In hard-rock aquifers, fractured zones constitute adequate drinking water exploitation areas but ... more In hard-rock aquifers, fractured zones constitute adequate drinking water exploitation areas but also potential contamination paths. One critical issue in hydrogeological research is to identify, characterize, and monitor such fractured zones at a representative scale. A tracer test monitored with surface electrical resistivity tomography (ERT) could help by delineating such preferential flow paths and estimating dynamic properties of the aquifer. However, multiple challenges exist including the lower resolution of surface ERT compared with crosshole ERT, the finite time that is needed to complete an entire data acquisition, and the strong dilution effects. We conducted a natural gradient salt tracer test in fractured limestones. To account for the high transport velocity, we injected the salt tracer continuously for four hours at a depth of 18 m. We monitored its propagation with two parallel ERT profiles perpendicular to the groundwater flow direction. Concerning the data acquisition, we always focused on data quality over temporal resolution. We performed the experiment twice to prove its reproducibility by increasing the salt concentration in the injected solution (from 38 to 154 g∕L). Our research focused on how we faced every challenge to delineate a preferential flow and solute transport path in a typical calcareous valley of southern Belgium and on the estimation of the transport velocity (more than 10 m∕hour). In this complex environment, we imaged a clear tracer arrival in both ERT profiles and for both tests. Applying filters (with a cutoff on the relative sensitivity matrix and on the background-resistivity changes) was helpful to isolate the preferential flow path from artifacts. Regarding our findings, our approach could be improved to perform a more quantitative experiment. With a higher temporal resolution, the estimated value of the transport velocity could be narrowed, allowing estimation of the percentage of tracer recovery.
Several techniques are available to estimate the depth of investigation or to identify possible a... more Several techniques are available to estimate the depth of investigation or to identify possible artifacts in dc resistivity surveys. Commonly, the depth of investigation (DOI) is mainly estimated using an arbitrarily chosen cut-off value on a selected indicator (resolution, sensitivity or DOI index). Ranges of cut-off values are recommended in the literature for the different indicators. However, small changes in threshold values may induce strong variations in the estimated depths of investigation. To overcome this problem, we developed a new statistical method to estimate the DOI of dc resistivity surveys based on a modified DOI index approach. This method is composed of 5 successive steps. First, two inversions are performed using different resistivity reference models for the inversion (0.1 and 10 times the arithmetic mean of the logarithm of the observed apparent resistivity values). Inversion models are extended to the edges of the survey line and to a depth range of three times the pseudodepth of investigation of the largest array spacing used. In step 2, we compute the histogram of a newly defined scaled DOI index. Step 3 consists in the fitting of the mixture of two Gaussian distributions ( and ) to the cumulative distribution function of the scaled DOI index values. Based on this fitting, step 4 focuses on the computation of an interpretation index defined for every cell of the model as the relative probability density that the cell belongs to , which describes the Gaussian distribution of the cells with a scaled DOI index close to 0.0. In step 5, a new inversion is performed using a third resistivity reference model (the arithmetic mean of the logarithm of the observed apparent resistivity values). The final electrical resistivity image is produced using as alpha blending values allowing the visual discrimination between well-constrained areas and poorly-constrained cells.
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).
In hard-rock aquifers, fractured zones constitute adequate drinking water exploitation areas but ... more In hard-rock aquifers, fractured zones constitute adequate drinking water exploitation areas but also potential contamination paths. One critical issue in hydrogeological research is to identify, characterize, and monitor such fractured zones at a representative scale. A tracer test monitored with surface electrical resistivity tomography (ERT) could help by delineating such preferential flow paths and estimating dynamic properties of the aquifer. However, multiple challenges exist including the lower resolution of surface ERT compared with crosshole ERT, the finite time that is needed to complete an entire data acquisition, and the strong dilution effects. We conducted a natural gradient salt tracer test in fractured limestones. To account for the high transport velocity, we injected the salt tracer continuously for four hours at a depth of 18 m. We monitored its propagation with two parallel ERT profiles perpendicular to the groundwater flow direction. Concerning the data acquisition, we always focused on data quality over temporal resolution. We performed the experiment twice to prove its reproducibility by increasing the salt concentration in the injected solution (from 38 to 154 g∕L). Our research focused on how we faced every challenge to delineate a preferential flow and solute transport path in a typical calcareous valley of southern Belgium and on the estimation of the transport velocity (more than 10 m∕hour). In this complex environment, we imaged a clear tracer arrival in both ERT profiles and for both tests. Applying filters (with a cutoff on the relative sensitivity matrix and on the background-resistivity changes) was helpful to isolate the preferential flow path from artifacts. Regarding our findings, our approach could be improved to perform a more quantitative experiment. With a higher temporal resolution, the estimated value of the transport velocity could be narrowed, allowing estimation of the percentage of tracer recovery.
Several techniques are available to estimate the depth of investigation or to identify possible a... more Several techniques are available to estimate the depth of investigation or to identify possible artifacts in dc resistivity surveys. Commonly, the depth of investigation (DOI) is mainly estimated using an arbitrarily chosen cut-off value on a selected indicator (resolution, sensitivity or DOI index). Ranges of cut-off values are recommended in the literature for the different indicators. However, small changes in threshold values may induce strong variations in the estimated depths of investigation. To overcome this problem, we developed a new statistical method to estimate the DOI of dc resistivity surveys based on a modified DOI index approach. This method is composed of 5 successive steps. First, two inversions are performed using different resistivity reference models for the inversion (0.1 and 10 times the arithmetic mean of the logarithm of the observed apparent resistivity values). Inversion models are extended to the edges of the survey line and to a depth range of three times the pseudodepth of investigation of the largest array spacing used. In step 2, we compute the histogram of a newly defined scaled DOI index. Step 3 consists in the fitting of the mixture of two Gaussian distributions ( and ) to the cumulative distribution function of the scaled DOI index values. Based on this fitting, step 4 focuses on the computation of an interpretation index defined for every cell of the model as the relative probability density that the cell belongs to , which describes the Gaussian distribution of the cells with a scaled DOI index close to 0.0. In step 5, a new inversion is performed using a third resistivity reference model (the arithmetic mean of the logarithm of the observed apparent resistivity values). The final electrical resistivity image is produced using as alpha blending values allowing the visual discrimination between well-constrained areas and poorly-constrained cells.
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).
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