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See detailQuantitative temperature monitoring of a heat tracing experiment using cross-borehole ERT
Hermans, Thomas ULg; Wildemeersch, Samuel ULg; Jamin, Pierre ULg et al

in Geothermics (2015), 53

The growing demand for renewable energy leads to an increase in the development of geothermal energy projects and heat has become a common tracer in hydrology and hydrogeology. Designing geothermal ... [more ▼]

The growing demand for renewable energy leads to an increase in the development of geothermal energy projects and heat has become a common tracer in hydrology and hydrogeology. Designing geothermal systems requires a multidisciplinary approach including geological and hydrogeological aspects. In this context, electrical resistivity tomography (ERT) can bring relevant, qualitative and quantitative information on the temperature distribution in operating shallow geothermal systems or during heat tracing experiments. We followed a heat tracing experiment in an alluvial aquifer using cross-borehole time-lapse ERT. Heated water was injected in a well while water of the aquifer was extracted at another well. An ERT section was set up across the main flow direction. The results of ERT were transformed into temperature using calibrated petrophysical relationships. These ERT-derived temperatures were then compared to direct temperature measurements in control piezometers collected with distributed temperature sensing (DTS) and groundwater temperature loggers. Spatially, it enabled to map the horizontal and vertical extent of the heated water plume, as well as the zones where maximum temperatures occurred. Quantitatively, the temperatures and breakthrough curves estimated from ERT were in good agreement with the ones observed directly during the rise and maximum of the curve. An overestimation, likely related to 3D effects, was observed for the tail of the heat breakthrough curve. The error made on temperature can be estimated to be between 10 to 20 %, which is a fair value for indirect measurements. From our data, we estimated a quantification threshold for temperature variation of 1.2°C. These results suggest that ERT should be considered when designing heat tracing experiments or geothermal systems. It could help also to assess the geometrical complexity of the concerned reservoirs. It also appears that ERT could be a useful tool to monitor and control geothermal systems once they are in operation. [less ▲]

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See detailProspection géophysique de la zone faillée de Hockai dans la région de Malmedy: Rapport des tomographies de résistivité électrique
Hermans, Thomas ULg; Nguyen, Frédéric ULg

Report (2014)

Ce rapport consiste en la présentation des résultats des prospections géophysiques par tomographie de résistivité électrique (ERT) menées sur la zone faillée de Hockai dans la région de Malmedy. Le but ... [more ▼]

Ce rapport consiste en la présentation des résultats des prospections géophysiques par tomographie de résistivité électrique (ERT) menées sur la zone faillée de Hockai dans la région de Malmedy. Le but principal de ces investigations est de juger de la fracturation de la roche dans et en dehors de la Zone de Faille de Hockai (ZFH) et de mettre en évidence les structures liées à cette zone de failles. [less ▲]

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See detailCase studies of incorporation of prior information in electrical resistivity tomography: comparison of different approaches
Caterina, David ULg; Hermans, Thomas ULg; Nguyen, Frédéric ULg

in Near Surface Geophysics (2014), 12(4), 451-465

Many geophysical inverse problems are ill-posed and their solution non-unique. It is thus important to reduce the amount of mathematical solutions to more geologically plausible models by regularizing the ... [more ▼]

Many geophysical inverse problems are ill-posed and their solution non-unique. It is thus important to reduce the amount of mathematical solutions to more geologically plausible models by regularizing the inverse problem and incorporating all available prior information in the inversion process. We compare three different ways to incorporate prior information for electrical resistivity tomography (ERT): using a simple reference model or adding structural constraints to Occam's inversion and using geostatistical constraints. We made the comparison on four real cases representing different field applications in terms of scales of investigation and level of heterogeneities. In those cases, when electromagnetic logging data are available in boreholes to control the solution, it appears that incorporating prior information clearly improves the correspondence with logging data compared to the standard smoothness constrain. However, the way to incorporate it may have a major impact on the solution. A reference model can often be used to constrain the inversion; however, it can lead to misinterpretation if its weight is too strong or the resistivity values inappropriate. When the computation of the vertical and/or horizontal correlation length is possible, the geostatistical inversion gives reliable results everywhere in the section. However, adding geostatistical constraints can be difficult when there is not enough data to compute correlation lengths. When a known limit between two layers exists, the use of structural constrain seems to be more indicated particularly when the limit is located in zones of low sensitivity for ERT. This work should help interpreters to include their prior information directly into the inversion process through an appropriate way. [less ▲]

