A Time Domain Reflectometry System for Characterizing the Dielectric Constant Versus Unfrozen Water Content of Glacial Ice

Thinning of the world's ice masses has contributed significantly to sea level rise over the last century and is likely to have a much more significant impact on sea level in the future. Hence, a key challenge is to predict the manner in which ice masses will respond to climate change, using dynamic models. These models require detailed knowledge of glacier mechanical properties, which depend on the proportion and distribution of unfrozen water at ice grain boundaries. Water content and distribution is also the strongest influence on the velocity of electromagnetic (radar) waves in ice, which makes radar a very useful tool in modern glaciology. In order to interpret radar data to give ice properties it is necessary to characterize the relationship between the unfrozen water content within ice, and its dielectric constant. This can be done using laboratory measurements of glacial ice-core dielectric constant coupled with thin section microscopic analysis of ice cores to measure porewater contents. Design and testing of the equipment for characterization of ice core dielectric constant using Time Domain Reflectometry (TDR) is reported here. TDR is a technique developed for characterization of the dielectric constant, and hence the water content, of soils. Conventionally, the TDR waveguides are embedded in the medium under investigation. However, it is difficult to insert TDR electrodes into solids such as rock and ice. To circumvent this difficulty, `press on' TDR waveguides have been developed for use in rock; here we present a modified version for use with ice cores. The problems associated with TDR measurements on glacial ice compared with soils are i) that the unfrozen water content is relatively low, leading to low dielectric constant and short two-way travel times, and ii) that some ice facies have relatively large crystal sizes compared with the sensed volume of conventional probe designs. Several press-on probe designs have been developed to counter these problems and aspects of their performance are reported.