Seismic characterisation of subglacial conditions beneath the margin of the West Greenland Ice Sheet

格陵兰冰盖对气候变暖的反应对于海平面上升的模型具有重要意义。因此，它受到媒体、公众和政府决策的极大关注。然而，它的行为受到一些复杂过程的影响，这些过程之间的相互作用尚不清楚。因此，冰盖是主要科学研究活动的焦点，探索对其稳定性的关键影响以及未来如何演变。最近的研究表明，水对冰盖稳定性有重要影响。存在于冰盖底部的水充当冰和其下物质之间的润滑剂，使冰以更快的速度移动。对于西格陵兰冰盖，基底水的来源储存在表面融水湖泊中。这些在夏季融化季节开始时形成，在6月和7月，通常在7月和8月经历一次或多次排水事件。据信，它们所含的水通过冰中的裂缝网络排出，最终到达冰盖的底部并引起动态响应。这些湖泊的排水是相当突然的（在一个例子中，一个面积约6平方公里的湖泊在2小时内被排干），因此大量的水可以在短时间内被引入冰盖的床。某些科学团体已经观察到融水湖的排水，并在不久之后发现当地冰动力的增加;这显然意味着融水到达冰盖床的理论是正确的。然而，融水的输送从未被直接成像;它只是从其他数据集的响应中推断出来的。在本提案中，我们打算在地震地球物理数据中探测、描绘和量化冰盖底部的融水通道，这些数据在地球科学和商业实践的其他领域中广泛用于此目的。我们提出的地球物理技术已被广泛应用于估计地下流体含量。水改变了地下的物理性质，地震能量对地下的物理性质特别敏感。因此，我们打算在7月和8月的整个融化和排水季节，利用地球物理学不断监测冰盖的底部。我们预计，我们的地震数据将显示基础水含量逐渐增加，因为越来越多的融水湖在调查现场附近排水。我们的特定现场位于一组湖泊和冰臼（垂直或近垂直的开口，便于将水排入冰体内）之间，并直接位于先前显示的冰盖区域上方，以响应湖泊排水事件。因此，我们相信，如果这种反应是由于融水进入冰盖床，我们的地球物理调查将能够探测到融水。我们的观察将得到一系列已经在现场进行的补充科学项目的结果的补充。这包括，例如，空中和地面雷达、被动地震和全球定位系统评估或监测，以及一个新的地表融化耦合动态反馈的流域尺度模型。融水是对冰的动态的重要影响，随着全球变暖的持续，在未来可能变得更加重要。因此，了解这种影响对于预测格陵兰冰盖将如何应对气候变化至关重要，而这项研究期间的观测无疑将有助于我们了解冰动力学。

The response of the Greenland Ice Sheet to a warming climate is of great significance for models of sea-level rise. As such, it receives significant attention from the media, among the general public and in government policy decisions. However, its behaviour is influenced by a number of complex processes, the interactions between which are not well understood. The ice sheet is therefore a focus of major scientific research campaigns, exploring the key influences on its stability and how it will evolve in the future. Recent research has suggested that water is a significant influence on ice sheet stability. Water that is present at the base of the ice sheet acts as a lubricant between the ice and the material beneath it, allowing the ice to move at a faster velocity. For the West Greenland Ice Sheet, a source of basal water is stored in surface meltwater lakes. These form at the start of the summer melt season, in June and July, and typically undergo one or more drainage events throughout July and August. It is believed that the water they contain drains through a network of cracks in the ice, eventually reaching the base of the ice sheet and causing a dynamic response. The drainage of these lakes is rather sudden (in one example, a lake with an area around 6 km2 was drained in under 2 hours) hence a large volume of water can be introduced to the bed of the ice sheet in a short space of time. Certain scientific groups have observed the drainage of a meltwater lake, and have detected increases in local ice dynamics very soon after; this clearly implies that the theory of meltwater reaching the bed of the ice sheet is correct. However, the delivery of meltwater has never been imaged directly; it has only been inferred from responses in other datasets. In this proposal, we intend to detect, delineate and quantify the passage of meltwater at the bed of the ice sheet in seismic geophysical data, which are widely used for this purpose in other areas of the earth sciences and commercial practice. The geophysical techniques we propose have been extensively applied to estimating the fluid content in the subsurface. Water modifies the physical properties of the subsurface to which seismic energy is particularly sensitive. We therefore intend to use geophysics to continually monitor the base of the ice sheet, throughout the melt-and-drainage season in July and August. We anticipate that our seismic data would show the basal water content to increase gradually, as an increasing number of meltwater lakes drain in the vicinity of the survey site. Our specific field site is located amongst a cluster of lakes and moulins (vertical or sub-vertical openings facilitating water drainage into the ice body), and located directly over an area of the ice sheet previously shown to respond to lake drainage events. We are therefore confident that if that response is due to the delivery of meltwater to the bed of the ice sheet, that meltwater will be detectable by our geophysical survey. Our observations will be complemented by a range of results from complementary scientific projects already underway at the site. This includes, e.g., airborne and ground-based radar, passive seismic and GPS assessment or monitoring, and a new catchment-scale model of surface-melt coupled dynamic feedbacks. Meltwater is an important influence on ice dynamics that, with continued global warming, is likely to become more significant in the future. Understanding that influence is therefore crucial for predicting how the Greenland Ice Sheet will respond to climate change, and the observations made during this study will undoubtedly contribute to our knowledge of ice dynamics.