Electrical resistivity monitoring of a collapsing meta-stable soil
P. JacksonK.J. NorthmoreDavid EntwisleDavid GunnA. E. MilodowskiD. I. BoardmanA. ZourmpakisC. D. F. RogersIan JeffersonNeil Dixon
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Abstract:
Amodel of electrical conduction through clay-coated, silt-sized quartz-grains inter-connected by clay-bridges (e.g. brickearth) is developed. Underpinned by SEM studies of brickearth, the model predicts resistivity to be proportional to the size of the quartz-grains, where the resistance afforded by clay grain-coatings and clay-bridges is comparable. The model accommodates resistivity that increases through bridge breakage and decreases through bridge compression. The resistivity of in-situ undisturbed brickearth was found to be in the range 15 to 35 ohm-m. At such low values we demonstrate that electrical flow is dominated by conduction within clay-coatings and their interconnecting clay-bridges, rather than in mobile pore-water. A small electrode array, buried at shallow depth beneath the load plate (1.0 m by 1.0 m) of a field collapse experiment, monitored resistivity to a depth of 1.5 m over a 260 hour period. While the water level beneath the load plate remained below 1.0 m depth, the resulting 3D inverted resistivity models detected water injected immediately beneath the plate; recording rapid increases, in stages over 90 minutes, in the depth interval 0.45 to 0.75 m directly under the plate, during what appears to be collapse. These increases are attributed to breaking of clay-bridges weakened by injected water.Keywords:
Electrical Resistivity Tomography
The amount and spatial distribution of plant roots are crucial ecological features, and methods based on soil electrical resistivity (r) tomography (ERT) have been proposed for their non-destructive measurement. ERT allows to map root systems in conditions where the contrast of ρ between soil and roots is high, but the electrical behaviour of resistive or heterogeneous soils may interfere with root-borne effects and requires investigation. We studied the spatial distribution of ρ in different soil-root conditions to test the hypothesis that ERT would allow to detect the spatial distribution of plant roots even when low contrast between roots and background soil variation was expected. High-resolution 2-D and 3-D DC (Direct Current) soil resistivity tomograms were used to compare areas of high and low vegetation density in containers where bare soil (LM), was compared to a Medicago sativa L. (HM) stand, and in resistive soils where a stand of Arundo plinii Turra (HA) was compared with a bare soil (LA) and the area under the canopy of Olea europaea L. (HO) was compared with interrow areas (LO). Destructive measurements of root biomass per unit soil volume (RD), soil electrical conductivity (EC), stone content (S) and water content (q) were made in all treatments. Soil resistivity was significantly affected by vegetation density, with a resistive response in HM, HA and HO. The response was related to RD with significant univariate relationships and the spatial pattern of soil resistivity was dominated by roots and other resistive features like stones in all soils. This allows to conclude that ERT is able to detect plant-root effects even in the presence of a resistive background but resistive features interfere with the mesasurements and need to be taken into account. Abbreviations: ρ = in-situ soil electrical resistivity; EC = electrical conductivity of soil samples; θ = volumetric water content; RD = root biomass per unit soil volume; ERT = electrical resistivity tomography; 2-D = Two-dimensional; 3-D = three-dimensional; DC = Direct Current.
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Abstract European forests are suffering considerably from the consequences of the droughts of recent years, and the exact reasons and influencing factors for this are still not fully understood. This study was conducted to characterize the changes and dynamics of soil moisture in a mixed forest in northern Bavaria within 1 year. Since electrical resistivity correlates well with soil water content, we used two‐dimensional electrical resistivity tomography (ERT) monitoring and time‐lapse analyses to supplement punctual measurements by sensors and soil analyses to show soil moisture changes throughout a whole year (2020–2021). While the topsoil dries out significantly from summer to autumn down to a depth of about 3 m, a clear increase in soil water content and a decrease in resistivity below 3 m can be observed during winter period. Anomalies in the topsoil (0–1 m) showing lower resistivities than the surrounding substrate could be related to tree positions by additional terrestrial laser scans. A significant relationship could be found between tree crown projection area and resistivity in 1–2 m depth. We found a trend that mean resistivity below pine is lower as below beech. ERT data were also used to estimate the soil water content via Archie's law and the results correlate strongly with the measured values, but the degree of correlation varies depending on the depth level. ERT as a noninvasive method, in combination with additional data, for example, on the vitality status of individual trees, could help to better understand root water uptake and water supply to trees, especially during periods of drought.
