Collaborative Research: Seismic Investigation of the Sub-ice Environment and Crustal Composition of Antarctica

第一部分：非技术南极冰盖下的地质为冰川系统的移动提供了一个关键条件：软弱的、多孔的、可能还含水的沉积层提供了一个滑层，而接触处的坚硬基岩可能会减少冰流。然而，由于非洲大陆偏远，冰盖很厚，了解地质情况极其困难。使用部署在冰盖顶部的地震台提供了一种通过冰层看到大陆的替代工具，因为它们记录的地震信号包含关于冰盖底部和下面地壳地质的敏感信息。例如，厚而多孔的沉积层会导致地震波传播得更慢。在这个项目中，研究人员将1)利用过去18年在南极洲200多个地点收集的地震数据，2)执行一套现代地震数据分析工具来产生各种地震信号，3)将它们转换成关于下伏地壳的物理性质(如地震波的速度)和化学成分(如二氧化硅含量)的一整套地图。这些地震速度图通过不确定度测量来量化精度，为冰下沉积层的分布提供了第一级评估。其他信息，如这些地点下方的其他地壳性质和推断的化学成分，将进一步阐明被厚厚的冰盖掩埋的大陆的地质演化。值得注意的是，确定地壳的二氧化硅含量将有助于更好地估计冰下地壳的热分布。第二部分：技术描述从冰冻圈和岩石圈的角度来看，南极冰盖的底层地壳都具有重要的意义：在局部和区域尺度上，南极冰盖底部的浅层物理性质为快速移动的冰盖的动力学提供了重要的边界条件；在更大的尺度上，整个大陆变化的地壳结构(厚度和泊松比)保留了过去地质演化的特征，其化学成分对地热结构有重要贡献。然而，由于非洲大陆的偏远和厚厚的冰盖，这些特性还没有得到系统的研究。在这项拟议的研究中，研究人员将分析来自200多个永久性和便携式地震台的18年来的地震记录，以限制冰下地震结构、地壳结构，并利用多种地震技术得出对块状地壳化学成分的影响。他们的分析将包括：1)环境噪声和地震的宽带瑞雷波水平垂直比(H/V)的系统测量；2)高频接收器函数波形和两层h-k叠加的分析，以提供泊松比和次冰不连续深度的信息；3)勒夫波频散的测量和径向各向异性的反演；4)环境噪声和地震的瑞雷波方位各向异性的测量；5)详细的亚冰各向同性结构的瑞雷波频散、H/V比和接收器函数波形的联合解释；6)所有地震各向异性测量的蒙特卡罗横观各向同性(TTI)模拟。这些分析将合并和综合，以产生一套地震成果，包括以分级分辨率确定冰下沉积物厚度图，更准确地确定地壳厚度和泊松比，以及量化地壳成分。拟议的工作将达到两个主要的科学目标。首先，它将提供有关南极冰盖底部接触和地震台地点沉积物厚度的详细信息，改善冰盖模拟的边界条件，确定南极境内主要沉积盆地的位置。其次，对南极地壳厚度和成分的限制将提高对大陆地质史的理解。值得注意的是，地壳结构和组成将进一步促进未来对塑造南极洲的各种构造过程的地质调查，如裂谷、岩石圈移除、造山和造山。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Part I: NontechnicalThe geology beneath the Antarctic Ice Sheet sets up a key condition to its moving glacial systems: a weak, porous, and possibly water bearing sedimentary layer provides a slippery bed, whereas hard basement rock at the contact may reduce the ice flow. Knowing the geology, however, has been extremely difficult since the continent is remote and the ice cover is thick. Using seismic stations deployed on top of the ice sheet provides an alternative tool to see the continent through the ice, as seismic signal they record contains sensitive information about the base of the ice-sheet and the geology of the crust below. For example, a thick and porous sedimentary layer will cause the seismic waves to travel more slowly. In this project, the investigators will 1) utilize the seismic data collected in the past 18 years at more than 200 sites in Antarctica, 2) perform a suite of modern seismic data analysis tools to produce a variety of seismic signals, and 3) translate them into sets of maps about the physical properties (such as the speed of the seismic wave) and chemical composition (such as silica content) of the underlying crust. These seismic speed maps, with accuracy quantified by uncertainty measurements, provide a first-order assessment to the distribution of the sub-ice sedimentary layer. Additional information such as other crustal properties and inferred chemical composition beneath these sites will further shed light on the geological evolution of the continent buried by the thick ice sheet. Notably, determining the silica content of the crust will help produce better estimates of the thermal profiles of the sub-ice crust. Part II: Technical DescriptionThe underlying crust of Antarctica Ice Sheet holds strong importance from perspectives of both cryosphere and lithosphere: at local and regional scale, the shallow physical properties at the base of the Antarctic Ice Sheet sets an important boundary condition for the dynamics of the fast moving ice sheet; at a greater scale, the architecture (thickness and Poissons ratio) of the crust that varies across the continent retains the signature of the past geological evolution and its chemical composition contributes significantly to the geothermal structure. However, these properties have not been systematically studied due to both the remoteness of the continent and its thick ice-coverage. In this proposed study, the investigators will analyze 18 years of seismic records from more than 200 permanent and portable seismic stations, to constrain the sub-ice seismic structure, crustal architecture, and draw implications on the chemical composition of the bulk crust using multiple seismic techniques. Their analysis will include: 1) systematic measurement of the broad-band Rayleigh wave horizontal-vertical ratios (H/V) from both ambient noise and earthquakes; 2) analysis of high frequency receiver function waveforms and 2-layer h-k stacking to provide information on Poissons ratio and the depth of sub-ice discontinuities; 3) measurement of Love wave dispersion and inversion for radial anisotropy; 4) measurement of Rayleigh wave azimuthal anisotropy from both ambient noise and earthquakes; 5) Joint interpretation of Rayleigh wave dispersion, H/V ratios, and receiver function waveforms for detailed sub-ice isotropic structure; 6) a Monte Carlo transversely isotropic (TTI) modeling of all seismic anisotropic measurements. These analyses will be combined and synthesized to produce a set of seismic deliverables including the determination of a map of thickness of the sub-ice sediments with hierarchical-resolution, more accurate crustal thickness and Poissons ratio determination, and quantification of crustal composition. The proposed work will meet two major scientific objectives. First, it will provide detailed information about the bottom contact of the Antarctic ice-sheet and sediment thickness at localities of seismic stations, improving the boundary conditions for the ice-sheet modeling and determining the locations of major sedimentary basins within Antarctica. Second, constraints on the thickness and composition of the Antarctic crust will improve the understanding of the geology history of the continent. Notably, the crustal architecture and composition will further facilitate future geological investigation to various tectonic processes that have shaped Antarctica, such as rifting, lithospheric removal, orogeny, and mountain building.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.