Electrical Resistivity Tomography Measurements Research Articles

A dual-modality electrical tomography sensor for measurement of gas–oil–water stratified flows

Multiphase flow measurement is a challenging work, especially for the measurement of gas–oil–water flows. In the past, researchers tried to measure gas–oil–water flows by using two separate electrical tomography (ET) sensors, e.g. electrical capacitance tomography (ECT) and electrical resistance tomography (ERT). In this paper, a typical ECT sensor with inner electrodes, as an ECT/ERT dual-modality sensor, is investigated for the measurement of gas–oil–water stratified flows. With this ECT sensor, the voltage-excitation and current-measurement strategy is adopted for both ECT and ERT measurement unlike the conventional ERT with current-injection and voltage-measurement, which cannot provide quantitative measurements because of serious fringe effect. To validate the effectiveness of the proposed measurement strategy, experiment was conducted with an impedance-analyser-based data acquisition system, which can take both capacitance and conductance measurements simultaneously. Based on the measured capacitance and conductance, the corresponding permittivity and conductivity distributions can be reconstructed for the same cross section. A simple data fusion method is proposed to extract the information regarding the oil distribution from the reconstructed images, which was proved to be effective. In the experiment, different excitation frequency and different conductivities of the water phase were examined to investigate their influence on the three-phase flow imaging and measurement. The proposed measurement strategy has advantages of simplified sensor design, reducing the complexity of the related electronics as well as acquiring all information from the same cross section.

Monitoring hillslope moisture dynamics with surface ERT for enhancing spatial significance of hydrometric point measurements

Abstract. Besides floodplains, hillslopes are basic units that mainly control water movement and flow pathways within catchments of subdued mountain ranges. The structure of their shallow subsurface affects water balance, e.g. infiltration, retention, and runoff. Nevertheless, there is still a gap in the knowledge of the hydrological dynamics on hillslopes, notably due to the lack of generalization and transferability. This study presents a robust multi-method framework of electrical resistivity tomography (ERT) in addition to hydrometric point measurements, transferring hydrometric data into higher spatial scales to obtain additional patterns of distribution and dynamics of soil moisture on a hillslope. A geoelectrical monitoring in a small catchment in the eastern Ore Mountains was carried out at weekly intervals from May to December 2008 to image seasonal moisture dynamics on the hillslope scale. To link water content and electrical resistivity, the parameters of Archie's law were determined using different core samples. To optimize inversion parameters and methods, the derived spatial and temporal water content distribution was compared to tensiometer data. The results from ERT measurements show a strong correlation with the hydrometric data. The response is congruent to the soil tension data. Water content calculated from the ERT profile shows similar variations as that of water content from soil moisture sensors. Consequently, soil moisture dynamics on the hillslope scale may be determined not only by expensive invasive punctual hydrometric measurements, but also by minimally invasive time-lapse ERT, provided that pedo-/petrophysical relationships are known. Since ERT integrates larger spatial scales, a combination with hydrometric point measurements improves the understanding of the ongoing hydrological processes and better suits identification of heterogeneities.

CO2 degassing in the Hartoušov mofette area, western Eger Rift, imaged by CO2 mapping and geoelectrical and gravity surveys

