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

Conference (2013, December 06)

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

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

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

Conference (2013, December 05)

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

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

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See detail3D ERT monitoring of the reactivation of waste biodegradation with fresh leachate injection
Dumont, Gaël ULg; Robert, Tanguy ULg; Pilawski, Tamara et al

Conference (2013, December 04)

The aim of this study is to monitor (bio) physical processes occurring in a landfill. The experiment consists in injecting leachate towards a drain in unsaturated and not yet digested waste to reactivate ... [more ▼]

The aim of this study is to monitor (bio) physical processes occurring in a landfill. The experiment consists in injecting leachate towards a drain in unsaturated and not yet digested waste to reactivate (or activate) waste biodegradation. The target is the first 15 meters of the studied landfill subsurface. The visualization of the wet front arrival (short term effect) is crucial because we want to ensure that waste is entirely humidified to allow the reactivation of waste digestion. The second process is a long term effect consisting in the increase of the internal temperature of the landfill which is synonymous of the reactivation of biodegradation processes. We use 3D time-lapse ERT on a monthly basis to capture the decrease of electrical resistivity related to the increasing temperature. We also collect ground truth data, including distributed temperatures in a borehole to validate results. For short term effects, we monitored the wet front arrival with three 2D ERT profiles composing the 3D image, during an entire day. Preliminary results, corroborated by ground truth data, show that leachate flow in anisotropic (more rapid horizontally than vertically). So far, waste was completely humidified and slight changes of temperature occurred. [less ▲]

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See detailGeophysical characterisation of a former waste disposal site in the context of landfill mining
Dumont, Gaël ULg; Robert, Tanguy ULg; Pilawski, Tamara et al

in EarthDoc - Near Surface Geoscience 2013 – 19th European Meeting of Environmental and Engineering Geophysics (2013, September 11)

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See detail3D ERT Monitoring of the Reactivation of Waste Biodegradation with Fresh Leachate Injection
Robert, Tanguy ULg; Dumont, Gaël ULg; Pilawski, Tamara ULg et al

in EarthDoc - Near Surface Geoscience 2013 – 19th European Meeting of Environmental and Engineering Geophysics (2013, September 11)

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See detailReliability of ERT-derived Temperature - Insights from Laboratory Measurements
Robert, Tanguy ULg; Hermans, Thomas ULg; Dumont, Gaël ULg et al

in EarthDoc - Near Surface Geosciences 2013 - 19th European Meeting of Environmental and Engineering Geophysics (2013, September)

We performed laboratory measurements on fully saturated sand samples in the context of deriving reliable temperature from time-lapse electrical resistivity tomography (ERT). The experiment consisted in ... [more ▼]

We performed laboratory measurements on fully saturated sand samples in the context of deriving reliable temperature from time-lapse electrical resistivity tomography (ERT). The experiment consisted in monitoring an increase of temperature in sand samples with electrical resistivity measurements. We neglected the effect of surface conductivity since experiments showed two orders of magnitude between surface and fluid conductivities. We show that using simple linear relationship between fluid electrical conductivity and temperature alone does not allow reliable temperature estimates. Indeed, chemical analyses highlight the importance of accounting chemical reactions occurring when temperature changes, including dissolution/precipitation processes. We performed two experiments based on typical in-situ conditions. We first simulated the injection of a less conductive tap water and second, the injection of heated formation water. In the second case, minerals solubility decreases and precipitation occurs, leading to an increase of bulk resistivity. This mechanism competes with dissolution of minerals when tap water is injected, since tap water is not in equilibrium with the medium. In any case, further research is needed to fully understand the mechanisms and to develop a fully integrated law to derive better temperature estimates. [less ▲]

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See detailHydrogeological processes in fractured and porous media: insights from geophysical case studies
Robert, Tanguy ULg; Hermans, Thomas ULg; Nguyen, Frédéric ULg

Conference (2013, January 18)

This presentation focuses on geophysical case studies with the aim to highlight the possibilities to study and monitor hydrogeological processes in the subsurface, including transport processes in ... [more ▼]

