Geophysical estimation of 2D hydraulic conductivity for groundwater assessment in hard rock

Abstract

Accurate evaluation of aquifer potential is essential for groundwater assessments. Aquifer potential is mainly evaluated by aquifer parameters. Hydraulic conductivity (K) is the main aquifer parameter extensively measured in groundwater studies. Traditionally, boreholes are used to determine K. However, the conventional approaches have several limitations, can only provide point-scale K measurements, and cannot be performed in the high topography areas. On the other hand, geophysical approaches are less time-consuming and non-invasive, more cost-effective, faster, and can assess the subsurface hydrogeological conditions over large areas. In the past, several empirical-based geophysical studies were carried out to estimate K. However, in such studies, VES (vertical electrical sounding) method was used to estimate 1D K mostly in the homogeneous setting. Given the natural heterogeneity of hard rock terrains, the borehole/VES-based K causes uncertainty in accurate assessment of aquifer potential related to the weathered layers and fractures/faults. To this end, this contribution uses ERT (electrical resistivity tomography) method to predict 2D K for more reliable and thorough evaluation of groundwater potential in the heterogeneous sites. The proposed approach assesses the water-bearing ability of geological layers via definite ranges of K and resistivity, i.e., high potential aquifer enclosed by completely weathered/fractured rock via resistivity below 400 Ωm and K between 1.0 × 10^–1–9.5 × 10 m/d, medium potential aquifer of partly weathered/fractured rock with resistivity from 400 to 1000 Ωm and K from 3.5 × 10^–5 to 1.0 × 10^–1 m/d, negligible-potential aquifer of integral rock having resistivity variations between 1000 and 3000 Ωm and K between 6.5 × 10^–8 and 3.5 × 10^–5 m/d, and no potential aquifer of fresh rock with resistivity above 3000 Ωm and K from 1.5 × 10^–10 to 6.5 × 10^–8 m/d. Two deeper zones of weathered rocks were evaluated by the localized fractures/faults namely F1 and F2. This is the rare study, which estimates 2D K using ERT or any geophysical method. The obtained results, compared with the traditional approaches, offer much better evaluation of aquifer potential for groundwater assessment in the complex geological settings of hard rock.