References of "Nguyen, Frédéric"
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See detailUsing geostatistical constraints in electrical imaging for improved reservoir characterization
Martin, Roland; Kemna, Andreas; Hermans, Thomas ULg et al

Conference (2010, December 14)

Developing predictive models of reservoirs is often complicated by the spatial heterogeneities and the different scales which control flow and transport processes. In numerous studies over the past two ... [more ▼]

Developing predictive models of reservoirs is often complicated by the spatial heterogeneities and the different scales which control flow and transport processes. In numerous studies over the past two decades, geophysical imaging techniques have proved very useful for reservoir characterization. However, the loss of resolution and the non-uniqueness of standard solutions to inverse problems strongly limit the use of such deterministic imaging approaches. On the other hand, the use of common geostatistical approaches for reservoir characterization, for instance from logging information, may be a difficult task, since accurate variogram information is difficult to obtain (dense sampling in the vertical and lateral directions), and also because a high number of conditioned simulations is needed to remove statistical bias. Combining the high spatial sampling of deterministic geophysical imaging methods with geostatistical constraints, valid in the whole image plane, appears as a very promising approach to enhance reservoir characterization. To do so, we use a parameterized model covariance matrix based on standard variogram functions and a prior model as regularization operator in the inversion of electrical resistance data. This way of including additional data is not restricted to electrical data but the variogram parameters may be also inferred from for example available textural or lithological information. The benefit of the presented approach is twofold: (i) It honors the spatial statistics of the reservoir and (ii) it alters the posterior model by further reducing model ambiguity inherent to the inversion compared to classical (smooth model) regularization. The proof of concept is given by synthetic studies carried out on random fields from Gauss simulations with varying (an)isotropic scale lengths using different model (co)variogram functions. We also demonstrate the approach on electrical field data combined with borehole electromagnetic data from two artificial sea inlets in the nature reserve "The Westhoek" near the French-Belgian border. The electromagnetic logs were used to calculate an experimental vertical variogram characteristic of the study site. The results enabled to determine the extension of the salt water plume laterally, and significantly enhance its extension in depth, but also in terms of total dissolved solid content. These observations are in agreement with the hydrogeological situation at the site. A comparison with borehole data shows that the results are much more plausible than results obtained with a traditional smoothness constraint used as regularization operator. In conclusion, the incorporation of geostatistical information, vertical variograms in our case, in the inverse process improves imaging capabilities for reservoir characterization significantly. [less ▲]

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See detailSUIVI D'UN TEST GEOTHERMIQUE PEU PROFOND DANS UN AQUIFERE SABLEUX PAR TOMOGRAPHIE DE RESISTIVITE ELECTRIQUE : ETUDE DE FAISABILITE
Hermans, Thomas ULg; Vandenbohede, Alexander; Nguyen, Frédéric ULg et al

in Bulletin du groupe francophone d'humidimétrie et transferts en milieux poreux (2010, November 24), 56(Novembre 2010), 21-26

A shallow thermal test was monitored with electrical resistivity tomography (ERT). Heated water (48°C) was injected in a sandy aquifer (10°C). Laboratory measurements indicated a change of 2.1 %/°C of ... [more ▼]

A shallow thermal test was monitored with electrical resistivity tomography (ERT). Heated water (48°C) was injected in a sandy aquifer (10°C). Laboratory measurements indicated a change of 2.1 %/°C of water electrical conductivity for injection water. Due to this change, the plume of heated water was detected by geophysical imaging. In parallel, a hydrogeological flow and heat transport model was calibrated. Geophysics brought important information to calibrate this model and to show the heat exchange through the unsaturated zone. The comparison of the two models shows that ERT was able to retrieve the position and the temperature of the plume. However, a distortion occurs due to the smoothing of the solution during the regularized inversion process. This work demonstrates the ability of ERT to help quantifying parameters governing heat transport in the subsurface non-invasively and therefore studying and designing heat and storage experiments for shallow reservoirs. [less ▲]

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See detailA Saline Tracer Test Monitored with ERT to Detect Preferential Flow/Transport Paths in Limestones
Robert, Tanguy ULg; Caterina, David ULg; Deceuster, John et al

in EarthDoc - Near Surface 2010 – 16th European Meeting of Environmental and Engineering Geophysics (2010, September 07)

The success of a tracer test highly depends on the number and the localisation of the sampling wells. When preferential solute transport paths are expected, one needs to set up carefully the tracer test ... [more ▼]

