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

许多南极冰川正在向海洋排放冰，并导致全球海平面加速上升。然而，未来的冰排放率仍然是不确定的，部分原因是在冰床界面发生的过程的不完整的特性。特别是，冰床界面过程敏感地依赖于冰下的有效压力，N（覆盖层压力减去基底水压力），但有限的知识如何N在空间和时间上的变化抑制了现实的纳入N到冰流量估计。N仅在少数几个地点直接测量。海洋声学研究人员提出了一种地震波传播理论，该理论将水饱和颗粒沉积物（类似于南极冰川下普遍存在的冰下冰碛）的N与地震波反射特性联系起来。该项目将进行新的实验室实验来约束和测试理论，然后根据实验中获得的见解，从现有的主动地震数据中研究南极洲N在空间和时间上的变化。这项工作的成果可以应用于大量现有和未来的主动地震数据，允许增加映射的N在空间和时间的可能性。这反过来又会导致更好地了解冰川和冰盖动态，并最终减少未来对源自南极冰盖或冰碛下任何其他冰块的海平面上升预测的不确定性。冰下有效压力N是估计冰川运动所需的关键参数之一，但众所周知很难测量。估算N的常用技术是劳动密集型的做法，即直接从钻孔和相连的冰臼中测量，或从表面速度反演中推断。该项目将测试、校准和实施地震波传播理论，该理论将为海洋沉积物开发的水饱和颗粒沉积物的N与冰下条件联系起来。一个大直径的环剪切装置将被用来剪切温带冰在一系列已知的直到类型在受控的N值，模拟冰下滑动的变形床。环剪将配备一个声学信号发生/传感系统，该系统将允许连续测量冰床界面的地震反射幅度。这些数据将用于在孔隙度和粒度分布可以测量的情况下将反射振幅直接与N相关联。冰碛物类型将包括端元细粒度和粗粒度冰碛物，以及为复制Whillans冰流而生成的合成冰碛物。即使发现N仅对反射振幅具有二阶效应，并且孔隙度是主导因素，实验仍将提供解释孔隙度方面的现有主动地震数据的急需的约束。实验结果将用于重新分析现有的主动地震数据，以研究N如何随空间和时间变化。具体来说，这项工作将重新分析收集的地震数据的Whillans，坎布和拉特福德冰流的粒度分布是已知的冰下沉积物的核心样本。该项目的结果可以提供一种新的技术，大大提高我们对冰下水文学和冰流对冰下有效压力的依赖性的理解。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

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.