Near Surface 2010 - 16th EAGE European Meeting of Environmental and Engineering Geophysics, 2010
Abstract:[en] The success of a tracer test highly depends on the number and the localisation of t... more Abstract:[en] The success of a tracer test highly depends on the number and the localisation of the sampling wells. When preferential solute transport paths are expected, one needs to set up carefully the tracer test to recover information such as the local groundwater flow direction and an estimate of the transport velocities. In this work, we used electrical resistivity tomography (ERT) to monitor a saline tracer test. This experiment was performed in fractured limestones where high transport velocities and strong dilution effects were expected. This ...
Permanent monitoring of changes in soil properties is of increasing interest in many engineering ... more Permanent monitoring of changes in soil properties is of increasing interest in many engineering applications such as management of groundwater contamination, landslide and sinkhole risks prevention, detection of saline water intrusion, comprehension of charge and discharge processes of subsurface aquifer. As geophysical investigations allow detecting contrasts in physical properties of the subsurface, field and lab experiments have been conducted for a few years to assess the reliability of these methods to monitor temporal changes in soil properties. Among the methods available, DC resistivity tomography is recognized as one of the most promising techniques. In order to assess the efficiency of electrical resistivity in monitoring charge and discharge processes of subsurface aquifer, and also to better model hydrological effects on the gravity measurements, an on-going field experiment is conducted at the Membach station located in the eastern part of Belgium. This geophysical station is equipped with an accelerometer, seismometers and a superconducting gravimeter, installed at the end of a 130 m long tunnel excavated in a low-porosity argillaceous sandstone mount at 48.5 m depth. Continuous gravimetric observations have been taken since August 1995. Since 2004 rainfall and soil moisture changes are measured in situ. In July 2010, an automated permanent geoelectrical acquisition system was installed to monitor subsurface resistivity variations during a test period of about 6 months. The aim of this experiment is to better understand charge and discharge processes of the subsurface aquifer, which are expected to be mainly due to rainfall variations. This aquifer is localized at the top of the weathered bedrock at a depth of 4 to 5 meters. The acquisition system consists in a straight profile of 48 buried electrodes (with a 2 meters spacing) connected to a Syscal R1 resistivimeter which is automatically controlled by a computer. Resistivity measurements are taken at least twice a day at fixed hours using a combination of dipole-dipole and Wenner-Schlumberger arrays. Acquired data are filtered in order to reject faulty measures. Time-lapse inversion (Loke (1999)) is then carried out to reconstruct a 2D model of resistivity changes. Preliminary results obtained during July show changes in inverted resistivities of about 30% in the first 4 to 5 meters layer. These observations are consistent with changes in measured gravimetric water content. This seems to indicate that subsurface aquifer charge and discharge processes are mainly due to rainfall, as expected. However, inversion errors remain high even after data filtering. This could be a consequence of weather occurring in July, leading to a poor contact between the electrodes and dry host soils near the surface. This problem should not happen anymore as the rest of the monitoring experiment is conducted during the wet season. Acknowledgments This work is conducted under the auspices of the Walloon Region Ministry under the First Spin-Off program (visa n° 916974).
Journal of Environmental & Engineering Geophysics, 2007
A field experiment was conducted over a Light Non-Aqueous Phase Liquid (LNAPL) contaminated site ... more A field experiment was conducted over a Light Non-Aqueous Phase Liquid (LNAPL) contaminated site located near a gas station where a tank had leaked its content. In this area, a dolomitic bedrock is overlain by 5 to 9 m of clayey sands. In order to delineate the plume and measure piezometric heads and depths to bedrock, ten boreholes were drilled and four cone penetration tests were conducted. Soil and water samples were collected and analyzed. Although these samplings highlight the presence of hydrocarbons in some boreholes, the plume is poorly outlined due to the limited number of drillings.