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See detailInverting Hydraulic Heads In An Alluvial Aquifer Constrained With Electrical Resistivity Tomography Data Through Multiple-Point Statistics And Probability Perturbation Method: A Case Study
Hermans, Thomas ULg; Scheidt, Celine; Caers, Jef et al

Conference (2014, July)

Solving spatial inverse problems in the Earth Sciences remains a considerable challenge given the large number of parameters to invert for, the non-linearity of forward models and as a result the ill ... [more ▼]

Solving spatial inverse problems in the Earth Sciences remains a considerable challenge given the large number of parameters to invert for, the non-linearity of forward models and as a result the ill-posedness of the problem. Geostatistics is therefore needed to specify prior models, more particularly, information to control the spatial features of the inverse solutions. We used multiple-point statistics (MPS) to build models of pre-defined hydrofacies: clay, sand and gravel facies constrained to geological data (hard data) and geophysical data (soft data). The electrical resistivity tomography method was chosen to bring relevant spatially distributed information on the presence of the facies, given its sensitivity to variations in lithology and porosity. The comparison of the geophysical signature of the deposits with direct observations in boreholes enables to derive the conditional probability of observing a facies given its electrical resistivity. This is used to produce probability maps for each facies and constrain stochastic simulations of the alluvial aquifer. Then, the probability perturbation method (PPM) is used to integrate hydraulic heads data, using MPS to generate models. This process enables us to obtain calibrated models of the aquifer. The PPM algorithm will automatically seek solutions fitting both hydrogeological data and training-image based geostatistical constraints. Only geometrical features of the model are affected by the perturbation, i.e. we do not attempt to directly find the optimal value of hydrogeological parameters (chosen a priori), but the optimal spatial distribution of facies whose prior distribution is quantified in a training image. The methodology is first tested with a synthetic benchmark. The tests performed show that the choice of the training image is a major source of uncertainty. Therefore, one first needs to select those training images consistent with the geophysical data (and hence reject the inconsistent ones). Then, we proceed with them to hydrogeological inversions. Geophysical data (soft constraints) acts as an accelerator of convergence by reducing prior uncertainty. The hydraulic conductivity of each facies is a sensitive parameter, but it can be easily optimized prior to the PPM process. The stochastic method is then successfully applied within the context of an alluvial aquifer submitted to a pumping experiment. We show how the integration of various sources of data (borehole logs, geophysics, hydraulic heads) aids in calibrating hydrogeological models, locating high hydraulic conductivity zones and reducing uncertainty. The developed methodology proposes a common framework (multiple-point statistics) to integrate various information sources with variable resolutions relevant for hydrogeology: geological, geophysical and hydrogeological data. The method can be extended to integrate tracer tests to enable the calibration of transport parameters as well. The originality of the method is to use geophysical data both to refine the choice of the training image and to constrain the inversion of hydrogeological models. [less ▲]

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See detailVadose zone studies at an industrial contaminated site: the vadose zone monitoring system and cross-hole geophysics
Fernandez de Vera, Natalia ULg; Beaujean, Jean ULg; Jamin, Pierre ULg et al

Poster (2014, April 29)

Poster presented at the European Geoscience Union General Assembly 2014. In this poster, the installation of the vadose zone experimental set up is presented along with first results

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See detailVadose zone studies at an industrial contaminated site: the vadose zone monitoring system and cross-hole geophysics
Fernandez de Vera, Natalia ULg; Pena Hernandez, Juan Angel; Beaujean, Jean ULg et al

Scientific conference (2014, March 05)

Oral presentation of the PhD project at the ENVITAM PhD day in Louvain-la-Neuve

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See detailVadose zone studies at an industrial contaminated site: the vadose zone monitoring system and cross-hole geophysics
Fernandez de Vera, Natalia ULg; Pena Hernandez, Juan; Beaujean, Jean ULg et al

Scientific conference (2014, January 15)

Oral presentation for the PhD geoscience day at the University of Liege.

