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

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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.