References of "Nguyen, Frédéric"
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See detailIncorporation of prior information in the regularized inversion of ERT data with CRTomo
Hermans, Thomas ULg; Caterina, David ULg; Martin, Roland et al

Conference (2012, February 03)

The electrical resistivity tomography (ERT) inverse problem is ill-posed leading to non-uniqueness of its solution. A possible way to reduce the amount of mathematical solutions to more geologically ... [more ▼]

The electrical resistivity tomography (ERT) inverse problem is ill-posed leading to non-uniqueness of its solution. A possible way to reduce the amount of mathematical solutions to more geologically plausible models is to incorporate priori information in the inversion process. In this presentation, we describe the implementation of three different techniques ways to go beyond standard Occam’s inversion for ERT with the code CRTomo : a simple reference model weighted by a user-defined parameter, a structural constraint defined by modifying the construction of the roughness matrix compared to standard Occam’s inversion and a geostatistical constraint based on a vertical correlation length deduced from borehole data to build the model covariance matrix and use it to regularize the inversion. With several examples, we show that the traditional smoothness constraint yield small contrasts of resistivity, far from the reality revealed by borehole measurements. Incorporating prior information from boreholes clearly or from another geophysical technique (like GPR or seismic refraction) improves the misfit with logging data. The choice of the prior information incorporation technique highly depends on which king of information is available and further developments are needed to extend the techniques to more complex cases. [less ▲]

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See detailGeoelectrical investigations (DC) on a contaminated site during biostimulation: monitoring results and resolution analysis
Caterina, David ULg; Nguyen, Frédéric ULg

Conference (2012, January 12)

In Belgium, as in many other countries, relatively anarchic economical and industrial development of the past century has resulted in a significant number of contaminated sites. When one of these sites ... [more ▼]

In Belgium, as in many other countries, relatively anarchic economical and industrial development of the past century has resulted in a significant number of contaminated sites. When one of these sites poses a risk to human or ecosystem, measures need to be taken to clean it up. Among these measures, methods using in situ bioremediation are beginning to become more important because of their ease of implementation and their relatively low cost. However, it is often difficult to ensure their effectiveness except by carrying out extensive drilling and sampling, which can be long and expensive while offering only punctual information. Thus, it becomes necessary to use other techniques to overcome these shortcomings. Recently, an increasing interest is being born to use geophysical methods as tools for remediation monitoring. As part of our work, we conducted several electrical resistivity tomography campaigns on a bus station located in Bassenge (Belgium) which has undergone a contamination of hydrocarbons (gasoline) for several years and on which a biostimulation remediation device was set up in order to clean it up. The aim of our investigations was to study the electrical response of the contaminated area during the remediation phase and whether electrical resistivity tomography allowed to monitor its effectiveness. After a year of monitoring, the time lapse images obtained show a significant decrease of electrical resistivities (up to -40%) during biostimulation at the location of the main contaminant plume and an increase again of resistivities from the time the biostimulation was stopped. The electrical response during the biostimulation is in agreement with the models presented by several authors in the literature. The increase again of resistivities after the stimulation is however more surprising and can be explained by several physico-chemical (sorption-desorption processes) or biological (decrease of conductive biofilms) assumptions. The results obtained tend to suggest that it is possible to use electrical resistivity tomography as a tool for qualitative control during the remediation of a contaminated site. However, for a more quantitative use of resistivity models, it is important to assess their reliability through the use of resolution indicators. We therefore developed a methodology to address this issue based on the creation of synthetic models representing simplified cases of field resistivities and we applied it on our case study. The results obtained provided us important information about the reliable parts of the resistivity models. These findings may lead in the future to the development of mathematical models that can link quantitatively geophysical properties to the level of (de)contamination of a site. [less ▲]

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See detailA shallow geothermal experiment in a sandy aquifer monitored using electric resistivity tomography
Hermans, Thomas ULg; Vandenbohede, Alexander; Lebbe, Luc et al

in Geophysics (2012), 77(1), 11-21

Groundwater resources are increasingly used around the world for geothermal exploitation systems. To monitor such systems and to estimate their governing parameters, we rely mainly on borehole ... [more ▼]

