Optical Fabric and Fiber Logging of Glacial Ice (1142010)

Abstract: This award supports a project to combine the expertise of both glaciologists and optical engineers to develop polarization- preserving optical scattering techniques for borehole tools to identify changes in high-resolution crystal structure (fabric) and dust content of glacial ice. The intellectual merit of this work is that the fabric and impurity content of the ice contain details on climate, volcanic activity and ice flow history. Such fabric measurements are currently taken by slicing an ice core into sections after it has started to depressurize which is an extremely time-intensive process that damages the core and does not always preserve the properties of ice in its in-situ state. In addition the ice core usually must be consumed in order to measure the components of the dust. The fabric measurements of this study utilize the concept that singly-scattered light in ice preserves most of its polarization when it is backscattered once from bubbles or dust; therefore, changes to the polarization of singly-backscattered light must originate with the birefringence. Measurements based on this concept will enable this program to obtain continuous records of fabric and correlate them to chronology and dust content. The project will also develop advanced borehole instruments to replace current logging tools, which require optical sources, detectors and power cables to be submerged in borehole fluid and lowered into the ice sheet at temperatures of -50oC. The use of telecommunications fiber will allow all sources and detectors to remain at the surface and enable low-noise signal processing techniques such as lock-in amplification that increase signal integrity and reduce needed power. Further, fiber logging systems would be much smaller and more flexible than current tools and capable of navigating most boreholes without a heavy winch. In order to assess fabric in situ and test fiber-optic borehole tools, field measurements will be made at WAIS Divide and a deep log will also be made at Siple Dome, both in West Antarctica. If successful, the broader impacts of the proposed research would include the development of new analytical methods and lightweight logging tools for ice drilling research that can operate in boreholes drilled in ice. Eventually the work could result in the development of better prehistoric records of glacier flow, atmospheric particulates, precipitation, and climate forcing. The project encompasses a broad base of theoretical, experimental, and design work, which makes it ideal for training graduate students and advanced undergraduates. Collaboration with schools and classroom teachers will help bring aspects of optics, climate, and polar science to an existing Middle School curriculum.

摘要：本资助项目旨在融合冰川学家与光学工程师的专业专长，研发适用于钻孔工具的保偏光学散射技术（polarization-preserving optical scattering techniques），以识别冰川冰的高分辨率晶体结构与冰晶组构（fabric）及尘埃含量变化。本研究的学术价值在于：冰的冰晶组构与杂质含量蕴含着气候、火山活动以及冰流历史的相关细节。当前此类组构测量需在冰芯开始减压后将其切片，该过程耗时极长，且会损坏冰芯，无法始终保留冰的原位状态特性；此外，测量尘埃成分通常需损耗整块冰芯。本研究的冰晶组构测量基于以下理念：冰中的单次散射光在经气泡或尘埃单次背散射后，仍可保留大部分偏振特性，因此单次背散射光的偏振变化必然源自双折射（birefringence）。基于此理念开发的测量方法，将使本项目能够获取冰的冰晶组构连续记录，并将其与年代学（chronology）及尘埃含量相关联。本项目还将研发先进的钻孔仪器以替代现有测井工具。当前测井工具需将光源、探测器与电缆浸没在钻孔流体中，并在-50℃的环境下下放至冰盖内。而采用通信光纤可使所有光源与探测器均置于地表，同时可运用锁相放大（lock-in amplification）等低噪声信号处理技术，提升信号完整性并降低所需功耗。此外，光纤测井系统相较现有工具体积更小、灵活性更强，无需重型绞车即可适配多数钻孔。为原位评估冰晶组构并测试光纤钻孔工具，研究团队将在西南极洲的WAIS Divide（韦斯特南极冰原分流点）开展野外测量，并在Siple Dome（锡普尔穹丘）进行深层测井。若本研究取得成功，其更广范围的影响将包括开发新型分析方法与轻量化测井工具，用于冰钻研究，可在冰上钻孔中作业。最终，本研究有望助力构建更完善的史前冰川流动、大气颗粒物、降水以及气候强迫相关记录。本项目涵盖大量理论、实验与设计工作，非常适合培养研究生与高年级本科生。团队还将与学校及课堂教师合作，将光学、气候学与极地科学相关内容融入现有初中课程体系。