Description: Over the last years AWI implemented a new ultra-wideband radar, developed by the Center for Remote Sensing of Ice Sheets (CReSIS), on AWI's polar airplane Polar 6, a Basler BT-67. After terminating the overall system tests and calibration/validation survey in 2015, the system has been in operational use in Greenland and Antarctica for several field seasons. We will present an overview of the results and experiences obtained over the last two years to illustrate the system performance in terms of achievable specifications for imaging and sounding ice sheets, and will discuss the requirements and opportunities for logistic deployment in Greenland as well as Antarctica.

Description: Fast flowing ice streams are known to contribute in a large proportion to continental ice sheet discharge and contribute therefore to sea level variability. In this study we investigate internal layering, stratigraphy and bedrock topography of an active ice stream in North East Greenland. In conjunction with data of physical properties of the EastGRIP Ice Core it enables us to expand insights from a selective data set over a broader area to gain insights into the complex mechanisms governing ice streams. We acquired airborne radar data at the North East Greenland Ice Stream in the vicinity of EastGRIP Drill Site. The data has been recorded in May 2018 in different acquisition modes with AWI’s Ultra-wideband multichannel radar installed on the AWI Polar6 Basler BT-67 aircraft. A total area of 16000 km^2 has been mapped with profiles along and perpendicular to ice flow direction. The area in direct vicinity to the drill site is has been covered with a profile spacing of 5 km, further downstream and upstream the profile distance is10 km. The survey area reaches from 150 km upstream to 150 km downstream of the drilling sites, and also includes both shear margins and parts of the slow flowing areas adjacent to the ice stream. The data have been processed and are currently evaluated for bedrock topography and 3D shape of distortion of internal layers. Our high resolution radargrams provide detailed insights into the bedrock topography, including the ~500 m step just upstream of the drill site and flow parallel bed ridges within the ice stream. Moreover, we mapped the distortion of internal reflectors within the shear margins of the ice stream in great detail, and captured the radar stratigraphy signature of a shift in the north-western shear margin. Outside of the ice stream large scale fold structures parallel to the main trunk of NEGIS were mapped.

Description: Antarctic ice shelves often contain narrow, curvilinear tracts of thin ice, termed ice-shelf channels, that impact ice-shelf stability. Their surface depressions appear prominently in satellite imagery and form an interesting morphology of unknown origin, the more so because the processes leading to ice-shelf channel formation are unclear. Here we investigate the origin of ice-shelf channels at the Roi Baudouin Ice Shelf, which have previously been attributed to ice overriding sediment ridges formed by long-term deposition in subglacial water conduits. However, due to a limited radar dataset at the time, the shape and upstream extent of the basal obstacles was unclear, making it difficult to pinpoint the exact type of the subglacial landform. In 2019, we revisted this location with an improved airborne ultra-wideband radar and collected a number of across-flow profiles upstream of the grounding line. Consistent with previous suggestions, we find that the basal obstacles shrink in size with increasing distance from the grounding line until they are invisible about 10 km upstream. However, the change in size is gradual in the first 8 km, and then very abrupt in the last 2 km. Variations in basal reflectivity indicate the existence of patches of subglacial water in all profile lines. Once fully evaluated, this rich dataset has the potential to classify the basal landform and hence shed light on the origin of ice-shelf channels and their impact on ice-shelf stability.

Description: In connection with the East Grip ice core drilling project airborne and groundbased multichannel radar systems were used to look in detail at the internal structures inside and outside of the Northeast Greenland Icestream. While the icestream margins surface expression is just delineated by slight depressions their internal structures are characterized by rather complicated multifold elements. These features hopefully can be used to better characterize the dynamics inherent in icestream systems.

Description: Ice streams are fast moving regions within the large ice sheets of Greenland and Antarctica. Recent developments of high resolution ice sounding radar systems for deployment from an aircraft or on the ground allow a detailed view of internal structures associated with the particular stress and strain fields related to the particular flow fields across the margins between slow and fast moving ice. Exemplary data will be shown from the margins of the North-East Greenland ice stream which were obtained in association with the EASTGRIP icecore drilling project.

