Low-frequency wave propagation in an ice-covered Arctic Ocean. Modeling with the spectral element package SPECFEM2D.

Abstract

The significance of sea ice on acoustic wave propagation in the Arctic Ocean is extensively investigated, yet not completely understood. The Arctic sea ice is characterized by ice roughness and ice ridges and to properly model this complex geometry is often a challenge for numerical wave propagation tools. The open-source spectral element package SPECFEM2D is a powerful tool for wave propagation problems in laterally varying domains and can handle coupled fluid-solid domains. In this thesis, SPECFEM2D is used to model low-frequency deep-water wave propagation in an ice-covered Arctic Ocean. Using the axisymmetric version of the software and a point source at the symmetry axis enables simulations with a realistic 3D geometrical spreading at a moderate computational cost. Simulations are performed for a 50 Hz source located at 30 m depth in models with an absorbing bottom at 500 m depth and a propagation range of 4 km. Different models of laterally varying ice-water interfaces are implemented. Ice roughness is modeled with a Gaussian power spectrum with correlation length of 19.1 m and RMS roughness of 0.6 m. Ice ridges of 4.9 and 7.1 m depth are also introduced. Results are compared for a uniform, a linearly increasing and a realistic upwards refracting sound velocity profile in the water. The results suggest that a relatively thin ice layer, whether rough or not, with or without ridges, only impacts Arctic transmission loss at 50 Hz and 30 m depth to a minor extent over shorter ranges. Instead, the sound velocity profile in the water is shown to be the most important parameter controlling acoustic transmission loss at 30 m depth. The wave field in the ice layer itself is much weaker than in the water but is shown to be more impacted by the presence of lateral ice thickness variations.