Groundwater resources are increasingly used around the world for geothermal exploitation systems. To monitor such systems and to estimate their governing parameters, we rely mainly on borehole observations of the temperature field at a few locations. Bulk electrical resistivity variations can bring important information on temperature changes in aquifers. In this paper, we demonstrate the ability of surface electrical resistivity tomography to monitor spatially temperature variations in a sandy aquifer during a thermal injection test. Heated water (48°C) was injected for 70 hours at the rate of 87 l/h in a 10.5°C aquifer. Temperature changes derived from time-lapse electrical images are in agreement with laboratory water electrical conductivity-temperature measurements. In parallel, a coupled hydrogeological saturated flow and heat transport model was calibrated on geophysical data for the conceptual model, and on hydrogeological and temperature data for the parameters. The resistivity images showed an upper flow of heated water along the well above the injection screens and lead to a new conceptualization of the hydrogeological source term. The comparison between the temperature models derived from resistivity images and from the simulations is satisfactory. Quantitatively, resistivity changes allowed estimating temperature changes within the aquifer, and qualitatively, the heated plume evolution was successfully monitored. This work demonstrates the ability of electrical resistivity tomography to study heat and storage experiments in shallow aquifers. These results could potentially lead to a number of practical applications, such as the monitoring or the design of shallow geothermal systems. [less ▲]

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See detailStatic Detection of Thin Layers into Concrete with Ground Penetrating Radar
Van der Wielen, Audrey ULg; Courard, Luc ULg; Nguyen, Frédéric ULg

in Restoration of Buildings and Monuments = Bauinstandsetzen und Baudenkmalpflege (2012), 18(3/4), 247-254

Ground Penetrating Radar (GPR) is a nondestructive technique particularly well adapted to the inspection of concrete structures and can help to determine the structure inner geometry or to detect damaged ... [more ▼]

Ground Penetrating Radar (GPR) is a nondestructive technique particularly well adapted to the inspection of concrete structures and can help to determine the structure inner geometry or to detect damaged areas. When the GPR is used on structures containing thin layers, for example the sealing layer of a concrete bridge deck or the void into a masonry wall, it is important for the radar user to know the minimum thickness required to detect and estimate the thickness of those layers. The theory of thin layer detection is based on a sine wave but, in reality, most commercial GPR systems emit a large frequency band wavelet, which undergoes attenuation into the layer. To analyze the influence of those realistic conditions on the reflection coefficient of a thin layer, we combined experimental measurements and numerical FDTD simulations. The experimental results matched the numerical predictions well, presenting a fast attenuation compared to the theoretical predictions. Nevertheless, for thicknesses inferior to λ/11, the reflection coefficient could still be considered as linearly dependent of the thickness to wavelength ratio. [less ▲]

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See detailShallow heat injection and storage experiment monitored with electrical resistivity tomography and simulated with heat transport model
Hermans, Thomas ULg; Daoudi, Moubarak ULg; Vandenbohede, Alexander et al

Poster (2011, December 06)

Groundwater resources are increasingly used around the world as geothermal systems. Understanding physical processes and quantification of parameters determining heat transport in porous media is ... [more ▼]

