Faulting and groundwater in a desert environment: constraining hydrogeology using time‐domain electromagnetic data

Abstract

Within the south‐western Mojave Desert, the Joshua Basin Water District is considering applying imported water into infiltration ponds in the Joshua Tree groundwater sub‐basin in an attempt to artificially recharge the underlying aquifer. Scarce subsurface hydrogeological data are available near the proposed recharge site; therefore, time‐domain electromagnetic (TDEM) data were collected and analysed to characterize the subsurface. TDEM soundings were acquired to estimate the depth to water on either side of the Pinto Mountain Fault, a major east‐west trending strike‐slip fault that transects the proposed recharge site. While TDEM is a standard technique for groundwater investigations, special care must be taken when acquiring and interpreting TDEM data in a two‐dimensional (2D) faulted environment. A subset of the TDEM data consistent with a layered‐earth interpretation was identified through a combination of three‐dimensional (3D) forward modelling and diffusion time‐distance estimates. Inverse modelling indicates an offset in water table elevation of nearly 40 m across the fault. These findings imply that the fault acts as a low‐permeability barrier to groundwater flow in the vicinity of the proposed recharge site. Existing production wells on the south side of the fault, together with a thick unsaturated zone and permeable near‐surface deposits, suggest the southern half of the study area is suitable for artificial recharge. These results illustrate the effectiveness of targeted TDEM in support of hydrological studies in a heavily faulted desert environment where data are scarce and the cost of obtaining these data by conventional drilling techniques is prohibitive.

Airborne Geophysical Imaging of Weak Zones on Iliamna Volcano, Alaska: Implications for Slope Stability

“…Both inversion approaches employ one‐dimensional (1‐D) physics, in which resistivity is assumed to only vary with depth. Although lateral contrasts in resistivity most certainly occur, the 1‐D modeling assumption is generally a valid approximation except within 100–300 m of a sharp lateral resistivity contact (Bedrosian et al., 2013). We used a deterministic spatially constrained inversion (SCI) and laterally constrained inversion (LCI) (Auken et al., 2015) carried out with Aarhus Workbench software (Aarhus Geosoftware) as well as a Bayesian Markov‐chain Monte Carlo (MCMC) inversion implemented in the open‐source U.S. Geological Survey code Geophysical Bayesian Inference in Python (GeoBIPy; Minsley, 2011; Minsley et al., 2020; Table S2).…”

Geometry and Structure of a Fault‐Bounded Extensional Basin by Integrating Geophysical Surveys and Seismic Anisotropy Across the 30 October 2016 Mw 6.5 Earthquake Fault (Central Italy): The Pian Grande di Castelluccio Basin

“…TDEM method provides the measure the electrical resistivity distribution of the ground through the induction of eddy currents in the subsoil (Christiansen et al, ; Nabighian & Macnae, ). It is widely used to image the electrical properties of subsurface geological structures over different lithological settings with the capability to reach significant penetration depths over moderately resistive environments (few hundreds of meters; Bedrosian et al, ). We follow the approach by Civico et al () to map the top bedrock depths using 1‐D vertical resistivity models.…”

Geometry and evolution of a fault‐controlled Quaternary basin by means of TDEM and single‐station ambient vibration surveys: The example of the 2009 L'Aquila earthquake area, central Italy

“…In fact, the ratio of penetration depth to loop size for TDEM can be much greater than 1, as opposed to electrical resistivity tomography, where great penetration depths require long electrode arrays [ Revil et al ., ; Pucci et al ., ]. Ground TDEM surveys were rarely used in the recent past to map fault zone [e.g., Bedrosian et al ., ] or for depth‐to‐bedrock investigations over a fault‐controlled area [e.g., Bedrosian et al ., ].…”