observed in laboratories (e.g. 0.09mm), suggesting that the particles formed locally. Frequency analyses were also used to estimate the volumetric suspended frazil concentration, which appeared to be on the order of 6ppm. Based on evidence suggested by the data and the volume backscattering coefficient at the two frequencies, this study also presents the complex sequence of processes that occurs during a typical supercooling frazil event. This paper concludes with future directions for research using acoustic instrumentation for further understanding of frazil ice dynamics. © 2010 John Wiley & Sons, Ltd.