Fractures and fabrics in glacier ice: Sensitivity of seismic anistropy for antarctic ice masses

要准确预测南极冰对海平面上升的影响，就需要对冰的动态如何演变作出可靠的估计。这发生在大气、冰和海洋系统之间一系列复杂的反馈中。对于冰系，我们需要了解内部应力状态。预计冰川流动将在更温暖的温度条件下加速，导致冰体内的应力状态变化。地球物理学是估算现今应力状态的一种有价值的手段，因此，它可以提供给未来冰体演化的预测计算模型。冰晶本身是强各向异性的，因此，当应力状态导致晶体排列时，形成大块各向异性织物。对于这样的组构，地震能量在垂直于地层传播时比沿沿着地层传播时传播得更快。这就定义了垂直横向各向同性（VTI）的状态，在这种状态下，观测到的速度变化只与入射角的垂直度有关，而与方位角无关。当存在优先排列的裂缝时，也可能出现各向异性。地震能量通过裂缝带传播的速度比通过完整冰层传播的速度要慢。这定义了水平横观各向同性（HTI），其中速度随相对于裂缝方向的方位角而变化。这些制度都是快速冰川流动的指标，因此，重要的是要制定有效的战略，监测这些regiments.Ice的各向异性的发展，可以从表面地震反射数据的特点，但一直很少有分析的灵敏度的采集几何形状，以不同的各向异性制度。考虑这一点尤其重要，因为外地后勤往往需要一个折衷的采购战略。建立可靠的模型englacial各向异性织物，并模拟地震响应，他们将导致一套地震采集准则，为任何给定的glacier target.ObjectivesIn这个项目中，你将考虑建模和可探测性的各向异性织物在两个特定的南极冰块，这两个被认为是至关重要的区域冰的稳定性。- HTI织物将探讨与强度的基础裂缝在拉森C冰架，在南极半岛。考虑到2017年冰山崩解与更宽大陆架的稳定性之间的潜在联系，拉森C的应力状况令人感兴趣。- 将模拟西南极冰盖的主要出口Thwaites冰川的流态VTI织物。具体来说，这些调查将集中在冰川的剪切边缘，这标志着快速冰川流动的开始。对各向异性进行可靠的测量将有助于了解对快速冰川流动的控制，将利用波动方程的离散裂缝公式建立各向异性模型，并在WAVE软件中实施;将把这些模型与真实的地震数据中的各向异性特征进行比较。拉森C冰架的方位地震数据集档案已经存在。Thwaites Glacier的数据将在两个实地活动中获得，您将于2022年加入，以记录新的三维地震反射数据。您将在利兹科学家团队的指导下开展该项目，他们是冰川学，地震建模和各向异性分析方面的领先专家。本项目的具体目标包括但不限于：1.利用地震模拟方法模拟各向异性冰组构和裂缝的响应.评估地震响应对冰组构和裂缝强度以及地震采集几何结构的敏感性。Larsen C冰架和Thwaites冰川新的现场数据分析。在后一种情况下，这种处理

Accurate predictions of the contribution of Antarctic ice to sea-level rise require reliable estimates of how ice dynamics will evolve. This takes place within a complex set of feedbacks between atmosphere, ice and ocean systems. For the ice system, we require knowledge of the internal stress regime. Glacier flow is expected to accelerate under warmer temperature regimes, leading to stress-state changes within the ice mass. Geophysics is a valuable means of estimating the present-day stress regime, such that it can then be supplied to predictive computational models of future ice mass evolution.Englacial stress-states can be measured from seismic anisotropy. The ice crystal is itself strongly anisotropic hence, a bulk anisotropic fabric is formed when a stress regime causes crystals to align. For such fabrics, seismic energy will propagate more quickly when travelling orthogonal to the layer, than when travelling along it. This defines a regime of vertical transverse isotropy (VTI), in which the observed variation of velocity is only with the obliquity of the incidence angle, rather than with azimuth.Anisotropy may also arise when there are preferentially-aligned crevasses. Seismic energy propagating through a crevassed zone will travel more slowly than that travelling through intact ice. This defines horizontal transverse isotropy (HTI), where velocity varies with the azimuth relative to the crevasse orientation. Each of these regimes is an indicator of fast glacier flow, hence it is important to develop effective strategies for monitoring the development of these regimes.Ice anisotropy can be characterised from surface seismic reflection data, however there has been little analysis of the sensitivity of the acquisition geometry to varying anisotropic regimes. This is especially important to consider since field logistics often demand a compromised acquisition strategy. Building reliable models of englacial anisotropic fabrics, and simulating the seismic response to them, will lead to a set of seismic acquisition guidelines for any given glacier target.ObjectivesIn this project, you will consider the modelling and detectability of anisotropic fabrics in two specific Antarctic ice masses, both of which are considered critical for regional ice stability. - HTI fabrics will be explored with relation to the intensity of basal crevassing in Larsen C Ice Shelf, on the Antarctic Peninsula. The stress regime of Larsen C is of interest given the potential link between the iceberg calving in 2017 and the stability of the wider shelf. - VTI fabrics will be simulated for flow regimes of Thwaites Glacier, a major outlet of the West Antarctic Ice Sheet. Specifically, these investigations will be focused around the shear margin of the glacier, which marks the onset of fast glacier flow. A robust measurement of anisotropy will contribute to the understanding of the controls on fast glacier flow.Anisotropy models will be developed using a discrete fracture formulation of the wave equation, implemented in WAVE software; these models will be compared against the signatures of anisotropy in real seismic data. An archive of azimuthal seismic datasets already exists for Larsen C Ice Shelf. Data from Thwaites Glacier will be acquired in two field campaigns, which you will join in 2022, to record novel 3-D seismic reflection data.You will undertake this project under the guidance of a team of Leeds scientists, who are leading experts in glaciology, seismic modelling and anisotropic analysis. Specific objectives of the project include, but are not limited to:1. Use of seismic modelling methods to simulate the response to anisotropic ice fabrics and crevasses.2. Assessment of the sensitivity of the seismic response to the intensity of the ice fabric and fractures, and the seismic acquisition geometry.3. Analysis of new field data from Larsen C Ice Shelf and Thwaites Glacier. In the latter case, this processing