This presentation focuses on geophysical case studies with the aim to highlight the possibilities to study and monitor hydrogeological processes in the subsurface, including transport processes in fractured or in porous media. The presentation emphasizes two geoelectrical methods, namely electrical resistivity tomography (ERT) which images the electrical resistivity distribution of the subsurface and self-potential (SP) whose measured signal is directly sensitive to groundwater fluxes. The first case study concerns the geophysical identification and characterization of large hydraulically-active fractured areas in calcareous synclines and in particular the assessment of the joint use of ERT and SP to set up new piezometers in fractured limestone. This assessment shows that piezometers drilled inside less resistive areas and/or in negative SP anomalies presented high hydraulic capacities. Inversely, piezometers drilled inside more resistive zones and/or outside an SP anomaly presented low hydraulic capacities. The SP anomaly related to preferential flow in fractures was thus demonstrated for the first time. All these fractures information, obtained with geophysics, improved the conceptualization and calibration of the groundwater flow model of the calcareous valley. A seasonal monitoring of SP signals proved to be a successful methodology to better understand the hydrodynamics of calcareous aquifers and in particular to follow the seasonal drawdown of the water table in the calcareous valley. Different methodologies to delineate the main groundwater flow direction were also tested. The latter can be achieved for example by drawing an SP map showing the main hydraulic gradients or by monitoring a salt tracer test with ERT to highlight preferential flow in fractures. The second case study concerns the ERT monitoring of a shallow geothermal test conducted in a porous medium (sand). The main objective of this study was to derive temperature from a series of electrical resistivity images since the electrical resistivity is directly sensitive to temperature changes. This field work demonstrates that surface electric resistivity tomography can monitor heat injection and storage experiments in shallow aquifers providing a number of practical applications, such as the monitoring or the design of shallow geothermal systems or the use of heated water to replace salt water in tracer tests. Through these two different case studies, this presentation also emphasizes in a practical way on the importance of data inversion and image appraisal since these issues are crucial to quantitatively study hydrogeological processes. [less ▲]

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See detailA comparison study of image appraisal tools for electrical resistivity tomography
Caterina, David ULg; Beaujean, Jean ULg; Robert, Tanguy ULg et al

in Near Surface Geophysics (2013)

To date, few studies offer a quantitative comparison of the performance of image appraisal tools. Moreover, there is no commonly accepted methodology to handle them even though it is a crucial aspect for ... [more ▼]

To date, few studies offer a quantitative comparison of the performance of image appraisal tools. Moreover, there is no commonly accepted methodology to handle them even though it is a crucial aspect for reliable interpretation of geophysical images. In this study, we compare quantitatively different image appraisal indicators to detect artefacts, estimate depth of investigation, address parameters resolution and appraise ERT-derived geometry. Among existing image appraisal tools, we focus on the model resolution matrix (R), the cumulative sensitivity matrix (S) and the depth of investigation index (DOI) that are regularly used in the literature. They are first compared with numerical models representing different geological situations in terms of heterogeneity and scale and then used on field data sets. The numerical benchmark shows that indicators based on R and S are the most appropriate to appraise ERT images in terms of the exactitude of inverted parameters, DOI providing mainly qualitative information. In parallel, we test two different edge detection algorithms – Watershed’s and Canny’s algorithms – on the numerical models to identify the geom-etry of electrical structures in ERT images. From the results obtained, Canny’s algorithm seems to be the most reliable to help practitioners in the interpretation of buried structures. On this basis, we propose a methodology to appraise field ERT images. First, numerical bench¬mark models representing simplified cases of field ERT images are built using available a priori information. Then, ERT images are produced for these benchmark models (all simulated acquisition and inversion parameters being the same). The comparison between the numerical benchmark mod¬els and their corresponding ERT images gives the errors on inverted parameters. These discrepan¬cies are then evaluated against the appraisal indicators (R and S) allowing the definition of threshold values. The final step consists in applying the threshold values on the field ERT images and to validate the results with a posteriori knowledge. The developed approach is tested successfully on two field data sets providing important information on the reliability of the location of a contamina¬tion source and on the geometry of a fractured zone. However, quantitative use of these indicators remains a difficult task depending mainly on the confidence level desired by the user. Further research is thus needed to develop new appraisal indicators more suited for a quantitative use and to improve the quality of inversion itself. [less ▲]

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See detailA salt tracer test monitored with surface ERT to detect preferential flow and transport paths in fractured/karstified limestones
Robert, Tanguy ULg; Caterina, David ULg; Deceuster, John et al

in Berichte der Geologischen Bundesanstalt (2012, September), 93

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See detailComparison of temperature estimates from heat transport model and electrical resistivity tomography during a shallow heat injection and storage experiment
Hermans, Thomas ULg; Daoudi, Moubarak ULg; Vandenbohede, Alexander et al

in Berichte der Geologischen Bundesanstalt (2012, September), 93

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. Geophysical methods may be useful in order to yield additional information 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 a 10.5°C aquifer. Since bulk electric resistivity variations can bring important information on temperature changes in aquifers (water electric conductivity increases about 2%/°C around 25°C), we monitored the test with surface electric resistivity tomography and demonstrate its ability to monitor spatially temperature variations. Time-lapse electric images clearly show the decrease and then the increase in bulk electric 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. 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 for the injection phase. This field work demonstrates that surface electric 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 or the use of heated water to replace salt water in tracer tests. [less ▲]

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See detailInversion of multi-temporal geoelectrical field data sets: insights on noise characterization and regularization
Nguyen, Frédéric ULg; Kemna, Andreas; Robert, Tanguy ULg et al

Poster (2012, July 11)

