Exploring the regolith with electrical resistivity tomography in large-scale surveys: electrode spacing-related issues and possibility
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Published:2021-04-07
Issue:4
Volume:25
Page:1785-1812
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ISSN:1607-7938
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Container-title:Hydrology and Earth System Sciences
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language:en
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Short-container-title:Hydrol. Earth Syst. Sci.
Author:
Gourdol Laurent, Clément RémiORCID, Juilleret JérômeORCID, Pfister LaurentORCID, Hissler ChristopheORCID
Abstract
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, larger ES – albeit
offering poorer shallow apparent resistivity data – is 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.
Publisher
Copernicus GmbH
Subject
General Earth and Planetary Sciences,General Engineering,General Environmental Science
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