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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/hess-2020-374
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2020-374
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  31 Jul 2020

31 Jul 2020

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This preprint is currently under review for the journal HESS.

Exploring the regolith with electrical resistivity tomography in largescale surveys: electrode spacing related issues and possibility

Laurent Gourdol1, Rémi Clément2, Jérôme Juilleret1, Laurent Pfister1, and Christophe Hissler1 Laurent Gourdol et al.
  • 1Catchment and Eco-hydrology Research Group (CAT), Luxembourg Institute of Science and Technology (LIST), Belvaux, L-4422, Luxembourg
  • 2REVERSAAL Research Unit, National Research Institute for Agriculture, Food and Environment (INRAE), Villeurbanne, F-69626, France

Abstract. Within the Critical Zone, regolith plays a key role in the fundamental hydrological functions of water collection, storage, mixing and release. Electrical Resistivity Tomography (ERT) is recognized as a remarkable tool for characterizing the geometry and properties of the regolith, overcoming limitations inherent to conventional borehole-based investigations. For exploring shallow layers, a small electrode spacing (ES) will provide a denser set of apparent resistivity measurements of the subsurface. As this option is cumbersome and time-consuming, smaller ES – albeit offering poorer shallow apparent resistivity data – are often preferred for large horizontal ERT surveys. To investigate the negative trade-off between larger ES and reduced accuracy of the inverted ERT images for shallow layers, we use a set of synthetic conductive/resistive/conductive three-layered soil–saprock/saprolite–bedrock models in combination with a reference field dataset. Our results suggest that an increase in ES causes a deterioration of the accuracy of the inverted ERT images in terms of both resistivity distribution and interface delineation and, most importantly, that this degradation increases sharply when the ES exceeds the thickness of the top subsurface layer. This finding, which is obvious for the characterization of shallow layers, is also relevant even when solely aiming for the characterization of deeper layers. We show that an oversized ES leads to overestimations of depth to bedrock and that this overestimation is even more important for subsurface structures with high resistivity contrast. To overcome this limitation, we propose adding interpolated levels of surficial apparent resistivity relying on a limited number of ERT profiles with a smaller ES. We demonstrate that our protocol significantly improves the accuracy of ERT profiles when using large ES, provided that the top layer has a rather constant thickness and resistivity. For the specific case of large-scale ERT surveys the proposed upgrading procedure is cost-effective in comparison to protocols based on small ES.

Laurent Gourdol et al.

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Short summary
Electrical Resistivity Tomography (ERT) is a remarkable tool for characterizing the regolith, but its use over large areas remains cumbersome due to the requirement of small electrode spacing (ES). In this study we document the issues of using oversized ES and propose a new approach to overcome this limitation. We demonstrate that our protocol significantly improves the accuracy of ERT profiles using large ES and offers a cost-effective means for carrying out large-scale surveys.
Electrical Resistivity Tomography (ERT) is a remarkable tool for characterizing the regolith,...
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