Collaborative Research: Towards improved imaging of the outermost core through determination of the effects of lowermost mantle heterogeneity and anisotropy

合作研究：通过确定最低地幔异质性和各向异性的影响来改善最外层地核的成像

基本信息

批准号：
2026917
负责人：
Maureen Long
金额：
$ 20.5万
依托单位：
Yale University
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2020
资助国家：
美国
起止时间：
2020-09-01 至 2024-08-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=2026917&HistoricalAwards=false
关键词：
Collaborative Research Towards improved imaging

项目摘要

Earth’s outer core is molten iron, plus roughly 10% of a lighter alloying material. With a radius that is slightly larger than the planet Mars, the core sits roughly ~2900 km below Earth’s surface. The fluid outer core holds high importance for a number of reasons, including the generation of Earth’s magnetic field, and being an important mechanism to transfer heat to the mantle, which helps to drive the convective engine responsible for plate tectonics. In the past few decades, geophysicists have detected a thin shell at the top of the outer core where seismic waves appear to slow down, suggesting some form of stratification in the fluid. However, the seismic models for this shell do not all agree, which motivates further study. A principle challenge in seismically imaging the outer core is that seismic waves from earthquakes that are used to study the core have to travel through the entire mantle of the Earth twice (down and then back), and the mantle is very heterogeneous, with seismic velocities that vary with position. Thus, how can seismologists determine if patterns in measured signals are due to anomalous outermost core structure versus heterogeneities in Earth’s mantle? The purpose of this project is to document the degree to which the heterogeneous mantle affects the data which are used to map the core. The team of four seismologists will collect seismic data from earthquakes and seismic sensors from all over the world, predict the observations using state of the art seismic wavefield computations, and conduct refined measurements on the signals that are sensitive to deep mantle and outer core parts of the planet. The expected outcome is a better understanding of the degree to which Earth’s heterogeneous mantle affects measurements of data to study the core, and to produce an improved model of the outermost core by using the best data which are demonstrated to be minimally affected by the mantle. Better seismic mapping of Earth’s outermost core will inform research that aims to understand the enigmatic nature of the magnetic field, heat flow from the core to the mantle, as well as possible chemical exchange between the mantle and core which is important for understanding the chemical evolution of the planet. All four seismologists on the team will share results with the public in a variety of venues (including the classroom, public presentations, and science fairs) to promote awareness of the importance of Earth and planetary interiors in shaping phenomena experienced at the surface. The project will train several graduate and undergraduate students.Seismically imaged P wave velocity (Vp) reductions in the outermost 50-400 km of the core imply the presence of a stably stratified layer overlying the deeper, separately convecting interior. The precise thickness and nature of the reduced velocity (and density) layer holds critical significance for geodynamo models that address making Earth’s magnetic field, as well as the ability to understand core composition and mineralogy. However, in over ~30 years of seismic studies, no consensus has emerged among seismologists on either the thickness of the layer or the velocity structure within it. This is likely due to the effects of mantle heterogeneity and anisotropy on the seismic data used to probe the uppermost outer core. This project will investigate travel time, wave path, waveform, and shear wave splitting anomalies of seismic waves that travel in the outermost core, including multiple reflections from the underside of the core-mantle boundary (“SmKS” waves), which are more sensitive to outer core structure than any other seismic wave, in order to identify and mitigate the effects of mantle structure on outer core models. To accomplish this, a method that sums seismograms at geographically localized seismic sub-arrays will be used to improve the clarity of weak signals relative to noise, which can improve upon identification of contamination from mantle heterogeneity and anisotropy. 3D wavefield synthetics will be used to benchmark how 1D outer core imaging tools are affected by mantle heterogeneity. New outermost core seismic models will subsequently be determined in forward and inverse experiments based on highest-quality data that have been corrected for the effects of mantle heterogeneity and anisotropy. Thus, this project will thus directly test the longstanding (but likely imperfect) assumption that differential SmKS travel times can be used to reliably retrieve outer core structure without explicitly considering the effects of mantle heterogeneity and anisotropy. In addition to producing new models of outer core structure based on high-quality, corrected data, this project will produce complementary data products that contain new insights into lower mantle velocity heterogeneity and anisotropy.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.

地球的外芯是熔融铁，大约有10％的较轻合金材料。半径比火星行星略大，核心位于地球表面以下约2900公里。流体外部芯具有很高的重要性，原因有很多，包括地球磁场的产生，并且是将热量转移到地幔的重要机制，这有助于驱动负责板块构造的对流引擎。在过去的几十年中，地球物理学家在外部芯的顶部检测到一个薄的壳，地震波似乎慢下来，这表明流体中的某种形式的分层。但是，该外壳的地震模型并不同意，这激发了进一步的研究。地震成像外部核心的一个原则挑战是，用于研究核心的地震中的地震波必须两次穿过地球的整个地幔（然后向后），地幔非常异构，地震速度与位置不同。这是，地震学家如何确定测量信号中的模式是否是由于地球地幔中最外面的核心结构而不是异质性引起的？该项目的目的是记录异质地幔影响用于绘制核心的数据的程度。四个地震学家的团队将从世界各地的地震和地震传感器中收集地震数据，使用最先进的地震波场计算预测观察结果，并对对地球的深壁炉和外部核心部分敏感的信号进行精制测量。预期的结果是更好地理解地球异质地幔影响数据测量以研究核心的程度，并通过使用最佳数据通过证明受地幔影响最小的最佳数据，从而产生最大核心的改进模型。地球最外部核心的更好地震映射将为旨在了解磁场的神秘性，从核心到地幔的热流以及地幔与核心之间的化学交换是否重要，这对于理解行星的化学演化很重要。团队中的所有四位地震学家将在各种场所（包括课堂，公共演讲和科学博览会）与公众分享成果，以提高人们对地面上经历的塑造现象的地球和行星内饰的重要性的认识。该项目将训练几名毕业生和本科生。核心最外层的50-400 km的p速度（VP）降低，这意味着存在稳定的分层层，上面层上更深的，分别对令人信服的内部。降低速度（和密度）层的精确厚度和性质对于解决地球磁场的地球模型以及理解核心组成和矿物学的能力而具有至关重要的意义。但是，在大约30年的地震研究中，地震学家之间在层的厚度或速度结构上都没有达成共识。这可能是由于地幔异质性和各向异性对用于探测最高外核的地震数据的影响。该项目将研究以最大核心行进的地震波的旅行时间，波道，波形和剪切波裂解的异常，包括来自核心掩体边界底面（“ SMKS”波）的多种反射，这些反射比任何其他地震波对外部核心的结构都更敏感，以识别和识别和衡量外部核心模型的影响。为此，将使用地震图在地理位置上的地震子阵列上进行地震图的方法来提高相对于噪声的弱信号的清晰度，从而在识别地幔异质性和各向异性污染的情况下可以改善弱信号的清晰度。 3D波场合成学将用于基准测试1D外核成像工具如何受地幔异质性的影响。随后将在正向和反相反的实验中确定最高质量的最高核心地震模型，这些模型基于最高质量数据，这些数据已校正了地幔异质性和各向异性的影响。因此，该项目将直接测试长期存在的（但可能不完美的）假设，即可以使用差分SMK的旅行时间可靠地检索外部核心结构而无需明确考虑地幔异质性和各向异性的影响。除了基于高质量，校正的数据生产新的外部核心结构模型外，该项目还将生产完整的数据产品，这些数据产品对较低的地幔速度异质性和各向异性的新见解。该奖项反映了NSF的法定任务，并通过使用该基金会的知识分子优点和广泛影响来评估NSF的法定任务，并被认为是宝贵的支持。