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.
Highlights
Within the critical zone, the architecture and properties of the regolith, as well as its distribution across the landscape, play a key role in how rainfall is collected, stored and released to generate streamflow (Schoeneberger and Wysocki, 2005; Lin, 2010; Ghasemizade and Schirmer, 2013; Brooks et al, 2015)
Note that since the results obtained for both arrays are very similar, we only present those obtained from the Wenner– Schlumberger array and that fit the field case study (Figs. 5– 7, Tables 1–2)
For both arrays and whatever the electrode spacing (ES) retained, that resistivity and thickness contrasts play a key role in the resulting inverted Electrical resistivity tomography (ERT) images
Summary
The architecture and properties of the regolith, as well as its distribution across the landscape, play a key role in how rainfall is collected, stored and released to generate streamflow (Schoeneberger and Wysocki, 2005; Lin, 2010; Ghasemizade and Schirmer, 2013; Brooks et al, 2015) Factors such as the depth and composition of the soil cover and the rock weathering determine water pathways, storage capacity, residence times in the subsurface and subsequent interactions with surface water bodies (Freer et al, 2002; Hopp and McDonnell, 2009; Graham et al, 2010; Gabrielli et al, 2012; Lanni et al, 2013; Ameli et al, 2016). Several authors have recently pointed out the subsurface as being the greatest knowledge gap in the understanding/modelling of hydrological processes, with a greater investment in “seeing” the subsurface needed to provide the Earth system modelling community with critical guidance on how to parameterize model subsurface structure depths and properties (Fan et al, 2019)
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