Strong, subcontinental mantle-dominated CO2 degassing occurs in the Hartousov and Bublak mofette fields in the western Eger Rift. The combination of CO2 gas flux and soil gas measurements as well as gravity and geoelectric surveys provides insight into the surface and subsurface of this unique mofette area. CO2 soil gas and gas flux measurements reveal that large amounts of carbon dioxide are released via channels with diameters below 1 m. Carbon dioxide emissions of several tens and up to more than 100 kg day−1 are ejected via these small seeps. Measurements with small spacings are necessary to account for the point like, focused gas discharge in the lesser degassing surrounding. We estimate that between 23 and 97 tons of CO2 are released over an area of about 350,000 m2 each day in the Hartousov mofette field. The application of widely used geostatistical tools leads to estimations of the CO2 discharge with very high standard deviations due to the strong positive skewness of the data distribution. Geophysical investigations via electrical resistivity tomography and gravity measurements were carried out over areas of strong seepage and reveal distinct anomalies in the subsurface below mofettes, indicating rock and sediment alterations and/or sediment transport by pressurised, ascending CO2 and water mobilised by it. This study reveals that the gas emanations only occur west of a morphological step which is related to a N–S-oriented fault zone, the Pocatky-Plesna fault zone. The results of CO2 mapping and the geophysical studies can track the course of this fault zone in this area. Our results fit into a tectonic model in which the mofette fields are in the centres of two independent pull-apart basin-like structures. We hypothesise that the sinistral strike-slip movement of the Pocatky-Plesna fault zone leads to a pull-apart basin-like opening, at which the strong, mantle-derived CO2 degassing occurs nowadays. Since the Hartousov and Bublak mofette fields only illustrate examples along the N–S-striking Pocatky-Plesna fault zone, its role and other N–S-striking faults’ roles of the Regensburg–Leipzig–Rostock zone for upper mantle degassing might have been underestimated previously.

Quantitative inverse modelling of a cylindrical object in the laboratory using ERT: An error analysis

Electrical resistivity tomography (ERT) is a geophysical technique that can be used to obtain three-dimensional images of the bulk electrical conductivity of the subsurface. Because the electrical conductivity is strongly related to properties of the subsurface and the flow of water it has become a valuable tool for visualization in many hydrogeological and environmental applications. In recent years, ERT is increasingly being used for quantitative characterization, which requires more detailed prior information than a conventional geophysical inversion for qualitative purposes. In addition, the careful interpretation of measurement and modelling errors is critical if ERT measurements are to be used in a quantitative way. This paper explores the quantitative determination of the electrical conductivity distribution of a cylindrical object placed in a water bath in a laboratory-scale tank. Because of the sharp conductivity contrast between the object and the water, a standard geophysical inversion using a smoothness constraint could not reproduce this target accurately. Better results were obtained by using the ERT measurements to constrain a model describing the geometry of the system. The posterior probability distributions of the parameters describing the geometry were estimated with the Markov chain Monte Carlo method DREAM(ZS). Using the ERT measurements this way, accurate estimates of the parameters could be obtained. The information quality of the measurements was assessed by a detailed analysis of the errors. Even for the uncomplicated laboratory setup used in this paper, errors in the modelling of the shape and position of the electrodes and the shape of the domain could be identified. The results indicate that the ERT measurements have a high information content which can be accessed by the inclusion of prior information and the consideration of measurement and modelling errors.

Surface‐to‐tunnel electrical resistance tomography measurements

ABSTRACTIn this work we examine the applicability of surface‐to‐tunnel electrical resistance tomography measurements for imaging subsurface targets. Various issues of this special arrangement are discussed and explored by means of numerical model simulations, experimental measurements in a tank, and a field application. The geometrical factor was considered as a filter during the generation of the protocols in order to reject electrode combinations and problematic measurements. Different electrode arrays are examined and compared for various targets, and we concluded that, for the tested cases, the pole–tripole electrode arrangement produced superior results. Additional tests showed that the maximum distance between the electrode lines is strongly related to the nature of the prospected target. Furthermore, the study showed that the tunnel size affects the measured data and needs to be taken into account when processing surface‐to‐tunnel data, and the full incorporation of the tunnel into a 3D model proved to be the optimum among the tested correction schemes. Finally, we present a real case study involving electrical resistance tomography surface‐to‐tunnel measurements, and results are in comparable agreement with the findings of the tests with numerical and experimental data.

