Donald D. Blankenship, Matthew E. Peters and David L. Morse

The grounding zones of ice streams are a sensitive indicator of ice sheet variability and sea-level change. These dynamic systems involve the interaction of the moving ice mass with the underlying materials, including liquid water, saturated lubricating tills, and rough or frozen bedrock sticky spots. In addition, bottom crevasses result from tidal flexure. Imaging and characterizing the subglacial environment of grounding zones is fundamental to understanding these complex systems. Airborne radar sounding is an increasingly valuable tool for investigations of polar ice sheets and glaciers, especially when studying the basal interface. We present results from airborne radar data acquired over ice stream C, West Antarctica, in 2001 using a uniquely configured airborne radar system. Our focus was on characterizing the basal interface within the grounding zone of this ice stream through radar reflection and scattering analyses. These new results are also used to extend the interpretation of data from regional surveys flown in 1988 over the downstream portions of both ice streams C and Whillans ice stream.

The newly integrated radar system uses a programmable signal source with a dual-channel coherent down-conversion receiver linked to a 10 kW transmitter. The radar operates in chirped pulse mode at 60 MHz and 15 MHz bandwidth. High and low-gain channels allow for recording both weak bed echoes and strong surface echoes simultaneously and without range-dependent gain control. Data acquisition includes integrations of 16 returned radar signals about every 15 cm along-track. Pulse compression and unfocussed synthetic aperture radar (SAR) processing using additional along-track integration were significant components of data analysis. The radar system used for the 1988 surveys operated in pulsed mode at 50 MHz and recorded both SAR (along-track integrated) and individual signal observations every second, or about every 60 m along-track.

Echoes from the basal interface generally consist of both specularly reflected and diffusely scattered energy. Unfocussed SAR narrows the along-track radar beam, thus increasing resolution and the likelihood of specular reflection from the bed. Basal reflection coefficients are computed from these data and used for inferring subglacial materials, most notably regions where significant quantities of liquid water are present immediately beneath the ice. However, echo strength statistics based on reflection and scattering theory show that diffuse scattering can still dominate these echoes. Scattering analysis includes imaging of the basal interface at very short along-track integration distances coupled with echo strength statistics and the rates of change of echo amplitude (fading) and phase with distance. Using the new radar data, we quantify off-nadir scattering from the subglacial interface to infer both the small-scale roughness and the distribution of slopes and facets associated with bottom crevasses and bedrock sticky spots. Finally, we extend these results to the less well-sampled radar surveys over the grounding zones of ice stream C and Whillans ice stream where we complement our previously identified distribution of water and saturated sediments with a distribution of anomalously rough areas that are most likely to contain sticky spots.