A simulation model for high-frequency under-ice reverberation

Abstract

A model has been developed to evaluate reverberation produced by the backscatter of a high-frequency acoustic pulse from pack ice regions characteristic of the interior Arctic. The model uses measured two-dimensional under-ice acoustic profile data and several empirical results that relate geometric parameters of the large-scale under-ice relief features, e.g., ice keels to construct a three-dimensional bimodal under-ice surface consisting of first-year ice keels and sloping flat ice regions. A first-year keel is modeled as an ensemble of randomly oriented ice blocks on a planar surface inclined at some slope angle with respect to a horizontal plane at sea level. The keel is characterized by length, draft, width, ice thickness, and aspect angle. A region of flat ice is modeled as a smooth planar surface whose slope angle is less than some critical angle that serves to distinguish a flat ice feature from an ice keel. The Kirchhoff approximation is used to evaluate the target strength of a facet of an ice block. The target strength of a keel is calculated in range increments as the coherent sum of the backscatter from all scattering facets contained within one-half the pulse length projected onto the keel. Reverberation at each range increment is calculated and compared with measured results from several Arctic sites. The model is used to show the effects of various ice and acoustic parameters on reverberation and target strength frequency distributions.