Electric Resistivity Tomography and Magnetic Susceptibility Measurements at the Baden Culture Site Stavenice-Úsov (Czech Republic)

Electric Resistivity Tomography and Magnetic Susceptibility Measurements at the Baden Culture Site Stavenice-Úsov (Czech Republic)

Towards more systematic perceptual model development: a case study using 3 Luxembourgish catchments

AbstractThe synthesis of experimental understanding of catchment behaviour and its translation into qualitative perceptual models is an important objective of hydrological sciences. We explore this challenge by examining the cumulative understanding of the hydrology of three experimental catchments and how it evolves through the application of different investigation techniques. The case study considers the Huewelerbach, Weierbach and Wollefsbach headwater catchments of the Attert basin in Luxembourg. Subsurface investigations including bore holes and pits, analysis of soil samples and Electrical Resistivity Tomography measurements are presented and discussed. Streamflow and tracer data are used to gain further insights into the streamflow dynamics of the catchments, using end‐member mixing analysis and hydrograph separation based on dissolved silica and electrical conductivity. We show that the streamflow generating processes in all three catchments are controlled primarily by the subsolum and underlying bedrock. In the Huewelerbach, the permeable sandstone formation supports a stable groundwater component with little seasonality, which reaches the stream through a series of sources at the contact zone with the impermeable marls formation. In the Weierbach, the schist formation is relatively impermeable and supports a ‘fill and spill’‐type of flow mechanism; during wet conditions, it produces a delayed response dominated by pre‐event water. In the Wollefsbach, the impermeable marls formation is responsible for a saturation‐excess runoff generating process, producing a fast and highly seasonal response dominated by event water. The distinct streamflow generating processes of the three catchments are represented qualitatively using perceptual models. The perceptual models are in turn translated into quantitative conceptual models, which simulate the hydrological processes using networks of connected reservoirs and transfer functions. More generally, the paper illustrates the evolution of perceptual models based on experimental fieldwork data, the translation of perceptual models into conceptual models and the value of different types of data for processes understanding and model representation. Copyright © 2014 John Wiley & Sons, Ltd.

Increasing the effectiveness of electrical resistivity tomography using γ11n configurations

ABSTRACTA new array type, i.e., the γ11n arrays, is introduced in this paper, in which the sequence of the current (C) and potential (P) electrodes is CPCP, and the distance between the last two electrodes is n times the distance between the first two ones and that of the second one and the third one. These arrays are called quasinull arrays because they are—according to their array and behaviour—between the traditional and null arrays. It is shown by numerical modelling that, in detecting small‐effect inhomogeneity, these configurations may be more effective than the traditional ones, including the optimized Stummer configuration. Certain γ11n configurations—especially the γ112, γ113, and γ114—produced better results both in horizontal and vertical resolution investigations. Based on the numerical studies, the γ11n configurations seem to be very promising in problems where the anomalies are similar to the numerically investigated ones, i.e., they can detect and characterize, e.g., tunnels, caves, cables, tubes, abandoned riverbeds, or discontinuity, in a clay layer with greater efficacy than those of the traditional configurations. γ11n measurements need less data than traditional configurations; therefore, the time demand of electrical resistivity tomography measurements can be shortened by their use.

An iterative particle filter approach for coupled hydro-geophysical inversion of a controlled infiltration experiment

The modeling of unsaturated groundwater flow is affected by a high degree of uncertainty related to both measurement and model errors. Geophysical methods such as Electrical Resistivity Tomography (ERT) can provide useful indirect information on the hydrological processes occurring in the vadose zone. In this paper, we propose and test an iterated particle filter method to solve the coupled hydrogeophysical inverse problem. We focus on an infiltration test monitored by time-lapse ERT and modeled using Richards equation. The goal is to identify hydrological model parameters from ERT electrical potential measurements. Traditional uncoupled inversion relies on the solution of two sequential inverse problems, the first one applied to the ERT measurements, the second one to Richards equation. This approach does not ensure an accurate quantitative description of the physical state, typically violating mass balance. To avoid one of these two inversions and incorporate in the process more physical simulation constraints, we cast the problem within the framework of a SIR (Sequential Importance Resampling) data assimilation approach that uses a Richards equation solver to model the hydrological dynamics and a forward ERT simulator combined with Archie's law to serve as measurement model. ERT observations are then used to update the state of the system as well as to estimate the model parameters and their posterior distribution. The limitations of the traditional sequential Bayesian approach are investigated and an innovative iterative approach is proposed to estimate the model parameters with high accuracy. The numerical properties of the developed algorithm are verified on both homogeneous and heterogeneous synthetic test cases based on a real-world field experiment.

