Monitoring Long-Term Subgrade Moisture Changes with Electrical Resistivity Tomography

<p>A full understanding of the moisture regimes in agricultural soils is critical when considering a climatic future that is potentially both drier and hotter. The study of near-surface hydrodynamics necessitates a high temporal frequency of measurements to allow the capture of fast-moving processes. In addition to this, heterogeneities in soils, coupled with spatially variable moisture contents, mean that point sensors alone can miss valuable information when studying soil hydrodynamics.</p><p>The sensitivity of Electrical Resistivity Tomography (ERT) to soil water content means that it is ideally suited to studying soil hydrodynamics. As a non-invasive technique, it is able to effectively capture spatial heterogeneities, while repeating ERT measurements at short time intervals enables the monitoring of rapid soil moisture changes.</p><p>In order to study Conservation Agriculture (CA) - an agricultural technique shown to have increased water and heat stress resilience - we collect frequent ERT measurements (≤ daily) over a long period of time (> 2 years) using PRIME - a low-cost, low-power, ERT monitoring instrument developed by the British Geological Survey. Our equipment is permanently installed at three agricultural observatories in southern Africa (Malawi, Zimbabwe, and Zambia), and is complemented by many co-located point sensors monitoring soil temperature, water content, and matric potential.</p><p>We compare the hydrodynamics - derived from ERT data - found under CA, with those under conventional agricultural tillage methods, to better understand the root of CA’s resilience to hot and dry weather conditions. We use laboratory measurements together with the high number of co-located point sensor measurements to build pedological / geophysical relationships for each site, comparing those derived from laboratory scale samples to those derived from in-field measurements.</p>

The aim of this work was study the effects of climate and vegetation on the moisture content present in soils that support a complex vegetation system which comprises a pasture and micro-forest area, at the Center for Innovation and Technology Transfer of the Technological University of Panama located in Aguadulce, Coclé. For such purposes, a geophysical monitoring was carried out through four electrical resistivity tomographies on a 23.5 m long profile developed in the rainy season from July 9 to October 29, 2019 with time lapses of 38, 34 and 39 days between them. The datasets were inverted using a time-lapse ERT code based on the Boundless Electrical Resistivity Tomography (BERT). With the results obtained in this work (electric resistivity tomographies), was possible to delimit the electrical anomalies related to the physical characteristics of the subsoil in both: the pasture and the micro-forest area. Geophysical monitoring and volumetric moisture content calculated via Archie's law, allowed quantifying the dynamics of moisture content in both areas with significant percentage differences between -50% and 100% over the micro-forest area. This leads us to evaluate the levels of moisture absorption in both areas and consider the problem of land use and the impact it has on the groundwater recharge processes in this important region of the country.

Abstract. Arctic regions are under immense pressure from a continuously warming climate. During the winter and shoulder seasons, recently deglaciated sediments are particularly sensitive to human-induced warming. Understanding the physical mechanisms and processes that determine soil liquid moisture availability contributes to the way we conceptualize and understand the development and functioning of terrestrial Arctic ecosystems. However, harsh weather and logistical constraints limit opportunities to directly observe subsurface processes year-round; hence automated and uninterrupted strategies of monitoring the coupled heat and water movement in soils are essential. Geoelectrical monitoring using electrical resistivity tomography (ERT) has proven to be an effective method to capture soil moisture distribution in time and space. ERT instrumentation has been adapted for year-round operation in high-latitude weather conditions. We installed two geoelectrical monitoring stations on the forefield of a retreating glacier in Svalbard, consisting of semi-permanent surface ERT arrays and co-located soil sensors, which track seasonal changes in soil electrical resistivity, moisture, and temperature in 3D. One of the stations observes recently exposed sediments (5–10 years since deglaciation), whilst the other covers more established sediments (50–60 years since deglaciation). We obtained a 1-year continuous measurement record (October 2021–September 2022), which produced 4D images of soil freeze–thaw transitions with unprecedented detail, allowing us to calculate the velocity of the thawing front in 3D. At its peak, this was found to be 1 m d−1 for the older sediments and 0.4 m d−1 for the younger sediments. Records of soil moisture and thermal regime obtained by sensors help define the conditions under which snowmelt takes place. Our data reveal that the freeze–thaw shoulder period, during which the surface soils experienced the zero-curtain effect, lasted 23 d at the site closer to the glacier but only 6 d for the older sediments. Furthermore, we used unsupervised clustering to classify areas of the soil volume according to their electrical resistivity coefficient of variance, which enables us to understand spatial variations in susceptibility to water-phase transition. Novel insights into soil moisture dynamics throughout the spring melt will help parameterize models of biological activity to build a more predictive understanding of newly emerging terrestrial landscapes and their impact on carbon and nutrient cycling.

Málaga town presents a serious problem caused by the insufficient environmental offer of the superficial water resource to supply the dotation corresponding to the needs of the urban area. Although the geographic and geological location generates optimal conditions for the occurrence of groundwater, studies on the existence, location and availability are incomplete and fragmented, and in general, there is no information about the hidryc resource. The main objective of the present paper pretends to build through the applied geophysical prospection Vertical Electric Sounding (VES) and Electrical Resistivity Tomography (ERT) the 2D model of electrical resistivity of the subsurface, from the processing of synthetic data to the treatment of geoelectrical information with hydrogeological purposes. The data acquisition and processing allowed the interpretation, from the geoelectric point of view and the basic analysis of the 1D profiles and the 2D models developed; demonstrating the presence of highly fractured aquifers that can provide the municipal supply in the future; results that establish the first step in the evaluation of Málaga’s water resources

Electrical resistivity tomography (ERT) is a non-destructive geophysical method that provides images of the soil electrical resistivity. In particular, this study focuses on small-scale soil subsurface areas (up to 1 m deep). Soil resistivity depends on several factors, particularly soil water content. Consequently, ERT has proven to be effective in detecting water flow pathways during infiltration tests. ERT has already been used for hydroecological and hydrogeological applications, but its use still requires further investigation and new developments, particularly for specific processes such as preferential flows. Indeed, water infiltration into the soil is all but homogeneous with the occurrence of preferential flows due to several factors (lithological heterogeneity, macropores, cracks, macrofauna galleries, and root channels). The presence of these preferential flows promotes the downward movement of water and solutes to deeper soil horizons. Thus, the detection of these flows using geophysical methods, such as ERT, should provide a better understanding of the dynamics of preferential flows. However, conventional ERT inversion methods have proven unable to provide insight into the processes at adequate scales (e.g., the macropore scale) and have shown difficulties in detecting sharp variations because of smoothing constraints. Information from apparent resistivity may be lost because of these constraints.To overcome these limitations, an inversion method based on Convolutional Neural Networks (CNNs) is proposed to detect small-scale resistivity heterogeneities. For the training step, we designed a specific generator to produce synthetic resistivity data for various 2D random electrical resistivity distributions mimicking different types of soil heterogeneity (earthworm and root induced macropore, layering, etc.) and for typical protocols for ERT acquisition. For each case, our database associates the true resistivity field with the apparent resistivity. This database contains a large number of training pairs that allow machine learning.The preliminary results show that while some predictions were able to predict properly the soil heterogeneities (shape and size), the values of the estimated true resistivity were far from the target values. To avoid unrealistic estimates, we added physical constraints during the training process. An additional forward calculation was performed based on the true resistivities predicted by the neural network, then the apparent resistivities corresponding to the predicted resistivities were compared to the apparent resistivities corresponding to the targeted true resistivities and a supplementary loss function was added to the initial loss function. At this stage, the neural networks were trained on heterogeneous resistivity distributions in the soil with neither time evolution nor link to hydrological processes. In the future, the resistivity generator will be coupled with hydrological models to simulate water infiltration dynamics and changes in soil water content over time. This perspective is essential for applying the proposed method to detect flow pathways during infiltration tests.

Climate change is altering the dynamics of groundwater fluctuations and posing new challenges for groundwater management worldwide. The decline in winter snow cover shifts precipitation infiltration more toward the winter season, while a prolonged vegetation period enhances evapotranspiration, leading to greater summer groundwater depletion. Extreme weather events such as floods and droughts, together with increasing water extraction driven by rising water demand, promote repeated cycles of drying and rewetting in near-surface, unconsolidated sediments. Over time, these cycles alter the hydromechanical properties of the subsoil and increase its susceptibility to deformation and subsidence.In this study, we investigate these subsidence and deformation processes at historical monuments in central Germany, which have experienced pronounced structural damage. Since 2024, five observation sites of historic churches in the federal states of Saxony and Saxony-Anhalt have been monitored. These sites were selected because they are predominantly located in rural regions, where groundwater systems are comparatively less affected by urban-related stressors, allowing climate-related groundwater fluctuations to be examined with reduced interference from superimposed anthropogenic signals. The monuments were constructed several centuries ago and have remained largely stable over time. However, after several years of extreme weather conditions, significant cracks began to appear around 2016. In some cases, the buildings were temporarily classified as being at risk of collapse. Since the damage did not occur immediately following individual extreme events but developed over an extended period, the long-term trend in subsurface water saturation needs to be investigated. To distinguish persistent drying trends from seasonal fluctuations, quarterly electrical resistivity tomography (ERT) measurements were conducted in the vicinity of the monuments along fixed profiles with lengths of up to 160 m during six field campaigns between April 2024 and November 2025. During the observation period, the electrical resistivity in the shallow subsurface increased significantly, indicating progressive desiccation to a depth of approximately 5 m, with wintertime rewetting insufficient to restore moisture levels. This prolonged desiccation likely induced further shrinkage and deformation, especially in the clay-rich layers. In contrast, a decrease in electrical resistivity was measured in the deeper layers, indicating a higher moisture content compared to the drier upper soil layers. Continued monitoring will further contribute to determining the long-term effects of climate variability on subsurface moisture dynamics, delineating zones with critical moisture changes, and linking these to settlement-prone areas of the monuments.

Earth Embankments constructed from compacted material are an integral part of any flood defence network. In the UK alone more than 34,000km of embankments protect housing and infrastructure from estuarine and coastal flooding. Many such embankments are in a poor state of repair and with rising sea levels and increased levels of precipitation predicted results of climate change, such flood defences will be tested with increasing regularity. Fissures forming due to desiccation in clayey soils have been shown as a potential source of embankment failure during overtopping. Current methods of visually identifying fissures in flood embankments are inadequate particularly when faced with dense vegetation. The extent of fissuring cannot be discovered without the use of trenching, which is costly and destructive to the embankment structure. Electrical Resistivity Tomography (ERT) offers a non‐invasive method for appraising the embankment structure. ERT has been shown to be sensitive to the presence of fissures in laboratory studies and an effective method for monitoring the onset of fissuring with inversion models mapping the fissure positions and allowing their growth to be observed. The presented study compares the use of a survey using ERT on the Thorngumbald embankment, near Kingston Upon Hull, UK, with previous laboratory studies on scale models using miniature resistivity arrays. Large scale scans across the slopes of the embankment showed areas of confirmed fissuring, while miniature arrays were used to fully map the fissures in the subsurface. The positions of surface fissures matched with those resolved from the resistivity data and the results compare favorably with those achieved in the laboratory. Additionally scans were taken across the embankment revealing serious fissuring on the landward face, the side most affected by heavy vegetation.

Cavities located in the near surface represent a major hazard especially in sites located in highly urbanized town centers. For this reason, the location, the determination of size and shape of the cavity and the knowledge of the elastic properties of the overlying subsoil are necessary to evaluate the risk of subsidence and to plan the intervention for safety restoration. In this study, electrical resistivity tomography and seismic refraction tomography were performed with the twofold purpose of detecting the cavities and of optimizing an experimental procedure to better identify and locate them. These methods were tested on both synthetic and real data and the integrated geophysical approach has proven successful to detect single cavities together with their physical and geometrical characteristics.

A 2D surface electrical resistivity is one of non-destructive methods to investigate groundwater. In this study, 2D resistivity method was used to produce subsurface imaging profiles which are known as ERT (Electrical Resistivity Tomography) at Block C, Sawah Sempadan, Tanjung Karang, Selangor. These ERT profiles were then used to create 3D model view of potential aquifer at the study area. Overall, there were 3 ERT profiles of 1600m for major survey lines and 6 ERT profiles of 400m for minor lines. ERT profiles were then compared to lithology log from nearby tube wells of same geological formation in order to locate potential aquifer location in these profiles. Results show that there were 7 locations of identified potential aquifer in this study area. Later, the resistivity data were added with longitude and latitude data before imported into Voxler to achieve the objective of the study. The 3D model view of skeleton shape was productively built in Voxler as it visibly illustrates the whole subsurface profiles of study area. Locations of potential aquifers were identified in this 3D model view to show the exact location and depth of each potential aquifer.