Groundwater resources are increasingly used around the world as geothermal systems. Understanding physical processes and quantification of parameters determining heat transport in porous media is therefore important. To monitor the geothermal behavior of groundwater systems and to estimate the governing parameters, we rely mainly on borehole observations of the temperature field at a few locations (temperature logs or thermal response test). In analogy to research in hydrogeophysics, geophysical methods may be useful in order to yield additional information for thermal properties estimation with greater coverage than conventional wells. We report a heat transport study during a shallow heat injection and storage field test. Heated water (about 50°C) was injected for 6 days at the rate of 80 l/h in 10.5°C aquifer. Since bulk electrical resistivity variations can bring important information on temperature changes in aquifers (water electrical conductivity increases about 2%/°C around 25°C), we monitored the test with surface electrical resistivity tomography and demonstrate its ability to monitor spatially temperature variations. Time-lapse electrical image clearly show the decrease and then the increase in bulk electrical resistivity of the plume of heated water, during respectively the injection and the storage phase. This information enabled to calibrate the conceptual flow and heat model used to simulate the test (using SEAWAT). Inverted resistivity values are validated with borehole electromagnetic measurements (EM39) and are in agreement with the temperature logs used to calibrate the parameters of the thermo-hydrogeological model. This field work demonstrates that surface electrical resistivity tomography can monitor heat and storage experiments in shallow aquifers. These results could potentially lead to a number of practical applications, such as the monitoring or the design of shallow geothermal systems. Moreover, sensitivity analyses and collinear diagnostic were used to assess the pertinence of the flow and heat model parameters. The most sensitive parameter is the conductivity of the solid followed by the porosity, heat capacity of the solid and the longitudinal dispersivity. This indicates the predominance of conductive transport during the storage phase over the convective transport during the injection phase. These values rely only on temperature logs and more parameters could be derived or more robust values could be achieved with the use of geophysical data in a coupled inversion scheme. [less ▲]

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See detailGeoelectrical monitoring on a contaminated site during biostimulation
Caterina, David ULg; Nguyen, Frédéric ULg

Poster (2011, December 01)

In Belgium, as in many other countries, relatively anarchic economical and industrial development of the past century has resulted in a significant number of contaminated sites. When one of these sites ... [more ▼]

In Belgium, as in many other countries, relatively anarchic economical and industrial development of the past century has resulted in a significant number of contaminated sites. When one of these sites poses a risk to human or ecosystem, measures need to be taken to clean it up. Among these measures, methods using in situ bioremediation are beginning to become more important because of their ease of implementation and their relatively low cost. However, it is often difficult to ensure their effectiveness except by carrying out extensive drilling and sampling, which can be long and expensive while offering only punctual information. Thus it becomes necessary to use other techniques to overcome these shortcomings. Recently, an increasing interest is being born to use geophysical methods as tools for remediation monitoring. As part of our work, we conducted several electrical resistivity tomography campaigns on a site contaminated by LNAPLs (gasoline) on which a biostimulation remediation device was set up. The aim of our investigations was to study the electrical response of the contaminated area during the remediation phase and whether electrical resistivity tomography allowed to monitor its effectiveness. After a year of monitoring, the time lapse images obtained show a significant decrease of the electrical resistivity (up to -50%) at the location of the main contaminant plume. This particular response during the biostimulation, in agreement with the models presented by several authors in the literature, tends to suggest that it is possible to use electrical resistivity tomography as a tool for qualitative control during the remediation. These findings may also lead in the future to the development of models to estimate more quantitatively the level of (de)contamination of a site. [less ▲]

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See detailInversion of multi-temporal geoelectrical data sets: insights from several case studies
Nguyen, Frédéric ULg; Kemna, Andreas; Robert, Tanguy ULg et al

Conference (2011, December)

Time-lapse inversion of geoelectrical data is increasingly growing as remote monitoring systems are being used in more applications such as seawater intrusions, landslides, bioremediation of contaminated ... [more ▼]

Time-lapse inversion of geoelectrical data is increasingly growing as remote monitoring systems are being used in more applications such as seawater intrusions, landslides, bioremediation of contaminated sites, landfill operations, shallow geothermal systems, or water resources. To date, several inversion strategies exist for taking into account the temporal dimension of the data. Among the most used ones are the independent inversion of multi-temporal data sets, the difference inversion, the temporally-constrained inversion, and the more recent process-based inversion. The success of a particular time-lapse inversion scheme depends on the validity of several assumptions made by these inversion schemes. Difference inversion schemes generally assume that part of the noise contained in the data cancels out when working with temporal data differences. Process-based inversion requires a more advanced knowledge of the system prior the inversion. Temporally-constrained inversion on the other hand assumes that the changes are localized and minor. We show in this paper using data sets with different time and spatial scales, and with different degrees of geological complexity and resistivity contrasts, that the particular success of a time-lapse inversion scheme is highly dependent on the temporal behaviour of the noise estimation in the time-lapse data set and of the model-dependent resolution pattern of the survey. We attempt to provide guidelines for successful quantitative interpretation of time-lapse data sets whenever possible. [less ▲]