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Abstract Simplification of tillage practices is often considered as a solution for reducing time constraints and costs, and also for limiting soil erosion. Simplified tillage practices adopted over several years progressively induce modifications of soil physical properties in the top soil layer at decimetric scale. The general aim of this study was to test the capacity of electrical resistivity tomography to characterize the spatial variability of soil structure, bulk density and water flow in a loamy Cambisol with two modalities of tillage (conventional tillage, no tillage). For each tillage treatment, the experiment combined: 1) a description of the soil structure profile, 2) measurements of bulk density and soil hydraulic conductivities; 3) 2D electrical resistivity tomographies measured before and after an irrigation to characterize the spatial variability of soil water fluxes. These geophysical data are correlated to the spatial variability of soil structure and hydrodynamics properties.
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Abstract Soil wetness is an important property in determining the variable disposition of hillslopes to shallow landslides. Recent studies have demonstrated the potential of in situ soil wetness information for landslide early warning. However, the spatial representativeness of in situ sensors may be affected by local heterogeneities of soil properties and hydrological processes, and their installation may be destructive. Electrical resistivity tomography (ERT) has been used in the past to estimate plot‐scale soil moisture variation and may overcome these limitations. In this study, we installed and operated an automated ERT monitoring system on a landslide‐prone hillslope in the Napf region (Switzerland). The system was operational during a period of 9 mo, and measurements were conducted at high temporal resolution and under different soil hydrological conditions. Electrical resistivity was measured along two perpendicular profile lines in Wenner–Schlumberger configuration at 0.25‐m electrode spacing. Soil saturation was calculated by the Archie's law and the parameters were fitted with colocated soil moisture sensors. Comparison of ERT‐derived soil moisture with soil wetness from in situ sensors showed a good correlation, and infiltration properties critical for landslide early warning could be reliably reproduced. Further, analysis of spatial saturation variation revealed that ERT was capable to detect heterogeneities of soil hydrological process. Under highly saturated conditions, the reliability of the saturation estimation was affected by an increased number of faulty measurements and the spatial heterogeneity of the infiltration process.
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There is a gap between lab experiments where resistivity-soil moisture relations are generally very good and field studies in complex environmental settings where relations are always less good and complicated by many factors. An experiment was designed where environmental settings are more controlled, the best outside laboratory, to assess the transferability from lab to outdoor. A field experiment was carried out to evaluate the use of electric resistivity tomography (ERT) for monitoring soil moisture dynamics over a period of 67 days. A homogeneous site in the central part of The Netherlands was selected consisting of grass pasture on an aeolian sand soil profile. ERT values were correlated to gravimetric soil moisture samples for five depths at three different dates. Correlations ranged from 0.43 to 0.73 and were best for a soil depth of 90 cm. Resistivity patterns over time (time-lapse ERT) were analyzed and related to rainfall events where rainfall infiltration patterns could be identified. Duplicate ERT measurements showed that the noise level of the instrument and measurements is low and generally below 3% for the soil profile below the mixed layer but above the groundwater. Although the majority of the measured resistivity patterns could be well explained, some artefacts and dynamics were more difficult to clarify, even so in this homogeneous field situation. The presence of an oak tree with its root structure and a ditch with surface water with higher conductivity may have an impact on the resistivity pattern in the soil profile and over time. We conclude that ERT allows for detailed spatial measurement of local soil moisture dynamics resulting from precipitation although field experiments do not yield accuracies similar to laboratory experiments. ERT approaches are suitable for detailed spatial analyses where probe or sample-based methods are limited in reach or repeatability.
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