With time‐lapse electrical resistivity tomography (ERT), transport processes in the subsurface can be imaged and monitored. The information content of obtained spatiotemporal data sets opens new ways to characterize subsurface spatial variability. Difficulties regarding a quantitative interpretation of the imaged transport may arise from the fact that data inversion used in ERT is generally underdetermined and that ERT data acquisition is often limited to two‐dimensional (2‐D) image planes. To address this problem, we conducted a numerical tracer experiment in a synthetic heterogeneous aquifer with preset variability and spatial correlation of the hydraulic conductivity and monitored the tracer breakthrough in a 2‐D image plane perpendicular to the mean flow direction using time‐lapse ERT. The tracer breakthrough patterns in the image plane were interpreted using equivalent transport models: an equivalent convection dispersion equation to characterize the spatially averaged breakthrough and a stream tube model to characterize local breakthrough curves. Equivalent parameters derived from simulated and from ERT inverted concentrations showed a good agreement, which demonstrates the potential of ERT to characterize subsurface transport. Using first‐order approximate solutions of stochastic flow and transport equations, equivalent model parameters and their spatial variability were interpreted in terms of the local‐scale dispersion and the spatial variability of the hydraulic conductivity. The spatial correlations of the stream tube velocity and of the hydraulic conductivity were found to be closely related. Consequently, the hydraulic conductivity spatial correlation may be inferred from stream tube velocity distributions, which can be observed with sufficiently high spatial resolution using ERT.

The aim of this study is to prove the effectiveness of two electrical geophysical prospecting techniques, namely electrical resistivity tomography (ERT) and induced polarization (IP), in locating thin vein structures of metal sulphides embedded in Palaeozoic materials underlying a sedimentary cover. For this purpose, a Quaternary basin known as La Garza was selected, located in the mining district of Linares-La Carolina (Southern Spain). Galena (PbS) veins appear abundantly throughout this area, hosted in the Palaeozoic granitic bedrock. The studied veins show thicknesses from 0.5 to 2.0 m, and most present a vertical planar distribution. The veins lose their continuity below the sedimentary cover due to normal fractures that control the subsidence of the basin. During the 1980s, geophysical research campaigns were carried out in La Garza using vertical electrical sounding and failed in detecting the hidden veins. For this reason, to carry out this study, a closed regular mesh was designed, composed by eight ERT and IP profiles, with variable lengths between 315 and 411 metres. An electrode spacing between 5 and 7 metres was selected, thus allowing the granite bedrock to be reached without significantly reducing the resolution capabilities of the method. Even though ERT and IP are well-known geophysical techniques for mapping ore deposits, this is a case study that shows the advantages of the simultaneous use of both techniques (ERT and IP), over their individual application. ERT allows for reconstructing the morphology of the basin and the fractures that control it due to high-resistivity contrast between the overlying sedimentary cover and the underlaying granitic basement. However, it cannot provide any insights about their degree of mineralization. At this point, it is the IP technique that makes it possible to differentiate which are the mineralized structures. Some of these fractures produce high (above 50 mV/V) and moderate (below 50 mV/V) chargeability values, suggesting the existence of several unexploited metal veins. Furthermore, the derived models enable researchers to analyse the morphology of this sedimentary basin controlled by normal faults.

Electrical resistivity as a binary classifier for bridge scour evaluation

ERT and salinity – A method to determine whether ERT-detected preferential pathways in brackish water-irrigated soils are water-induced or an artifact of salinity

Monitoring soil volume wetness in heterogeneous soils by electrical resistivity. A field-based pedotransfer function

Cross‐well electrical resistivity tomography (ERT) was used to monitor the migration of a saline tracer in a two‐well pumping‐injection experiment conducted at the Massachusetts Military Reservation in Cape Cod, Massachusetts. After injecting 2200 mg/L of sodium chloride for 9 hours, ERT data sets were collected from four wells every 6 hours for 20 days. More than 180,000 resistance measurements were collected during the tracer test. Each ERT data set was inverted to produce a sequence of 3‐D snapshot maps that track the plume. In addition to the ERT experiment a pumping test and an infiltration test were conducted to estimate horizontal and vertical hydraulic conductivity values. Using modified moment analysis of the electrical conductivity tomograms, the mass, center of mass, and spatial variance of the imaged tracer plume were estimated. Although the tomograms provide valuable insights into field‐scale tracer migration behavior and aquifer heterogeneity, standard tomographic inversion and application of Archie's law to convert electrical conductivities to solute concentration results in underestimation of tracer mass. Such underestimation is attributed to (1) reduced measurement sensitivity to electrical conductivity values with distance from the electrodes and (2) spatial smoothing (regularization) from tomographic inversion. The center of mass estimated from the ERT inversions coincided with that given by migration of the tracer plume using 3‐D advective‐dispersion simulation. The 3‐D plumes seen using ERT exhibit greater apparent dispersion than the simulated plumes and greater temporal spreading than observed in field data of concentration breakthrough at the pumping well.