Description: We compared elastic moduli in polar firn derived from diving wave refraction seismic velocity analysis, firn-core density measurements and microstructure modelling based on firn-core data. The seismic data were obtained with a small electrodynamic vibrator source near Kohnen Station, East Antarctica. The analysis of diving waves resulted in velocity–depth profiles for different wave types (P-, SH- and SV-waves). Dynamic elastic moduli of firn were derived by combining P- and S-wave velocities and densities obtained from firn-core measurements. The structural finite-element method (FEM) was used to calculate the components of the elastic tensor from firn microstructure derived from X-ray tomography of firn-core samples at depths of 10, 42, 71 and 99 m, providing static elastic moduli. Shear and bulk moduli range from 0.39 to 2.42 GPa and 0.68 to 2.42 GPa, respectively. The elastic moduli from seismic observations and the structural FEM agree within 8.5% for the deepest achieved values at a depth of 71 m, and are within the uncertainty range. Our observations demonstrate that the elastic moduli of the firn can be consistently obtained from two independent methods which are based on dynamic (seismic) and static (tomography and FEM) observations, respectively, for deeper layers in the firn below ∼10 m depth.

Description: The densification of firn depends on the elastic properties of firn, processes which are still not fully explained by the usual models. Geophysical methods provide spatially distributed data, while the analysis of firn cores is restricted to finite locations, but with a different vertical resolution. In this study, we compared elastic moduli in polar firn derived from refraction seismic velocity analysis and vertical density profiles from the firn-core measurements to elastic properties derived from microstructure modelling based on firn-core data. The seismic data were obtained with a small electrodynamic vibrator source (ElViS) near Kohnen Station, East Antarctica. The analysis of divingwaves resulted in velocity–depth profiles for P-, SH- and SV-wave velocities. Elastic moduli of firn were derived by combining P- and S-wave velocities and densities obtained from firn-core measurements. P-wave velocities derived from diving-wave analysis range from 2060 m s−1at 10 m depth to 3400 m s−1at 70 m depth, S-wave velocities from 1250 m s−1 to 1700 m s−1, respectively. The structural finite-element method (FEM) was used to calculate the components of the elastic tensor from firn microstructure derivedfrom X-ray tomography of firn-core samples at depths of 10, 42, 71 and 99 m. Shear and bulk moduli range from 0.39 GPa to 2.42 GPa and 0.68 GPa to 2.42 GPa, respectively. The elastic moduli from seismic observations and the structural FEM agree within 8.5% for the values derived at a depth of 71 m, and are within the uncertainty range. Our study demonstrates that elastic moduli of firn can be consistently obtained from two independent methods, which are based on dynamic (seismic) and static (tomography and FEM) observations, respectively. The agreement of the results for both methods indicates that elastic properties in firn can be acquired as spatially distributed data with the seismic approach, supported by local density information. Thus, information about elastic properties can be derived over larger lateral distances than would be possible with the static method. This enables the analysis of the firn and conclusions of the densification models might be drawn from observations of spatial and temporal changes in elastic properties.

Description: The Antarctic Roadmap Challenges (ARC) project identified critical requirements to deliver high priority Antarctic research in the 21st century. The ARC project addressed the challenges of enabling technologies, facilitating access, providing logistics and infrastructure, and capitalizing on international co-operation. Technological requirements include: i) innovative automated in situ observing systems, sensors and interoperable platforms (including power demands), ii) realistic and holistic numerical models, iii) enhanced remote sensing and sensors, iv) expanded sample collection and retrieval technologies, and v) greater cyber-infrastructure to process ‘big data’ collection, transmission and analyses while promoting data accessibility. These technologies must be widely available, performance and reliability must be improved and technologies used elsewhere must be applied to the Antarctic. Considerable Antarctic research is field-based, making access to vital geographical targets essential. Future research will require continent- and ocean-wide environmentally responsible access to coastal and interior Antarctica and the Southern Ocean. Year-round access is indispensable. The cost of future Antarctic science is great but there are opportunities for all to participate commensurate with national resources, expertise and interests. The scope of future Antarctic research will necessitate enhanced and inventive interdisciplinary and international collaborations. The full promise of Antarctic science will only be realized if nations act together.

Description: In this paper, we present the development of a multi-channel VHF/UHF ultra-wideband airborne radar sounder and imager for measurements of polar ice sheets. The radar was developed at the Center for Remote Sensing of Ice Sheets (CReSIS) for operation onboard the German Alfred Wegener Institute (AWI) Basler BT-67 aircraft. The system operates from 150 to 600 MHz corresponding to a vertical resolution of 33 cm in free space. The radar is equipped with three 4-m long 8-element antenna subarrays installed under the fuselage and both wings to support 8 transmit and 24 receive channels. The radar waveform from each transmit channel can be configured individually to enable real-time transmit beamforming for wide-swath ice bed imaging of up to 10 km wide. The radar system was deployed to Greenland in the spring of 2016 as a part of the joint AWI/CReSIS test campaign to conduct measurements over glaciers. Sample radar data from this field campaign are presented to illustrate the capability of the radar. Index Terms— Airborne radar, remote sensing, ultra-wideband, cryosphere, ice.