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See detailIntegration of near-surface geophysical, geological and hydrogeological data with multiple-point geostatistics in alluvial aquifers
Hermans, Thomas ULg; Nguyen, Frédéric ULg

Scientific conference (2011, October 20)

This work takes place in the context of the following PhD research topic. Alluvial plains constitute essential geological bodies for environmental studies such contaminated sites remediation, low-enthalpy ... [more ▼]

This work takes place in the context of the following PhD research topic. Alluvial plains constitute essential geological bodies for environmental studies such contaminated sites remediation, low-enthalpy geothermal energy or groundwater resources. The general objective of this work is to contribute to a better quantification of the heterogeneity of the parameters governing flow and transport processes by combining near-surface geophysics and geostatistics. Two-points geostatistical approaches (variogram based) have been developed to quantify the heterogeneity of one geological formation but fail to reproduce the heterogeneity of fluvial deposits with multiple facies. Multiple-points geostatistics introduced the training image concept to replace the variogram within an extended sequential simulation framework. The role of the training image is to depict the conceptual geological patterns. Previous studies have demonstrated the need for more conditioning data to generate an efficient training image. The use of geophysics in this context has been studied in the petroleum research with wave-based methods (seismic reflection data). However, little research has been done to assess the use of near-surface geophysical measurements, relying on potential and wave methods, to condition multiple-point geostatistics for environmental studies. This research project will focus on three specific objectives. The first will consist in computing conceptual training images from the combination of near-surface geophysics images, geological information and borehole data. The second objective will use these images to build models of the subsurface properties with multiple-point geostatistics conditioned to site-specific near-surface geophysical data and borehole information. Finally, groundwater model parameter estimation will be performed using a geologically consistent perturbation approach based on tracer experiments, using well sampling and time-lapse geophysical data. The approach will be demonstrated on both synthetic benchmarks and real field sites. This presentation shows preliminary research results on the use of combined geophysical techniques to improve geophysical models, the building of geological scenarios (training images) in the Meuse alluvial aquifer and the use of time-lapse ERT data to improve our understanding of hydrogeological reservoirs. [less ▲]

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See detailGeophysical monitoring of a thermal response test
Daoudi, Moubarak; Hermans, Thomas ULg; Vandenbohede, Alexander et al

Conference (2011, October 20)

Geothermal energy systems efficiency relies on the quantification of the thermal properties of the ground. The estimation of those properties can be done by means of thermal response tests (TRT) or thanks ... [more ▼]

Geothermal energy systems efficiency relies on the quantification of the thermal properties of the ground. The estimation of those properties can be done by means of thermal response tests (TRT) or thanks to values found in literature tables; the first approach provides relatively limited information since the measures are done in boreholes and the values from general tables can present a lack of accuracy. Geophysical methods might be useful in order to yield additional information for thermal properties estimation on higher investigation scales. In this study, electrical resistivity tomography (ERT) is used to monitor the evolution of heated water injected into an unconfined aquifer. Both heat injection and storage were monitored during a total period of 17 days. The principal conclusion is that the use of ERT is efficient to monitor the progressive temperature increase and decrease in the aquifer due to the presence of the heated plume. The resistivity distributions calculated by inversion were compared with electromagnetic borehole measurements and were consistent in the case of the storage phase, whereas a higher gap remained between measured and calculated resistivities for the injection phase. A conversion of the calculated resistivities into temperature values enabled a validation with temperature logs for the storage phase, while the temperature deduced for the injection profiles were too low. Those differences are justified by the fact that different error models were used to proceed to the imaging of the heat injection and storage. This work demonstrates the ability of ERT to monitor geothermal experiment in shallow aquifer. Keywords: geothermal energy, electrical resistivity, electromagnetic method, heat transfer, inversion process, thermal response test. [less ▲]

