Grounding zone subglacial properties from calibrated active source seismic methods

• Main Authors: Horgan, Huw J., Haastrecht, Laurine, Alley, Richard B., Anandakrishnan, Sridhar, Christianson, Knut, Muto, Atsuhiro
• Format: Text
• Language: English
• Published: 2020
• Subjects: West Antarctica; Whillans; Whillans Ice Stream; Antarc*; Antarctica; Ice Shelf
• Online Access: https://doi.org/10.5194/tc-2020-147; https://tc.copernicus.org/preprints/tc-2020-147/

The grounding zone of Whillans Ice Stream, West Antarctica, exhibits an abrupt transition in basal properties from the grounded ice to the ocean cavity over distances of less than 0.5–1 km. Active source seismic methods reveal the grounded portion of the ice stream is underlain by a relatively stiff substrate (relatively high shear wave velocities) compared to the deformable till found elsewhere beneath the ice stream. Several kilometers upstream of the grounding zone, layers of subglacial water are detected, as are regions that appear to be water layers less than the thickness resolvable by our technique. The presence of stiff subglacial sediment and thin water layers upstream of the grounding zone supports previous studies that have proposed the dewatering of sediment within the grounding zone and the possibility that ocean water is pumped into the subglacial system and upstream. The setting enables calibration of our methodology using returns from the floating ice shelf. This allows a comparison of different techniques used to estimate the sizes of the seismic sources. We find a strong correlation (coefficient of determination = 0.45) between our calibrated method and a commonly used amplitude ratio method, but our results also highlight the incomplete knowledge of other factors affecting the amplitude of seismic sources and reflections in the cryosphere.