UC Berkeley

Distributed Acoustic Sensing (DAS) is an emerging tool in array seismology, which uses high frequency interferometry of pulsed laser backscattering inside optical fiber to analyze the axial strain induced on the fiber cable commonly buried in horizontal trenches and vertical wells at the surface of the Earth. This technology was developed over the last decade for hydrocarbon and carbon sequestration reservoir imaging and monitoring, but the focus in this thesis is to explore its application to problems in earth science, broadly defined. Combining DAS with telecommunications optical fiber networks offers meter-scale, long-term observations of ground motion over watershed apertures in sectors of the planet where traditional geophysics has been hindered by cost and field logistics, such as offshore and in urban areas. The thesis is organized as follows. I introduce the motivation for using fiber-optic seismology in Chapter 1. In Chapter 2, I define principles of the methodology and describe how the instrument works. In Chapter 3, I use continuous DAS recordings of ambient vehicular seismic noise generated on a local road to study degrading permafrost over a two-month period of artificial warming. In Chapter 4, I focus on earthquake ground motions recorded on horizontal DAS arrays. Unlike classic inertial seismometers, there is presently a limited amount of information about DAS instrument response, thus in Chapter 5 the aim is to use natural signals to quantify the broadband frequency range of DAS instruments and deduce the related amplitude and phase response functions. Lastly, in Chapter 6, I use fiber-optics on the seafloor of Monterey Bay, CA inside of an unused science cable to investigate the production of nearcoast primary and secondary microseisms, identify unmapped seafloor faults, and observe quasi-geodetic hydrodynamic phenomena in the milli-Hertz frequency range. In Chapter 7, I summarize findings and speculate about future directions in this field.