Characterizing Near-Surface Fractured-Rock Aquifers: Insights Provided by the Numerical Analysis of Electrical Resistivity Experiments

Abstract

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.

Determination of Soil Properties from Electrical Measurements in Agricultural Plots, Villa de Arriaga, San Luis Potosí, Mexico

“…One of the geoelectrical methods widely used in near surface studies is EP. The EP method is commonly used to study aquifers and rock properties (Ucha et al, 1984), to support archaeological surveys (Perdomo, 2013), geotechnical studies (Adebisi et al, 2016), geological mapping, and the detection of fractures (Demirel et al, 2018). The principle of the method consists of performing resistivity measurements through a four-electrode array AMNB, similar to the one used in the ERT survey, along a line or profile on the surface.…”

Using electrical resistivity tomography to evaluate the relationship between groundwater potencial and tectonism in northeast Mexico

“…The lateral resistivity changes in the Cuatrociénegas sector indicate the possible upward and downward blocks through normal faults. Instead of downward blocks, there may also be fractured limestones (Robert et al, 2011;Demirel et al, 2018) or paleo-channel zones (dissolution zones; Nimnate et al, 2017). Low resistivity values in limestone (15-30 Ωm) indicate water saturation, and therefore, indicate the location of the carbonate aquifer (Figures 4 and 5).…”

“…Particularly, in transect 4, slightly vertical block movements are inferred from the interpreted normal faults (Figure 7). The subsided blocks can indicate highly fractured rocks or a paleo-channel zone (Robert et al, 2011;Nimnate et al, 2017;Demirel et al, 2018).…”

Caracterização de Suscetibilidade ao Colapso por Análise Estrutural e Geofísica em Área Cárstica no Município de Sete Lagoas - MG

“…Posteriormente, os dados foram tratados no software OpenStereo (Grohmann et al, 2011). Os dados de fraturas foram organizados em diferentes famílias e classificados quanto a sua frequência e abertura (ISRM, 1983), uma vez que tais parâmetros são relevantes para a detecção de descontinuidades por eletrorresistividade (Demirel et al, 2018).…”