Electromagnetic measurements of multi-year sea ice using impulse radar

Abstract

Sounding of multi-year sea ice, using impulse radar operating in the 80- to 500-MHz frequency band, has revealed that the bottom of this ice cannot always be detected. This paper discusses a field program aimed at finding out why this is so, and at determining the electromagnetic (EM) properties of multi-year sea ice. It was found that the bottom of the ice could not be detected when the ice structure had a high brine content. Because of brine's high conductivity, brine volume dominates the loss mechanism in first-year sea ice, and the same was found true for multi-year ice. A two-phase dielectric mixing formula, used by the authors to describe the EM properties of first-year sea ice, was modified to include the effects of the gas pockets found in the multi-year ice. This three-phase mixture model was found to estimate the EM properties of the multi-year ice studied over the frequency band of interest. The latter values were determined by: (1) vertical sounding to a subsurface target of known depth, where the two-way travel time of the EM wavelet in the ice is measured; (2) cross-borehole transmission, where the transit time of the EM wavelet is measured through a known thickness of sea ice; and (3) a wide-angle or common-depth-point reflection method. Preliminary findings also indicate that a representative value for the apparent bulk dielectric constant of multi-year sea ice over 2 1 2 m thick is 3.5. This represents an effective EM wavelet velocity of 0.16 m/ns, which may be used to estimate multi-year sea ice thickness in cases where the ice bottom is detected in ice profile data.