Probing Water Below the Surface: Insights from Seismic Interferometry with Conventional and DAS Array

Citation

Shen, Zhichao (2022) Probing Water Below the Surface: Insights from Seismic Interferometry with Conventional and DAS Array. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5vtn-1c34. https://resolver.caltech.edu/CaltechTHESIS:05022022-204308721

Abstract

Water is essential to our daily lives, yet its subsurface behavior remains challenging to track using remote observations. By extracting seismic waves traveling through the Earth, seismic interferometry is a powerful tool to image the Earth’s interior, in particular the long-term and short-term behavior of water circulation. With conventional dense seismic networks and emerging distributed acoustic sensing (DAS), I demonstrate how seismic interferometry brings new insights on water below the surface ranging from the depths of the mantle transition zone (MTZ) to the subsurface aquifers of our planet.

By applying a novel inter-source interferometry method that turns deep earthquakes into virtual seismometers, I not only present evidence for an intermediate-scale metastable olivine wedge and small-scale intra-slab scatterers in the MTZ beneath the Japan Sea, but also reveal their dimensions and velocity perturbations more accurately than before. Beyond the relative independent scales of slab structures, these results point toward a consistent picture of transformational faulting of metastable olivine as the initiation mechanism of deep earthquakes, petrologic processes associated with dehydration of subducting slabs, and an extremely dry slab core below 410-km. Borrowing the idea from inter-source interferometry, I develop a slab operator method by utilizing the waveform broadening due to the high-velocity anomaly. With synthetic tests and real data, I illustrate the feasibility of this method for accurately determining large-scale slab velocity perturbations.

Shifting from the Earth’s interior to the subsurface, I investigate the feasibility of vadose zone water monitoring with DAS. DAS provides an affordable and scalable solution for deploying ultra-dense seismic arrays by transforming existing optic-fiber cables into thousands of seismic sensors. With two years of ambient noise recorded on the Ridgecrest DAS array, the time-lapse images of seismic changes (dv/v) reveal an unprecedented high-resolution spatiotemporal evolution of water saturation in vadose zone. A striking correlation between the dv/v amplitude and the sedimentary thickness is observed, while the frequency analysis of dv/v measurements suggests an uppermost 10 m hydrologic source as the cause for dv/v temporal variability. The results demonstrate the great potential of DAS for long term subsurface water monitoring.

Thesis Files