ACOUSTIC SCATTERING FROM ELEMENTAL ARCTIC ICE FEATURES - NUMERICAL MODELING RESULTS

In this paper acoustic scattering from Arctic ice is considered. No an alytic scattering theories are able to explain the observed loss at lo w frequency (10-100 Hz) in long-range propagation experiments. A finit e difference method is used to solve the heterogeneous elastodynamic e quations in two dimensions; this technique permits arbitrary roughness , unrestricted in slope, displacement, or radius of curvature and prov ides direct, physical insight into the rough ice scattering mechanism. Broadband numerical scattering simulations are conducted on pressure ridges. The specular loss due to a ridge is affected by three paramete rs: cross-sectional area or mass of the ridge, excitation of plate wav es, and a material-dependent power law. The first two affect the magni tude of the loss, while the last affects the frequency dependence. Mul ti-year ridges are completely frozen and are best modeled as elastic s tructures yielding a loss frequency dependence of almost-equal-to f9/2 . Observed loss in field data, with a frequency dependence of almost-e qual-to f3/2, is not explained by scatter from multi-year ridges. In c ontrast, young pressure ridges are modeled as fluid structures since t hey are loose aggregations of ice blocks and cannot support shear stra in. Scatter from fluid ridges has a loss frequency dependence of almos t-equal-to f3/2 and yields a good match to the observed frequency depe ndence in field data. These results suggest that observed long-range p ropagation loss is best explained by scatter from large, young pressur e ridges.