References of "Nguyen, Frédéric"
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See detailFractured bedrock investigation by using high-resolution borehole images and the Distributed Temperature Sensing technique
Radioti, Georgia ULg; Delvoie, Simon ULg; Radu, Jean-Pol ULg et al

in ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics (2015, May)

In order to investigate the fracturing of the bedrock and its possible heterogeneous distribution in situ, four boreholes equipped with double-U geothermal pipes of 100 m long were installed on the campus ... [more ▼]

In order to investigate the fracturing of the bedrock and its possible heterogeneous distribution in situ, four boreholes equipped with double-U geothermal pipes of 100 m long were installed on the campus of the University of Liege (Liege, Belgium) over a surface area of 32 m². The bedrock, which starts at a depth approximately of 8 m, is quite fractured and consists mainly of siltstone and shale interbedded with sandstone. Different geophysical methods are applied at two different phases, after drilling the boreholes and after injecting the grouting material. The first approach consists in lowering an ultrasonic borehole imager (borehole televiewer; Zemanek, Glenn, Norton, & Caldwell, 1970), an instrument that acts as an ultrasonic transducer and receiver, into the boreholes to obtain high-resolution, continuous images with 360° coverage of the local geology and fracturing. Moreover gamma-ray logs of the four boreholes are obtained and inclinometry is conducted. After drilling the boreholes fiber optic cables are attached along the pipe loops and the double-U pipes are installed inside the boreholes. Then the grouting material is injected. The second approach consists in measuring the temperature along the fibers by applying the Distributed Temperature Sensing technique (Soto, Sahu, Faralli, Bolognini, Di Pasquale, Nebendahl, & Rueck, 2007). A laser pulse is injected into the optical fiber and the temperature along the fiber is determined by the intensity of Raman stokes and anti-stokes reemitted signals. Temperature evolution is measured during hardening of the grouting material. Local maxima of the temperature curve are probably due to a local lower thermal conductivity and/or a local larger quantity of grouting material due to gathering of fractures. A detailed fracture characterisation (position, opening, orientation, dip angle) is obtained based on the acoustic signal travel time and amplitude. The fractures are characterised by the same dipping and orientation but significantly vary in number and location in the four boreholes, despite the close distance between them. Gamma-ray data and observation of the cuttings during drilling result in rock identification through depth as well as in determination of the layer dipping. The inclination of the four boreholes tends to be perpendicular to the dipping. The combination of the two geophysical methods as presented provides information useful for the hydro-thermo-mechanical behaviour of the bedrock. The contribution of the thermal behaviour of borehole heat exchangers to bedrock investigation will be further studied by conducting Distributed Thermal Response tests (Fujii, Okubo, & Itoi, 2006). During the tests we will measure the temperature variation thanks to the installed fiber optics. These data will allow us to correlate any anisotropic thermal behaviour to the geological characteristics. The available information could be used for a detailed numerical model. [less ▲]

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See detailExperimental design to monitor the influence of crop residue management on the dynamics of soil water content
Chelin, Marie ULg; Parvin, Nargish ULg; Hiel, Marie-Pierre ULg et al

Poster (2015, April 15)

Choices related to crop residue management affecting soil structure determine spatio-temporal dynamics of water content and eventually crop yields. In this contribution, we discuss the experimental design ... [more ▼]

Choices related to crop residue management affecting soil structure determine spatio-temporal dynamics of water content and eventually crop yields. In this contribution, we discuss the experimental design we adopted to study the influence of three different agricultural management strategies (tillage and residue management) on the soil water dynamics under maize in a Cutanic Siltic Luvisol in Gembloux, Belgium. In order to limit soil disturbance, we opted for the use electrical resistivity tomography (ERT) and we use the bulk electrical conductivity as a proxy for soil moisture content. ERT is collected every week on a surface of two square meters corresponding to three rows of seven maize plants through surface stainless steel electrodes. Four additional sticks with stainless steel electrodes will be vertically inserted into the soil up to 1.20 m to get more detailed information near to the central maize row. In each of the monitoring plots, two time-domain reflectometry (TDR) probes will be installed for data validation. In order to calibrate the relationship between electrical resistivity and soil water content under highly variable field conditions (changes in soil structure, variable weather conditions, plant growth, fertilization), a trench will be dug, in which a set of four electrodes, one TDR probe and one temperature sensor will be placed at four different depths. In addition, two suction cups will be installed in each of the plots to quantify changes in ion composition and electrical conductivity of the soil solution at two different depths. Within the framework of the multidisciplinary research platform AgricultureIsLife, regular assessment of pore structure and crop developement will be conducted using X-ray images. Combining this wide range of data, we will be able to investigate and quantify the effect of simultaneously changing pore water conductivity, soil porosity, soil temperature and soil moisture on the effectiveness of time-lapse ER measurements as a proxy for soil moisture changes. [less ▲]

