Experimental examination of growing and newly submerged sea ice including acoustic probing of the skeletal layer

Abstract

Results of an in-situ experiment to examine acoustic parameters and scattering properties of growing and newly submerged arctic sea ice are presented. The primary emphasis is on the acoustic properties of highly porous sea ice. High resolution (to about 1 cm) vertical profiles of the longitudinal wave speed in growing and submerged sea ice are derived from the experimental data. The results indicate a skeletal layer about 3 cm thick at the bottom of the growing ice. The time dependence of the vertical sound-speed profile in a submerged block reveals a long time scale process of up to 80 h and a short process of a few hours. The former process is block warming, the latter is thought to be due to the disturbance of the hydrostatic equilibrium as the ice is submerged. Concurrently with the wave speed measurements, scattering from the underside of the ice was measured at several frequencies. A comparison with predictions based on the sound-speed data demonstrate the ability to predict normal incidence reflectivity from sound-speed profiles as well as the viability of using scattering data in inversions to obtain sound-speed profiles. The data were also used to calculate the absorption of a longitudinal wave propagating vertically. The peak absorption rate in the skeletal layer was between 2 and 5 dB/cm at 92 kHz. The temperature dependence of the absorption seen in the submerged ice suggests that the McCammon–McDaniel equation is useful away from the skeletal layer. [Work supported by the Office of Naval Research (N00014-91-J-1647).]