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See detailShallow heat injection and storage experiment : heat transport simulation and sensitivity analysis.
Vandenbohede, Alexander; Hermans, Thomas ULg; Nguyen, Frédéric ULg et al

in Journal of Hydrology (2011), 409(1-2), 262-272

Interest in heat transport in porous media has increased because of its many applications such use as tracer or in geotechnical engineering solutions. Understanding of the physical processes and ... [more ▼]

Interest in heat transport in porous media has increased because of its many applications such use as tracer or in geotechnical engineering solutions. Understanding of the physical processes and parameters determining heat transport is therefore important. In this paper, heat transport is studied during a shallow heat injection and storage field test. The test is simulated using SEAWAT. Sensitivity analyses and collinear diagnostics are used to derive which parameters can be derived from the test and how reliable these values are. Heat transport during the test is compared with heat transport in the surficial zone at the same field site to compare parameter values. The most sensitive parameter is the thermal conductivity of the solid followed by the porosity, heat capacity of the solid and the longitudinal dispersivity. This indicates the predominance of conductive transport during the storage phase over the convective transport during the injection phase. Whereas heat transport in the surficial zone is insensitive to the longitudinal dispersivity, this parameter must be included to simulate the field test. This indicates that dispersivity can not be ignored simulating convective heat transport in aquifers. [less ▲]

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See detailImproving groundwater flow model conceptualisation and calibration with electrical resistivity tomography and self-potential methods
Robert, Tanguy ULg; Therrien, René; Lemieux, Jean-Michel et al

Conference (2011, September 19)

Developing a conceptual model for groundwater flow requires knowledge on the distribution of geological materials, which generally comes from geological observations on outcrops and boreholes, from the ... [more ▼]

Developing a conceptual model for groundwater flow requires knowledge on the distribution of geological materials, which generally comes from geological observations on outcrops and boreholes, from the interpretation of hydraulic tests or from geophysical surveys. The identification of spatial structures in the subsurface, such as preferential flow paths created by fractured zones, is also critical in developing a reliable conceptual model but it is difficult to achieve. Geophysical methods have been widely used to map the subsurface distribution of geological materials. Recent developments in geophysics, such as the increased use of joint inversion of geophysical and hydrogeological data, have further allowed to quantify the hydraulic conductivity of geological materials. The objective of our work is to demonstrate that the electrical resistivity tomography (ERT) and the self-potential (SP) methods can improve both the conceptual model developed for groundwater flow systems and the calibration of the corresponding groundwater flow model. The use of the two geophysical methods, combined with a groundwater flow model, is presented for a fractured limestone aquifer. The self-potential method relies on passive measurements of the ambient electrical potential at ground surface or in boreholes. One of the mechanisms responsible for the measured signal measured is the transport of dissolved ions with groundwater flow. When this electrokinetic effect is the dominant contribution, the resulting signal is called the streaming potential and it contains information about groundwater fluxes that can be useful to calibrate groundwater flow models. The solution to the SP forward problem was added to the HydroGeoSphere model, which simulates 3D groundwater flow and solute transport in porous media, including fractured geological formations. With this addition, the model can calculate the self-potential signal associated with groundwater flow, given the distribution of Darcy fluxes resulting from the forward flow solution and the electrical resistivity that is, for example, outputted by ERT data inversion. Darcy fluxes are transformed into sources of electrical current by using the streaming potential coupling coefficient. This parameter can be measured either in the laboratory or in-situ from the self-potential signal between two locations where the depth of the water table is known, such as observation wells. We used here both ERT and SP to develop a conceptual model for groundwater flow in a typical carboniferous limestone syncline in South Belgium. The rolling topography in the investigated area results from a succession of calcareous valleys (synclines) and sandstone crests (anticlines). The calcareous synclines form aquifers that are very complex since they are highly fractured and even karstified. A typical calcareous syncline has a width of about 800 m and, using ERT, we could subdivide the syncline into zones of different hydraulic conductivity, based on the degree of fracturation. The zones are oriented along the axis of the syncline and their width ranges between 10 and 40 m. The ERT profiles showed that there is a highly conductive zone, in terms of electrical conductivity, near the syncline fold axis. That zone is interpreted as being highly fractured. Other conductive zones are located symmetrically along both flanks of the calcareous syncline, with respect to the syncline fold axis. The main flow direction is along the axis of the syncline, towards a nearby river. The SP raw signals also showed that, locally, there is a second flow component perpendicular to the axis of the syncline, with groundwater flowing from the flanks of the syncline towards the axis. The conceptual groundwater flow model developed here includes the zones identified with ERT, which were then incorporated into the numerical model. The SP signals were inverted with PEST to calibrate the hydraulic conductivity value of the different zones. HydroGeoSphere was therefore used to simulate first groundwater flow and then the associated self-potential signals in an iterative process. At the start of an iteration, HydroGeoSphere solves the groundwater flow equation given one particular set of hydraulic conductivities and calculates the resulting Darcy fluxes. These fluxes are transformed into sources of electrical current assuming that the electrokinetic effect is the dominant contribution of the SP signals. HydroGeoSphere then calculates the distribution of self-potential given the sources of electrical current and the distribution of electrical resistivity. The hydraulic conductivity values of the zones are then modified and the iteration continues until the model reproduces the measured self-potential signal. [less ▲]

