Ice Core Analysis Benches for Density and dielectric Properties
The initial idea for measuring conductivity of ice cores was: "Volcanic eruptions deposit acids in the ice core, the acid's protons enhance the ice's conductivity and thus the conductance of the ice should be enhanced". The conductance was first measured with the electrical conductivity measurement (ECM) [Hammer, 1980]. Two electrodes spaced a few centimeters are moved along the ice core's freshly microtomized surface. The approx. 1250 V DC voltage between the electrodes drives a current accross the ice core which ic recorded with the depth of the core. The enhanced conductance - i.e. current divided by voltage - spikes can be interpreted in terms of recorded volcanic eruptions and thus provide a handy tool for dating the ice core.
The disadvantage of a required surface preparation and cutting the ice core was eliminated by having the ice core serving as a capacitor's dielectric. This DiElectric Profiling Method (DEP) is applied by moving two curved electrodes along the ice core [Moore & Paren, 1987; Moore, 1989]. The dielectric profiling not only can be used for the determination of acidity, but for sea salt aswell [Moore et. al., 1992] While the DEP's standard application focuses on measuring the ice core's conductivity the benches used at the AWI are optimized for the measurement of the permittivity of the ice aswell [Wilhelms et. al., 1998]. A material's permittivity is by definition the factor by which the free air capacitance of a capacitor is enhanced by filling it with the material. In the ice core application the case is a little more complicated because of air gaps between ice core and electrodes, but the basic principles are taken from a parallel plated capacitor setup. The curved capacitor setup for applications in ice cores follows the same design, but is more complicated to be treated. For some years the dielectric properties are measured in the high frequency range with a scanning frequency of 250 kHz on a first version of a DEP-bench thus being representative for radar frequencies up to the GHz range aswell. The latest version of the combined DEP-gamma-bench is designed to center the core and thus being able to eliminate the influence of the remaining air gaps between core and electrode thus allowing measurements in the low frequency range.
Density of Ice Cores
Besides the standard density measurement by volume and weighing which yields an on the order of dm averaged density, there is a high resolution density measurement by attenuation of a gamma beam applied at AWI. The first densimeter was operated on the GRIP ice core analyses bench. A newer combined DEP-gamma-bench unites both methods on one single bench.
