HR: 0800h
AN: NS11A-0157    [Abstracts]
TI: Tracking the Evolution of Sea Ice Properties With In-Situ Dielectric Probes and Cross- borehole Resistivity Tomography
AU: * Pringle, D
EM: pringle@arsc.edu
AF: Arctic Region Supercomputing Center, University of Alaska, Fairbanks, PO Box 756020, Fairbanks, AK 99775, United States
AU: * Pringle, D
EM: pringle@arsc.edu
AF: Geophysical Institute, University of Alaska, Fairbanks, PO Box 757320, Fairbanks, AK 99775, United States
AU: Ingham, M
EM: malcolm.ingham@vuw.ac.nz
AF: School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington, 6001, New Zealand
AU: Eicken, H
EM: hajo.eicken@gi.alaska.edu
AF: Geophysical Institute, University of Alaska, Fairbanks, PO Box 757320, Fairbanks, AK 99775, United States
AU: Dubuis, G
EM: guy.dubuis@epfl.ch
AF: Ecole Polytechnique Federale de Lausanne, 1015, Lausanne, CH-1015, Switzerland
AU: Backstrom, L
EM: larsg@gi.alaska.edu
AF: School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington, 6001, New Zealand
AB: The physical properties of sea ice depend on its temperature, which is easily measured in situ, and salinity, which is typically determined on melted core samples. The ice matrix and the brine inclusions in sea ice have a very large electrical conductivity contrast which makes electrical methods attractive for in-situ salinity measurements. However, the connection between bulk electrical properties and salinity are confounded by the complex, anisotropic and multi-scale microstructure for which only highly simplified models exist. We report on recent measurements of the complex dielectric permittivity at 50 MHz and the DC resistivity of sea ice. Our aims are to develop in-situ salinity measurements and by deriving brine volume fractions (v_b) from concurrent temperature measurements to gain insight into microstructural effects. Specifically we seek to identify evidence of critical transitions in transport properties, such as a sudden onset of brine percolation attributable to non-linear increases in the connectivity of the brine inclusions. Cross-borehole resistivity tomography measurements in sea ice were made over the growth and melt season in landfast first-year sea ice near Barrow, Alaska. 2-D and 3-D inversions resolved the seasonal evolution of the horizontal resistivity (ρ_H) structure with a better resolution than previous, surface-based approaches. When v_b is low, ρ_H obeys Archie's Law with m ≈ 2.9 but a departure to a stronger increase in conductivity is seen for v_b > 7-10 % indicating enhanced horizontal brine inclusion connectivity. Laboratory measurements with 50 MHz Vitel Hydraprobes established conditions for accurate measurements of sea-ice dielectric permittivity. Results from single-crystals show anisotropy with respect to the vertically-oriented, sub-parallel intra-crystalline brine layers. In natural sea ice, the real part of the permittivity (`dielectric constant') can be used to estimate salinity and the imaginary part (`loss factor') shows signatures of brine motion during spring warming once v_b > 5-6 %.
DE: 0770 Properties
DE: 0794 Instruments and techniques
DE: 1863 Snow and ice (0736, 0738, 0776, 1827)
DE: 4540 Ice mechanics and air/sea/ice exchange processes (0700, 0750, 0752, 0754)
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting