Mapping water flow pathways in the Fengjiaping landslide using self-potential and electrical resistivity tomography

The cultivation of date palms (Phoenix dactylifera) is widespread in hyper-arid regions and relies on high-frequency irrigation to achieve satisfactory yields. Adequate irrigation management is of great importance, and requires understanding of the dynamics of sap flow and water storage within the date palm stem. Traditionally, sap flow estimates are obtained using heat dissipation probes. This method provides point estimates that may not represent the spatial distribution of sap flow within the date palm stem. The aim of this study is to investigate whether electrical resistivity tomography (ERT) measurements on date palm stems can be used to obtain information on the spatial distribution of sap flow in order to obtain improved estimates of transpiration. In a first step, laboratory experiments were used to improve understanding of the electrical and hydraulic properties of date palm stems. A laboratory set-up was developed that induced flow in a date palm stem segment using vacuum pressure while making time-lapse ERT measurements. It was found that such ERT monitoring allows to visualize changes in radial flow variability due to different flow conditions. In addition, the electrical conductivity of the outflow was considerably higher than that of the introduced solution, which suggest the presence of stored salt in the stem segment. The relationship between bulk electrical conductivity and water content of date palm stem segments was investigated on smaller samples using multi-step-outflow experiments combined with bulk electrical conductivity measurements. The results showed that the water redistribution in the sample was slow after the initial desaturation, which suggests that the water is tightly bound as in a clay soil. The observed relationship between bulk electrical conductivity and saturation could be described with models established for porous media. In a second step, field experiments were performed that combined ERT and sap flow measurements on both juvenile date palm trees growing in lysimeters and mature date palm trees. For this, a custom-made measurement system was used to acquire high-speed ERT measurements with a temporal resolution of several minutes. The high-resolution monitoring of both the juvenile and mature date palms showed a high spatial variability in electrical conductivity within both the juvenile and mature date palm stems. This has obvious implications for the installation of sap flow sensors, where low-conductivity areas likely indicating regions without flow should be avoided. ERT monitoring also revealed diurnal changes in the spatial distribution of the electrical conductivity that are associated with the tree response to irrigation. An induced drought period for the juvenile date palm in the lysimeter also resulted in a noticeable decrease in the mean electrical conductivity on the second day after irrigation was stopped, suggesting that ERT may also provide an early indicator of water stress.

Quantitative characterization of solute transport processes in the laboratory using electrical resistivity tomography

Trees are thought to use internal water stored within the stem as a key buffer against short-term drought. Together with deep root water uptake and stomatal regulation, this mechanism helps trees to maintain critical physiological functions alive. Stem moisture content and water potential is generally observed using electrical conductivity (EC), moisture sensors, or microtensiometers installed in the sap wood of living trees. While these sensors offer precise information at the point scale, applying imaging techniques such as Electrical Resistivity Tomography (ERT) can help the investigation of spatial patterns and internal changes in electrical conductivity across larger portions of the trunk. Although ERT monitoring was mainly developed for hydrogeological applications to track soil moisture or groundwater dynamics at scales of tens to hundreds of meters, it has more recently been adopted in forest ecohydrological research for smaller-scale applications. ERT imaging of tree stems has proven its ability to inform on internal structures of trunks, while time-lapse ERT has shown promise to inform on stem water content variations.Here, we present results from a field experiment conducted on a mature beech tree (Fagus sylvatica L.) at the Weierbach Experimental Catchment (WEC) in Luxembourg. The tree has been equipped with 4 rings of 30 stainless-steel screw electrodes each, with 5 cm electrode spacing and 50 cm vertical spacing between rings. ERT data was acquired during the growing season, from March to November, at a 4-hour temporal resolution. Additional tree sensors installed on the same tree, including sap flux, radial growth, moisture, water potential and temperature, provide complementary measurements for comparison with the ERT results.At the seasonal scale, observations indicate spatially consistent changes in resistivity, with progressive resistivity decrease in the sap wood during the growing season. At the diel scale, pronounced daily variations in electrical resistivity are also observed, which seem to follow physiological processes also picked up by sap flow sensors and dendrometers. We will discuss challenges linked with the downscaling of the time-lapse ERT technique, both in time and space. These include: (i) accounting for strong temperature effects within the stem that influence reconstructed resistivity models and require advanced correction methods, and (ii) accurately determining electrode geometry at high resolution, including electrode orientation and seasonal changes in stem diameter. Finally, we address the interpretation of resistivity changes in terms of wood moisture dynamics and potential variations in sapwood chemical composition.

Combining time-lapse electrical resistivity tomography and air injection to detect agricultural subsurface drains

This study underscores the need for subsurface imaging and monitoring techniques to offer timely information on railway embankment condition and to contribute to the decision-making processes needed to minimise the risks of catastrophic slope failure. We investigate electrical resistivity tomography (ERT) as a means of providing railway earthwork asset condition assessment information through the deployment of a bespoke ERT monitoring system (PRIME – the Proactive Infrastructure Monitoring and Evaluation system), which has been specifically developed for geotechnical monitoring applications.We focus on two test sites, Botley and Withy Beds, which are situated on mainline railway embankments in the UK near Southampton and London respectively. Both embankments have long histories of slope instability and are constructed from London Clay (a high plasticity clay widely associated with ground deformation problems). Long-term ERT monitoring infrastructure has been deployed across both sites to enable imaging of subsurface heterogeneity and to monitor subsurface moisture content variations. At Botley a grid of electrodes extending from the embankment shoulder to toe, over an area of ~20 by 30 m, was deployed to enable time-lapse 3D imaging of a progressive rotational failure at the site, whilst at Withy Beds a line of electrodes was deployed along the embankment toe to enable time-lapse 2D imaging for a ~300m length of susceptible embankment.  Manual geodetic (total station and LiDAR) monitoring of the slope geometry and electrode positions, and conventional geotechnical monitoring using temperature, soil moisture and matric suction sensors have also been used at the sites to validate the results of the ERT monitoring. In additional, laboratory petrophysical testing of samples from the sites has been used to establish relationships between resistivity, moisture content and matric suction.More than three-years of ERT monitoring data have been collected from the sites. Initial analyses of the results have shown strong correlations between the conventional geotechnical monitoring results and ERT derived estimates of soil moisture. At the site scale, a remarkably clear low-resistivity layer can be seen in the middle embankment segment of Botley, which suggests a high clay content and likely limited hydraulic permeability. The properties of this layer, in conjunction with time-lapse ERT observations made during periods of heavy rainfall, have revealed the hydrological functioning of the slope and the strong influence of evapotranspiration associated with clusters of mature trees. On the other hand, the Withy Beds embankment shows less intense drying and wetting patterns, even though noticeable fluctuations in resistivity suggest the presence of localised zones of moisture build-up. The sandy sections at the Withy Beds site are consistently dry even after rainfall, which permits water to seep into the clay layer beneath. On the other hand, the clay lands have higher moisture content and exhibit summertime surface drying.In this study we have provided unprecedented insights, in terms of ERT monitoring duration and spatiotemporal resolution, into the structure and moisture dynamics of mainline railway embankments. ERT has been demonstrated as novel means of providing operationally relevant condition monitoring information to support the management of vulnerable railway earthworks associated with complex ground conditions.

Revealing the dynamics of groundwater movement in the vadose zone is crucial to groundwater management and artificial recharge development. In this study, the groundwater flow characterization of the pumping process is monitored by the time-lapse electrical resistivity tomography (ERT) and self-potential (SP)data tomography. The ERT data invert the resistivity distribution. Then, we combine the SP data and resistivity result to invert the apparent current density and estimate the permeability based on the Poisson equation. A total of 24 hours of time-lapse surveys are taken during the pumping and recharge of groundwater. The results show a significant increase in in resistivity and permeability during pumping water, which suggests groundwater drawdown. Similarly, significant decrease resistivity and permeability are observed during the recovery indicating groundwater recharge. These results have a significant agreement with the groundwater table monitoring result. Our experiment test suggests that combine ERT and SP data could provide a reliable way in groundwater or other hydrogeological surveys.

Electrical resistivity tomography (ERT) monitoring provides time-lapse images of the subsurface. These images can be used to assess spatiotemporal variation in moisture content, which is a key driver of slope failure, making ERT monitoring an effective tool to evaluate precursory conditions of failure. This work presents the results of ERT monitoring on a slope above a major highway located on the border between England and Wales. During highway construction in the 1960s the slope was subject to several large landslide events which resulted in the re-design of the carriageway and installation of engineered mitigation measures. A section of the slope known as the ‘partially slipped area’ exhibited partial displacement during this time but did not progress to full slope failure, and therefore presents an ongoing risk to the highway, even though it does not experience ongoing displacement. An ERT monitoring system was installed across this area to monitor subsurface variations in moisture content. The results show a complex pattern of subsurface moisture dynamics within the partially slipped area when compared to the adjacent area of stable slope. This is most likely a result of the uneven and hummocky terrain in the partially slipped area and its effects on rainfall infiltration, storage and drainage, combined with the displacement-induced jointing present in the underlying sandstone units. The ERT results are used to assess the volume of unstable ground, placing the volume at the upper end of estimates from previous studies. Furthermore, analysis of the ERT dataset for surface displacements shows no movement at the site, which is confirmed by analysis of differential LiDAR plots and ground motion data derived from InSAR. This study demonstrates the application of ERT monitoring on a low activity, high risk slope, highlighting the need to understand subsurface processes at the slope-scale to inform long-term slope management.

Integrated hydrogeophysical modelling and data assimilation for geoelectrical leak detection

Excessive groundwater abstraction in coastal areas exacerbates saltwater intrusion (SWI), a widespread global issue. Characterization of mechanisms delivering saltwater to wells can assist in developing suitable SWI mitigation strategies for reducing the risk of groundwater degradation. This paper presents findings from hydrogeological monitoring, time-lapse electrical resistivity tomography (ERT) and self-potential (SP) measurements to investigate SWI under natural and artificially perturbed conditions in a quasi-homogeneous pristine coastal sand aquifer, affected by large tidal ranges (>2m). Time-lapse ERT surveys conducted under undisturbed conditions identified an upper saline recirculation cell (IRC) beneath the intertidal zone, arising due to seawater infiltrating into an underlying ∼20m thick sand sequence containing fresher groundwater, with resistivity variations noted between spring and neap tides. Measurements taken during a 69-h constant-rate pumping test, discharging at 10.2L/s, revealed that pumping drew saline water from the IRC towards abstraction wells. This resulted in saltwater contributions to discharge increasing from 1.4 to 4.1%, consistent with the decrease in resistivity detected in ERT profiles between 3m and 7m below surface. Over the same period, SP signals fell by between 20 and 30mV with greater declines occurring at locations nearer to the high-water mark. Monitoring data suggest that these changes in SP are primarily due to saline water intrusion from the IRC, rather than pressure changes resulting from pumping. Research findings provide further evidence that SP monitoring could act as a key geophysical early warning parameter for SWI, while ERT data further highlight the potential for monitoring SWI in shallow coastal aquifers. This study also demonstrates that optimal groundwater abstraction strategies in tidal-influenced coastal aquifers can be achieved by targeting deeper zones.

In France, 10% of total arable land is equipped with subsurface drainage systems, to control winter and spring waterlogging due to a temporary perched water table. Most of these systems were installed in the1980s and have aged since then and may now need maintenance. Sometimes, the location of the systems is known, but the standard situation in France is that the original as-built master sketches are no longer available. Performance assessment of drainage systems and curative actions are complicated since drain location is unknown. In this article, the authors test the application of a non-destructive drain detection method which consists in water injection at the outfall of the drainage network combined with time-lapse electrical resistivity tomography (ERT) monitoring. To assess the performance of this methodology, which consists in measuring electrical resistivity from electrodes placed at the nodes of a 1.2-m regular mesh, the authors interpreted the signal using a two-step approach. The first step is based on 3D ERT numerical modelling during a scenario of surface infiltration processes (forward modelling followed by geophysical inversion); this step optimizes the ERT method for locating the infiltration at depths below 1m. The second step is the validation of the results obtained by numerical modelling with an experimental data set, using water injection into the drainage network combined with time-lapse ERT monitoring on an experimental field site. The results showed the relevance of time-lapse ERT monitoring on a small agricultural plot for locating the drainage network. The numerical results also showed several limitations of the combined methodology: (i) it is necessary to use an electrode spacing unit less than 1.20m, which does not facilitate investigation on large agriculture plots, (ii) measurements must be taken when resistivity contrast is the strongest between the infiltration area and the soil and (iii) the volume of water needed for injection can limit the extension of the method.

Water saturation in the bedrock or colluvium is highly related to most landslide hazards, and rainfall is likely a crucial factor. The dynamic processes of onsite rock/soil mechanics could be revealed via monitoring using the electrical resistivity tomography (ERT) technique and Archie’s law. This study aims to investigate water saturation changes over time using time-lapse ERT images, providing a powerful method for monitoring landslide events. A fully automatic remote resistivity monitoring system was deployed to acquire hourly electrical resistivity data using a nontraditional hybrid array in the Lantai area of Yilan Taiping Mountain in Northeast Taiwan from 2019 to 2021. Six subzones in borehole ERT images were examined for the temporal and spatial resistivity variations, as well as possible pathways of the groundwater. Two representative cases of inverted electrical resistivity images varying with precipitation may be correlated with water saturation changes in the studied hillslope, implying the process of rainfall infiltration. Layers with decreased and increased electrical resistivity are also observed before sliding events. Accordingly, we suggest that high-frequency time-lapse ERT monitoring could play a crucial role in landslide early warning.

In this paper, we discuss the results of long-term electrical resistivity tomography (ERT) monitoring of a critical slope located on an important high-speed railway gallery. Data were acquired by a customized ERT system from 24 March 2022 until 31 august 2023 and were analysed with the final objective of defining thresholds of attention for resistivity changes derived from water table fluctuations after heavy rainfalls. This helps the authorities in reducing the hydrogeological risk impacts related to potential slope instabilities triggered by extreme meteorological conditions. In order to continuously observe water level changes, five piezometers were also integrated with the ERT monitoring system which is also accompanied by a meteorological station. All datasets were inverted using a time-lapse algorithm that was optimized to minimize artifacts generated by the subsurface complex geology of the site. Due to the long period considered, seasonal temperature corrections on resistivity values were also explored by calibrating a seasonal model of soil temperature versus depth and evaluating the corresponding effects on the resistivity tomographic maps. Finally, the correlation between resistivity values and piezometric levels was studied by producing scatterplot graphs for a selected subzone of the ERT sections. Based on this analysis, a preliminary threshold of attention was defined.

A filtering method to correct time-lapse 3D ERT data and improve imaging of natural aquifer dynamics

Time-lapse electrical resistivity tomography (ERT) monitoring of used engine oil contamination in laboratory setting

Electrical resistivity tomography (ERT) monitoring experiments are being conducted more often to image spatiotemporal changes in soil properties. When conducting long-term ERT monitoring, the identification of suspicious electrodes in a permanent spread is of major importance because changes in electrode contact properties of a single electrode may affect the quality of many measurements on each time-slice. An automated methodology was developed to detect these temporal changes in electrode contact properties, based on a Bayesian approach called “weights of evidence.” Contrasts (CW) and studentized contrasts (CS) are estimators of the influence of each electrode in the global data quality. A consolidated studentized contrast (CCS) is introduced to consider the proportion of rejected quadripoles which contain a single electrode. These estimators are computed for each time-slice using Q-factor (coefficient of variation of repeated measurements) threshold values, from 0 to 10%, to discriminate between selected and rejected quadripoles. An automated detection strategy is proposed to identify suspicious electrodes by comparing the CCS to the MECS (maximum expected CS values when every electrode is good for the given data set). These MECS are computed using Monte-Carlo simulations of a hundred random draws where the distribution of Q-factor values follows a Weibull cumulative distribution, with k=0.421 and λ=1.303, fitted on a background data set filtered using a 5% threshold on absolute reciprocal errors. The efficiency of the methodology and its sensitivity to the selected reciprocal error threshold are assessed on synthetic and field data. Our approach is suitable to detect suspicious electrodes and slowly changing conditions affecting the galvanic contact resistances where classical approaches are shown to be inadequate except when the faulty electrode is disconnected. A data-weighting method is finally proposed to ensure that only good data will be used in the inversion of ERT monitoring data sets.