Dielectric permittivity measurements on ice cores: Implications for interpretation of radar to yield glacial unfrozen water content

摘要

The dielectric permittivity of glaciers and ice caps can be measured in field or airborne surveys using ice-penetrating radar. Permittivity contrasts in polar ice caps indicate ice stratigraphy and age, whereas those in temperate glaciers have been interpreted as changes in unfrozen water content, which is an important control on glacier mechanics. Many previous workers have assumed simple relationships between permittivity (inferred from radar velocity) and unfrozen water content, but these relationships have never been verified in the laboratory. Here, we present measurements of the dielectric properties of ice cores from a temperate glacier in Switzerland, using the Time Domain Reflectometry (TDR) technique, which has a measurement frequency close to that of radar. The objectives of the measurement were to quantify the effects of intercrystalline unfrozen water and air content on ice dielectric permittivity. TDR probes were specially designed and built for ice core measurement to allow them to be pressed onto the ice core surface, and to maximize the signal travel time. The dielectric permittivity of the ice cores was primarily dependent on air content. Most cores had dielectric permittivities of between 3.1 and 3.3, which correspond with literature values for air-free ice above the Debye dispersion frequency (which is typically in the kHz range). Ice cores from accumulation areas of the glacier, from ice which had not been deeply buried, showed lower dielectric permittivity values owing to significant air content. The effects of unfrozen water present within the intercrystalline vein system on dielectric permittivity were too small to detect at temperatures below -2 degrees C. This is in accord with theoretical predictions, which suggest unfrozen water contents of 1% for freshwater ice at such temperatures. Significant water contents inferred from radar surveys of glaciers must indicate well rotted ice at the pressure melting point or the presence macroscale water bodies.