EAR-PF: Characterizing small changes in the Earth from time-reversed multiply-scattered Rayleigh waves

EAR-PF：通过时间反转多重散射瑞利波表征地球的微小变化

• 批准号: 1144883
• 负责人: Thomas Mikesell
• 金额: $ 17万
• 依托单位: Mikesell Thomas D
• 依托单位国家: 美国
• 项目类别: Fellowship Award
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-06-01 至 2015-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1144883&HistoricalAwards=false
• 关键词: EAR; PF; Characterizing; small; changes

Dr. Dylan Mikesell has been awarded an NSF Earth Sciences Postdoctoral Fellowship to develop an integrated research and education plan at the Earth, Atmospheric and Planetary Science Department at the Massachusetts Institute of Technology (MIT). Current passive seismic methods rely heavily on a priori velocity models to accurately quantify and locate small velocity changes. This project aims to theoretically extend a 1D acoustic wave method to 2D elastic waves. We borrow from recent multiple scattering studies, which use incoherent reflections to locate isolated changes in a 1D medium. These waves are called incoherent reflections because there is no acoustic impedance contrast between the isolated change and the background medium. With this approach we eliminate the need for a priori velocity information. Furthermore, the input data for this new method borrow from recent seismic noise studies. We will use Green's functions derived from multi-component seismic noise crosscorrelations. This approach ensures that we use surface waves, which dominate the seismic noise field and propagate in 2D rather than 3D.We will test the new method with laboratory experiments and then monitor a recent eruption at Pavlof Volcano, Alaska.In all fields of Earth science we seek methods to characterize potential hazards. Detecting geologic changes using seismic waves is a useful tool for understanding a range of phenomena in the Earth's subsurface, which in turn help us to better understand hazards themselves. Seismic waves are sensitive to changes in Earth properties, in particular seismic wave speed, which is related to more fundamental properties such as density and stiffness. Over the past decade, methods using multiply scattered seismic waves (i.e., waves that reflect more than once from subsurface heterogeneities) have been developed and applied to detect small subsurface velocity changes. These methods have shown promise for detecting real-time changes; however, obstacles remain and there is room for improvement. This project expands upon recent developments that link these multiple-scattering techniques to time-reversal techniques in order to exploit the ability of the Earth to re-focus waves, allowing us to locate small subsurface velocity changes with less reliance on our a priori knowledge of subsurface parameters. This project has potential applications in monitoring changes in volcanoes over time (as well as monitoring other heterogeneous materials) and pre- and post-earthquake changes along active seismic faults. More generally, it has the potential to impact the fields of nondestructive material testing and medical imaging. Over the course of the project, Dr. Mikesell will work with MIT's K12 outreach center to develop STEM content focused on seismic hazards. Using local classrooms and informal learning centers, undergraduate and graduate students will have the opportunity to deliver this content, thus developing their communication skills with non-technical audiences.

Dylan Mikesell博士被授予美国国家科学基金会地球科学博士后奖学金，在麻省理工学院（MIT）的地球、大气和行星科学系制定一个综合研究和教育计划。目前的被动地震方法严重依赖于先验速度模型来精确量化和定位小的速度变化。本项目旨在从理论上将一维声波方法扩展到二维弹性波。我们借鉴了最近的多项散射研究，这些研究使用非相干反射来定位一维介质中的孤立变化。这些波被称为非相干反射，因为在孤立的变化和背景介质之间没有声阻抗对比。通过这种方法，我们消除了对先验速度信息的需要。此外，这种新方法的输入数据借鉴了最近的地震噪声研究。我们将使用由多分量地震噪声相互关系导出的格林函数。这种方法确保了我们使用表面波，表面波控制着地震噪声场，并且以二维而不是三维的方式传播。我们将用实验室实验来测试这种新方法，然后监测阿拉斯加巴夫洛夫火山最近的一次喷发。在地球科学的所有领域中，我们都在寻找描述潜在危险的方法。利用地震波探测地质变化是了解地球地下一系列现象的有用工具，这反过来又有助于我们更好地了解灾害本身。地震波对地球性质的变化很敏感，尤其是地震波速度的变化，而地震波速度与密度和刚度等更基本的性质有关。在过去的十年里，利用多次散射地震波（即从地下非均质中反射多次的地震波）的方法已经被开发出来，并应用于探测地下速度的小变化。这些方法已经显示出检测实时变化的希望；然而，障碍仍然存在，还有改进的余地。该项目扩展了将这些多重散射技术与时间反转技术联系起来的最新发展，以利用地球重新聚焦波的能力，使我们能够定位小的地下速度变化，减少对地下参数先验知识的依赖。该项目在监测火山随时间的变化（以及监测其他非均质物质）和地震前后沿活动地震断层的变化方面具有潜在的应用价值。更广泛地说，它有可能影响无损材料检测和医学成像领域。在项目过程中，Mikesell博士将与麻省理工学院的K12外展中心合作，开发以地震危害为重点的STEM内容。利用当地的教室和非正式的学习中心，本科生和研究生将有机会提供这些内容，从而发展他们与非技术观众的沟通技巧。