Inversion of geoelectrical time-lapse data sets is increasingly growing as monitoring systems are being used in more applications such as seawater intrusion, landslides, remediation of contaminated sites ... [more ▼]

Inversion of geoelectrical time-lapse data sets is increasingly growing as monitoring systems are being used in more applications such as seawater intrusion, landslides, remediation of contaminated sites, landfill operation, shallow geothermal systems, or management of water resources. To date, several inversion strategies exist for taking into account the temporal dimension of the data. The most used nowadays are the independent inversion of multi-temporal data sets, the difference inversion, the temporally-constrained inversion, and the more recent process-based inversion. However, difference inversion schemes generally assume that part of the noise contained in the data cancels out when working with temporal data differences. Temporally-constrained inversion on the other hand assumes that the changes are localized and minor. Process-based inversion requires a more advanced knowledge of the system prior the inversion. In this study we demonstrate that the resolution of the time-lapse inversion scheme is mostly dependent on the quantification of the temporal behavior of the data error, on the resolution of the model-dependent pattern of the survey, and not on the regularization strategy. Our study is based on the imaging results of different data sets with different time and spatial scales, and with different degrees of geological complexity and resistivity contrast, The considered sites are a shallow sandy aquifer and a fractured hard rock aquifer where tracer experiments were performed and monitored using surface arrays. The two studied transport processes are advection, with velocities on the order of 10 m/hour and slower advection/diffusion processes. The strongest improvements were brought by using the data difference and a quantitative estimation of the data error. We found in particular a dependence of the time-lapse data error to the measured resistance (i.e., signal-to-noise-ratio), permitting to formulate an error model to describe the data error present in time-lapse data sets. We used minimum gradient support regularization to invert for model changes with enhanced contrast and found this technique more suited to time-lapse studies than for static images. Noise characterization and error models appear therefore as essential and the most impacting for a successful inversion both for static and time-lapse data whereas different spatio-temporal regularization techniques allowed to decrease artefacts but needs to be coherent with the process. [less ▲]

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See detailGeophysical identification, characterization, and monitoring of preferential groundwater flow paths in fractured media
Robert, Tanguy ULg

Doctoral thesis (2012)

This thesis investigates fractured zones leading to preferential groundwater flow paths. In this context, we used the electrical resistivity tomography (ERT) and the self-potential (SP) methods to ... [more ▼]

This thesis investigates fractured zones leading to preferential groundwater flow paths. In this context, we used the electrical resistivity tomography (ERT) and the self-potential (SP) methods to identify, characterize, monitor, and finally model preferential flow in hydraulically-active fractured zones at a scale representative of real world applications. From an experimental point of view, we first identified the magnitude of self-potential signature, a -15 mV anomaly that could be associated with preferential groundwater flow in a shallow quartzite aquitard whereas the streaming potential theory was originally developed for porous media. This signature was confirmed experimentally in limestone aquifers at greater depths. The joint use of surface ERT and SP allowed the identification of water-bearing fractured areas which were electrically more conductive, presenting contrasts from 1 to 10 and which were hydraulically-active presenting negative SP anomalies ranging from -10 to -30 mV. We were also able to correlate hydraulic heads and SP gradients during a low and a high groundwater level period leading to interesting perspectives in understanding the dynamics of complex groundwater flow systems. Finally, a preferential flow and rapid transport path, over 10 m/h, was highlighted in a 20 m deep fractured and karstified limestone valley by monitoring a salt tracer test with only surface ERT. This methodology was being mostly used for relatively shallow and homogeneous aquifers up to now. Such information is crucial to set up new monitoring wells or to define the sampling rates of classic tracer test. From a methodological point of view, we quantitatively assessed the efficiency of blocky and minimum-gradient-support regularizations in electrical imaging to recover sharp interfaces on numerical benchmarks and with field data. The usefulness of resolution indicators such as the cumulative sensitivity matrix and the resolution matrix were also assessed in this context. We demonstrated that noise characterization is crucial in time-lapse inversion and may supplant the choice of the time-lapse inversion scheme, calling for a systematic analysis of reciprocal measurements (or a subset of them). We also showed that, when using data differences in an inversion scheme, the data error, as estimated by time-lapse reciprocal measurements, depends on the mean measured resistance. These error characterization studies should always be performed if one wants to avoid wrong interpretations about the hydrodynamics. We further showed that focused inversion techniques (blocky inversion, minimum-gradient-support) may offer great perspectives when recovering model changes in time-lapse inversion. Finally, ERT and SP were jointly used to conceptualize a physically-based and spatially distributed hydrogeological model, in particular to characterize the preferential flow paths. Predicted hydraulic heads and SP-derived hydraulic heads using the water table model showed a clear correlation, leading to perspectives in terms of hydrogeological model calibration. Further experiments are however needed to fully estimate the streaming potential apparent coupling coefficient, but the use of the full SP signals for hydrogeological model calibration is a clear perspective to this work. [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 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 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 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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