The role of geophysical ERT method to evaluate the leakproofness of diapragm wall of deep foundation trenches on the example of the construction of retail and office complex in Lublin, Poland

Abstract This paper addresses the problem of assessing the leakproofness of the bottom of a deep foundation trench, secured by cavity wall, using geophysical methods of electrical resistivity tomography. The study was conducted on a large construction project in Lublin, in a place where there are complicated soil-water conditions: the groundwater level is above the proposed depth of foundation trench, the subsoil is heterogeneous, and there are karsted and weathered carbonate sediments with confined aquifer below the bottom of the trench. A hydraulic fracture occurred at the bottom of the trench during the engineering works, which caused the water flow into the trench. In order to recognize the soil-water conditions the first stage of geophysical measurements of electrical resistivity tomography (ERT) was made. The applied methodology allowed to determine the extent of the hydraulic fracture zone within the bottom of foundation trench. In order to assess the leakproofness of Diaphragm Wall the geophysical ERT measurements were repeated (stage 2) A clear reduction in the value of the electrical resistivity of soils in the area of hydraulic fracture was caused by clay injection. The results of ERT measurements are discussed and graphically presented.

Soil moisture variability in a temperate deciduous forest: insights from electrical resistivity and throughfall data

In deciduous forests, soil moisture is an important driver of numerous physical, microbial, and biogeochemical processes. Therefore, characterizing the interactions between vegetation and soil moisture is critical to forecast long-term water resources and ecosystem health. However, these interactions are difficult to measure, both in time and space. Recent studies have shown the ability of electrical resistivity imaging (ERI) to characterize the spatial and temporal dynamics of soil moisture at a range of scales. However, no study has yet attempted to use ERI to describe spatiotemporal variability of soil water in relation to vegetation structure and throughfall. In this study, at a mature forest site in Michigan, USA, we captured spatial and temporal dynamics of soil moisture using weekly ERI measurements augmented with throughfall and soil temperature measurements, and a detailed vegetation survey for five adjacent quadrats. Our results show that throughout the growing season, the soil moisture gradually declined despite strong variations in cumulative monthly rainfall. This decline was occasionally halted, but not reversed, during weeks with high precipitation. Spatial variability of electrical resistivity and soil moisture was not related to soil temperature differences but showed a strong correlation with canopy variables.

Uncertainty quantification of CO2 saturation estimated from electrical resistance tomography data at the Cranfield site

A parametric bootstrap approach is presented for uncertainty quantification (UQ) of CO2 saturation derived from electrical resistance tomography (ERT) data collected at the Cranfield, Mississippi (USA) carbon sequestration site. There are many sources of uncertainty in ERT-derived CO2 saturation, but we focus on how the ERT observation errors propagate to the estimated CO2 saturation in a nonlinear inversion process. Our UQ approach consists of three steps. We first estimated the observational errors from a large number of reciprocal ERT measurements. The second step was to invert the pre-injection baseline data and the resulting resistivity tomograph was used as the prior information for nonlinear inversion of time-lapse data. We assigned a 3% random noise to the baseline model. Finally, we used a parametric bootstrap method to obtain bootstrap CO2 saturation samples by deterministically solving a nonlinear inverse problem many times with resampled data and resampled baseline models. Then the mean and standard deviation of CO2 saturation were calculated from the bootstrap samples. We found that the maximum standard deviation of CO2 saturation was around 6% with a corresponding maximum saturation of 30% for a data set collected 100 days after injection began. There was no apparent spatial correlation between the mean and standard deviation of CO2 saturation but the standard deviation values increased with time as the saturation increased. The uncertainty in CO2 saturation also depends on the ERT reciprocal error threshold used to identify and remove noisy data and inversion constraints such as temporal roughness. Five hundred realizations requiring 3.5h on a single 12-core node were needed for the nonlinear Monte Carlo inversion to arrive at stationary variances while the Markov Chain Monte Carlo (MCMC) stochastic inverse approach may expend days for a global search. This indicates that UQ of 2D or 3D ERT inverse problems can be performed on a laptop or desktop PC. The parametric bootstrap method provides a promising alternative to the MCMC approach for fast uncertainty quantification of the underlying geophysical inverse problems.