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See detailParameter estimation using slug tests : application to a heat injection and storage experiment
Vandenbohede, Alexander; Louwyck, Andy; Daoudi, Moubarak et al

Poster (2011, September 19)

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See detailImproving Groundwater Flow Model Conceptualisation and Calibration with ERT and Self-potential Methods
Robert, Tanguy ULg; Therrien, René; Lemieux, Jean-Michel et al

in EarthDoc - Near Surface 2011 – 17th European Meeting of Environmental and Engineering Geophysics (2011, September 14)

The self-potential (SP) method relies on passive measurements of the ambient electrical potential at the ground surface or in boreholes. When the electrokinetic effect is the dominant contribution, the ... [more ▼]

The self-potential (SP) method relies on passive measurements of the ambient electrical potential at the ground surface or in boreholes. When the electrokinetic effect is the dominant contribution, the resulting signal is called the streaming potential and contains information about groundwater fluxes that can be useful for calibration of groundwater flow models. The streaming potential forward equation was implemented in the HydroGeoSphere model, which simulates 3D groundwater flow and solute transport in porous media, including fractured geological formations. HydroGeoSphere is able to calculate the streaming potential given a distribution of Darcy velocity and electrical resistivity. Since groundwater flow modelling relies on a conceptual model, prior information on the distribution of the geological units and hydraulic conductivity at the site is mandatory. However, this information is often scarce or missing. In this work, we use the electrical resistivity tomography (ERT) and the SP methods as an additional source of information for building the groundwater flow model. ERT is used to identify the location of fractured zones in a fractured and karstified calcareous aquifer of South Belgium. The SP signal is used with PEST in order to calibrate the groundwater flow model and better constrain the hydraulic conductivity of the fractured zones. [less ▲]

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See detailHow to incorporate prior information in geophysical inverse problems: deterministic and geostatistical approaches.
Hermans, Thomas ULg; Caterina, David ULg; Martin, Roland et al

in EarthDoc - Near Surface 2011 - 17th European Meeting of Environmental and Engineering Geophysics (2011, September 13)

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

Many geophysical inverse problems are ill-posed leading to non-uniqueness of the solution. 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 go beyond standard Occam’s inversion for electrical resistivity tomography (ERT) using electromagnetic logging data in the context of salt water infiltration: a simple reference model, a structural constraint and a geostatistical constraint based on a vertical correlation length. Results with the traditional smoothness constraint yield small contrasts of resistivity, far from the reality revealed by borehole measurements. Incorporating prior information from boreholes clearly improves the misfit with logging data. If a good reference model can always be used, it can lead to misinterpretation if its weight is too strong. When the computation of the correlation length is possible, the geostatistical inversion gives satisfactory results everywhere in the section. In this specific case, the geostatistical approach seems to be a more robust way to incorporate prior information. The structural constraint seems to be more indicated when integrating information from other geophysical methods such as GPR or seismic. [less ▲]

