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

冰从大冰原流入海洋的速度直接影响海平面。驱动这种流动的力量是由日益为人所熟知的冰盖几何形状控制的，但对流动的阻力取决于冰的粘性。冰有一种特殊的性质，它的粘度取决于冰的变形速率。这种灵敏度通常用格伦指数来描述。最近的理论研究表明，我们对格伦指数的了解还不够充分，无法(1)准确预测冰川循环期间海洋冰盖变化的非常基本的结果；（ii）以比目前卫星测量更高的精度预测冰流中地表响应的空间维度。格伦指数可以在实验室或现场测量。实验室测量与现场测量不同，并且很难在现场观察到的低应变率下进行。在许多野外测量中，很难很好地描述应力特征，也很难知道冰的来源如何影响测量结果。我们将去划分那些可以很好地描述应力场和很好地限制物源的位置。雷达层在冰层内提供标记，它们在相对较短时间内的垂直位移可以使用干涉相敏雷达技术测量。这将提供瞬时垂直速度场和应变率场在三分之一到一半的冰原。GPS技术也将用于测量表面应变率，这可以与雷达得出的垂直应变率进行比较。测量将在GRIP和NEEM（格陵兰）进行，这两个地点每隔一年进行一次冰芯钻探。我们进行了一项概念验证研究，在NGRIP（冰层厚度为3000米）进行了为期两周的测量。在这里，我们能够以30%的精度测量小于10^-4/yr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%。在分隔位置，速度场对格伦指数特别敏感，在冰的上部尤其如此。我们将使用全系统建模来确定最适合数据的Glen指数，从而在控制良好的位置测量现场的Glen指数。这些基本上是平面流动实验，流动是二维的。冰的粘度取决于三维流动效应。对三维流动进行测量是不可行的，但流变模型可以用分层法对三维流动进行测试。三联路口，也就是三个山脊交汇的地方，也是控制得很好的地方。我们在南极洲的两个三重路口（伯克纳岛和弗莱彻冰脊）进行了广泛的雷达层测量，并将使用这些数据来约束流场，并确定它们与我们的流变测量结果的一致性。

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.