Reservoir monitoring of steam‐assisted gravity drainage using borehole measurements

ABSTRACTThis paper presents the results from a research project focusing on permanent cross‐well geophysical methods for reservoir monitoring during steam‐assisted gravity drainage. A feasibility study indicated detectable differences in seismic and electrical reservoir properties based on expected changes in temperature and fluid saturation during the production of extra heavy oil. As a result of this, a permanent cross‐well system was installed at the Leismer Demonstration Area, located in the Athabasca Oil Sands region in Alberta, Canada. Baseline data sets, including cross‐well seismic, three‐dimensional vertical seismic profiling and cross‐well electrical resistivity tomography, have been acquired. Comparisons between conventional surface seismic and downhole seismic data show an increase in resolution and frequency content as expected. Steam‐assisted gravity drainage‐induced time‐lapse effects are clearly visible in the 3D vertical seismic profiling and electrical resistivity tomography data sets, even after a few months of oil production. In general, the 3D vertical seismic profiling images show a higher resolution than the surface seismic data, in particular when dealing with vertical positioning of the time‐lapse events. The electrical resistivity tomography baseline shows clear separation between zones of high and low electrical resistivity, and during 23 months of electrical resistivity tomography measurements the maximum reduction of resistivity is 85%. Time‐lapse observations from acoustic and electrical borehole data correspond well, and are also supported by temperature measurements in the two observation wells. Emerging technologies, updated models, improved flexibility, and reduced costs will allow future reservoir monitoring with surface and borehole data in combination, or even with borehole data exclusively.

On the value of combining surface and cross‐borehole ERT measurements to study artificial tile drainage processes

Many articles have shown the usefulness of electrical resistivity tomography (ERT) where characterizing spatial electrical resistivity variations over time associated with water content variations. In a case study of artificial drainage processes in comparison to isolated measurements of the temporal variation in water content, ERT could be used to obtain additional qualitative spatial information (2D or 3D). Indeed, there have been no articles relating the advantages of using ERT to study tile drainage processes. The aim of this article is to explore the effectiveness of the ERT method for delineating soil moisture spatial patterns, following their temporal variations in the context of artificial drainage. First, in the numerical description, taking into account the current knowledge on artificial drainage processes, the authors evaluate the combination of surface and cross‐borehole ERT measurements and different array types. In a field application, the surface and cross‐borehole combination helps to obtain better information on the soil structure and to optimize electrical resistivity monitoring during tile drainage processes. In the second part of the article, the comparison between the ERT data sets and ancillary data from TDR sensors provides a useful critical evaluation of ERT for studying water transfer in waterlogged soil influenced by artificial drainage. The field results showed that ERT contributes additional 2D information on resistivity as related to the water content, which complements single TDR measurements.

Spatial considerations on electrical resistance tomography measurements

This work reports experimental evidence of the effect of certain geometrical parameters on the accuracy of electrical resistance tomography (ERT) measurements in two-phase systems. Emphasis is given to millimetre-size electrodes that suffer from significant electric field distortion due to (a) extension of the measuring volume beyond the electrode plane (fringe effect) and (b) the non-uniform distribution of field strength at the electrode plane. Water constitutes the continuous (conductive) phase, whereas Teflon rods constitute the dispersed (non-conductive) phase. The examined parameters include the diameter of the cylindrical test vessel, the size of electrodes and the number and size (radius and length) of the submerged Teflon rods. The variable in these tests is the axial and radial position of the Teflon rods inside the test cell. It is found that for homogeneously (axially and radially) dispersed rods, the void fraction measured by a set of electrodes at a plane agrees pretty well with 2D theoretical predictions. However, in cases of non-homogeneously dispersed rods, void fraction measurements deviate considerably from theoretical values. This is a manifestation of severe electric field distortion associated with the employed small electrodes. Moreover, evidence is provided that in the examined system, the fringe effect is more significant than the topography of the field strength in distorting measurements. To allow a quantitative analysis of the present data, nonlinear regression combined with dimensional analysis is conducted to derive an expression that describes the void fraction measurements by a set of electrodes at a plane for different axial and radial positions of the submerged rods. A parametric analysis of this expression illustrates the significance of different parameters on ERT measurements.

