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.
Highlights
Groundwater, which represents about 98% of all liquid freshwater on Earth, is the most extracted raw material from the subsurface in the world
Cases 1–6, in order to assess (i) how the curves are influenced by the particular details of the fracture configuration; and (ii) whether the sensitivity of the electrical resistivity (ER)-profiling data to fracture dip angle and length is high enough to suggest that we might be able to eventually invert for these parameters
In contrast to analytical solutions for ER methods that have been developed for simple configurations related to fractures, for example an infinite low-resistivity vertical dyke embedded in a homogeneous half-space [27], the use of numerical modeling in our paper allows for consideration of more complex and realistic scenarios involving various fracture angles and lengths, as well as different background properties including the presence of a layer of overburden
Summary
Groundwater, which represents about 98% of all liquid freshwater on Earth, is the most extracted raw material from the subsurface in the world. Numerous studies have focused on the development of methods to identify and characterize subsurface fractures and fracture networks, in order to improve conceptual and numerical models of groundwater flow and contaminant transport [7,8,9,10] In this context, applied geophysical methods are promising, because many of the physical properties to which these methods are sensitive are strongly influenced by the presence of fractures, and the corresponding measurements can be acquired rapidly and non-invasively from the ground surface and/or from boreholes. The presented results and their detailed interpretation lead to improved understanding of (i) the signature of a buried fracture in ER-profiling data and how it changes as a function of the fracture and overburden characteristics; (ii) the importance of this signature in comparison with the inherent noise in ER measurements; and (iii) the impact of the electrode array on fracture detectability and resolution with ER methods
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