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See detailDetection of Defects in Concrete With Ground Penetrating Radar
Van der Wielen, Audrey ULg; Courard, Luc ULg; Nguyen, Frédéric ULg

in ESPSC 2011: European Symposium on Polymers in Sustainable Construction (Czarnecki Symposium) (2011, September)

Ground Penetrating Radar (GPR) is a nondestructive technique particularly well adapted to the inspection of concrete structures and can help to determine the structure inner geometry or to detect damaged ... [more ▼]

Ground Penetrating Radar (GPR) is a nondestructive technique particularly well adapted to the inspection of concrete structures and can help to determine the structure inner geometry or to detect damaged areas. When the GPR is used on structures containing thin layers, for example the sealing layer of a bridge or the void into a masonry wall, it is important for the radar user to know the minimum thickness required to detect and estimate the thickness of those layers. The theory of thin layer detection is based on a sine wave but, in reality, the GPR emits a complicated pulse, which undergoes attenuation into the layer. To see the influence of those realistic conditions on the reflection coefficient of a thin layer, we combined experimental measurements and numerical FDTD simulations. The experimental results matched the numerical predictions well, presenting a fast attenuation compared to the theoretical predictions. Nevertheless, for thicknesses inferior to λ/11, the reflection coefficient could still be considered as linearly dependent of the thickness to wavelength ratio. [less ▲]

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See detailImproved automatic calibration of groundwater flow models using self-potential measurements
Robert, Tanguy ULg; Therrien, René; Lemieux, Jean-Michel et al

Conference (2011, April 11)

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See detailGPR Limits for Thin Layers in Concrete Detection: Numerical and Experimental Evaluation
Van der Wielen, Audrey ULg; Courard, Luc ULg; Nguyen, Frédéric ULg

in Progress In Electromagnetics Research Symposium Proceedings, Marrakesh, Morocco, March 20-23, 2011 (2011, March)

The Ground Penetrating Radar (GPR) is a nondestructive technique increasingly used in civil structures inspection. In those structures, thin layers, presenting a relatively small thickness compared to ... [more ▼]

The Ground Penetrating Radar (GPR) is a nondestructive technique increasingly used in civil structures inspection. In those structures, thin layers, presenting a relatively small thickness compared to their area, are common and induce a complex multiple reflection scheme of the GPR waves on the two interfaces limiting the layer. The theoretical relationships predicting the reflected amplitude are based on multiple simplifying assumptions that are not matched by most commercial GPR impulse machines. In the first part of this study, we confronted the theoretical curves with numerical finite difference simulations performed with GprMax2D, with both a continuous sine wave and a realistic pulse derived from measurements. In the second part, we performed experiments on two concrete slabs, separated with an air space of variable thickness. The measured amplitude appeared to be different from the predictions, probably due to surface noise. [less ▲]

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See detailImagerie d'une infiltration artificielle d'eau salée à la côte belge par tomographie de résistivité électrique contrainte par des données géostatistiques
Hermans, Thomas ULg; Nguyen, Frédéric ULg

Scientific conference (2011, January 14)

Electrical resistivity tomography is a geophysical method widely used for salt water intrusion studies, thanks to its ability to detect salinity contrasts. In this context, boreholes are often available ... [more ▼]

Electrical resistivity tomography is a geophysical method widely used for salt water intrusion studies, thanks to its ability to detect salinity contrasts. In this context, boreholes are often available to improve the interpretation of inverted models. The main objective of this study is to incorporate a priori information contained in borehole data to constrain the geophysical inversion with a geostatistical constraint based on the variogram. A synthetic benchmark showed that the choice of the prior model played a great role in the inversion process, especially in low sensitivity zones. The use of simulations conditioned with borehole data costs a lot of time because it is necessary to run several inversions to obtain a mean solution. The methodology was applied to artificial salt water infiltrations in the Westhoek nature reserve. In this case, a homogeneous prior model deduced from borehole data seemed judicious. Inversion results enabled to determine the extension of the salt water plume laterally, in depth, but also in terms of total dissolved solid content (TDS). These observations are in agreement with the hydrogeological situation of the site. A comparison with borehole data showed that the results are much more satisfying than a traditional smoothness constraint used as a regularization tool. [less ▲]

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