Influence of the geomembrane on time-lapse ERT measurements for leachate injection monitoring

Leachate recirculation is a key process in the operation of municipal waste landfills as bioreactors. To quantify the water content and to evaluate the leachate injection system, in situ methods are required to obtain spatially distributed information, usually electrical resistivity tomography (ERT). However, this method can present false variations in the observations due to several parameters. This study investigates the impact of the geomembrane on ERT measurements. Indeed, the geomembrane tends to be ignored in the inversion process in most previously conducted studies. The presence of the geomembrane can change the boundary conditions of the inversion models, which have classically infinite boundary conditions. Using a numerical modelling approach, the authors demonstrate that a minimum distance is required between the electrode line and the geomembrane to satisfy the good conditions of use of the classical inversion tools. This distance is a function of the electrode line length (i.e. of the unit electrode spacing) used, the array type and the orientation of the electrode line. Moreover, this study shows that if this criterion on the minimum distance is not satisfied, it is possible to significantly improve the inversion process by introducing the complex geometry and the geomembrane location into the inversion tools. These results are finally validated on a field data set gathered on a small municipal solid waste landfill cell where this minimum distance criterion cannot be satisfied.

Hydraulic Modeling and in situ Electrical Resistivity Tomography to Analyze Ponded Infiltration into a Water Repellent Sand

Water repellency (WR) might affect the spatial and temporal dynamics of a wetting front during infiltration and redistribution in a way that is difficult to predict with standard approaches. Therefore, the objectives of this study were to simulate the wetting plume geometry with a three‐dimensional numerical model and to test whether electrical resistivity tomography (ERT) is able to illustrate the geometry under highly dynamic conditions. At our study site under agricultural use (Gleyic Podzol, groundwater affected), persistent WR in the subsoil to the 120‐cm depth was responsible for a conical plume geometry observed after ponded tracer application with Brilliant Blue (BB) and bromide. The process was invasively observed with hydraulic sensors. At the same time, ERT was used to monitor a second ponded infiltration event under equal boundary conditions at the same site. Numerical simulation of the process showed that hysteresis in the water retention curve is needed to describe the specific infiltration plume geometry correctly. The main wetting function was derived from scaling the main drying curve with measured contact angle data. A comparison of wetting front arrival times among the hydraulic model, sensors, and independent ERT observations indicates an overall good agreement and shows the usefulness of ERT measurements under highly dynamic in situ conditions. Our results confirm the need to include strong hysteresis effects scaled with independent contact angle data when simulating infiltration dynamics in a water repellent soil to avoid an underestimation of the wetting front arrival.

A spatial and temporal analysis of different periglacial materials by using geoelectrical, seismic and borehole temperature data at Murtèl–Corvatsch, Upper Engadin, Swiss Alps

Abstract. Different geophysical investigations, such as electrical resistivity tomography (ERT) and refraction seismic tomography (RST), allow for an improved characterization of subsurface conditions in mountain permafrost areas. The knowledge of the permafrost internal composition constitutes a major prerequisite for climate-related modelling studies, for detailed hazard or local infrastructure assessments. To detect the small-scale variations of permafrost characteristics and its varying sensitivity to climate influencing factors, two ERT and RST monitoring profiles were installed in 2009 at two different sites called Chastelets and Murtèl forefield located in the Murtèl–Corvatsch area, Upper Engadin, eastern Swiss Alps. The geophysical profiles extend over four existing boreholes and are characterized by strong small-scale variations of surface as well as subsurface structures such as bedrock, fine material or coarse debris. Here we present ERT measurements carried out in a bimonthly interval during the years of 2009 to 2012 and RST measurements which were performed once a year, normally in August, during the same period. Based on these data sets the so-called four-phase model, based on petrophysical relationships, was applied to determine the volumetric fractions of ice, water and air within the heterogeneous ground, resulting in a relatively precise description of the subsurface material around the existing boreholes. The observations revealed a permafrost occurrence at the Chastelets rock glacier with an estimated ice-saturated layer of at least 10 m thickness and the detection of a thawed layer with increased water content in the lower frontal part of the rock glacier within an area of fine material. In the area of the Murtèl forefield the analysis revealed strongly weathered bedrock, which is in the upper part covered by a pronounced layer of coarse debris establishing a thermal regime which is able to sustain permafrost beneath. In addition, the high temporal ERT measurements revealed a seasonal formation of ice during wintertime within the coarse- as well as the fine-grained active layer zones. It can be concluded that the combination of existing borehole temperature measurements, the ERT/RST measurements and the application of the four-phase model resulted in an in-depth view of the investigated area, which is a major prerequisite for future modelling studies allowing for a better treatment of the present small-scale spatial ground variabilities.

