Laboratory measurements of acoustic backscattering from polystyrene pseudo-ice particles as a basis for quantitative characterization of frazil ice

Abstract

Measurements of volume backscattering coefficients in brine suspensions of neutrally buoyant, uniformly-sized, polystyrene spheres and disks were used to quantify size, shape and concentration dependences expected in equivalent measurements on freshwater suspensions of frazil ice. The results confirmed expectations of qualified compatibility with existing scattering theory. Specifically, the measured logarithmic backscattering coefficients were expressible, to within 1–2dB, as products of particle concentration and individual cross sections as calculated from the modal expansion of Faran (1951) applied to spheres with radii equal to either their actual values or (for hexagonal disks) to “effective” spherical radii. The limitations on the applicability of the theory were closely linked to: acoustic frequency, target-volume-determined effective radii; the linear dimensions of the faces of the individual target disks; and to particle concentrations. For combinations of particle dimensions, acoustic frequencies and concentrations outside of a well-defined Rayleigh Linear (RL) concentration dependence range, the relationships between measured volume backscattering coefficients and particle properties were complex and not amenable to a simple interpretation: reflecting, it is believed, polystyrene-specific scattering features. The features of the observed extensive Anomalous Linear (AL) and Non-Linear (NL) concentration dependence regimes were documented in terms of both their parameter dependences and the forms and parameters associated with their deviations from RL regime dependences. Analyses focused on identifying the boundaries of the latter regimes which currently provide the only measurement environments compatible with accurate extraction of particle concentration and size parameters. These identifications, made on the basis of polystyrene in brine data, were used, after insertion of frazil and freshwater material parameters, to identify equivalent RL boundaries for ABS (Acoustic Backscattering Sonar) frazil measurements. In practical terms, this framework led to estimates of upper and lower limits for usable ABS acoustic frequencies and established upper limits on the concentrations of larger frazil particles contributing significantly to backscattering signal returns. These results formed the basis for a measurement framework for obtaining validated frazil characterizations. This framework was tested on mixtures of differently-sized pseudo-frazil species to show compatibility between mixture data and equivalent backscattering coefficients calculated for each mixture on the basis of its composition and measured single species coefficients. Applications of this framework to actual frazil field data are essential for both further testing of effectiveness and to allow refinements of imposed, intentionally conservative, restrictions. Results of such applications are described and interpreted in a second publication.