权益分类	功能权益	普通用户	{{item.name}}会员
{{category.name}}	{{benefitItem.name}}

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：
2026931
负责人：
Hatice Bozdag
金额：
$ 26.94万
依托单位：
Colorado School of Mines
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=2026931&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.

地球的外核是熔融的铁，外加大约10%的较轻的合金材料。其核心半径略大于火星，位于地球表面以下约2900公里处。流体外核具有高度重要性的原因有很多，包括地球磁场的产生，以及作为将热量传递到地幔的重要机制，这有助于驱动负责板块构造的对流引擎。在过去的几十年里，地球物理学家在外核顶部发现了一个薄壳，地震波似乎在那里减速，这表明流体中存在某种形式的层化。然而，这个壳体的地震模型并不都是一致的，这促使了进一步的研究。对外核进行地震成像的一个主要挑战是，用于研究地核的地震波必须穿过整个地幔两次(向下再回来)，而地幔是非常不均匀的，地震速度随着位置的不同而变化。因此，地震学家如何才能确定测量到的信号中的模式是否是由于异常的最外层核心结构和地幔的不均质性？该项目的目的是记录非均质地幔对用于绘制岩心图的数据的影响程度。由四名地震学家组成的团队将收集来自世界各地的地震和地震传感器的地震数据，使用最先进的地震波场计算来预测观测结果，并对对地球深部地幔和外核部分敏感的信号进行精细测量。预期的结果是更好地了解地球不均匀地幔对数据测量的影响程度，以研究核心，并通过使用被证明受地幔影响最小的最佳数据来产生最外层核心的改进模型。更好地绘制地球最外层地核的地震图将为旨在了解磁场的神秘性质、从地核到地幔的热流以及地幔和地核之间可能的化学交换的研究提供信息，这对理解地球的化学演化非常重要。团队中的所有四位地震学家将在各种场所(包括课堂、公开演讲和科学博览会)与公众分享成果，以促进人们对地球和行星内部在塑造地表现象方面的重要性的认识。该项目将培训几名研究生和本科生。地震图像显示，在核心最外面50-400公里处的P波速度(Vp)降低意味着在更深的、单独对流的内部存在稳定的层状层。降低的速度(和密度)层的精确厚度和性质对于地球发电机模型具有至关重要的意义，这些模型解决了地球磁场的形成以及理解岩心成分和矿物学的能力。然而，在近30多年的地震研究中，地震学家对地层的厚度和内部的速度结构还没有达成共识。这可能是由于地幔的非均质性和各向异性对用于探测最上部外核的地震数据的影响。该项目将研究在最外层传播的地震波的旅行时间、波径、波形和横波分裂异常，包括从核-地幔边界底部的多次反射(“SmKS”波)，它比任何其他地震波对外核结构更敏感，以确定和缓解地幔结构对外核模型的影响。为了实现这一点，将使用一种在地理上定位的地震子阵列对地震记录进行求和的方法来提高弱信号相对于噪声的清晰度，这可以在识别地幔非均质性和各向异性的污染时得到改善。3D波场合成将被用来对一维外核成像工具如何受到地幔非均质性的影响进行基准测试。新的最外层地震模型随后将在正演和反演实验中根据最高质量的数据确定，这些数据已经过地幔非均质性和各向异性的校正。因此，该项目将直接检验长期(但可能不完美)的假设，即差分SmKS旅行时可用于可靠地反演外核结构，而无需明确考虑地幔非均质性和各向异性的影响。除了基于高质量、正确的数据产生新的外核结构模型外，该项目还将产生补充数据产品，其中包含对低地幔速度不均质性和各向异性的新见解。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。