Scattering from stratified media with a rough surface: application to sea ice ridges

Bobby, Pradeep (2020) Scattering from stratified media with a rough surface: application to sea ice ridges. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Sea ice ridging is the dominant factor contributing to sea ice thickness, which has impacts on climate change and transportation. It is important to know the age of sea ice ridges, since ice age affects the strength of the ice and its ability to persist through the summer melt season. However, information on the age of sea ice ridges is not commonly available. The goal of this thesis is to develop a method to distinguish between first year and multi-year sea ice ridges using simulations of scattering signatures in the range 100-500 MHz. This goal is achieved by modifying existing scattering models, developing a sea ice model and comparing simulation results. The research is based on Walsh’s scattering approach, which was originally developed to model high frequency (HF) radar propagation across a rough surface or through stratified media and three updates to the scattering model are made. In the first update, Walsh’s method is modified from assuming the surface is a good conductor to be applicable to scattering from general dielectrics. Secondly, Walsh used a simplified scattering geometry, which implicitly assumed small surface slopes. By using the correct scattering geometry the method is extended to general surface slopes. The vertical component of the electric field is the most important for propagation across the surface, but the horizontal components of the field are relevant for penetration through the surface. The third update to the model is the derivation of the x-component of the electric field. Sea ice ridges are modeled as having a rough surface over stratified media. The total scatter is the sum of the surface and subsurface scatter. The subsurface scatter is a function of the field transmitted through the surface, the scatter from the layers and the transmission up through the underside of the rough surface. The subsurface scatter is found by considering all the scattering events in terms of scattering coefficients. The field transmitted down through the rough surface is found using a novel application of the boundary conditions at the surface. Due to the overlying rough surface, the scatter from the layers may be simplified to have the same structure as the Fresnel reflection coefficients for parallel and perpendicular radiation. Determining the field transmitted up through the underside of the surface may be found in a similar way as the first transmitted field, except that the underside of the surface has an inverted shape requiring that the rough surface scattering equations be rederived. To this point in the research sea ice ridges have been described in a general manner as having a rough surface over stratified media. To justify this approach and provide sufficient details for comparing scattering behaviour, a model describing the structure and internal characteristics of sea ice ridges is developed. The objective is not to fully describe sea ice ridges, but to include the factors that contribute to scattering in the frequency range from 100−500 MHz. Both first year and multi-year ridges have three layers consisting of the top of sail, remainder of sail and consolidated layer. Due to the lossy nature of sea ice, the salinity in the top layer of the ice dominates the scattering behaviour, but changes in the density, porosity and temperature of the ice also impact the scattered field. Since the ridge surface is assumed to have a sinusoidal profile with a long correlation length with respect to the radar wavelength, the surface and subsurface scatter may be separated spatially. However, simulations based on the characteristics of first year and multi-year ridges indicate that the total scatter is greater for multi-year ridges due to the subsurface contribution. This suggests that it should be possible to discriminate between first year and multi-year ridges for realistic surface geometries.

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