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See detailRegularized focusing inversion of time-lapse electrical resistivity data: an approach to parametrize the minimum gradient support functional
Nguyen, Frédéric ULg; Hermans, Thomas ULg

Poster (2015, April 15)

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

Inversion of time-lapse resistivity data allows obtaining ‘snapshots’ of changes occurring in monitored systems for applications such as aquifer storage, geothermal heat exchange, 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 and associated processes and controlling parameters. However, the reliability of the analysis is dependent on survey geometry, measurement schemes, data error, and 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 time but it is often needed to obtain a sharp temporal anomaly, for example in fractured aquifers. We here propose to use the alternative regularization approach based on minimum gradient support (MGS) (Zhdanov, 2002) for time-lapse surveys which will 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. A common difficulty encountered by practitioners in this type of regularization is the choice of an additional parameter, the so-called , required to define the MGS functional. To the best of our knowledge, there is no commonly accepted or standard methodology to optimize the MGS parameter . The inversion algorithm used in this study is CRTomo (Kemna 2000). It uses a Gauss-Newton scheme to iteratively minimize an objective function which consists of a data misfit functional and a model constraint functional. A univariate line search is performed at each Gauss-Newton iteration step to find the optimum value of the regularization parameter  which minimizes the data misfit as a function of  while the data misfit is above the desired value and yields the desired target misfit (root-mean square value of error-weighted data misfit equal to 1) at the last iteration for a maximum value of . We propose here to optimize the  of the MGS functional by considering a univariate line search at the first iteration to find the  that minimizes the data misfit. The parameter is then kept constant during the Gauss-Newton iterative scheme. In this contribution, we validate our approach on a numerical benchmark and apply it successfully on a case study in the context of salt tracers in fractured aquifers. Zhdanov M.S. 2002. Geophysical Inverse Theory and Regularization Problems. Elsevier, Amsterdam, 628 p. Kemna A. 2000. Tomographic Inversion of Complex Resistivity - Theory and Application. PhD Thesis, Ruhr University Bochum. [less ▲]

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See detailVariogram-based inversion of time-lapse electrical resistivity data: development and application to a thermal tracing experiment
Hermans, Thomas ULg; Nguyen, Frédéric ULg

Poster (2015, April 15)

Electrical resistivity tomography (ERT) has become a popular imaging methodology in a broad range of applications given its large sensitivity to subsurface parameters and its relative simplicity to ... [more ▼]

Electrical resistivity tomography (ERT) has become a popular imaging methodology in a broad range of applications given its large sensitivity to subsurface parameters and its relative simplicity to implement. More particularly, time-lapse ERT is now increasingly used for monitoring purposes in many contexts such as water content, permafrost, landslide, seawater intrusion, solute transport or heat transport experiments. Specific inversion schemes have been developed for time-lapse data sets. However, in contrast with static inversions for which many techniques including geostatistical, minimum support or structural inversion are commonly applied, most of the methodologies for time-lapse inversion still rely on non-physically based spatial and/or temporal smoothing of the parameters or parameter changes. In this work, we propose a time-lapse ERT inversion scheme based on the difference inversion scheme. We replace the standard smoothness-constraint regularization operator by the parameter change covariance matrix. This operator takes into account the correlation between changes in resistivity at different locations through a variogram computed using independent data (e.g., electromagnetic logs). It may vary for subsequent time-steps if the correlation length is time-dependent. The methodology is first validated and compared to the standard smoothness-constraint inversion using a synthetic benchmark simulating the injection of a conductive tracer into a homogeneous aquifer inducing changes in resistivity values of known correlation length. We analyze the influence of the assumed correlation length on inversion results. Globally, the method yields better results than the traditional smoothness constraint inversion. Even if a wrong correlation length is assumed, the method performs as well as the smoothness constraint since the regularization operator balances the weight given to the model constraint functional in the objective function. Then the methodology is successfully applied to a heat injection and pumping experiment in an alluvial aquifer. The comparison with direct measurements in boreholes (temperature loggers and distributed temperature sensing optic fibres) shows that ERT-derived temperatures and breakthrough curves image reliably the heat plume through time (increasing part of the curve, maximum and tail are correctly retrieved) and space (lateral variations of temperature are observed) with less spatial smoothing than standard methods. The development of new regularization operators for time-lapse inversion of ERT data is necessary given the broad range of applications where ERT monitoring is used. In many studies, independent data are available to derive geostatistical parameters that can be subsequently used to regularize geophysical inversions. In the future, the integration of spatio-temporal variograms into existing 4D inversion schemes should further improve ERT time-lapse imaging. [less ▲]

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See detailResearch Self-Evaluation Report of the Department ArGEnCo
Collin, Frédéric ULg; Dewals, Benjamin ULg; Elsen, Catherine ULg et al

Report (2015)

Evaluation is nowadays a general trend in the European Universities. At the University of Liège, the evaluation process is threefold: fist there is the evaluation of the bachelor and master degrees, then ... [more ▼]

Evaluation is nowadays a general trend in the European Universities. At the University of Liège, the evaluation process is threefold: fist there is the evaluation of the bachelor and master degrees, then the second evaluation concerns the internal administrations and finally Research activities are being evaluated. This quality policy at the different levels of the Institution is organized by the Vice-Rector for Quality, Pr. Freddy Coignoul and the SMAQ service (Service de Management et d’Accompagnement à la Qualité), which aims to promote, coordinate and disseminate within the University of Liège a culture of the Quality, founded on the values of the Institution. The Research assessment is a procedure undertaken by the University in order to promote the quality of the research in the Departments or research centres. Within the Faculty of Applied Sciences, the evaluation of the Bachelor and Master degrees in Engineering was carried out in 2013 by the AEQES (Agence pour l’Evaluation de la Qualité de l’Enseignement) and CTI (Commission des Titres d’Ingénieur - France). The present report is dedicated to the evaluation of research led in the Education and Research Department (DER) ArGEnCo. The present evaluation report proposes a global presentation of all our research activities. At the Department level, one of the elected members of the Executive Bureau is in charge of the Research, covering both evaluation and quality policy. In October 2013, the Executive bureau appointed a research steering committee, whose fist task was to coordinate the present evaluation report, following the guidelines of the SMAQ service. The draft of such a document arises from the work of the steering committee but also from the contributions of all Department members, who were asked to contribute to the report or to complete databases. The progress of this evaluation report was presented by the Steering committee to the Department Council and during the General Assembly of the Department. Moreover, two Focus groups were organized by the SMAQ service, in order to get the feedback of technician and administrative members on one hand and scientific members on the other hand, on the general organization of the research in the ArGEnCo Department. The reports of these two Focus groups (Appendix 4 in French) underpinned our thinking about our research management. This report is therefore the fist evaluation of the research led in the Department, the present effort should be pursued, and deepened, within the coming years. Some of the databases necessary for the evaluation have been developed in the frame of this procedure. This has resulted in a huge amount of work for every Department members to collect all the data. New procedures in the management of the Department will help establishing the next research evaluation reports systematically, in particular for the collection of the complete dataset. Following the guidelines of the SMAQ service, this evaluation report is divided into two main parts: the fist one focuses on a self-evaluation report of the research activities and the second part presents the Action Plan of the Department for the next five years to come. The self-evaluation report is composed of four Sections: Section one presents the Research organization and topics of the Department, Section two focuses on the Research activities, Section three gives an overview of the mobility (in and out), the attractiveness and the scientific recognition of the Department and finally Section four is dedicated to the resources available in ArGEnCo Department. Each Section ends up with a SWOT analysis. The Action Plan proposed by the Steering Committee results from the analysis of the self-evaluation report, insofar as it synthesizes and prioritizes the actions of the Department for the next fie years. These priorities are formulated as proposals following a contractual approach between research entities and Institution authorities, to whom the report will be presented. [less ▲]

