Collaborative Research: Estimating Subglacial Effective Pressure with Active-source Seismic Data

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Many Antarctic glaciers are discharging ice to the sea and contributing to global sea-level rise at an accelerating pace. However, future rates of ice discharged remain uncertain in part because of incomplete characterization of processes occurring at the ice-bed interface. In particular, ice-bed interface processes depend sensitively on the subglacial effective pressure, N (overburden pressure minus basal water pressure), but limited knowledge of how N changes in space and time have inhibited the realistic incorporation of N into ice discharge estimates. N has only been directly measured in a few locations. Marine-acoustics researchers have proposed a seismic-wave propagation theory that relates N of water-saturated granular sediments, similar to the subglacial tills that are prevalent under Antarctic glaciers, to the seismic-wave reflection characteristics. This project will conduct novel lab experiments to constrain and test the theory, then investigate how N varies in space and time in Antarctica from the existing active-seismic data with the insights gained from the experiments. The outcome of this work could be applied to a large volume of existing and future active-seismic data, allowing for the possibility of increased mapping of N both in space and time. This could in turn lead to improved understanding of glacier and ice-sheet dynamics and ultimately reduce uncertainties in future projections of sea-level rise originating from the Antarctic Ice Sheet, or any other ice mass underlain by till. Subglacial effective pressure, N, is one of the key parameters required for estimating glacial motion but is notoriously hard to measure. Common techniques for estimating N have been the labor-intensive practice of measuring it directly from boreholes and connected moulins or inferring it from surface-velocity inversions. This project will test, calibrate and implement the theory of seismic-wave propagation that relates N of water-saturated granular sediments, developed for marine sediments, to subglacial conditions. A large-diameter ring-shear device will be used to shear temperate ice over a range of known till types at controlled N values, simulating subglacial slip over a deformable bed. The ring shear will be outfitted with an acoustic signal generating/sensing system that will allow continuous measurements of the seismic reflection amplitude of the ice-bed interface. These data will be used to relate reflection amplitudes directly to N in a situation where porosity and grain-size distribution can be measured. Till types will include end member fine- and course-grained tills, as well as a synthetic till generated to replicate Whillans Ice Stream. Even if N is found to only have a second-order effect on reflection amplitude and that porosity is the dominant factor, the experiments will still provide a much-needed constraint for interpreting existing active-seismic data in terms of porosity. The findings from the experiments will be used to reanalyze existing active-seismic data to investigate how N varies with space and time. Specifically, this work will reanalyze seismic data collected on Whillans, Kamb and Rutford Ice Streams where grain-size distributions are known from subglacial sediment-core samples. The results of this project could provide a novel technique to greatly increase our understanding of subglacial hydrology and dependency of ice flow on subglacial effective pressure.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.