The success of a tracer test highly depends on the number and the localisation of the sampling wells. When preferential solute transport paths are expected, one needs to set up carefully the tracer test to recover information such as the local groundwater flow direction and an estimate of the transport velocities. In this work, we used electrical resistivity tomography (ERT) to monitor a saline tracer test. This experiment was performed in fractured limestones where high transport velocities and strong dilution effects were expected. This required a continuous injection and fast ERT acquisition. Two different salt concentrations (40 and 160 g/l) were injected to deal with dilution effects. We also tested the resolution and the depth of investigation of our dipole-dipole sequence by changing the electrode spacing. Two transversal (and a longitudinal) profiles were placed every 20 m from the injection well. During the first (second) test, a maximum of -8 % (-16 %) change of electrical resistivity was observed in the nearest ERT profile while no change occurred in the other ones. We were then able to estimate the transport velocities in addition to the local groundwater flow direction even if the dilution effects were important. [less ▲]

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See detailUtilisation du GPR pour la détection des discontinuités dans les tabliers de ponts
Van der Wielen, Audrey ULg; Nguyen, Frédéric ULg; Courard, Luc ULg

Scientific conference (2010, August 27)

Le Ground Penetrating Radar (GPR) est une technique non destructive utilisée depuis des décennies dans l’auscultation des sols. Depuis quelques années, l’apparition sur le marché d’antennes à hautes ... [more ▼]

Le Ground Penetrating Radar (GPR) est une technique non destructive utilisée depuis des décennies dans l’auscultation des sols. Depuis quelques années, l’apparition sur le marché d’antennes à hautes fréquences a permis d’appliquer cette méthode à des structures de moindre épaisseur, et notamment aux tabliers de ponts en béton. A l’Université de Liège, des travaux sont menés afin d’améliorer la détection des discontinuités dans les tabliers de ponts en béton. Dans une première étude expérimentale, 12 échantillons en béton contenant des défauts simulant des délaminations ont été réalisés et scannés par radar. Nous avons ainsi pu représenter les défauts en 3D et constater que leur visibilité était dépendante de leur épaisseur. Actuellement, une seconde étude théorique et expérimentale porte sur la détermination exacte de la limite de détection des couches minces. [less ▲]

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See detailUse of image appraisal tools and covariance matrix
Caterina, David ULg; Beaujean, Jean ULg; Nguyen, Frédéric ULg

Conference (2010, July)

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See detailJoint and sequential inversion of geophysical and hydrogeological data to characterize seawater intrusion models
Beaujean, Jean ULg; Nguyen, Frédéric ULg; Kemna, Andreas

in Teresa Condesso de Melo, Maria; Lebbe, Luc; Virgílio Cruz, José (Eds.) et al Proceedings SWIM21, 21st Salt Water Intrusion Meeting (2010, June)

The integrated water resource management problems require studying efficiently seawater intrusion at local and regional scale and identifying in real time the seawater/freshwater interface dynamic ... [more ▼]

The integrated water resource management problems require studying efficiently seawater intrusion at local and regional scale and identifying in real time the seawater/freshwater interface dynamic. Hydrogeological modeling is widely used to predict seawater intrusion if additional natural or man‐made factors are modified. These models are currently calibrated using measured heads and salt mass fractions in boreholes, which generally result in sparse data coverage. Within this scope, non to minimally invasive geophysical techniques like electrical resistivity tomography (ERT) or time‐domain electromagnetic method are becoming increasingly popular, given their relatively greater spatial horizontal resolution compared to borehole observations. We present a comparison between both a sequential and joint approach to calibrate seawater intrusion models using ERT. The former consists of constraining hydrogeological parameters using ERT derived parameters and relies on sequential inversions of the geophysical and hydrogeological data using a given petrophysical relationship. The second approach in based on simultaneous inversion of petrophysical and hydrogeological data using electrical resistance data as data. It is performed by coupling an inversion code with two hydrogeological and geophysical modeling codes through a petrophysical conversion. This investigation was performed on a densitydependent flow and transport numerical (three/two‐dimensional) simulation study from complex and realistic heterogeneous models. In the sequential approach, the simulations showed that only the shallow salt concentration of the seawater/freshwater transition zone could be recovered for different time‐lapse, due to poorly resolved regions in depth. The capability of image appraisal indicators (cumulative sensitivity and resolution) has been analyzed to emphasize the discrepancy between the targeted and imaged parameter values. On the other hand, the preliminary coupled inversion avoids the regularization bias introduced by ERT and addresses the non‐stationarity of the petrophysical relationship. [less ▲]

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See detailMonitoring a shallow geothermal experiment in a sandy aquifer using electrical resistivity tomography: a feasibility study
Hermans, Thomas ULg; Vandenbohede, Alexander; Nguyen, Frédéric ULg et al