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See detailUtilisation de la géostatistique multi-points pour l'intégration de données de tomographie de résistivité électrique aux modèles hydrogéologiques
Hermans, Thomas ULg; Scheidt, Céline; Caers, Jef et al

Scientific conference (2014, January 15)

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See detailDétermination des propriétés de couches dans le béton à l'aide d'un géoradar commercial à hautes fréquences: approche pic-à-pic et analyse fréquentielle du coefficient de réflexion
Van der Wielen, Audrey ULg; Nguyen, Frédéric ULg; Courard, Luc ULg

in Annales du Bâtiment et des Travaux Publics (2014)

The Ground Penetrating Radar (GPR) is an efficient tool for the non-destructive inspection of concrete structures. It is widely used for the detection of rebars or humid zones or for evaluating the ... [more ▼]

The Ground Penetrating Radar (GPR) is an efficient tool for the non-destructive inspection of concrete structures. It is widely used for the detection of rebars or humid zones or for evaluating the thickness of elements. But when an element contains a thin layer, the radar waves are submitted to multiple reflections on the interfaces and the layer appears in the radargram as a single reflection, whose detailed analysis can allow determining the thickness and the permittivity of the thin layer. Two approaches were considered in this paper. In the first one, the analysis is based on the peak-to-peak reflection amplitude. The second approach uses a frequency analysis of the reflection coefficient, whose amplitude and phase can then be calculated for several frequencies. With this method, the thickness and permittivity of the layer can in theory be simultaneously determined. Both methods were numerically validated through finite difference simulations and experimentally tested on concrete samples containing an air layer of variable thickness. We showed that the frequency analysis allowed to reach a higher precision in the parameters estimation for a limited additional computing cost. The method efficiency depends on the conditions and is optimal for layers with a high permittivity presenting a large contrast with the matrix. [less ▲]

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See detailCoupling heat and chemical tracer experiments for estimating heat transfer parameters in shallow alluvial aquifers
Wildemeersch, Samuel ULg; Jamin, Pierre ULg; Orban, Philippe ULg et al

in Journal of Contaminant Hydrology (2014)

Geothermal energy systems, closed or open, are increasingly considered for heating and/or cooling buildings. The efficiency of such systems depends on the thermal properties of the subsurface. Therefore ... [more ▼]

Geothermal energy systems, closed or open, are increasingly considered for heating and/or cooling buildings. The efficiency of such systems depends on the thermal properties of the subsurface. Therefore, feasibility and impact studies performed prior to their installation should include a field characterization of thermal properties and a heat transfer model using parameter values measured in situ. However, there is a lack of in situ experiments and methodology for performing such a field characterization, especially for open systems. This study presents an in situ experiment designed for estimating heat transfer parameters in shallow alluvial aquifers with focus on the specific heat capacity. This experiment consists in simultaneously injecting hot water and a chemical tracer into the aquifer and monitoring the evolution of groundwater temperature and concentration in the recovery well (and possibly in other piezometers located down gradient). Temperature and concentrations are then used for estimating the specific heat capacity. The first method for estimating this parameter is based on a modeling in series of the chemical tracer and temperature breakthrough curves at the recovery well. The second method is based on an energy balance. The values of specific heat capacity estimated for both methods (2.30 and 2.54 MJ/m3/K) for the experimental site in the alluvial aquifer of the Meuse River (Belgium) are almost identical and consistent with values found in the literature. Temperature breakthrough curves in other piezometers are not required for estimating the specific heat capacity. However, they highlight that heat transfer in the alluvial aquifer of the Meuse River is complex and contrasted with different dominant process depending on the depth leading to significant vertical heat exchange between upper and lower part of the aquifer. Furthermore, these temperature breakthrough curves could be included in the calibration of a complex heat transfer model for estimating the entire set of heat transfer parameters and their spatial distribution by inverse modeling. [less ▲]

