Near-Surface Full-Waveform Inversion Reveals Bedrock Controls on Critical Zone Architecture

For decades, seismic imaging methods have been used to study the critical zone, Earth’s thin, life-supporting skin. The vast majority of critical zone seismic studies use traveltime tomography, which poorly resolves heterogeneity at many scales relevant to near-surface processes, therefore, limiting progress in critical zone science. Full-waveform inversion can overcome this limitation by leveraging more of the seismic waveform and enhancing the resolution of geophysical imaging. In this study, we apply full-waveform inversion to elucidate previously undetected heterogeneity in the critical zone at a well-studied catchment in the Laramie Range, Wyoming. In contrast to traveltime tomograms from the same data set, our results show variations in depth to bedrock ranging from 5 to 60 meters over lateral scales of just tens of meters and image steep low-velocity anomalies suggesting hydrologic pathways into the deep critical zone. Our results also show that areas with thick fractured bedrock layers correspond to zones of slightly lower velocities in the deep bedrock, while zones of high bedrock velocity correspond to sharp vertical transitions from bedrock to saprolite. By corroborating these findings with borehole imagery, we hypothesize that lateral changes in bedrock fracture density majorly impact critical zone architecture. Borehole data also show that our full-waveform inversion results agree significantly better with velocity logs than previously published traveltime tomography models. Full-waveform inversion thus appears unprecedently capable of imaging the spatially complex porosity structure crucial to critical zone hydrology and processes.