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，并加速推动全球海平面上升。然而，未来的冰排放速率仍然不确定，部分原因是对冰床界面上发生的过程的描述不完整。特别是，冰床界面过程敏感地依赖于冰层下的有效压力N(覆盖层压力减去基础水压力)，但对N在空间和时间上如何变化的了解有限，阻碍了将N实际纳入冰流量估计。仅在少数几个位置直接测量了N。海洋声学研究人员提出了一种地震波传播理论，将饱和水颗粒沉积物的N与地震波反射特性联系起来，这种颗粒沉积物类似于南极冰川下普遍存在的冰下碎屑。该项目将进行新颖的实验室实验，以约束和测试该理论，然后根据现有的活动地震数据和实验获得的见解，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活动地震数据，从而有可能在空间和时间上增加N的测绘。这反过来可能导致对冰川和冰盖动力学的更好理解，并最终减少对南极冰盖或TIL所覆盖的任何其他冰块未来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，它很难测量。估算N的常用技术一直是劳动密集型的做法，即直接从钻孔和相连的山口测量N，或从表面速度反演推断N。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饱和水颗粒沉积物的N与冰下条件有关。一个大直径的环形剪切装置将被用来在一系列已知的冰川类型上以受控的N值剪切温带冰，模拟在可变形的冰床上的冰下滑动。环形剪切机将配备一个声音信号产生/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。这些数据将被用来在可以测量孔隙度和颗粒尺寸分布的情况下将反射幅度直接与N相关联。收银机类型将包括终端成员细粒度和过程粒度的收银机，以及为复制威兰冰流而生成的合成收银机。即使发现N对反射幅度只有二阶影响，孔隙度是主导因素，这些实验仍将为从孔隙度解释现有的活动地震数据提供亟需的约束。这些实验的结果将被用来重新分析现有的活跃地震数据，以调查N如何随空间和时间变化。具体地说，这项工作将重新分析在惠兰冰川、坎布冰川和拉特福德冰川上收集的地震数据，这些冰川的粒度分布是从冰下沉积物岩芯样本中得知的。这一项目的成果可以提供一种新的技术，极大地提高我们对冰川下水文学和冰流对冰下有效压力的依赖性的了解。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.