ACOUSTIC SCATTERING FROM ELEMENTAL ARCTIC ICE FEATURES - NUMERICAL MODELING RESULTS

In this paper acoustic scattering from Arctic ice is considered. No analytic scattering theories are able to explain the observed loss at low frequency (10-100 Hz) in long-range propagation experiments. A finite difference method is used to solve the heterogeneous elastodynamic equations in two dimensions; this technique permits arbitrary roughness, unrestricted in slope, displacement, or radius of curvature and provides 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 parameters: cross-sectional area or mass of the ridge, excitation of plate waves, and a material-dependent power law. The first two affect the magnitude of the loss, while the last affects the frequency dependence. Multi-year ridges are completely frozen and are best modeled as elastic structures yielding a loss frequency dependence of almost-equal-to f9/2. Observed loss in field data, with a frequency dependence of almost-equal-to f3/2, is not explained by scatter from multi-year ridges. In contrast, young pressure ridges are modeled as fluid structures since they are loose aggregations of ice blocks and cannot support shear strain. Scatter from fluid ridges has a loss frequency dependence of almost-equal-to f3/2 and yields a good match to the observed frequency dependence in field data. These results suggest that observed long-range propagation loss is best explained by scatter from large, young pressure ridges.