High-frequency acoustic scattering from under-ice microscale surface roughness

Abstract

The under-ice surface, characteristic of the interior Arctic, may be partitioned into three more or less distinct roughness regimes. Large-scale under-ice surface roughness is produced by discrete large-scale relief features, e.g., ice keels, and may be parametrized by ice depth that varies from several meters to several tens of meters. Small-scale surface roughness is produced by ice blocks and/or ice rubble, and may be parametrized by the maximum linear dimension of the ice blocks and/or rubble that varies from several tens of centimeters to several meters. Microscale surface roughness is present on the various surfaces of the ice blocks and/or rubble and is produced on the upper ice surface by snow and/or erosion, on vertical surfaces by the fracture of ice, and at the sea water-ice boundary by ice dendrites formed when saline ice freezes, and may be parametrized by an rms roughness parameter that varies from several millimeters to several centimeters. The micro-scale surface roughness produced by ice dendrites has been modeled by an array of close-packed, infinite and parallel semicylinders whose cross section can be varied from circular to elliptical. A Fredholm integral equation of the second kind has been developed and solved numerically to calculate the pressure field scattered from the microscale surface roughness. The effects of various boundary conditions and geometric parameters on the scatter of a high-frequency (>2 kHz) plane wave have been determined. [Research supported by the Office of Naval Technology and Naval Underwater Systems Center IR/IED.]