Apparent Coherent Energy Loss of Ice-Reflected, High-Frequency LFM Pulses.

摘要

Measurements of the effects of interaction of waterborne, high frequency (HF, 14 to 59 kHz), linear FM (LFM) acoustic signals with the bottom of multiyear pack ice are presented. The underside of arctic sea ice is a random, extended reaction scatterer of incident waterborne sound. At high frequencies, the problem is magnified because ice relief may be many orders of magnitude larger than a wavelength, resulting in significant out-of-plane scattering and multiple scattering. Also, at shorter wavelengths, the complicated water-to-ice transition region, typically about 20 centimeters thick, and other ice inhomogeneities and anisotropy become more important. Interaction of coherent, incident sound with such a complicated elastic medium results in a scattered field of unknown coherence, especially near the reflecting interface. In April 1986 and April 1987, forward reflection measurements were made at deep-water ice camps in the Beaufort Sea. A linear array of hydrophones was mounted on a rigid strongback and deployed beneath a multi-year ice floe each year. Sixteen phones were distributed along an aperture of 16 m. The data sets reported here were taken with the array deployed horizontally at a depth of 61 m and a source located normal to the array at depths of 55, 61, and 67 m, at ranges about 450, 900, and 1800 m in 1986 and 970 m in 1987. Keywords: Acoustic propagation; Sea ice; Noise; Reprints. (jhd)