Poster (2010, May 06)

Geothermal resources, especially groundwater resources, are increasingly used around the world. It is therefore necessary to improve the characterisation of thermal parameters of the aquifers. This is ... [more ▼]

Geothermal resources, especially groundwater resources, are increasingly used around the world. It is therefore necessary to improve the characterisation of thermal parameters of the aquifers. This is done mostly with thermal response tests. In this work, we monitored with electrical resistivity tomography a geothermal test on the campus of Ghent University (Belgium). We injected warm water (45°C) into a sandy aquifer where the groundwater has a temperature of 10°C at a rate of 100 liter/hour during three days. Laboratory measurements indicated that we could expect at most a change of 2%/°C of the water electrical conductivity. The time-lapse series of electrical images show clearly the thermal plume corresponding to the injected water with a maximum change of minus 20% after 72 hours of injection. A comparison with a geothermal model shows that the anomaly is well detected but also distorded due to the inversion regularization (smoothness constraint). ERT enabled to follow the evolution of the injected heated water with more spatial coverage that traditionnal techniques. ERT is thus an interesting tool to monitor the exploitation of geothermal resources. In the future, it will be necessary to improve the inversion process to use quantitavely such results in a calibration process. [less ▲]

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See detailImage Appraisal Tools for Electrical Resistivity Tomography
Caterina, David ULg; Beaujean, Jean ULg; Robert, Tanguy ULg et al

Poster (2010, January 12)

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See detailElectrical resistivity tomography and self-potential methods as a tool for fractured aquifer characterization and monitoring
Robert, Tanguy ULg; Nguyen, Frédéric ULg

Poster (2010, January 12)

Electrical resistivity tomography (ERT) and self-potential (SP) investigations are currently conducted in carboniferous limestones in Belgium. The aims of this study are (1) to characterize and (2) to ... [more ▼]

Electrical resistivity tomography (ERT) and self-potential (SP) investigations are currently conducted in carboniferous limestones in Belgium. The aims of this study are (1) to characterize and (2) to monitor the groundwater flow in fractured or karstic area and (3) to use these geophysical data as a help for groundwater flow model conceptualisation and inverse calibration. Large ERT profiles (320 meters) allow us to image the electrical resistivity distribution of the first 60 meters of the subsurface and to detect and characterize (in terms of direction, width and depth) some fractured and/or karstic zones expected to be less resistive. Data errors as well as indicators of resolution (resolution matrix, sensitivity matrix and DOI index) are analysed in order to calculate the depth of investigation of ERT and to avoid the misinterpretation of the resulting images. Self-potential measurements are performed along the electrical profiles and permit us to find some negative anomalies possibly related with groundwater preferential flow pathways. Since SP signals are related with the groundwater flow when taking into account the electrokinetic effect, we are able to estimate a first distribution of the water table along our profiles. These geophysical data concurrently with ‘ground truth’ geological and hydrogeological data give us a way to better understand the groundwater flow in limestone synclines of the Dinant Synclinorium geological structure. Still, further efforts are needed (1) to fully cover the syncline area and (2) to couple the SP, ERT and hydrogeological data through a more complex forward model of the electrokinetic effect of the self-potential method. [less ▲]

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See detailRelation between bond quality and impact-echo frequency spectrum
Piotrowski, Tomasz; Garbacz, Andrzej; Van der Wielen, Audrey ULg et al

in Restoration of Buildings and Monuments = Bauinstandsetzen und Baudenkmalpflege (2010), 16(4/5), 303-314

According to EN 1504-10 and ACI Concrete Repair Manual, bond strength and interface quality are the main features of repair system necessary to be assessed. Pull-off test is most commonly used for bond ... [more ▼]

According to EN 1504-10 and ACI Concrete Repair Manual, bond strength and interface quality are the main features of repair system necessary to be assessed. Pull-off test is most commonly used for bond strength evaluation but growing interest in nondestructive techniques (NDT) is recently noted. Impact-echo (IE) is treated as the most promising one for this purpose. The aim of this paper is to analyze an effect of bond quality on stress wave propagation in repair systems. A group of samples has been prepared in order to obtain repair systems of different bond quality. Prior to repair, quality of concrete substrates has been characterized according different techniques: compressive strength, superficial cohesion, surface roughness index and cracking quantification. Than a polymer-modified repair mortar has been applied. After hardening, IE signals have been recorded and pull-off bond strength determined. The relationships between parameters characterizing surface quality, bond strength, IE frequency spectrum and results of wavelet analysis of IE signal have been analyzed. [less ▲]