Description: Drilling an ice core through an ice sheet (typically 2000 to 3000 m thick) is a technical challenge that nonetheless generates valuable and unique information on palaeo-climate and ice dynamics. As technically the drilling cannot be done in one run, the core has to be fractured approximately every 3 m to retrieve core sections from the bore hole. This fracture process is initiated by breaking the core with core-catchers which also clamp the engaged core in the drill head while the whole drill is then pulled up with the winch motor. This standard procedure is known to become difficult and requires extremely high pulling forces (Wilhelms et al. 2007), in the very deep part of the drill procedure, close to the bedrock of the ice sheet, especially when the ice material becomes warm (approximately -2°C) due to the geothermal heat released from the bedrock. Recently, during the EastGRIP (East Greenland Ice coring Project) drilling we observed a similar issue with breaking off cored sections only with extremely high pulling forces, but started from approximately 1800 m of depth, where the temperature is still very cold (approximately -20°C). This has not been observed at other ice drilling sites. As dependencies of fracture behaviour on crystal orientation and grain size are known (Schulson & Duval 2009) for ice, we thus examined the microstructure in the ice samples close to and at the core breaks. First preliminary results suggest that these so far unexperienced difficulties are due to the profoundly different c-axes orientation distribution (CPO) in the EastGRIP ice core. In contrast to other deep ice cores which have been drilled on ice domes or ice divides, EastGRIP is located in an ice stream. This location means that the deformation geometry (kinematics) is completely different, resulting in a different CPO (girdle pattern instead of single maximum pattern). Evidence regarding additional grain-size dependence will hopefully help to refine the fracturing procedure, which is possible due to a rather strong grain size layering observed in natural ice formed by snow precipitation. --------------------- Wilhelms, F.; Sheldon, S. G.; Hamann, I. & Kipfstuhl, S. Implications for and findings from deep ice core drillings - An example: The ultimate tensile strength of ice at high strain rates. Physics and Chemistry of Ice (The proceedings of the International Conference on the Physics and Chemistry of Ice held at Bremerhaven, Germany on 23-28 July 2006), 2007, 635-639 Schulson, E. M. & Duval, P. Creep and Fracture of Ice. Cambridge University Press, 2009, 401

Description: The internal layer architecture and crystal anisotropy in ice are two main parameters to take into account for analysing particle signals within the IceCube and follow-on arrays. Using airborne and ground-based radar measurements we present a new approach to especially define the horizontal anisotropy of the bulk fabric in ice and suggest implementation during pre-site surveys in particular for the RNO-G test array near Summit, Greenland.

Description: Smith-Johnsen et al. (The Cryosphere, 14, 841–854, https://doi.org/10.5194/tc-14-841-2020, 2020) model the effect of a potential hotspot on the Northeast Greenland Ice Stream (NEGIS). They argue that a heat flux of at least 970 mW m−2 is required to have initiated or to control NEGIS. Such an exceptionally high heat flux would be unique in the world and is incompatible with known geological processes that can raise the heat flux. Fast flow at NEGIS must thus be possible without the extraordinary melt rates invoked in Smith-Johnsen et al. (2020).

Description: Advances in radio-echo sounding technology over the last two decades made it possible to map complex englacial structures in the lower part of the Greenland and Antarctic Ice Sheet. Deformation structures are made visible by distorted isochrones acting as radar reflectors. Decoding the formation history of these structures offers an excellent possibility to reconstruct past ice movements, and thus provides an additional archive about processes on the earth's surface in the past. In this study, we use ultra-wideband ice-penetrating radar data to map the deformation of the radar stratigraphy in Northern Greenland. We construct 3-dimensional horizons from folded radar layers of features which show no apparent link to the current velocity field or the regional bed topography. Furthermore, we are able to constrain the geometry and spatial extend of folds, which suggests that they were formed in several stages and in a different ice-dynamic setting than the present one in Northern Greenland.