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See detailCharacterization of superficial deposits using electrical resistivity tomography (ERT) and horizontal-to-vertical spectral ratio (HVSR) geophysical methods: A case study
Sauret, Elie; Beaujean, Jean; Nguyen, Frédéric ULg et al

in Journal of Applied Geophysics (2015), 121

In developing countries, superficial aquifers are potential water resources for irrigation in agriculture. Cost effective methodologies are required to characterize those deposits in order to identify ... [more ▼]

In developing countries, superficial aquifers are potential water resources for irrigation in agriculture. Cost effective methodologies are required to characterize those deposits in order to identify better locations for groundwater abstraction. This study has investigated the potential use of combined electrical resistivity tomography (ERT) and horizontal-to-vertical spectral ratio (HVSR) to characterize the heterogeneity and thickness of superficial deposits deployed along of a river. The ERT and HVSRmethods are non-invasive geophysical techniques, simple, efficient, robust and easy-to-use in alluvial environment contexts. Using these geophysicalmethods in the Kou basin in Burkina Faso (West Africa), a good correspondence is obtained between ERT images and resonance frequencies determined on theHVSR profiles perpendicular to the Kou River. The superficial deposits and the bedrock depth have been characterized and mapped. The role of faulting andmagmatic intrusions in the accumulation of fractured, deconsolidated andweathered bedrockmaterials and the filling of superficial deposits have been observed and highlighted. Froma hydrogeological point of view, the thickness of (clay-free) superficial deposits presents a relatively important groundwater reservoir and potentially high productivity. The ERT and HVSR were proven to be efficient and complementary methods in superficial deposit environment characterization, and a viable option for exploration of superficial deposits in terms of groundwater reservoir in environments where the bedrock exhibits strong lateral variation due to faulting or volcanic activities. [less ▲]

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See detailVadose zone characterisation at industrial contaminated sites
Fernandez de Vera, Natalia ULg; Dahan, Ofer; Dassargues, Alain ULg et al

in CL:AIRE bulletin (2015)

In order to improve risk characterization and remediation measures for soil and groundwater contamination, there is a need to improve in situ vadose zone characterization. However, most available ... [more ▼]

In order to improve risk characterization and remediation measures for soil and groundwater contamination, there is a need to improve in situ vadose zone characterization. However, most available technologies have been developed in the context of agricultural soils. Such methodologies are not applicable at industrial sites, where soils and contamination differ in origin and composition. To overcome such difficulties, a vadose zone experiment has been setup at a former industrial site in Belgium. Industrial activities carried out on site left a legacy of soil and groundwater contamination in BTEX, PAH, cyanide and heavy metals. The experiment comprises the combination of two techniques: the Vadose Zone Monitoring System (VMS) and cross-hole geophysics. The VMS allows continuous measurements of water content at different depths of the vadose zone, as well as the possibility of water sampling at different depths. The system is formed by a flexible sleeve containing monitoring units along its depth which is installed in a slanted borehole. The flexible sleeve contains three types of monitoring units in the vadose zone: Time Domain Transmissometry (TDT), which allows water content measurements; Vadose Sampling Ports (VSP), used for collecting water samples coming from the matrix; the Fracture Samplers (FS), which are used for retrieving water samples from the fractures. Cross-hole geophysics techniques consist in the injection of an electrical current using electrodes installed in vertical boreholes. From such injections, spatial patterns related with subsurface heterogeneities, water content and solute concentrations are inferred. Two VMS were installed in two slanted boreholes on site, together with four vertical boreholes containing electrodes for geophysical measurements. The site was monitored under natural recharge conditions. Results show the reaction of the vadose zone to rainfall events, as well as chemical evolution of soil water with depth and rainfall infiltration. [less ▲]

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See detailUncertainty in training image-based inversion of hydraulic head data constrained to ERT data: workflow and case study
Hermans, Thomas ULg; Nguyen, Frédéric ULg; Caers, Jef

in Water Resources Research (2015), 51

In inverse problems, investigating uncertainty in the posterior distribution of model parameters is as important as matching data. In recent years, most efforts have focused on techniques to sample the ... [more ▼]

