ALBERT

Beschreibung: The European Project for Ice Coring in Antarctica (EPICA) ice core was drilled between 2001 and 2006 at the Kohnen Station, Antarctica. During the drilling process the borehole was logged repeatedly. Repeated logging of the borehole shape is a means of directly measuring the deformation of the ice sheet not only on the surface but also with depth, and to derive shear strain rates for the lower part, which control the volume of ice transported from the inner continent towards the ocean. The logging system continuously recorded the tilt of the borehole with respect to the vertical (inclination) as well as the heading of the borehole with respect to magnetic north (azimuth) by means of a compass. This dataset provides the basis for a 3-D reconstruction of the borehole shape, which is changing over time according to the predominant deformation modes with depth. The information gained from this analysis can then be evaluated in combination with lattice preferred orientation, grain size and grain shape derived by microstructural analysis of samples from the deep ice core. Additionally, the diameter of the borehole, which was originally circular with a diameter of 10 cm, was measured. As the ice flow velocity at the position of the EDML core is relatively slow (about 0.75 m/a), the changes of borehole shape between the logs during the drilling period were very small and thus difficult to interpret. Thus, the site has been revisited in the Antarctic summer season 2016 and logged again using the same measurement system. The change of the borehole inclination during the time period of 10 years clearly reveals the transition from a pure shear dominated deformation in the upper part of the ice sheet to shear deformation at the base. We will present a detailed analysis of the borehole parameters and the deduced shear strain rates in the lower part of the ice sheet. The results are discussed with respect to ice microstructural data derived from the EDML ice core. Microstructural data directly reflect the deformation conditions, as the ice polycrystal performs the deformation which leads e.g. to characteristic lattice orientation distributions and grain size and shape appearance. Though overprinted by recrystallization (due to the hot environment for the ice) and the slow deformation,analysis of statistically significant grain numbers reveals indications typical for the changing deformation regimes with depth. Additionally we compare our results with strain rates derived from a simulation with a model for large scale ice deformation, the Parallel Ice Sheet Model (PISM).

Beschreibung: Dielectric profiling (DEP) is a fast, non-destructive method to precisely scan the dielectric properties of an ice core in as high depth resolution as a few millimetres. Initially being proposed to acquire conductivity profiles, the method has been extended to also interpret relative permittivity and ultimately determine the firn’s density and its pure ice phase’s conductivity by inversion of the measured properties with the specifically developed mixing model DECOMP. The conductivity profile itself exhibits time markers from volcanic eruptions and facilitates, by integrating the density along depth, the calculation of average accumulation rates in between these time markers. The modelling of synthetic radar traces is another application of dielectric profiling data, that establishes a high precision link between the icecore and geophysical ground penetrating radar surveys in the vicinity of the drilling site. In the light of these applications the aim to log the dielectric properties directly in the borehole has been out there for a few decades. Mainly motivated in acquiring records without missing data due to core breaks and to even better extent the depth scale beyond sections of possibly missing core sections. Along with the development of rapid access drilling, where no ice core but only cuttings are taken, there is an even more urgent desire to acquire the high quality electrical data directly from in-situ measurements in the borehole. The traditional DEP method, as described in the literature, uses commercial auto-balancing-bridge devices to measure the electrical circuit properties of the DEP capacitor with the icecore as a dielectric filling. The commercial devices typically have 19” rack size dimensions and extensive circuit design would have been required to adapt the electronics to fit into a typically 100 mm diameter tube of a borehole logger. We developed a lock-in amplifier based electronics with a small circuit layout and successfully tested it in an icecore DEP device . We will inter-compare the records with the results of an auto-balancing-bridge based measurement and estimate the expected measurement performance of our new lock-in amplifier circuit. The electrostatic theory to determine the borehole-wall ice properties has been described before and allows to predict the precision of the ice parameters as measured in a borehole log. We will also lay out the mechanical and electrical design to build and operate a borehole DEP in a dry and a liquid filled hole.