Laboratory calibration of upward looking sonars for measuring suspended frazil ice concentration

This paper describes a series of laboratory experiments conducted at the University of Alberta Cold Room Facility, investigating the use of upward looking sonars to measure frazil ice concentration. Two upward looking sonars, one high (546 kHz) frequency and one low (235 kHz) frequency were deployed on the bottom of a custom built frazil ice tank located in the cold room. A step by step procedure is provided for processing the raw sonar signal to compute the volume backscatter strength. A sieving technique was used to make direct measurements of frazil ice concentrations in the tank and sieved frazil ice particles were examined under a microscope to determine their average shape and size. Measured mass concentrations ranged from 0.012% to 0.135% and the majority of observed frazil particles were disk shaped, varying in diameter from 0.25 to 4.25 mm. The sonar measurements showed that the high frequency model is more sensitive to the presence of suspended frazil particles than the low frequency model, especially at lower concentrations. The depth averaged volume backscatter strength, computed from both the high and low frequency data, was found to be correlated with the frazil ice concentration and the resulting regression equations are provided. These correlations need to be tested in the field prior to being used for quantitative measurements of frazil ice concentration in rivers. The applicability of simplified theoretical backscatter models to estimate frazil ice properties is also discussed.

► Frazil ice concentrations ranged from 0.012% to 0.135%.
► Frazil ice particles were disk shaped with diameters from 0.25 to 4.25 mm.
► The 546 kHz sonar is more sensitive to suspended frazil ice than the 235 kHz sonar.
► Depth averaged signal strengths were closely correlated with the frazil concentration.