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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 detailA heat injection and pumping experiment in a gravel aquifer monitored with crosshole electrical resistivity tomography
Hermans, Thomas ULg; Wildemeersch, Samuel ULg; Jamin, Pierre ULg et al

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

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

Thermal tracing experiments are becoming common in hydrogeology to estimate parameters governing heat transport processes and to study geothermal reservoirs. Electrical resistivity tomography (ERT) has proven its ability to monitor salt tracer tests, but few studies have investigated its performances in thermal tracing experiments. In this study, we monitor the injection and pumping of heated water using crosshole ERT in a panel crossing the main flow direction. Difference inversion time-lapse images clearly show the heterogeneous pattern of resistivity changes, and thus temperature changes, highlighting the existence of preferential flow paths in the aquifer. Comparison of temperature estimates from ERT and direct measurements in boreholes show the ability of ERT to quantify the temperatures in the aquifer and to draw the breakthrough curves of the thermal tracer with a relative accuracy. Such resistivity data may provide important information to improve hydrogeological models. Our study proves that ERT, especially crosshole ERT, is a reliable tool to follow thermal tracing experiments. It also confirms that ERT should be included to in situ techniques to characterize heat transfer in the subsurface and to monitor geothermal resources exploitation. [less ▲]

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See detailProbability perturbation method applied to the inversion of groundwater flow models using HydroGeoSphere
Hermans, Thomas ULg; Scheidt, Céline; Caers, Jef et al

Conference (2013, April 04)

Solving spatial inverse problems in Earth Sciences remains a big challenge given the high number of parameters to invert for and the complexity of non-linear forward models. Techniques were developed to ... [more ▼]

Solving spatial inverse problems in Earth Sciences remains a big challenge given the high number of parameters to invert for and the complexity of non-linear forward models. Techniques were developed to reduce the number of parameters to invert for or to produce geologically consistent simulations from an initial guess. These techniques ask for a prior model to constrain the spatial distribution of the solution. Geostatistical models contain, by nature, information to control the spatial features of the inverse solutions, but the integration of dynamic data into such models remains difficult. We adapted, the “probability perturbation algorithm” (PPM) using Matlab® to invert hydrogeological data using multiple-point geostatistics to build models of pre-defined hydrofacies. The algorithm uses HydroGeoSphere (HGS) to compute the forward response of the model and SGems to produce geostatistical realizations. The algorithm only needs the proper definition of all the parameters to be used by HydroGeoSphere (grid matching with SGems, position of the wells, pumping rate, facies properties, boundary conditions, etc.). The PPM algorithm will automatically seek solutions fitting both hydrogeological data and geostatistical constraints. Through the inversion process, the initial geostatistical realization is perturbed. Only geometrical features of the model are affected, i.e. we do not attempt to directly find the optimal value of hydrogeological parameters, but the optimal spatial distribution of facies whose prior distribution is quantified in a training image. The algorithm can be divided in three steps. In the first step, we use SGems to generate an initial facies model with the multiple-point geostatistical algorithm SNESIM (single normal equation simulation). The facies model is composed of several categories representing hydrological facies (e.g. gravel, sand and clay). It can be conditioned using hard data (borehole data) and/or soft data (e.g. geophysical data). We then run a first flow simulation with HydroGeoSphere. This requires defining hydrogeological parameters (porosity, hydraulic conductivity, etc.) for each category of the facies model to create a hydrogeological model. The response of the latter model is compared to the expected one through an objective function. In the second step, a perturbation to the facies model is computed using a single parameter called rD. This perturbation is used to generate a new facies model with SGems and calculate a new objective function value via HGS, as done in the first step. An inner loop optimizes the value of rD. In the third step, we verify if the objective function of the best fitting model is smaller than a predefined value. If it is the case, we stop the algorithm, otherwise we go back to step 2 until convergence. We illustrate the methodology with a synthetic example in an alluvial aquifer. The model is based on a training image depicting gravel channels and clay lenses in a coarse sand aquifer. We simulate a pumping test and inverse water level data recorded at 9 wells using our implementation of the PPM algorithm. Using this method, it is possible to generate multiple solutions and to derive a posterior probability of the facies distribution. [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 detailThe state of the science and vision of the future: Report from the Hydrogeophysics Workshop
Knight, Rosemary; Cannia, James; Doetsch, Joseph et al

in Leading Edge (2013), 32(7), 814-818

In July 2012, 72 hydrogeophysicists from around the world gathered at the Hydrogeophysics Workshop in Boise, Idaho, USA. This was the first workshop to be jointly sponsored by the Society of Exploration ... [more ▼]

