Propagation modeling of under-ice transmissions using the parabolic equation

Abstract

A recent reformulation of the RAM Parabolic Equation model (OASIS’ CRAM model) permits efficient Nx2D propagation modelling, including ACOMMS performance estimation. This model is currently set up as a 3-layer propagation model. The 3-layers commonly used are water-column, sediment and basement - where the water-column and sediment have a depth dependent compressional speed, density and attenuation. The basement is an acoustic half-space with a sponge. To model under-ice propagation, the 3-layers can set up as sea-ice, water-column and seafloor. The only loss of generality is the single half-space seafloor, although bottom interaction is not as important a feature in the iso-thermal, upward refracting sound speed profiles in northern latitudes. The ice is modeled as an iso-speed (1700 m/s, rho = .988 kg/m3, alpha= 0.3dB/lambda) fluid on-top of the sea-water. The ice-water boundary is input into the code the same way a complex bathymetric profile is used. Although this approach doesn’t include shear propagation - an important source of frequency dependent attenuation, it does accurately model the scattering within the water column induced by interactions with the complex under-ice morphology. Comparison of measurements with models will be made for 900 Hz broadband transmissions to ranges of 50 km.