Reflections from a block of sea ice

Abstract

At an Arctic ice camp in the Beaufort Sea, monostatic acoustic reflections near normal incidence were measured from the lower face of an isolated block of natural sea ice. The measurements were repeated with the transition layer sawed off to leave a surface of solid ice. A comparison of the two results showed that the 15‐cm‐thick skeletal layer and transition zone cause a reduction in reflectivity, beyond that predicted from the impedance mismatch, that increases from 10 db at 20 kHz to 30 dB at 80 kHz. The system was calibrated using the air‐water interface formed at the open bottom of a shallow hollow metal cylinder. Air was forced into the cylinder when it was in place to fill it and thus provided a flat pressure‐release reflecting surface. As the measuring transducer was moved away from normal incidence, the response pattern was found to agree with the sidelobe pattern predicted for a flat, perfectly reflecting surface. These measurements of the effect of the skeletal layer on ice reflectivity will be helpful in modeling the reflections from ice keels. [Work supported by the Office of Naval Technology with technical management by NORDA.]