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 vertical sound-speed profiles in a submerged ice block reveals a long time scale process of up to 80 h and a short process of a few hours. The former process is the warming of the block; the latter is thought to be due to the disturbance of hydrostatic equilibrium as the ice is submerged. Concurrently with the wave-speed measurements, scattering from the undersurface of the ice was measured at several frequencies. A comparison with predictions based on the sound-speed data demonstrates the ability to predict normal incidence ice reflectivity from sound-speed profiles as well as the viability of using scattering data in inversions to obtain sound-speed profiles. Absorption of a longitudinal wave propagating vertically in ice was also measured. The peak absorption rate found in the skeletal layer was between 2 and 5 dB/cm at 92 kHz. The temperature dependence of absorption seen in the submerged ice suggests that the McCammon–McDaniel equation (∼T−2/3) is useful away from the skeletal layer. The measurements of ice temperature as a function of time and depth allow a calculation of the thermal diffusivity of the upper region of growing sea ice (0.0080 cm2/s) and an ‘‘effective’’ thermal diffusivity of the submerged ice block (about 0.0015 cm2/s ). Measurements of salinity profiles in the ice, along with the temperature profiles, are used to calculate porosity, which is then used in models of the longitudinal wave speed as a function of porosity. A comparison between models and experiment is employed that suggests a possible mechanism for the increased speed seen experimentally in the bottom portion of the ice after submergence.