In inverse problems, investigating uncertainty in the posterior distribution of model parameters is as important as matching data. In recent years, most efforts have focused on techniques to sample the posterior distribution with reasonable computational costs. Within a Bayesian context, this posterior depends on the prior distribution. However, most of the studies ignore modeling the prior with realistic geological uncertainty. In this paper, we propose a workflow inspired by a Popper-Bayes philosophy, that data should first be used to falsify models, then only be considered for matching. We propose a workflow consisting of three steps: (1) in defining the prior, we interpret multiple alternative geological scenarios from literature (architecture of facies) and site specific data (proportions of facies). Prior spatial uncertainty is modeled using multiple-point geostatistics, where each scenario is defined using a training image. (2) We validate these prior geological scenarios by simulating electrical resistivity tomography (ERT) data on realizations of each scenario and comparing them to field ERT in a lower dimensional space. In this second step, the idea is to probabilistically falsify scenarios with ERT, meaning that scenarios which are incompatible receive an updated probability of zero while compatible scenarios receive a non-zero updated belief. (3) We constrain the hydrogeological model with hydraulic head and ERT using a stochastic search method. The workflow is applied to a synthetic and a field case studies in an alluvial aquifer. This study highlights the importance of considering and estimate prior uncertainty (without data) through a process of probabilistic falsification. [less ▲]

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See detailGeophysical investigation of the Hockai Fault Zone, Eastern Belgium
Havenith, Hans-Balder ULg; Nguyen, Frédéric ULg; Halleux, Lucien ULg et al

in EGU General Assembly Conference Abstracts (2015)

In the frame of a regional project evaluating the geothermal potential of the Wallonian Region of Belgium, the Hockai Fault Zone has been identified as one of the most interesting targets. It is a ... [more ▼]

In the frame of a regional project evaluating the geothermal potential of the Wallonian Region of Belgium, the Hockai Fault Zone has been identified as one of the most interesting targets. It is a seismically active fault zone that hosted the largest historical earthquake in Northwestern Europe, the M6-6.5 Verviers event in 1692 as well as a swarm of small earthquakes that was recorded in 1989-90. On the surface, the presence of the fault zones is marked by a series of geomorphic features, such as several landslides near the borders in the northern part, repeated NW-SE oriented scarps all along the Eastern border (over a distance of 40 km), river diversions and captures with formation of paleo-valleys.... [less ▲]

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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 detailUncertainty in Training-Image Based Inversion of Hydraulic Head Data Constrained to ERT Data: Workflow and Case Study
Hermans, Thomas ULg; Nguyen, Frédéric ULg; Caers, Jef

Conference (2014, December 18)

In inverse problems, investigating the relationship between data and prior models and the uncertainty related to the posterior distribution of model parameters are as important as matching the data. In ... [more ▼]

In inverse problems, investigating the relationship between data and prior models and the uncertainty related to the posterior distribution of model parameters are as important as matching the data. In recent years, many efforts have been done to assess the posterior distribution of a given problem with reasonable computational costs through inversion techniques such as McMC. The derived posterior distribution is always dependent on the prior distribution. However, most of the studies ignore modeling the prior with realistic uncertainty. In this paper, we propose a workflow to assess the uncertainty of inversion of hydraulic heads data through the addition of electrical resistivity tomography (ERT) constraining data. The workflow is divided in three successive steps: 1) Construction of prior: we generate multiple alternative geological scenarios from literature data (architecture of facies) as well as site specific data (proportions of facies). Spatial uncertainty within each scenario is integrated hierarchically through geostatistics (multiple-point statistics simulation of facies constrained by ERT data as soft data). 2) Validation of prior scenarios: we transform prior facies scenarios into resistivity distribution scenarios through forward and inverse modeling. The scenarios are validated by comparison with field ERT data. The comparison is made through distance calculation and projection into a low dimensional space to calculate the probability of each scenario given field ERT data. 3) Matching dynamical data: we use the probability perturbation method, within each scenario, to integrate hydraulic heads to our models. We account for scenario probabilities, calculated in 2, in determining how many models per scenario we have to consider for building a reliable posterior distribution. As an illustration, the method is applied on a field case study in an alluvial aquifer (Belgium) where we consider prior uncertainty related to the type of elements (gravel channels or bars) and to their size. This study shows the importance of considering the uncertainty of the prior in inverse problems as it has a strong influence on model predictions and decision-making problems. [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 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

