Radar‐Sounding Characterization of the Subglacial Groundwater Table Beneath Hiawatha Glacier, Greenland

Radar-sounding surveys associated with the discovery of a large impact crater beneath Hiawatha Glacier, Greenland, revealed bright, flat subglacial reflections hypothesized to originate from a subglacial groundwater table. We test this hypothesis using radiometric and hydrologic analysis of those radar data. The dielectric loss between the reflection from the top of the basal layer and subglacial reflection and their reflectivity difference represent dual constraints upon the complex permittivity of the basal material. Either ice-cemented debris or fractured, well-drained bedrock explain the basal layer's radiometric properties. The subglacial reflector's geometry is parallel to isopotential hydraulic head contours, located 7.5–15.3 m below the interface, and 11 ± 7 dB brighter than the ice–basal layer reflection. We conclude that this subglacial reflection is a groundwater table and that its detection was enabled by the wide bandwidth of the radar system and unusual geologic setting, suggesting a path for future direct radar detection of subglacial groundwater elsewhere

Radar absorption, basal reflection, thickness and polarization measurements from the Ross Ice Shelf, Antarctica

Radio-glaciological parameters from the Moore’s Bay region of the Ross Ice Shelf, Antarctica, have been measured. The thickness of the ice shelf in Moore’s Bay was measured from reflection times of radio-frequency pulses propagating vertically through the shelf and reflecting from the ocean, and is found to be 576 ± 8 m. Introducing a baseline of 543 ± 7m between radio transmitter and receiver allowed the computation of the basal reflection coefficient, R, separately from englacial loss. The depth-averaged attenuation length of the ice column, 〈L〉 is shown to depend linearly on frequency. The best fit (95% confidence level) is 〈L(ν)〉= (460±20) − (180±40)ν m (20 dB km−1), for the frequencies ν = [0.100–0.850] GHz, assuming no reflection loss. The mean electric-field reflection coefficient is (1.7 dB reflection loss) across [0.100–0.850] GHz, and is used to correct the attenuation length. Finally, the reflected power rotated into the orthogonal antenna polarization i

Estimation of Ice Basal Reflectivity of Byrd Glacier using RES data

Ice basal reflectivity is much needed for the determination of ice basal conditions and for the accurate modeling of ice sheets to estimate future global mean sea level rise. Reflectivity values can be determined from the received radio echo sounding data if the power loss caused by different components along the two-way transmission of electromagnetic waves is accurately compensated. For the large volume of received radio echo sounding data collected over Byrd glacier in 2011-2012 with a multichannel radar system, the spherical spreading loss caused due to two-way propagation, power reduction due to roughness and relative englacial attenuation is compensated to estimate the relative reflectivity values of the Byrd glacier ice base. In order to estimate the scattered incoherent power component due to roughness, the distributions of echo amplitudes returned from the air-firn interface and from the ice – bed interface are modeled to estimate RMS height variations. The englacial attenuation rate for two-way propagation along the ice depth is modeled using the collected radar data. The estimated air-firn interface roughness parameters are relatively cross verified using Neal’s method and with correlations to the Landsat image mosaic of Antarctica. Estimated relative basal reflectivity values are validated using cross-over analysis and abruptness index measurements. From the Byrd relative reflectivity map, the corresponding echograms at the locations of potential subglacial water systems are checked for observable lake features. The results are checked for correlations with previously predicted lake locations and subglacial flow paths. While the results do not exactly match with the previously identified locations with elevation changes, high relative reflectivity values are observed close to those locations, aligning exactly or close to previously predicted flow paths providing a new window into the subglacial hydrological network. Relative reflectivity values are clustered to indicate the different potential basal conditions beneath the Byrd glacier

Basal Conditions of Petermann Glacier and Jakobshavn Isbrae derived from Airborne Ice Penetrating Radar Measurements

Understanding ice dynamics and ice basal conditions is important because of their impacts on sea level rise. Radio echo sounding has been extensively used for characterizing the ice sheets. The radar reflectivity of the ice bed is of special importance because it can discriminate frozen and thawed ice beds. The knowledge of the spatial distribution of basal water is crucial in explaining the flow velocity and stability of glaciers and ice sheets. Basal echo reflectivity used to identify the areas of basal melting can be calculated by compensating ice bed power for geometric losses, rough interface losses, system losses and englacial attenuation. Two important outlet glaciers of Greenland, Petermann glacier and Jakobshavn isbrae have been losing a lot of ice mass in recent years, and are therefore studied to derive its basal conditions from airborne radar surveys in this thesis. The ice surface and bed roughness of these glaciers are estimated using Radar Statistical Reconnaissance (RSR) method and validated using roughness derived from NASA’s Airborne Topographic Mapper (ATM) and Ku band altimeter. Englacial attenuation is modeled using Schroeder’s variable attenuation method. After compensating for these losses, the basal reflectivity for the two glaciers is estimated and validated using cross over analysis, geophysics, hydraulic potential, abruptive index and coherence index. The areas of basal melting i.e. areas with higher reflectivity are identified. Petermann glacier is found to have alternate frozen and thawed regions explaining the process of ice movement by friction and freezing. Due to the lack of topographic pinning the glacier is subject to higher ice flow speed. Jakobshavn glacier has several areas of basal melting scattered in the catchment area with most concentration near the glacier front which is likely due to surface water infiltration into ice beds via moulins and sinks. The ice bed channels and retrograde slope of this glacier are also important in routing subglacial water and ice mass. The basal conditions of these two glaciers presented in this study can help in modeling the behavior of these glaciers in the future