Application of ERT, Saline Tracer and Numerical Studies to Delineate Preferential Paths in Fractured Granites

Abstract

Accurate quantification of in situ heterogeneity and flow processes through fractured geologic media remains elusive for hydrogeologists due to the complexity in fracture characterization and its multiscale behavior. In this research, we demonstrated the efficacy of tracer-electrical resistivity tomography (ERT) experiments combined with numerical simulations to characterize heterogeneity and delineate preferential flow paths in a fractured granite aquifer. A series of natural gradient saline tracer experiments were conducted from a depth window of 18 to 22 m in an injection well (IW) located inside the Indian Institute of Technology Hyderabad campus. Tracer migration was monitored in a time-lapse mode using two cross-sectional surface ERT profiles placed in the direction of flow gradient. ERT data quality was improved by considering stacking, reciprocal measurements, resolution indicators, and geophysical logs. Dynamic changes in subsurface electrical properties inferred via resistivity anomalies were used to highlight preferential flow paths of the study area. Temporal changes in electrical resistivity and tracer concentration were monitored along the vertical in an observation well located at 48 m to the east of the IW. ERT-derived tracer breakthrough curves were in agreement with geochemical sample measurements. Fracture geometry and hydraulic properties derived from ERT and pumping tests were further used to evaluate two mathematical conceptualizations that are relevant to fractured aquifers. Results of numerical analysis conclude that dual continuum model that combines matrix and fracture systems through a flow exchange term has outperformed equivalent continuum model in reproducing tracer concentrations at the monitoring wells (evident by a decrease in RMSE from 199 to 65 mg/L). A sensitivity analysis on model simulations conclude that spatial variability in hydraulic conductivity, local-scale dispersion, and flow exchange at fracture-matrix interface have a profound effect on model simulations.

Monitoring the Drainage Efficiency of Infiltration Trenches in Fractured and Karstified Limestone via Time-Lapse Hydrogeophysical Approach

“…However, in fractured rock systems, electrical techniques are a powerful tool to detect preferential pathways [41,42] and provide estimations of the main rock parameters, such as secondary porosity, dip and strike angle [43].…”

On the Performance of Pilot‐Point Based Hydraulic Tomography with a Geophysical a Priori Model

“…A one‐dimensional (1D) ERT experiment (line survey) was performed at the surface using a dipole–dipole configuration with a spread of 75 m and an electrode spacing of 1 m. The resistivity profile was synthetically generated in RES2DMOD using the known soil profile (Sreeparvathy et al 2019) for the aquifer. We considered a horizontally extended section to capture the complete profile because the depth of the analysis was constrained by the total spread at the surface.…”

“…A 2D aquifer analog representing typical granitic formations of southern India, in particular, in the Kandi region of Telangana, was generated using both the geological map and hydrogeological fence diagrams (Sreeparvathy et al 2019). Lithological profiles from 40 exploratory wells at the Indian Institute of Technology Hyderabad (17 • 29 N, 78 • 08 E, 542 m above sea level), Telangana, India formed the basis for the construction of the synthetic aquifer (Sreeparvathy et al 2019).…”

“…We did not resort to complicated rock physics to obtain a petrophysical relationship between the resistivity profile and the hydraulic parameters. Instead, we used field data from the Kandi region in Telangana obtained by Sreeparvathy et al (2019), to deduce the geological stratification of the aquifer. Using known ranges for the parameters for different strata, we imposed linear constraints on the hydraulic parameters during inversion.…”

“…These methods are noninvasive, relatively simple, and cost‐effective, but are limited in their application to map hydro‐stratigraphic units and potential groundwater‐bearing zones, owing to the inaccuracy of the correlation functions linking proxy parameters to hydraulic parameters that is primarily due to the complexity of rock physics (Slater 2007). Of the available geophysical techniques, electrical resistivity tomography (ERT) has been most widely used for hydrogeological characterization and to delineate potential groundwater‐bearing zones (Day‐Lewis et al 2006; Robert et al 2011; Hermans et al 2015; Sreeparvathy et al 2019).…”