The influence of water‐to‐ice transition layers on the reflection coefficient and high‐frequency acoustic backscatter from an ice keel

Abstract

The physical properties of ice have been included in a plane‐wave reflection coefficient calculation that is then used in a model to predict high‐frequency acoustic backscatter from an ice keel. Reflection coefficients were computed using a plane‐wave model which had parallel layers of varying thicknesses. Each layer was homogeneous in density, compressional velocity and attenuation, and shear velocity and attenuation. Roughness could be included on any layer. A profile of the ice was used [Branch and Posey, J. Acoust. Soc. Am. Suppl. 1 77, S56 (1985)] that allowed a gradual transition from water to multilayered ice. The ice was backed by water to simulate conditions that exist on ice blocks, which form ice keels. An ice‐keel model [Bishop et al., J. Acoust. Soc. Am. Suppl. 1 77, S56 (1985)] was modified to accept oblique incidence signals and different types of ice‐block rotations. The reflection coefficients were used in the ice‐keel model to calculate target strength. Examples are presented that indicate that the reflection coefficient can, at low grazing angles, be highly dependent upon the water‐to‐ice transition layers. This can give higher returns from the ice‐keel model than expected. Moreover, the reflection coefficient's influence on the ice‐keel model is dependent upon the type of ice‐block rotation used. [Work supported by NAVSEA.]