1 2 ERT and SP investigations were conducted in carbonate rocks of the Dinant Synclinorium 3 (Wal... more 1 2 ERT and SP investigations were conducted in carbonate rocks of the Dinant Synclinorium 3 (Walloon Region of Belgium) to find suitable locations for new water wells in zones with 4 little hydrogeological data. Since measured information in the boreholes needed to be 5 representative of the area, large fractured zones were searched for the drillings. Large ERT 6 profiles (320 to 640 meters) allowed us to image the resistivity distribution of the first 60 7 meters of the subsurface and to detect and characterize (in terms of direction, width and 8 depth) fractured zones expected to be less resistive. Data errors, depth of investigation (DOI) 9
Electrical resistivity tomography (ERT) monitoring experiments are being conducted more often to ... more Electrical resistivity tomography (ERT) monitoring experiments are being conducted more often to image spatiotemporal changes in soil properties. When conducting long-term ERT monitoring, the identification of suspicious electrodes in a permanent spread is of major importance because changes in electrode contact properties of a single electrode may affect the quality of many measurements on each time-slice. An automated methodology was developed to detect these temporal changes in electrode contact properties, based on a Bayesian approach called "weights of evidence." Contrasts ðC W Þ and studentized contrasts ðC S Þ are estimators of the influence of each electrode in the global data quality. A consolidated studentized contrast ðCC S Þ is introduced to consider the proportion of rejected quadripoles which contain a single electrode. These estimators are computed for each time-slice using Q-factor (coefficient of variation of repeated measurements) threshold values, from 0 to 10%, to discriminate between selected and rejected quadripoles. An automated detection strategy is proposed to identify suspicious electrodes by comparing the CC S to the MEC S (maximum expected C S values when every electrode is good for the given data set). These MEC S are computed using Monte-Carlo simulations of a hundred random draws where the distribution of Q-factor values follows a Weibull cumulative distribution, with k ¼ 0.421 and λ ¼ 1.303, fitted on a background data set filtered using a 5% threshold on absolute reciprocal errors. The efficiency of the methodology and its sensitivity to the selected reciprocal error threshold are assessed on synthetic and field data. Our approach is suitable to detect suspicious electrodes and slowly changing conditions affecting the galvanic contact resistances where classical approaches are shown to be inadequate except when the faulty electrode is disconnected.
Abstract Permanent monitoring of changes in soil properties is of increasing interest in many eng... more Abstract Permanent monitoring of changes in soil properties is of increasing interest in many engineering applications such as management of groundwater contamination, landslide and sinkhole risks prevention, detection of saline water intrusion, comprehension of charge and discharge processes of subsurface aquifer. As geophysical investigations allow detecting contrasts in physical properties of the subsurface, field and lab experiments have been conducted for a few years to assess the reliability of these methods to monitor temporal ...
Near Surface 2010 - 16th EAGE European Meeting of Environmental and Engineering Geophysics, 2010
Abstract:[en] The success of a tracer test highly depends on the number and the localisation of t... more Abstract:[en] The success of a tracer test highly depends on the number and the localisation of the sampling wells. When preferential solute transport paths are expected, one needs to set up carefully the tracer test to recover information such as the local groundwater flow direction and an estimate of the transport velocities. In this work, we used electrical resistivity tomography (ERT) to monitor a saline tracer test. This experiment was performed in fractured limestones where high transport velocities and strong dilution effects were expected. This ...
Permanent monitoring of changes in soil properties is of increasing interest in many engineering ... more Permanent monitoring of changes in soil properties is of increasing interest in many engineering applications such as management of groundwater contamination, landslide and sinkhole risks prevention, detection of saline water intrusion, comprehension of charge and discharge processes of subsurface aquifer. As geophysical investigations allow detecting contrasts in physical properties of the subsurface, field and lab experiments have been conducted for a few years to assess the reliability of these methods to monitor temporal changes in soil properties. Among the methods available, DC resistivity tomography is recognized as one of the most promising techniques. In order to assess the efficiency of electrical resistivity in monitoring charge and discharge processes of subsurface aquifer, and also to better model hydrological effects on the gravity measurements, an on-going field experiment is conducted at the Membach station located in the eastern part of Belgium. This geophysical station is equipped with an accelerometer, seismometers and a superconducting gravimeter, installed at the end of a 130 m long tunnel excavated in a low-porosity argillaceous sandstone mount at 48.5 m depth. Continuous gravimetric observations have been taken since August 1995. Since 2004 rainfall and soil moisture changes are measured in situ. In July 2010, an automated permanent geoelectrical acquisition system was installed to monitor subsurface resistivity variations during a test period of about 6 months. The aim of this experiment is to better understand charge and discharge processes of the subsurface aquifer, which are expected to be mainly due to rainfall variations. This aquifer is localized at the top of the weathered bedrock at a depth of 4 to 5 meters. The acquisition system consists in a straight profile of 48 buried electrodes (with a 2 meters spacing) connected to a Syscal R1 resistivimeter which is automatically controlled by a computer. Resistivity measurements are taken at least twice a day at fixed hours using a combination of dipole-dipole and Wenner-Schlumberger arrays. Acquired data are filtered in order to reject faulty measures. Time-lapse inversion (Loke (1999)) is then carried out to reconstruct a 2D model of resistivity changes. Preliminary results obtained during July show changes in inverted resistivities of about 30% in the first 4 to 5 meters layer. These observations are consistent with changes in measured gravimetric water content. This seems to indicate that subsurface aquifer charge and discharge processes are mainly due to rainfall, as expected. However, inversion errors remain high even after data filtering. This could be a consequence of weather occurring in July, leading to a poor contact between the electrodes and dry host soils near the surface. This problem should not happen anymore as the rest of the monitoring experiment is conducted during the wet season. Acknowledgments This work is conducted under the auspices of the Walloon Region Ministry under the First Spin-Off program (visa n° 916974).