Detection of landfill cover damage using geophysical methods

ABSTRACTIn closed hazardous waste (i.e. industrial) landfills, impermeable covers are used to seal in the waste so as to minimise water infiltration and the accumulation of leachate inside the waste. In this paper, we present a geophysical study performed on a French landfill where the cover was designed using impermeable (clay) and drainage (sand) layers along with a Geosynthetic Clay Liner (GCL) in between. As observed, the quantity of leachate increases after rain events making the leachate treatment more expensive. This could be due to areas of weakness in the cover that have developed over time or which date back to its installation. Three different geophysical methods were employed and confronted to detect and assess such damage, and, if necessary, to help choose the most suitable remediation of the cover: Automatic Resistivity Profiling (ARP) which enables a rapid mapping of the entire landfill; the self‐potential method (SP) as one of its sources is water drainage; and electrical resistivity tomography (ERT) to obtain 2D resistivity models. Based on the ARP results revealing high lateral heterogeneities of the superficial cover materials, the SP and ERT measurements were conducted on a limited area. A negative SP anomaly was observed at the top of the landfill, where the cover is the thinnest and the GCL the most damaged, suggesting preferential infiltration under the GCL. The 2D ERT profile showed both vertical and horizontal variations. Comparison with manual auger holes showed that the alternating clayey and sandy layers did not conform to plans for the original cover when it was installed at this site about twenty years ago. Variations in electrical resistivity noticed at the depth of the GCL were attributed to possible damage to the GCL and a greater than expected permeability since the GCL is normally very resistive. The results obtained in this study were important to understand the damage to the cover due to the use of different types and thicknesses of cover materials compared to what was originally planned, and deterioration of the GCL.

Characterizing Soil–Pipe Networks with Pseudo‐Three‐Dimensional Resistivity Tomography on Forested Hillslopes with Restrictive Horizons

Soil pipes are important hillslope–hydrology features affecting mechanistic processes such as subsurface drainage, streamflow, erosion, hillslope stability, and discharge response to rain fall and snow melt. However, the limited capacity to characterize these pathways in situ remains a major limitation for process‐based understanding and incorporation into coupled hydrologic models. The goal of this study was to investigate the use of pseudo‐three‐dimensional electrical resistivity tomography (ERT) as a field method for characterizing soil–pipe spatial distribution and connectivity on a forested hillslope with a specific interest in detecting the formation of soil pipes from root decay and combustion. We conducted detailed ERT surveys on three 6.12‐ by 6.12‐m plots before and after a prescribed burn located on a 2‐ha clearcut of coniferous forest underlain by a fragipan. Excavation following the ERT measurements showed that soil pipes were mostly empty with 86% containing <25% decaying root matter. The mean diameter of pipes was 4.3 cm with a mean length of 1.6 m. Such empty pipes would resolve as elongated high‐electrical resistivity features. However, a high resistivity layer corresponding to the Bw and BE soil horizons overlaying a fragipan was observed on all plots with the majority of soil pipes larger than 1 cm diameter occurring within these horizons. The lack of observable contrast between pipes and the high resistivity layer as well as reduced data coverage within the vicinity of large soil pipes may have limited our ability to effectively characterize soil pipes occurring within these hillslopes. We conclude that the use of ERT surveys to detect large macropores pathways may be more suitable to settings where soil pipes are not occurring within high restrictive layers.