In July 2012, 72 hydrogeophysicists from around the world gathered at the Hydrogeophysics Workshop in Boise, Idaho, USA. This was the first workshop to be jointly sponsored by the Society of Exploration Geophysicists (SEG) and the American Geophysical Union (AGU), and brought together members from both societies, primarily from the Near-Surface Geophysics Section of SEG and the Near-Surface Focus Group and Hydrology Section of AGU. The intent of the workshop was to address current hydrogeophysical approaches for determining, predicting, and studying hydrologic properties and processes in both the saturated and unsaturated zones, at scales ranging from centimeters to watersheds. [less ▲]

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See detailContribution de la tomographie et du bruit sismique à la characterisation des dépôts alluviaux dans le bassin du Kou (Burkina Faso)
Sauret, Elie ULg; Beaujean, Jean ULg; Nguyen, Frédéric ULg et al

Conference (2012, November)

Abstract The alluvial plain of the Kou basin is located in the southwest of Burkina Faso. In this basin, the main surface water resource for agricultural needs is constituted by a small perennial river ... [more ▼]

Abstract The alluvial plain of the Kou basin is located in the southwest of Burkina Faso. In this basin, the main surface water resource for agricultural needs is constituted by a small perennial river, but in recent years this resource is insufficient to satisfy the uses in agriculture. The alluvial plain which extends from either side of this river banks is expected to have the potential for constituting an alternative water supply for agricultural needs. However, the characterisation of the alluvial plain is still superficial though the plain extension and the nature of the deposits are roughly known. The objective of this study is to improve the characterisation of the alluvial plain, in particular the heterogeneity and the thickness of the deposits, using geophysical methods, namely Electrical Resistivity Tomography (ERT) and Horizontal and Vertical Spectral Ratio (H/V SR). The ERT and H/V SR methods are non invasive geophysical techniques, simple, efficient, robust and easy-to-use geophysical tools in alluvial environment based respectively on the soil resistivity and the resonant frequency of superficial materials. In upstream of the alluvial plain, near the river, these methods were used to map the sandy to sandy-thin deposits (0-5m) and the unfractured bedrock. Downstream they highlight fractured and deconsolidated bedrock drawing a V-shaped geometry of deposits. This geometry is due to the faults and the magmatic intrusions. The bottom of the V-shaped would be filled mainly by fractured/deconsolidated bedrock materials and the edges by the clay and laterites deposits. The alluvial plain would be relatively thicker downstream of the study area (approximately 30 to 50m). A correlation is obtained between ERT images and resonance frequencies determined on the H/V profiles. From a hydrogeological point of view, downstream of the study area, the alluvial plain would constitute an important aquifer with a high porosity and thick deposits. This aquifer could be easily accessible with rudimentary structures (such as sumps) and could constitute a supplementary water source, for irrigation activities in this second region of Burkina Faso. [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 detailGPR detection of saturated areas into concrete in the presence of a water gradient
Louis, Arnaud ULg; Van der Wielen, Audrey ULg; Courard, Luc ULg et al

in 2012 14th International Conference on Ground Penetrating Radar (GPR) (2012, June)

In the concrete, saturated areas are most of the time limited by a transition zone, presenting a water gradient. This transition zone can affect the GPR wave’s reflection and decrease the reflection ... [more ▼]

In the concrete, saturated areas are most of the time limited by a transition zone, presenting a water gradient. This transition zone can affect the GPR wave’s reflection and decrease the reflection coefficient by comparison to the coefficient that would be obtained on a sharp interface. To quantify the impact of the water gradient on the reflection coefficient, we performed finite differences simulations. They showed that the reflection coefficient was reduced by 70% if the thickness of the transition zone was larger than 2/5 of the wavelength. Laboratory experiments, using hygrometric sensors for the water content control, confirmed this trend. [less ▲]

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See detailDetection of thin layers into concrete with static and CMP measurements
Van der Wielen, Audrey ULg; Courard, Luc ULg; Nguyen, Frédéric ULg

in 2012 14th International Conference on Ground Penetrating Radar (GPR) (2012, June)

Most concrete bridge decks contain a thin waterproofing layer, whose complex GPR signature can affect the detection of disorders or delaminations into the slab. In this study, we characterized the ... [more ▼]

Most concrete bridge decks contain a thin waterproofing layer, whose complex GPR signature can affect the detection of disorders or delaminations into the slab. In this study, we characterized the detection limits of our 2.3 GHz antenna for the detection of thin layers, with static and CMP measurements. In this last configuration, we showed that the radiation pattern and the reflection coefficient estimation are key parameters to use the inversion of the amplitude versus offset (AVO) curves for the estimation of the layer parameters. The theoretical results were compared to the results of FDTD simulations, performed with GprMax2D, and to laboratory measurements, performed on concrete slabs or sand containing thin layers. [less ▲]

