Fault Zone Hydraulic Parameter Estimation by Passive Methods Using Natural Forces.

Hydrogeological conditions within fracture zones control subsurface fluid, solute, and heat flux and storage. While conventional groundwater methods using induced disturbances often require elaborate and expensive equipment, passive methods that use observed responses of groundwater levels to atmospheric pressure and Earth tides provide the opportunity to estimate hydrogeologic properties without having to conduct field testing. We present estimates of hydraulic properties of a fault‐damage zone embedded within crystalline rock using responses to natural perturbations, and then compare these to estimates obtained using conventional methods. We calculate the skin factor using the exhaustive algorithm to improve the consistency between the tidal and conventional methods and compare it with different barometric‐response methods in the frequency and time domains. For the tidal method, the hydraulic conductivity is 16% larger than the estimate obtained from a conventional aquifer test, the barometric‐response method in the frequency domain is 6% larger, and the inverse, Hvorslev's, CBP, and Valois's methods in the time domain are, respectively, 71%, 72%, and 97% smaller and 624% larger. Estimated fault‐damage zone properties yield a hydraulic diffusivity of D ≈ 3 m2 s−1 and a permeability of k ≈ 4 × 10−13 m2 (0.4 Darcy), which confirms the presence of a highly damaged zone. The combined tidal‐ and barometric‐response methods provide a new and robust approach for understanding and characterizing hydrogeological properties of fault‐damage zones. Plain Language Summary: Fault zones play key roles in hydrogeological flow and transport processes (e.g., fluid, solute, heat). Yet, obtaining hydraulic parameters using conventional methods (e.g., hydrogeological, structural‐geological investigations) in these systems remains challenging due to the cost and logistics involved. Alternatively, passive methods using groundwater‐level responses to natural forcing (e.g., barometric‐pressure variation, Earth tides) may be more practical, robust, and efficient. In this study, we evaluate multiple methods for estimating aquifer parameters in both the time and frequency domains, using data from a well‐aquifer system in the Xianshui River fault zone. We demonstrate that hydraulic parameters estimated by passive methods in the frequency domain are more similar to those estimated using conventional aquifer tests than time‐domain methods. We show that responses to natural forcing provide a reliable tool for evaluating hydraulic and hydrogeomechanical parameters in fracture‐zone areas and are especially helpful for monitoring the continuous evolution of fault damage‐zone permeability. Key Points: Methods of water level in response to barometric pressure and earth tide were employed to determine the fault zone hydraulic propertiesBoth time‐ and frequency‐domain responses of the barometric pressure and tidal response method produced similar results with pumping testThe hydraulic diffusivity was around ∼3 m2 s‐1 which indicated the presence of a highly damaged zone in the study area [ABSTRACT FROM AUTHOR]