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See detailGeophysical Methods for Monitoring Temperature Changes in Shallow Low Enthalpy Geothermal Systems
Hermans, Thomas ULg; Nguyen, Frédéric ULg; Robert, Tanguy et al

in Energies (2014), 7

Low enthalpy geothermal systems exploited with ground source heat pumps or groundwater heat pumps present many advantages within the context of sustainable energy use. Designing, monitoring and ... [more ▼]

Low enthalpy geothermal systems exploited with ground source heat pumps or groundwater heat pumps present many advantages within the context of sustainable energy use. Designing, monitoring and controlling such systems requires the measurement of spatially distributed temperature fields and the knowledge of the parameters governing groundwater flow (permeability and specific storage) and heat transport (thermal conductivity and volumetric thermal capacity). Such data are often scarce or not available. In recent years, the ability of electrical resistivity tomography (ERT), self-potential method (SP) and distributed temperature sensing (DTS) to monitor spatially and temporally temperature changes in the subsurface has been investigated. We review the recent advances in using these three methods for this type of shallow applications. A special focus is made regarding the petrophysical relationships and on underlying assumptions generally needed for a quantitative interpretation of these geophysical data. We show that those geophysical methods are mature to be used within the context of temperature monitoring and that a combination of them may be the best choice regarding control and validation issues. [less ▲]

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See detailDetection of near-field, low permittivity layers with Ground Penetrating Radar: analytical estimation of the reflection coefficient
Van der Wielen, Audrey; Courard, Luc ULg; Nguyen, Frédéric ULg

in LAMBOT, Sébastien (Ed.) Proceedings of 15th International Conference on ground Penetrating Radar (2014)

The reflection coefficient of GPR waves encountering embedded thin layers is commonly estimated using a plane wave, far field approximation. But when the thin layer is situated in the near field of the ... [more ▼]

The reflection coefficient of GPR waves encountering embedded thin layers is commonly estimated using a plane wave, far field approximation. But when the thin layer is situated in the near field of the antenna, the spherical nature of the waves and the possible propagation of a lateral wave into the layer may have a strong influence on the measured reflected amplitude. In this work, we studied through 2D FDTD simulations the behavior of a radar wave interacting with thin layers of different thicknesses. The snapshots and radargrams showed a large influence of the layer thickness on the wave propagation. For the very thin layers, the evanescent wave plays a major role and the plane wave approximation gives a good estimation of the reflection coefficient. For thicker layers, the specific inclination of each multiple reflection has to be taken into account, as well as the lateral wave propagation. On the basis of these observations, we determined which analytical method should be used for the analytical prediction of the reflection coefficient, as a function of the layer thickness. [less ▲]

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See detailDétermination des propriétés de couches minces dans le béton à l’aide d’un géoradar commercial à hautes fréquences: approche pic-à-pic et analyse fréquentielle du coefficient de réflexion
Van der Wielen, Audrey; Courard, Luc ULg; Nguyen, Frédéric ULg

in Balayssac, Jean-Paul (Ed.) Compte-rendus de Diagnobeton 2014 (2014)

The Ground Penetrating Radar (GPR) is an efficient tool for the non-destructive inspection of concrete structures. It is widely used for the detection of rebars or humid zones or for evaluating the ... [more ▼]

The Ground Penetrating Radar (GPR) is an efficient tool for the non-destructive inspection of concrete structures. It is widely used for the detection of rebars or humid zones or for evaluating the thickness of elements. But when an element contains a thin layer, the radar waves are submitted to multiple reflections on the interfaces and the layer appears in the radargram as a single reflection, whose detailed analysis can allow determining the thickness and the permittivity of the thin layer. Two approaches were considered in this paper. In the first one, the analysis is based on the peak-to-peak reflection amplitude. The second approach uses a frequency analysis of the reflection coefficient, whose amplitude and phase can then be calculated for several frequencies. With this method, the thickness and permittivity of the layer can in theory be simultaneously determined. Both methods were numerically validated through finite difference simulations and experimentally tested on concrete samples containing an air layer of variable thickness. We showed that the frequency analysis allowed to reach a higher precision in the parameters estimation for a limited additional computing cost. The method efficiency depends on the conditions and is optimal for layers with a high permittivity presenting a large contrast with the matrix. [less ▲]