Conference (2014, September 03)

In situ vadose zone characterization is essential to improve risk characterization and remediation measures for soil and groundwater contamination. However, most available technologies have been developed ... [more ▼]

In situ vadose zone characterization is essential to improve risk characterization and remediation measures for soil and groundwater contamination. However, most available technologies have been developed in the context of agricultural soils. Most of these methodologies are not applicable at industrial sites, where soils and contamination differ in origin and composition. In addition, they are applicable only in the first meters of soils, leaving deeper vadose zones with lack of information, in particular on field scale heterogeneity. To overcome such difficulties, a vadose zone experiment has been setup at a former industrial site in Belgium. Industrial activities carried out on site left a legacy of soil and groundwater contamination in BTEX, PAH, cyanide and heavy metals. The experiment involves the combination of two techniques: the Vadose Zone Monitoring System (VMS) and cross-hole geophysics. The VMS allows continuous measurements of water content at different depths of the vadose zone (Dahan et al., 2009). In addition, it provides the possibility of pore water sampling at different depths. The system is formed by a flexible sleeve installed in a slanted borehole (Fig. 1) and containing monitoring units along its depth (Fig. 2). The flexible sleeve contains three types of monitoring units in the vadose zone: Time Domain Transmissometry (TDT), which allows water content measurements; Vadose Sampling Ports (VSP), used for collecting water samples coming from the matrix; and the Fracture Samplers (FS), which are used for retrieving water samples from the fractures. Cross-hole electrical tomography measurements are carried providing detailed spatial patterns about electrical properties of the subsurface. Such properties are related with subsurface heterogeneities, water content and solute concentrations. Two VMS were installed on site, together with four vertical boreholes containing electrodes for geophysical measurements. The site has been monitored under natural recharge conditions during the summer, autumn and winter. Results show reactions in the soil at depths up to 6m as a consequence of rainfall infiltration and groundwater level fluctuations. In addition, the chemistry of the soil water changes with depth and water infiltration. Background images obtained from geophysical measurements show a highly conductive subsurface due to the lithologies and the high mineralization of the water in the vadose zone. The combination of cross-hole geophysics with the VMS has provided an effective tool for characterizing the chemistry and the structure of the vadose zone. [less ▲]

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See detailCalibration of seawater intrusion models: Inverse parameter estimation using surface electrical resistivity tomography and borehole data
Beaujean, Jean ULg; Nguyen, Frédéric ULg; Kemna, A. et al

in Water Resources Research (2014), 50(8), 6828-6849

Electrical resistivity tomography (ERT) can be used to constrain seawater intrusion models because of its high sensitivity to total dissolved solid contents (TDS) in groundwater and its relatively high ... [more ▼]

Electrical resistivity tomography (ERT) can be used to constrain seawater intrusion models because of its high sensitivity to total dissolved solid contents (TDS) in groundwater and its relatively high lateral coverage. However, the spatial variability of resolution in electrical imaging may prevent the correct recovery of the desired hydrochemical properties such as salt mass fraction. This paper presents a sequential approach to evaluate the feasibility of identifying hydraulic conductivity and dispersivity in density-dependent flow and transport models from surface ERT-derived mass fraction. In the course of this study, geophysical inversion was performed by using a smoothness constraint Tikhonov approach, whereas the hydrological inversion was performed using a gradient-based Levenberg-Marquardt algorithm. Two synthetic benchmarks were tested. They represent a pumping experiment in a homogeneous and heterogeneous coastal aquifer, respectively. These simulations demonstrated that only the lower salt mass fraction of the seawater-freshwater transition zone can be recovered for different times. This ability has here been quantified in terms of cumulative sensitivity and our study has further demonstrated that the mismatch between the targeted and the recovered salt mass fraction occurs from a certain threshold. We were additionally able to explore the capability of sensitivity-filtered ERT images using ground surface data only to recover (in both synthetic cases) the hydraulic conductivity while the dispersivity is more difficult to estimate. We attribute the latter mainly to the lack of ERT-derived data at depth (where resolution is poorer) as well as to the smoothing effect of the ERT inversion. [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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