Grounding zone subglacial properties from calibrated active-source seismic methods

Abstract

Abstract. 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 downglacier-most grounded portion of the ice stream is underlain by a relatively stiff substrate (relatively high shear wave velocities of 1100±430 m s−1) compared to the deformable till found elsewhere beneath the ice stream. Changes in basal reflectivity in our study area cannot be explained by the stage of the tide. Several kilometres upstream of the grounding zone, layers of subglacial water are detected, as are regions that appear to be water layers but are 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 trapping of subglacial water upstream of the ocean cavity. 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, a constraint essential for the accurate recovery of subglacial properties. We find a strong correlation (coefficient of determination=0.46) between our calibrated method and a commonly used multiple-bounce method, but our results also highlight the incomplete knowledge of other factors affecting the amplitude of seismic sources and reflections in the cryosphere.

Sensitivity of the Ross Ice Shelf to environmental and glaciological controls

“…(2021) found relatively stiff till at the grounding line of Whillans Ice Stream compared to the main ice trunk. This stiffer till at 160 the grounding line thickens the ice and stabilizes the position of the grounding line (Joughin et al, 2005;Anandakrishnan et al, 2007;Horgan et al, 2021). A softening of the till at the grounding line through basal friction changes would change the position of the grounding line and discharge rates.…”

The Subglacial Lake That Wasn't There: Improved Interpretation From Seismic Data Reveals a Sediment Bedform at Isunnguata Sermia

“…Horgan et al. (2021) investigated several methods concluding the multiple bounce method is preferable. We compared two methods, one quantifying the exponential decay of the direct wave and extrapolating this to the source location with the multiple bounce method.…”

“…Determination of A 0 can be done by several methods: either using the exponential decay of the direct wave (direct wave method) or comparing the primary and first multiple reflection (Holland & Anandakrishnan, 2009) (multiple bounce method). Horgan et al (2021) investigated several methods concluding the multiple bounce method is preferable. We compared two methods, one quantifying the exponential decay of the direct wave and extrapolating this to the source location with the multiple bounce method.…”

Antarctic Sedimentary Basins and Their Influence on Ice‐Sheet Dynamics

“…Acoustic-impedance measurements along profiles can be used to discriminate between regions of hard bedrock from sediments or water at the bed. Of particular significance is the ability to discriminate different structures associated with tills and tillites that have a direct link to subglacial processes at the bed (Anandakrishnan et al, 1998;H. J. Horgan et al, 2021;Muto et al, 2016Muto et al, , 2019bPeters et al, 2006;A.…”