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See detailElectrical resistivity tomography and self-potential case studies for fractured aquifer characterization and monitoring
Robert, Tanguy ULg; Dassargues, Alain ULg; Nguyen, Frédéric ULg

in Geophysical Research Abstracts (2010), 12

Electrical resistivity tomography (ERT) and self-potential (SP) investigations have been conducted in complex carboniferous limestones aquifers in Belgium (synclinorium structures). The aims of this study ... [more ▼]

Electrical resistivity tomography (ERT) and self-potential (SP) investigations have been conducted in complex carboniferous limestones aquifers in Belgium (synclinorium structures). The aims of this study were (1) to detect and characterize fractured zones in hard rock aquifers, (2) to monitor groundwater flow/water content in these fractured or karstic areas and (3) to use geophysical data to support groundwater flow model set-up and calibration. The investigated areas lie in calcareous synclines. Electrical images allowed us to detect and characterize (in terms of direction, width and depth) several less resistive anomalies, which are interpreted in terms of fractured and/or karstic zones. To interpret the ERT images, data errors as well as image appraisal indicators (resolution matrix, sensitivity matrix and DOI index) were analysed and compared. This allowed us to determine the depth of investigation of ERT and to avoid the misinterpretation of the resulting images. Inversions based on focusing scheme are tested against smoothness-constraint inversion on these field data to provide more realistic images on the basis of prior geological knowledge. Self-potential measurements were performed along the electrical profiles and allowed us to find negative anomalies possibly related with groundwater preferential flow pathways. By taking the assumption that only the electrokinetic effect plays a role in the SP signals, we were able to estimate a first distribution of the water table along our profiles. The SP data showed that in this particular tectonic structure, two perpendicular hydraulic gradients are present. The first gradient is related with the main fold axis direction and is the major drainage system. The second hydraulic gradient is related with the flanks of the calcareous valley. Geophysical data concurrently with ‘ground truth’ geological and hydrogeological data allowed us to better understand the groundwater flow in these calcareous synclines and to verify the conceptual groundwater flow model. [less ▲]

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See detailOn the relation between bond quality and impact-echo frequency spectrum
Piotrowski, Tomasz; Garbacz, Andrzej; Van der Wielen, Audrey ULg et al

in Proceedings of the 13th International Congress on Polymers in Concrete ICPIC 2010 (2010)

According to EN 1504-10 and ACI Concrete Repair Manual, bond strength and interface quality are the main features of repair system necessary to be assessed. Pull-off test is most commonly used for bond ... [more ▼]

According to EN 1504-10 and ACI Concrete Repair Manual, bond strength and interface quality are the main features of repair system necessary to be assessed. Pull-off test is most commonly used for bond strength evaluation but growing interest in nondestructive techniques (NDT) is recently noted. Impact-echo (IE) is treated as the most promising one for this purpose. The aim of this paper is to analyze an effect of bond quality on stress wave propagation in repair systems. A group of samples has been prepared in order to obtain repair systems of different bond quality. Prior to repair, quality of concrete substrates has been characterized according different techniques: compressive strength, superficial cohesion, surface roughness index and cracking quantification. Than a polymer-modified repair mortar has been applied. Af-ter hardening, IE signals have been recorded and pull-off bond strength determined. The rela-tionships between parameters characterizing surface quality, bond strength, IE frequency spec-trum and results of wavelet analysis of IE signal have been analyzed. [less ▲]

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See detailNondestructive Detection of Delaminations in Concrete Bridge Decks: A First Experimental Study
Van der Wielen, Audrey ULg; Courard, Luc ULg; Nguyen, Frédéric ULg

in Proceedings of the XIII International Conference on Ground Penetrating Radar (2010)

To detect delaminations in concrete bridge decks, nondestructive techniques (NDT) permit a frequent and large inspection of the slabs without damaging structures. This research was devoted to detect ... [more ▼]

To detect delaminations in concrete bridge decks, nondestructive techniques (NDT) permit a frequent and large inspection of the slabs without damaging structures. This research was devoted to detect simulated defects in twelve repaired concrete slabs. These were scanned with high frequency ground penetrating radar (GPR) with the common offset (CO) and common midpoint (CMP) methods. The electromagnetic waves speed was determined from CMPs. A 3D visualization program was also created to display the CO measurements. The visibility of the inserted defects revealed to be dependent on their lateral extension, their thickness and their constitutive material. [less ▲]

