Probing Solid-Earth, Ocean, and Structural Dynamics with Distributed Fiber-Optic Sensing

Author: Williams, Ethan Francis

Year: 2023

Degree: Dissertation (Ph.D.)

Advisor: Zhan, Zhongwen

Committee Members: Clayton, Robert W.; Zhan, Zhongwen; Heaton, Thomas H.; Kohler, Monica D.; Callies, Joern; Simons, Mark

Option: Geophysics

DOI: 10.7907/vehm-dd85

Abstract

Observational geophysics conventionally relies on point sensors to document and monitor Earth’s dynamic processes, from locating earthquakes and imaging subsurface structure with seismometers to forecasting coastal wave heights and detecting tsunamis with buoys. Distributed acoustic sensing (DAS) offers a fundamentally different paradigm: distributed instead of point sensing. DAS converts fiber-optic cables into dense arrays of broadband, linear strainmeters, with spatial resolution as fine as one meter and temporal resolution up to several thousand samples per second. The first four chapters of this thesis concern ocean-bottom DAS, repurposing pre-existing telecommunications and power cables as distributed seafloor sensing networks for seismology and physical oceanography. In Chapter 2, we analyze one of the first ocean-bottom DAS datasets, demonstrating that seismic and ocean waves observed on the same array are related by a classic theory of double-frequency microseism generation. We also extract the principal body-wave phases of a M8.2 deep earthquake, demonstrating the earthquake detection capabilities of DAS even in a shallow water environment. In Chapter 3, we apply ambient noise interferometry to a one-hour of ocean-bottom DAS data and derive a shallow shear-wave velocity model. We also isolate spurious arrivals in noise cross-correlations associated with nearby offshore wind turbines, suggesting potential for remote monitoring. In Chapter 4, we adapt ambient noise interferometry to the ocean surface gravity wavefield, and estimate the tidal current velocity along a short cable segment in the Strait of Gibraltar with a waveform stretching method. In Chapter 5, we explore the application of DAS as a temperature sensor at long periods, documenting temperature signals up to 4 K associated with internal wave and boundary layer dynamics. We demonstrate that while ocean-bottom DAS exhibits sufficient strain sensitivity to record seafloor geodetic processes, oceanic temperature transients may overprint such signals. The last part of this thesis concerns a different frontier in geophysical instrumentation: long time-series. With a 20-year continuous record of ambient vibrations from a single accelerometer located on the ninth floor of a concrete building, we document long-term, passive changes in the building’s natural frequencies as well as complex, time-dependent nonlinear elasticity during earthquakes.

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