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See detailImaging artificial salt water infiltration using electrical resistivity tomography constrained by geostatistical data
Hermans, Thomas ULg; Vandenbohede, Alexander; Lebbe, Luc et al

in Journal of Hydrology (2012), 438-439

Electrical resistivity tomography is a well-known technique to monitor fresh-salt water transitions. In such environments, boreholes are often used to validate geophysical results but rarely used to ... [more ▼]

Electrical resistivity tomography is a well-known technique to monitor fresh-salt water transitions. In such environments, boreholes are often used to validate geophysical results but rarely used to constrain the geoelectrical inversion. To estimate the extent of salt water infiltration in the dune area of a Natural Reserve (Westhoek, Belgium), electrical resistivity tomography profiles were carried out together with borehole electromagnetic measurements. The latter were used to calculate a vertical variogram, representative of the study site. Then, a geostatistical constraint, in the form of an a priori model covariance matrix based on the variogram, was imposed as regularization to solve the electrical inverse problem. Inversion results enabled to determine the extension of the salt water plume laterally and at depth, but also to estimate the total dissolved solid content within the plume. These results are in agreement with the hydrogeological data of the site. A comparison with borehole data showed that the inversion results with geostatistical constraints are much more representative of the seawater body (in terms of total dissolved solids, extension and height) than results using standard smoothness-constrained inversion. The field results obtained for the Westhoek site emphasize the need to go beyond standard smoothness-constrained images and to use available borehole data as prior information to constrain the inversion. [less ▲]

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See detailTraining image scenarios for the Meuse alluvial aquifer and consistency with geophysical data
Hermans, Thomas ULg; Caers, Jef; Nguyen, Frédéric ULg

Poster (2012, February 08)

Recently, multiple-point statistics (MPS) introduced the training image (TI) concept to replace the variogram within an extended sequential simulation in order to describe more accurately multimodal ... [more ▼]

Recently, multiple-point statistics (MPS) introduced the training image (TI) concept to replace the variogram within an extended sequential simulation in order to describe more accurately multimodal distributions, with interconnected and curvilinear structures, such as those of alluvial plains. The role of the TI is to depict the conceptual geological patterns and it should be representative of the geological heterogeneity. MPS consists in extracting patterns from the training image, and anchoring them to subsurface data (e.g. well-log, seismic and production data). The construction of TI is one of the most critical and important step of MPS. Sedimentological studies may not be always available in a particular area. In this work, the alluvial aquifer of the Meuse river in the area of Liege is being investigated and there is few sedimentological data to build directly 3D conceptual model of the aquifer. In this context, we used general features of the Meuse river (slope, rate of flow, type of fluvial system) to select hydrofacies (based on a lithological classification in clay, sand and gravel) and their geometrical characteristics (channels and lobes of different sizes). Then several scenarios were built using these parameters to represent the uncertainty related to different possible geological scenarios. To verify the consistency of these TIs, a comparison with 2D electrical resistivity tomography data was carried out. 2D sections were randomly selected in the TIs and several cases were analyzed including the size of channels and lobes, the influence of surface resistivity in the results and the influence of the electrical resistivity of each facies. Forward and inverse electrical resistivity modelling was conducted on these synthetic models and the results were compared to field cases. The approach followed for the comparison is based on the calculation of a Euclidean distance between models and the visualization in a 2D or 3D space using multidimensional scaling (MDS). This technique allows verifying if field cases fall in the distribution represented by synthetic cases. In a second step, a cluster analysis was achieved on the MDS-map to provide a sensitivity analysis and to highlight which parameters were the most important for building training images. Then, the probability of each scenario was evaluated for the field cases using conditional probability. Conditional probability requires the calculation of the density function corresponding to the probability of the data given a geological scenario. This density function was obtained using a kernel density estimation technique based on the observations of the 2D MDS-map. Both the cluster analysis and the calculation of conditional probabilities for uncertain geological scenarios show that some parameters are not very sensitive (size of clay lenses, surface resistivity distribution) and that we can narrow the range of variations of some parameters (facies electrical resistivity values, gravel bodies size is more likely small, etc.). It shows that the investigation of the consistency of TI is an important step in each study including MPS. The next steps of the study are to generalize the analysis of the consistency of geological scenario with 3D geophysical data instead of 2D sections and to incorporate geophysical data as soft conditioning data for MPS simulations. [less ▲]

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