Journal of Environmental & Engineering Geophysics, 2007
A field experiment was conducted over a Light Non-Aqueous Phase Liquid (LNAPL) contaminated site ... more A field experiment was conducted over a Light Non-Aqueous Phase Liquid (LNAPL) contaminated site located near a gas station where a tank had leaked its content. In this area, a dolomitic bedrock is overlain by 5 to 9 m of clayey sands. In order to delineate the plume and measure piezometric heads and depths to bedrock, ten boreholes were drilled and four cone penetration tests were conducted. Soil and water samples were collected and analyzed. Although these samplings highlight the presence of hydrocarbons in some boreholes, the plume is poorly outlined due to the limited number of drillings.
1 2 ERT and SP investigations were conducted in carbonate rocks of the Dinant Synclinorium 3 (Wal... more 1 2 ERT and SP investigations were conducted in carbonate rocks of the Dinant Synclinorium 3 (Walloon Region of Belgium) to find suitable locations for new water wells in zones with 4 little hydrogeological data. Since measured information in the boreholes needed to be 5 representative of the area, large fractured zones were searched for the drillings. Large ERT 6 profiles (320 to 640 meters) allowed us to image the resistivity distribution of the first 60 7 meters of the subsurface and to detect and characterize (in terms of direction, width and 8 depth) fractured zones expected to be less resistive. Data errors, depth of investigation (DOI) 9
Electrical resistivity tomography (ERT) monitoring experiments are being conducted more often to ... more Electrical resistivity tomography (ERT) monitoring experiments are being conducted more often to image spatiotemporal changes in soil properties. When conducting long-term ERT monitoring, the identification of suspicious electrodes in a permanent spread is of major importance because changes in electrode contact properties of a single electrode may affect the quality of many measurements on each time-slice. An automated methodology was developed to detect these temporal changes in electrode contact properties, based on a Bayesian approach called "weights of evidence." Contrasts ðC W Þ and studentized contrasts ðC S Þ are estimators of the influence of each electrode in the global data quality. A consolidated studentized contrast ðCC S Þ is introduced to consider the proportion of rejected quadripoles which contain a single electrode. These estimators are computed for each time-slice using Q-factor (coefficient of variation of repeated measurements) threshold values, from 0 to 10%, to discriminate between selected and rejected quadripoles. An automated detection strategy is proposed to identify suspicious electrodes by comparing the CC S to the MEC S (maximum expected C S values when every electrode is good for the given data set). These MEC S are computed using Monte-Carlo simulations of a hundred random draws where the distribution of Q-factor values follows a Weibull cumulative distribution, with k ¼ 0.421 and λ ¼ 1.303, fitted on a background data set filtered using a 5% threshold on absolute reciprocal errors. The efficiency of the methodology and its sensitivity to the selected reciprocal error threshold are assessed on synthetic and field data. Our approach is suitable to detect suspicious electrodes and slowly changing conditions affecting the galvanic contact resistances where classical approaches are shown to be inadequate except when the faulty electrode is disconnected.
Abstract Permanent monitoring of changes in soil properties is of increasing interest in many eng... more Abstract Permanent monitoring of changes in soil properties is of increasing interest in many engineering applications such as management of groundwater contamination, landslide and sinkhole risks prevention, detection of saline water intrusion, comprehension of charge and discharge processes of subsurface aquifer. As geophysical investigations allow detecting contrasts in physical properties of the subsurface, field and lab experiments have been conducted for a few years to assess the reliability of these methods to monitor temporal ...
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