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See detailAssessing the Probability of Training Image-Based Geological Scenarios Using Geophysical Data
Hermans, Thomas ULg; Caers, Jef; Nguyen, Frédéric ULg

in Pardo-Iguzquiza, Eulogio; Guardiola-Albert, Carolina; Heredia, Javier (Eds.) et al Mathematics of Planet Earth - Proceedings of the 15th Annual Conference of the International Association for Mathematical Geosciences (2014)

In multiple-point statistics (MPS), the construction of training im-ages (TIs) is one of the most critical steps. Reliable geological studies may not always be available to depict with certainty what ... [more ▼]

In multiple-point statistics (MPS), the construction of training im-ages (TIs) is one of the most critical steps. Reliable geological studies may not always be available to depict with certainty what geological patterns or heterogeneity are present. In this context, geophysical techniques may provide additional information to reduce the possible large uncertainty in the understanding of prior geological scenarios. To overcome this problem, we developed a methodology to verify the consistency of geophysical data with independently-built TIs representing different plausible geological scenarios. If a TI is deemed consistent with the field geophysical survey, then in a sec-ond step we calculate a likelihood probability for each consistent TI. Our methodology starts by creating subsurface models with each TI. From these models we create synthetic geophysical data and from this synthetic data, synthetic inverted models. These models are now compared with a single inverted model obtained from the field sur-vey, allowing for our definition of what is “consistent”. To that ex-tent, we calculate the Euclidean distance between any two inverted models as well as field data and visualize the results in a 2D or 3D space using multidimensional scaling (MDS). With this technique, it is possible to verify if field cases fall in the distribution represented by synthetic cases, and thus are consistent with them. In a second step, we present a cluster analysis on the MDS-map to highlight which parameters are the most sensitive for the construction of TI. Based on this analysis, a probability of each geological scenario is computed through kernel smoothing of the densities in reduced pro-jected metric space. This approach was tested using electrical resistivity tomography as geophysical data to analyze TI scenarios for the Meuse alluvial aqui-fer (Belgium), where the lack of reliable sedimentological data lead to the definition of a multitude of geological scenarios, hence TIs. [less ▲]

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See detailA modified DOI-based method to statistically estimate the depth of investigation of dc resistivity surveys
Deceuster, J.; Etienne, A.; Robert, Tanguy et al

in Journal of Applied Geophysics (2014), 103

Several techniques are available to estimate the depth of investigation or to identify possible artifacts in dc resistivity surveys. Commonly, the depth of investigation (DOI) is mainly estimated by using ... [more ▼]

Several techniques are available to estimate the depth of investigation or to identify possible artifacts in dc resistivity surveys. Commonly, the depth of investigation (DOI) is mainly estimated by using an arbitrarily chosen cut-off value on a selected indicator (resolution, sensitivity or DOI index). Ranges of cut-off values are recommended in the literature for the different indicators. However, small changes in threshold values may induce strong variations in the estimated depths of investigation. To overcome this problem, we developed a new statistical method to estimate the DOI of dc resistivity surveys based on a modified DOI index approach. This method is composed of 5 successive steps. First, two inversions are performed by using different resistivity reference models for the inversion (0.1 and 10 times the arithmetic mean of the logarithm of the observed apparent resistivity values). Inversion models are extended to the edges of the survey line and to a depth range of three times the pseudodepth of investigation of the largest array spacing used. In step 2, we compute the histogram of a newly defined scaled DOI index. Step 3 consists of the fitting of the mixture of two Gaussian distributions (G1 and G2) to the cumulative distribution function of the scaled DOI index values. Based on this fitting, step 4 focuses on the computation of an interpretation index (II) defined for every cell j of the model as the relative probability density that the cell j belongs to G1, which describes the Gaussian distribution of the cells with a scaled DOI index close to 0.0. In step 5, a new inversion is performed by using a third resistivity reference model (the arithmetic mean of the logarithm of the observed apparent resistivity values). The final electrical resistivity image is produced by using II as alpha blending values allowing the visual discrimination between well-constrained areas and poorly-constrained cells. The efficiency of the proposed methodology is assessed on synthetic and field data. By using synthetic benchmark analysis, we demonstrate that the selected well-constrained cells are well-reconstructed in size and shape as well as in resistivity contrasts. Compared to the existing image appraisal tools, the proposed statistical method allows the identification of the statistically well-constrained cells of the model without using any arbitrary cut-off value. Using this statistical method in combination with the resolution, when interpreting dc resistivity surveys, provides the geophysicist valuable information to avoid over- or misinterpretation of ERT images. [less ▲]

