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.