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贡献，需要可靠地估计冰动力学将如何演变。这发生在大气、冰和海洋系统之间的一系列复杂的反馈中。对于冰系统，我们需要了解内部应力状态。在温度变暖的情况下，冰川流动预计会加速，从而导致冰块内部应力状态的变化。地球物理是估计当今应力状态的一种有价值的手段，这样它就可以被提供给未来冰块演化的预测计算模型。英国的应力状态可以从地震的各向异性中测量出来。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，就形成了大块的各向异性组构。对于这类织物，垂直于地层传播时，地震能量的传播速度会比沿地层传播时更快。这定义了垂直横向各向同性(VTI)的区域，其中观测到的速度变化仅随入射角的倾角变化，而不是随方位角变化。当存在优先排列的裂缝时，也可能出现各向异性。地震能量通过裂隙带传播的速度将比通过完整冰层传播的速度慢。这定义了水平横向各向同性(HTI)，其中速度随相对于决口方向的方位角而变化。每一种冰川状态都是冰川快速流动的指示器，因此制定有效的策略来监测这些冰川状态的发展是很重要的。冰层的各向异性可以从地表地震反射数据中得到特征，但很少有人分析采集几何形状对不同的各向异性状态的敏感性。这一点尤其需要考虑，因为实地物流通常需要一种折衷的收购战略。建立可靠的各向异性组构模型，并模拟它们的地震响应，将产生一套针对任何给定冰川目标的地震采集指南。目标在本项目中，您将考虑两个特定南极冰块中各向异性组构的建模和可探测性，这两个冰块都被认为对区域冰稳定至关重要。-将探索HTI织物与南极半岛拉森C冰架底部破裂强度的关系。考虑到2017年冰山崩解与更广泛大陆架稳定性之间的潜在联系，拉森C的应力状态令人感兴趣。-将为西南极冰盖的主要出口Thwaites冰川的流动状况模拟VTI织物。具体地说，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。各向异性的稳健测量将有助于理解对快速冰川流动的控制。各向异性模型将使用波动方程的离散裂缝公式来开发，并在WAVE软件中实现；这些模型将与实际地震数据中的各向异性特征进行比较。拉森C冰架已经有了方位地震数据集的档案。来自Thwaites Glacier的数据将在两个现场活动中获得，您将于2022年加入这两个活动，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进行这个项目，他们是冰川学、地震建模和各向异性分析方面的领先专家。该项目的具体目标包括但不限于：1.使用地震建模方法来模拟各向异性冰织物和裂缝的响应。评估地震响应对冰组构和裂缝的强度以及地震采集几何形状的敏感性。拉森C冰架和斯韦茨冰川新观测资料的分析。在后一种情况下，此处理

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