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See detailCoupling of hydrogeological models with hydrogeophysical data to characterize seawater intrusion and shallow geothermal systems
Beaujean, Jean ULg; Kemna, Andreas; Engesgaard, Peter et al

Conference (2013, December 12)

While coastal aquifers are being stressed due to climate changes and excessive groundwater withdrawals require characterizing efficiently seawater intrusion (SWI) dynamics, production of geothermal energy ... [more ▼]

While coastal aquifers are being stressed due to climate changes and excessive groundwater withdrawals require characterizing efficiently seawater intrusion (SWI) dynamics, production of geothermal energy is increasingly being used to hinder global warming. To study these issues, we need both robust measuring technologies and reliable predictions based on numerical models. SWI models are currently calibrated using borehole observations. Similarly, geothermal models depend mainly on the temperature field at few locations. Electrical resistivity tomography (ERT) can be used to improve these models given its high sensitivity to TDS and temperature and its relatively high lateral resolution. Inherent geophysical limitations, such as the resolution loss, can affect the overall quality of the ERT images and also prevent the correct recovery of the desired hydrochemical property. We present an uncoupled and coupled hydrogeophysical inversion to calibrate SWI and thermohydrogeologic models using ERT. In the SWI models, we demonstrate with two synthetic benchmarks (homogeneous and heterogeneous coastal aquifers) the ability of cumulative sensitivity-filtered ERT images using surface-only data to recover the hydraulic conductivity. Filtering of ERT-derived data at depth, where resolution is poorer, and the model errors make the dispersivity more difficult to estimate. In the coupled approach, we showed that parameter estimation is significantly improved because regularization bias is replaced by forward modeling only. Our efforts are currently focusing on applying the uncoupled/coupled approaches on a real life case study using field data from the site of Almeria, SE Spain. In the thermohydrogeologic models, the most sensitive hydrologic parameters responsible for heat transport are estimated from surface ERT-derived temperatures and ERT resistance data. A real life geothermal experiment that took place on the Campus De Sterre of Ghent University, Belgium and a synthetic case are tested. They consist in a thermal injection and storage of water in a shallow sandy aquifer. The use of a physically-based constraint accounting for the difference in conductivity between the formation and the tap injected water and based on the hydrogeological model calibrated first on temperatures is necessary to improve the parameter estimation. Results suggest that time-lapse ERT data may be limited but useful information for estimating groundwater flow and transport parameters for both the convection and conduction phases. [less ▲]

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See detailMonitoring temperature changes during heat tracing experiments using electrical resistivity tomography
Hermans, Thomas ULg; Wildemeersch, Samuel ULg; Nguyen, Frédéric ULg

Conference (2013, December 06)

Thermal tracing experiments are becoming common in hydrogeology to estimate parameters governing heat transport processes and to study geothermal reservoirs. Electrical resistivity tomography (ERT) has ... [more ▼]

Thermal tracing experiments are becoming common in hydrogeology to estimate parameters governing heat transport processes and to study geothermal reservoirs. Electrical resistivity tomography (ERT) has proven its ability to monitor salt tracer tests, but few studies have investigated its performances, both qualitatively and quantitatively, in thermal tracing experiments. In this study, we monitored a heat injection and pumping experiment in an alluvial aquifer using both surface and crosshole ERT. The data sets of the surface profile, located along the main direction of flow, are distorted during injection by an electrical short-circuit through the external pumping-heating-injection experimental set-up. Current is flowing outside the subsurface leading to bad data for electrode dipoles located near the pumping and injection wells. The crosshole ERT panel is perpendicular to the main direction of flow. Difference inversion time-lapse images clearly show a preferential flow path in the bottom of the aquifer related to the presence of a coarse and clean gravel layer. Direct temperature measurements are available in control piezometers during the experiment to validate the ERT-derived temperatures and confirm the spatial pattern of temperature observed with ERT. Breakthrough curves are correctly retrieved in time and difference of 10 to 20% are observed for temperature estimation. The latter requires site-specific petrophysical laws and chemical stability assumptions that must be carefully verified. Our study proves that ERT, especially crosshole ERT, is a reliable tool to follow thermal tracing experiments but also to characterize heat transfer in the subsurface and to monitor geothermal resource exploitations. We also show that surface ERT may be impacted by the survey layout in unsuspected ways. [less ▲]

