New Frontiers in Seismic Imaging of the Critical Zone

Abstract

The critical zone (CZ) is the life-supporting skin of our planet, spanning the top of vegetation to the base of weathered bedrock. This thin layer is the only place in the known universe where biota, water, atmosphere, and geologic materials interact and transform. Direct observations of the critical zone's subsurface structure typically require invasive methods such as drilling boreholes or excavating soil pits. Seismic imaging has been used for decades to circumvent direct measurements of the subsurface, but traditional approaches often rely on a limited subset of the information contained within the seismic wavefield, such as the timing of first arrival waveforms. By leveraging more of the data contained in the seismic wavefield, new facets of critical zone evolution can be revealed. This thesis develops and applies advanced geophysical techniques, specifically full-waveform inversion (FWI) and multi-component surface wave analysis, to constrain high-resolution models of p-wave velocity, s-wave velocity, and radial anisotropy. In the first manuscript, a workflow is developed to implement 2D full waveform inversion (FWI) within the critical zone. The workflow involves inverting surface waves and body waves separately to ensure that high-amplitude surface waves do not dominate and overprint the information contained in body waves. When applied to a site near Laramie Wyoming, the resulting FWI models reveal that bedrock fracture density serves as an important bottom-up control on CZ architecture. These findings show that the transition from saprolite to intact bedrock is sharp in areas with low fracture density but more diffuse where the underlying rock exhibits higher fracture density. Additionally, the FWI models show better agreement with borehole data as compared to previously published first arrival travel time tomography models. The second study explores the role of inherited rock fabric in the development of critical zone porosity by measuring radial anisotropy with surface waves. This novel method utilizes multi-component surface seismic data associated with Rayleigh and Love waves to quantify radial anisotropy at the hillslope scale. Field data and in situ measurements from the South Carolina Piedmont demonstrate a strong correlation between seismic anisotropy and porosity, with both properties developing concurrently as rock undergoes in situ weathering. This empirical evidence suggests that weathering processes do not act stochastically, and instead, are guided by the geologic fabric of the parent material. Moreover, this research provides further evidence that inherited rock fabric plays a major role in dictating the form and function of landscapes. The final study investigates subsurface structure and water stores beneath giant sequoias in Yosemite National Park. By employing dense arrays for multicomponent nodal geophones, a revised time-frequency-phase FWI algorithm, and geostatistical rock physics modeling, this research estimates volumetric water content beneath giant sequoias at different landscape positions. The results indicate that giant sequoias located on ridges and hillslopes lack sufficient shallow soil moisture and must instead rely on deep rock moisture from depths exceeding 2 meters to avoid water stress during arid summers. As such, this work underscores the importance of rock moisture to valued species in arid landscapes. These three studies present several avenues for seismic imaging to catalyze research in the critical zone. The advent and integration of multicomponent, dense nodal data sets with advanced processing methods such as FWI means that previously undiscernible subsurface characteristics can now be elucidated. By contextualizing novel images of the shallow subsurface within the vibrant field of critical zone science, we can better understand how Earth supports life.