Description: Microstructures from deep ice cores reflect the dynamic conditions of the drill location as well as the thermodynamic history of the drill site and catchment area in great detail. Ice core parameters (crystal lattice-preferred orientation (LPO), grain size, grain shape), mesostructures (visual stratigraphy) as well as borehole deformation were measured in a deep ice core drilled at Kohnen Station, Dronning Maud Land (DML), Antarctica. These observations are used to characterize the local dynamic setting and its rheological as well as microstructural effects at the EDML ice core drilling site (European Project for Ice Coring in Antarctica in DML). The results suggest a division of the core into five distinct sections, interpreted as the effects of changing deformation boundary conditions from triaxial deformation with horizontal extension to bedrock-parallel shear. Region 1 (uppermost approx. 450 m depth) with still small macroscopic strain is dominated by compression of bubbles and strong strain and recrystallization localization. Region 2 (approx. 450{\textendash}1700 m depth) shows a girdle-type LPO with the girdle plane being perpendicular to grain elongations, which indicates triaxial deformation with dominating horizontal extension. In this region (approx. 1000 m depth), the first subtle traces of shear deformation are observed in the shape-preferred orientation (SPO) by inclination of the grain elongation. Region 3 (approx. 1700{\textendash}2030 m depth) represents a transitional regime between triaxial deformation and dominance of shear, which becomes apparent in the progression of the girdle to a single maximum LPO and increasing obliqueness of grain elongations. The fully developed single maximum LPO in region 4 (approx. 2030{\textendash}2385 m depth) is an indicator of shear dominance. Region 5 (below approx. 2385 m depth) is marked by signs of strong shear, such as strong SPO values of grain elongation and strong kink folding of visual layers. The details of structural observations are compared with results from a numerical ice sheet model (PISM, isotropic) for comparison of strain rate trends predicted from the large-scale geometry of the ice sheet and borehole logging data. This comparison confirms the segmentation into these depth regions and in turn provides a wider view of the ice sheet.This article is part of the themed issue {\textquoteleft}Microdynamics of ice{\textquoteright}.

Description: Ice cores at a fist glance are almost ideal paleoclimate archives. They not only record temperatures of the past in a relatively direct way, they also keep the atmospheric composition intact. In the end it is only a question how smart we are in using appropriate analytic techniques to unravel the complete atmospheric record – greenhouse gases, aerosols and dust. Another advantage of ice cores is that we can count on very high resolution in time and although we have no direct dating method we can fairly confidently ascribe an absolute time to samples retrieved from a certain depth. From the first ice cores drilled in the sixties until today results from ice cores have helped us to better understand the climate system in particular its inherent global teleconnections. Ice cores are also our climate conscience – they show us without doubt how humankind is altering climate boundary conditions i.e. the greenhouse gas concentrations at an unprecedented rate. In my presentation I will briefly touch on techniques from drilling to analytics and show some of the most important results.

Description: 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).

Description: The large Ice sheets, Greenland and Antartica, are two key players for understanding the future effects of climate change when it comes to sea level rise. A considerable uncertainty is their dynamic response to changing boundary conditions, e.g. increased melting at the surface percolating to their base or warmer ocean water temperatures underneath ice shelves. To decipher ongoing processes and reveal their development, glaciologists often turn to the observation of internal structures in and basal conditions of ice sheets using radio-echo sounding (RES) techniques. Over the last twenty years the radar techniques employed for these observation have seen a considerable improvement, such that today’s ice-penetrating radar has little in common with the former RES techniques, where data were still stored on photographic films. The newest of such systems, in operation since 2016, is AWI’s ultrawideband radar 〈https://www.awi.de/im-fokus/eisschilde/das-neue-awi-eisradar.html〉 , operating in the radio (150-600 MHz) and microwave frequency range, an advancement from the original MCoRDS/I system developed by the Center for Remote Sensing of Ice Sheets (CReSIS). With 24 elements in use for the AWI UWB, the lateral and vertical resolution for imaging the interior of ice sheets at kilometers depth has been brought to the range of the sub-meter scale. This talk introduces the scientific objectives motivating the development of this system, presents its technical aspects, including data recording and processing, and finally shows first results from the last Greenland and Antarctic field seasons.