HR: 14:45h
AN: NS43A-06    [Abstracts]
TI: Dielectric Properties of Ice-Water Systems: Laboratory Characterization and Modeling
AU: * West, J
EM: jared@earth.leeds.ac.uk
AF: Institute of Earth and Biosphere, School of Earth and Environment, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT United Kingdom
AU: Rippin, D M
EM: D.M.Rippin@leeds.ac.uk
AF: School of Geography, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT United Kingdom
AU: Endres, A L
EM: alendres@sciborg.uwaterloo.ca
AF: Department of Earth Sciences, University of Waterloo, Waterloo, ON ON N2L 3G1 Canada
AU: Murray, T
EM: tavi@geography.leeds.ac.uk
AF: School of Geography, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT United Kingdom
AB: Glacier mechanical properties, and hence their response to climatic change, depend strongly on the proportion and distribution of unfrozen water at ice grain boundaries. Glaciologists have characterized unfrozen water content in several ways, notably via thin section microscopic analysis of ice cores to measure porewater contents, and field surveys of electromagnetic properties using radar. Water content has a very strong influence on the velocity of electromagnetic (radar) waves in ice, because of the high dielectric constant of water (~80) in comparison with ice (~3). However, there is a strong discrepancy between the two methods of measurement, with field radar surveys on glaciers giving unfrozen water contents of several volumetric percent, whereas ice-core microscopy gives values of less than one percent. This discrepancy has called into question the approach used to obtain the unfrozen water content from radar wave velocity. This approach assumes that the ice-water mixture is a lossless medium. Here, we report a laboratory and modeling based investigation of the relationship between dielectric properties and unfrozen water content of ice cores from the Glacier de Tsanfleuron, Switzerland, aimed at resolving the discrepancy. The laboratory study uses the technique of Time Domain Reflectometry to characterize the dielectric properties of ice cores from a range of ice facies. `Press on' TDR waveguides have been developed specifically for use on ice cores. Several press-on probe designs have been developed and aspects of their performance are reported. An independent estimate of unfrozen water content is determined from temperature and total pore fluid ionic strength. The results allow the establishment of relationships between the high frequency (~500MHz) dielectric properties and water content for various ice-crystal geometries. Mathematical modeling of the dependence of dielectric constant on frequency and water phase conductivity has been undertaken using Effective Medium Theory (EMT). The water phase in glacial ice has very high conductivity, despite the overall low ionic strength of glacier water, because dissolved ions preferentially remain in the liquid water phase during freezing. The EMT approach shows that the high conductivity of the water phase increases the apparent dielectric constant of the mixture at radar frequencies (~100MHz). It is proposed that this conductivity effect leads to the erroneously high water contents reported from field radar surface using conventional dielectric mixture relationships.
DE: 1827 Glaciology (1863)
DE: 1863 Snow and ice (1827)
DE: 1894 Instruments and techniques
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly