Geological applications of seismic full waveform inversion: Insight from a novel approach to evaluating the seismic properties of ice

地震勘测使人们能够深入了解冰块的冰内和冰下特性的物理特性。某些量，例如冰密度，可以与气候模型相关，因此可以为冰川的气候演变提供替代指标（Kuipers Munneke等人，2014; Hubbard等人，2016）。这些信息对于预测气候变暖时冰团的稳定性至关重要;这些冰团包括南极大陆边缘的大型冰架，人们普遍认为这是南极冰流对全球海平面上升的长期贡献的基础（DeContro和Pollard，2016年）。从地震速度导出密度的主要限制是速度：密度转换几乎完全是经验性的并且因此具有可疑的准确性（Booth等人，2013年）。广泛应用的Kohnen（1974）转换是基于南极冰芯的实验室分析，因此冰从其原始温度/压力环境中移除，在任何情况下，这可能与其他冰团不同。虽然密度的一般趋势可能是特征，更大的信心，反演参数的准确性将是有益的。地震数据集的全波反演（FWI）（Virieux and Operto，2009）代表了一种有前途的属性估计方法，值得在冰川学应用中进行研究。从地震波场本身。FWI已经在烃领域中看到了显著的发展（Brittan等人，2013; Bai和Yingst，2014; Jones，2015），但其在冰川学环境中的适用性尚未得到广泛证明。FWI在冰川学中的发展将代表地震能力的一个步骤变化，这可以在整个社区中广泛采用。该奖学金旨在探索FWI经验在工业和冰川学环境之间的转移，分三个阶段进行研究计划：第一阶段）使用现有的冰川学地震数据档案，审查目前的地震实践是否可以产生与FWI兼容的数据;第2阶段）制定优化的采集策略，以实现FWI;第3阶段）进行现场采集，以验证FWI性能。第1阶段可用的数据包括在NERC资助的MIDAS项目现场活动期间对南极洲拉森C冰架进行的地震和密度测量（Kulessa等人，2016; Hubbard等人，2016; Ashmore等人，此外，英国南极调查局（BAS）将提供他们在南极松岛冰川上的活动数据。在第二阶段，BAS将与学生联络，在即将到来的实地活动中指导符合FWI的收购，其中包括在Thwaites冰川和Rutford冰流上的进一步部署。在第三阶段，学生将进行符合FWI标准的地震采集，以确定哈哈什约库伦冰帽不断变化的密度（Giesen和Oerlemans，2010年）。通过南极科学合作计划（卡斯），也可能有机会在BAS的南极罗瑟拉站周围进行收购。这些新的收购将伴随着声波速度和密度的钻孔测量，以校准反演参数，并将其精度与标准的解释approaches.It也预计FWI在冰川学的经验可以受益于工业方法。例如，与典型油气区的地质情况相比，冰川在结构上比较简单;此外，在有钻孔控制的情况下，它从地面延伸到目标深度，因此提供了深度连续约束。

Seismic surveys give insight into the physical properties of the englacial and subglacial properties of ice masses. Certain quantities, e.g. ice density can be related to climate models and therefore provide proxies for the climatic evolution of a glacier (Kuipers Munneke et al., 2014; Hubbard et al., 2016). Such information is critical for predicting the stability of ice masses in a warming climate; these ice masses include the large ice shelves which fringe the Antarctic continent, widely believed to underpin the long-term contribution of Antarctic ice streams to global sea-level rise (DeContro and Pollard, 2016).A major limitation of deriving density from seismic velocities is that velocity:density conversions are almost entirely empirical and therefore of questionable accuracy (Booth et al., 2013). The widely-applied Kohnen (1974) conversion is based on lab analysis of a South Pole ice core hence ice is removed from its original temperature/pressure context which, in any case, is likely different for other ice masses. While the general trend of density is likely characterised, greater confidence in the accuracy of inverted parameters would be beneficial. Full wavefrom inversion (FWI) of a seismic dataset (Virieux and Operto, 2009) represents a promising approach to property estimation that deserves investigation for glaciological applications.FWI methods circumvent the need for empirical methods by deriving the underlying physical properties (seismic velocity, density, etc.) from the seismic wavefield itself. FWI has seen significant development in the hydrocarbons field (Brittan et al., 2013; Bai and Yingst, 2014; Jones, 2015), but its applicability in the glaciological setting is not widely proven. The development of FWI in glaciology would represent a step-change in seismic capabilities, which could be widely adopted throughout the community.This studentship aims to explore the transfer of FWI experience between the industrial and glaciological settings, in a three-phase research programme: Phase 1) using existing glaciological archives of seismic data, review whether current seismic practice can produce data that are FWI-compatible; Phase 2) develop an optimised acquisition strategy to enable FWI; Phase 3) undertake field acquisitions to validate FWI performance.Data available for Phase 1 include seismic and density measurements made on Antarctica's Larsen C Ice Shelf, during field campaigns of the NERC-funded MIDAS project (Kulessa et al., 2016; Hubbard et al., 2016; Ashmore et al., 2016); additionally, the British Antarctic Survey (BAS) will contribute data from their campaigns on the Antarctic Pine Island Glacier. During Phase 2, BAS will liaise with the student for guidance on FWI-compliant acquisitions in forthcoming field campaigns, which include a further deployment on Thwaites Glacier and on Rutford Ice Stream. Here, seismic velocities have important implications for the thermal regime and internal crystal fabric.In Phase 3, the student will undertake FWI-compliant seismic acquisitions to establish the evolving density of the Hardangerjokulen ice cap (Giesen and Oerlemans, 2010). There may also be the opportunity, via the Collaborative Antarctic Science Scheme (CASS) for acquisition in the environs of BAS's Antarctic Rothera Station. These new acquisitions will be accompanied by borehole measurements of sonic velocity and density to calibrate the inverted parameters, and compare their accuracy to standard interpretative approaches.It is also expected that experiences of FWI in glaciology can benefit industrial approaches. For example, glaciers are structurally simple compared to the geology of a typical hydrocarbon province; furthermore, where borehole control is available, it extends from the ground surface to the target depth therefore offering depth-continuous constraint.