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See detailReliability of resistivity-derived temperature: insights from laboratory measurements
Robert, Tanguy ULg; Hermans, Thomas ULg; Dumont, Gaël ULg et al

Conference (2013, December 06)

This contribution consists in studying the reliability of resistivity-derived temperature, for example from time-lapse electrical resistivity tomography (ERT) surveys. The idea of using temperature as a ... [more ▼]

This contribution consists in studying the reliability of resistivity-derived temperature, for example from time-lapse electrical resistivity tomography (ERT) surveys. The idea of using temperature as a quantitative tracer is growing in the hydrogeophysical community, especially to simulate geo/hydrothermal systems. However, plenty of physico-chemical processes are influenced by temperature and most of them impact directly resistivity measurements. Therefore, one needs to take them into account to retrieve quantitative temperature estimates from resistivity measurements but, up to now, it is seldom the case. The experiment we conducted consisted in simulating an ERT monitoring of heat storage in a sandy aquifer. We show that using experimental relationships between fluid electrical conductivity and temperature alone does not allow reliable temperature estimates, simply because rock-water interactions are neglected. Worst, from a certain temperature (45°C here), the bulk resistivity starts to increase with temperature although this is not expected from the experimental law. Chemical analyses made on water samples collected during the experiment highlight the importance of accounting chemical reactions (e.g. calcite precipitation with increasing temperature) occurring when temperature changes as well as their kinetics. Finally, other parameters as surface conductivity cannot always be neglected when estimating temperature from resistivity measurements. This means that retrieving reliable temperatures from bulk resistivity measurements (e.g. time-lapse ERT) requires the knowledge of water mineralization as well as the rock / soil mineralogy in order to fully integrate physico-chemical reactions between groundwater and the host rock, for example with a joint inversion scheme. [less ▲]

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See detailMinimum gradient support and geostatistics regularization approaches for inverting time-lapse data
Nguyen, Frédéric ULg; Hermans, Thomas ULg; Robert, Tanguy ULg

Conference (2013, December 05)

Inversion of time-lapse resistivity data allows obtaining ‘snapshots’ of changes occurring in monitored systems for applications such as aquifer storage, site remediation or tracer tests. Based on these ... [more ▼]

Inversion of time-lapse resistivity data allows obtaining ‘snapshots’ of changes occurring in monitored systems for applications such as aquifer storage, site remediation or tracer tests. Based on these snapshots, one can infer qualitative information on the location and morphology of changes occurring in the subsurface but also quantitative estimates on the degree of changes in certain property such as temperature or total dissolved solid content. Analysis of these changes can provide direct insight into flow and transport processes and controlling parameters. However, the reliability of the analysis is dependent on survey geometry, measurement schemes, data error, or regularization. Except regularization, survey design parameters may be optimized prior to the monitoring survey. Regularization, on the other hand, may be chosen depending on available information collected during the monitoring. Common approaches consider smoothing model changes both in space and/or time. We here propose to use two alternative regularization approaches which may be better suited to invert time-lapse data. The first approach is the minimum gradient support (MGS) regularization, which focus the changes in tomograms snapshots. MGS will limit the occurrences of changes in electrical resistivity but will also restrict the variations of these changes inside the different zones. The second approach is based on geostatistics and requires first to derive variogram parameters for the model changes. In this contribution, we demonstrate the benefits and limitations of these regularization approaches to time-lapse data on numerical benchmarks and three case studies. [less ▲]

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