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

Dylan Mikesell博士被授予NSF地球科学博士后奖学金，在马萨诸塞州理工学院（MIT）的地球、大气和行星科学系制定综合研究和教育计划。当前的被动地震方法严重依赖于先验速度模型来精确地量化和定位微小的速度变化。本项目旨在从理论上将一维声波方法扩展到二维弹性波。我们借用最近的多次散射研究，它使用非相干反射定位孤立的变化在一维介质。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。我们将使用从多分量地震噪声互相关导出的绿色函数。这种方法确保了我们使用的是面波，它在地震噪声场中占主导地位，并且以二维而不是三维传播。我们将用实验室实验来测试这种新方法，然后监测阿拉斯加帕夫洛夫火山最近的一次喷发。在地球科学的所有领域，我们都在寻找表征潜在危险的方法。使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敏感，特别是地震波速度，这与更基本的性质有关，如密度和刚度。在过去的十年中，使用多次散射地震波的方法（即，从地下不均匀性反射不止一次的波）已经被开发并应用于检测小的地下速度变化。这些方法已显示出检测实时变化的希望;然而，障碍仍然存在，并且还有改进的空间。该项目扩展了最近的发展，将这些多次散射技术与时间反演技术联系起来，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，而较少依赖于我们对地下参数的先验知识。该项目在监测火山随时间的变化（以及监测其他异质材料）和地震前和地震后沿着活动地震断层的变化方面具有潜在的应用价值。更一般地说，它有可能影响无损材料测试和医学成像领域。在该项目的过程中，Mikesell博士将与麻省理工学院的K12外展中心合作，开发专注于地震灾害的STEM内容。利用当地的教室和非正式学习中心，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受众的沟通技巧。

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.