Dielectric Permittivity Measurements on Glacier Ice Cores Using TDR

Glacier mechanical properties, and hence their response to climatic change, depend strongly on the proportion and distribution of unfrozen water at ice grain boundaries. Water content also 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). Hence, field-based radio- echo sounding (RES) techniques have been used to provide estimates of the amount of unfrozen water present within glacier ice. However, estimates of water content obtained in this way are often of the order of several volumetric percent: much higher than unfrozen water contents measured in ice cores. In order to investigate the source of this discrepancy, we have developed a series of press-on Time Domain Reflectometry (TDR) waveguides specifically for the measurement of the electromagnetic properties of ice cores, and used these to determine the dielectric permittivity of cores taken from the temperate Glacier de Tsanfleuron, Switzerland. Experiments were conducted using two waveguide designs on eight cores from Glacier de Tsanfleuron at temperatures below -2°C where unfrozen water content was low. Cores showed distinct permittivity values related to their composition, but no significant change with temperature. Relatively shallow ice from the accumulation area (`firn') has a relatively low permittivity on account of its substantial air content. Glacier ice from greater depths also showed significant variations in dielectric permittivity, which may be related to variations in ice crystal geometry, sediment content, or water chemistry. These findings mean that caution should be exercised when attributing changes in permittivity, detected in RES, to variations the unfrozen water content of glacier ice alone, without considering compositional factors.