Measuring and modelling the Raymond Effect for to infer low strain-rate ice rheology

冰从大冰原流入大海的速度直接影响海平面。驱动这种流动的力是由日益众所周知的冰盖的几何形状控制的，但流动阻力取决于冰的粘性。冰有一种特殊的性质，即粘性取决于冰变形的速度。这种敏感性通常用Glen指数来描述。最近的理论研究表明，我们对格伦指数的了解还不足以（i）准确地预测冰川循环期间海洋冰盖变化的基本结果;（ii）预测冰流中表面响应的空间维度，比目前的卫星测量更准确。格伦指数可以在实验室或现场测量。实验室测量偏离现场测量，并且在现场观察到的低应变率下非常难以进行。在许多野外测量中，很难很好地测量应力，也很难知道冰的来源如何影响测量。我们将划分应力场可以很好地表征的位置，并且物源受到很好的限制。雷达层提供了冰内的标记，它们在相对较短的时间内的垂直位移可以使用干涉相敏雷达技术来测量。这将提供瞬时的垂直速度场和应变率场在上三分之一到一半的冰领域。全球定位系统技术还将用于测量表面应变率，可与雷达测得的垂直应变率进行比较。测量将在GRIP和NEEM（格陵兰）进行，这两个地点是冰芯钻探的地点，每隔一年进行一次。我们已经进行了一项概念验证研究，在NGRIP（3000米厚的冰）进行了两周的时间间隔测量。在这里，我们能够以30%的精度测量小于10^-4/年的垂直应变率。一年的时间间隔将大大提高准确性，可能低于5%。在分水岭位置的速度场是特别敏感的格伦指数，这是特别是在上部的冰的情况下。我们将使用全系统建模来确定最适合数据的格伦指数，从而在控制良好的位置测量现场的格伦指数。这些基本上是平面流动实验，其中流动是二维的。冰的粘性取决于三维流动效应。对三维流动进行测量是不可行的，但可以使用分层法对三维流动的流变模型进行测试。三个分水岭交汇的三重交汇点同样是控制良好的位置。我们有广泛的雷达层测量在南极洲的两个三重路口（伯克纳岛和Flecher冰隆），并将使用这些来约束流场，并确定其一致性，我们的流变测量。

The rate at which ice flows into the sea from the large ice-sheets directly affects sea-level. The forces which drive this flow are controlled by the increasingly well-known geometry of the the ice-sheets, but the resistance to flow depends upon the viscous properties of ice. Ice has the peculiar property that the the viscosity depends upon the rate at which the ice is deforming. This sensitivity is usually described with the Glen index. Recent theoretical studies have shown that our knowledge of the Glen index is not sufficiently well known to (i) accurately predict very basic outcomes of marine ice-sheet change during glacial cycles; and (ii) predict the spatial dimensions of surface response in ice-streams to a better accuracy than current satellite measurements. The Glen index can be measured in the laboratory or the field. Laboratory measurements diverge from field measurements, and are very difficult to make at the low strain-rates observed in the field. In many field measurements it is difficult to characterise the stresses very well and to know how the provenance of the ice has affected measurements. We will go to divide locations where the stress field can be characterised well and the provenance is very well constrained. Radar layers provide markers within the ice, and their vertical displacement over relatively short time periods can be measured using interferometric phase-sensitive radar techniques. This will provide instantaneous vertical velocity fields and strain-rate fields in the upper third to a half of the ice field. GPS techniques will also be used to measure surface strain-rates, which can be compared with the vertical strain-rates derived from the radar. Measurements will be made at GRIP and NEEM (Greenland), sites for ice-core drilling at intervals of one year. We have carried out a proof-of-concept study making measurements with a time interval of a fortnight at NGRIP (a 3000m thickness of ice). Here we were able to measure vertical strain-rates of less than 10^-4/yr with 30% accuracy. Time intervals of a year will improve the accuracy considerably, likely below 5%. At divide locations the velocity field is especially sensitive to the Glen index, and this is particularly the case in the upper part of the ice. We will use full-system modelling to determine the Glen index which best fits the data, and thereby measure the Glen index in the field in a well-controlled location. These are essentially plane-flow experiments, where flow is two-dimensional. The viscosity of ice is dependent on three-dimensional flow effects. It is not feasible to measure carry out the procedure for three-dimensional flows, but rheological models can be tested for three dimensional flows using layering. Triple junctions, where three divide ridges meet, are similarly well-controlled locations. We have extensive radar layer measurements from two triple junctions in Antarctica (Berkner Island and Flecher Ice Rise), and will use these to constrain the flow field and determine their consistency of the our rheological measurements.