Characterizing near-surface fractured-rock aquifers: Insights provided by the numerical analysis of electrical resistivity experiments

Fractured-rock aquifers represent an important part of the groundwater that is used for domestic, agricultural, and industrial purposes. In these natural systems, the presence and properties of fractures control both the quantity and quality of water extracted, meaning that knowledge about the fractures is critical for effective water resource management. Here, we explore through numerical modeling whether electrical resistivity (ER) geophysical measurements, acquired from the Earth's surface, may potentially be used to identify and provide information about shallow bedrock fractures. To this end, we conduct a systematic numerical modeling study whereby we evaluate the effect of a single buried fracture on ER-profiling data, examining how the corresponding anomaly changes as a function of the fracture and domain characteristics. Two standard electrode configurations, the Wenner-Schlumberger (WS) and dipole-dipole (DD) arrays, are considered in our analysis, with three different spacing factors. Depending on the considered electrode array, we find that the fracture dip angle and length will impact the resistivity anomaly curves differently, with the WS array being better adapted for distinguishing between sub-horizontal and sub-vertical fractures, but the DD array leading to larger overall anomaly magnitudes. We also find that, unsurprisingly, the magnitude of the resistivity anomaly, and thus fracture detectability, is strongly affected by the depth of overburden and its electrical resistivity, as well as the fracture aperture and contrast between the fracture and bedrock resistivities. Further research into the electrical properties of fractures, both above and below the water table, is deemed necessary.