Fault Zone Hydraulic Parameter Estimation by Passive Methods Using Natural Forces

Abstract

AbstractHydrogeological 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.