High‐frequency under‐ice reflection losses and spatial coherence

Abstract

Propagation of high‐frequency acoustic signals under a multiyear pack ice results in a scattered acoustic field of fluctuating amplitude and coherence. High‐frequency scattering from under‐ice surfaces is complicated by out‐of‐plane and multiple scattering because acoustic wavelengths may be many orders of magnitude smaller than ice relief. In addition, the transition layer between water and ice, ice inhomogeneities, and ice anisotropy become important scattering sources. Measurements are presented of horizontal spatial coherence at 24 and 42 kHz from linear FM slide pulses of 800‐Hz bandwidth and 100‐ms duration. Measurements were made with a linear array of 16 hydrophones distributed over a 16‐m aperture and deployed 61 m below the ice surface. The data presented here were taken at a nominal range of 1000 m and results from array bearings at endfire, broadside, and 45° are compared. Results are presented for a fixed source at depths of 30, 61, and 91 m. Deconvolution of the source waveform is used to separate direct and reflected arrivals. Relative amplitudes of these arrivals are compared with what would be expected for a perfect reflector. The amplitude differences may be as high as 30 dB and the ice reflected coherence may increase slightly with range. In addition to hydrophone spacing, frequency, and source depth, the reflected arrival coherence is found to be sensitive to the physical location of the hydrophone in the aperture. This sensitivity is due to multipath interference from the scattering region. [Work supported by the Office of Naval Technology.]