USING NEAR-SURFACE GEOPHYSICS TO CHARACTERIZE THE HYDROGEOLOGIC ROLE OF THE RUBYS INN THRUST FAULT IN THE BRYCE CANYON REGION, UTAH

Recent commercial and domestic development pressures in the Bryce Canyon area could affect groundwater usage, availability, and dependent resources, so the Utah Geological Survey is conducting ongoing hydrogeologic studies in the region. A feature of interest is the south-directed Rubys Inn thrust fault, along the southern boundary of Emery Valley. Fault zones are an important control on local and regional groundwater flow and have the capacity to act as both barriers and conduits to groundwater movement. Various fault zone processes can reduce or enhance permeability, and heterogeneity within fault zones is common.

We combined transient electromagnetic (TEM) surveys, electrical resistivity tomography (ERT), seismic refraction, and outcrop observations to investigate the field-scale influence of the Rubys Inn thrust fault on groundwater location and movement along the southern boundary of Emery Valley. We collected TEM, ERT, and seismic refraction data along three transects orthogonal to the fault strike in May 2022. Where available, we used groundwater level data to constrain geophysical data. Outcrop observations west of our study area suggest the fault zone has reduced permeability and may act as a barrier to flow. Geophysical data suggest a contrast in resistivity values along the fault core relative to the surrounding materials. We infer that relatively low-resistivity regions are saturated zones within sediments and bedrock. Preliminary TEM and ERT inversions suggest a groundwater level offset across the fault zone, up to as much as 5 meters. Based on the variability in offset, it is likely the fault zone acts as a complex barrier-conduit system. Future work includes processing and interpretation of recently collected repeat transect measurements to investigate the possibility of seasonal variability.