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See detailUsing Electrical Resistivity Tomography and Self-potential Methods for Wells Implementations in Fractured Limestones
Robert, Tanguy ULg; Dassargues, Alain ULg; Brouyère, Serge ULg et al

in EarthDoc - Near Surface 2009 – 15th European Meeting of Environmental and Engineering Geophysics (2009, September 09)

Electrical resistivity tomography (ERT) and self-potential (SP) investigations were conducted in fractured limestones in Belgium. The aim of this study was to find suitable positions for high yield water ... [more ▼]

Electrical resistivity tomography (ERT) and self-potential (SP) investigations were conducted in fractured limestones in Belgium. The aim of this study was to find suitable positions for high yield water wells. Large ERT profiles (640 meters) allowed us to image the resistivity distribution of the first 60 meters of the subsurface and to detect and characterize (in terms of direction, width and depth) fractured zones expected to be less resistive. Data errors, DOI indexes and sensitivity models were analysed in order to calculate the depth of investigation of ERT and to avoid the misinterpretation of the resulting images. Self-potential measurements were performed along electrical profiles to narrow the possible locations given by the electrical images. Some negative anomalies possibly related to preferential flow were detected. ‘Ground truth’ geological data as well as pumping tests information gave us a way to assess the contribution of geophysics to a drilling programme. Wells implemented in low resistivity zones associated with SP anomalies have very high yields. Inversely, wells drilled in resistive zones or outside SP anomalies have poorer capacities. An apparent coupling coefficient between SP signals and differences in hydraulic heads was also estimated in order to image the water table. [less ▲]

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See detailProspection géophysique de la nappe alluviale dans la région de Nasso, Bodo – Dioulasso (Burkina Faso)
Beaujean, Jean ULg; Nguyen, Frédéric ULg; Sauret, Elie ULg et al

Report (2009)

Afin d’appuyer la gestion et la protection des ressources en eaux souterraines au Burkina Faso, dans la région de Bobo-Dioulasso, une mission géophysique relative à la caractérisation du site de la plaine ... [more ▼]

Afin d’appuyer la gestion et la protection des ressources en eaux souterraines au Burkina Faso, dans la région de Bobo-Dioulasso, une mission géophysique relative à la caractérisation du site de la plaine alluviale dans la localité de Nasso a été effectuée. Plusieurs méthodes géophysiques ont été employées afin de déterminer l’hétérogénéité des terrains superficiels, l’épaisseur de la nappe alluviale, ainsi que la profondeur du bedrock. Les trois méthodes déployées pour atteindre ces objectifs sont la tomographie électrique, le bruit de fond H/V, et le radar géologique. Ce dernier n’a pas donné de résultats satisfaisant tandis que les deux autres méthodes se sont montrées concluantes pour déterminer la profondeur du bedrock et détecter des hétérogénéités dans les terrains superficiels et profonds. [less ▲]

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See detailCharacterization of seawater intrusions using 2D electrical tomography
Nguyen, Frédéric ULg; Kemna, A.; Antonsson, A. et al

in Near Surface Geophysics (2009), 7

We have investigated the potential of 2D electrical imaging for the characterization of seawater intrusion using field data from a site in Almeria, SE Spain. Numerical simulations have been run for ... [more ▼]

We have investigated the potential of 2D electrical imaging for the characterization of seawater intrusion using field data from a site in Almeria, SE Spain. Numerical simulations have been run for several scenarios, with a hydrogeological model reflecting the local site conditions. The simulations showed that only the lower salt concentrations of the seawater-freshwater transition zone could be recovered, due to the loss of resolution with depth. We quantified this capability in terms of the cumulative sensitivity associated with the measurement setup and showed that the mismatch between the targeted and imaged parameter values occurs from a certain sensitivity threshold. Similarly, heterogeneity may only be determined accurately if located in an adequately sensitive area. At the field site, we identified seawater intrusion at the scale of a few kilometres down to a hundred metres. Borehole logs show a remarkable correlation with the image obtained from surface data but indicate that the electrically derived mass fraction of pure seawater could not be recovered due to the discrepancy between the in-situ and laboratory-derived petrophysical relationships. Surface-to-hole inversion results suggest that the laterally varying resolution pattern associated with such a setup dominates the image characteristics compared to the laterally more homogeneous resolution pattern of surface only inversion results and hence, surface-to-hole images are not easily interpretable in terms of larger-scale features. Our results indicate that electrical imaging can be used to constrain seawater intrusion models if image appraisal tools are appropriately used to quantify the spatial variation of sensitivity and resolution. The most crucial limitation is probably the apparent non-stationarity of the petrophysical relationship during the imaging process [less ▲]

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