Véronique Naudet

Particle swarm optimization applied to solving and appraising the streaming-potential inverse problem

Waterflowinthesubsoilgenerateselectricalcurrentsmeasurable at the ground surface with the self-potential SP method. These measured potentials, which result from hydroelectric coupling, are called streaming potentials and are wellcorrelatedwiththegeometryofthewatertable.Theparticleswarmalgorithmcanbeusedtoestimatethewater-table elevation from SP data measured at the ground surface. The basic idea behind particle swarm optimization PSO is that each model searches the model space according to its misfit history and the misfit of the other models particles of the swarm. PSO is a simple, robust, and versatile algorithm with a very good convergence ratetypically before 3000 forward runs, and it can explore a large model space without being time consuming. Based on samples gathered in a low-misfit area,wehavecomputedafastapproximationoftheposterior distribution of the water table, the electrokinetic coupling constant, and the reference hydraulic head.Although PSO is astochasticsearchtechnique,ourconvergenceresults,based onthestabilityofparticletrajectories,specifyclearcriteriato tunePSOparameters.

Estimation of water table from self‐potential data using particle swarm optimization (PSO)

Water flow in the subsoil generates electrical current measurable at the ground surface and terms self-potential (SP) anomalies. Due to this hydro-electric coupling called the electrokinetic effect, SP anomalies are well correlated with the geometry of the water table. In this contribution, we present the application of Particle Swarm algorithm to estimate the water table elevation from SP data measured at the ground surface. We present both synthetic and field data obtained from the literature.

3D electrical resistivity tomography to locate DNAPL contamination around a housing estate

A 3D electrical resistivity survey is carried out downstream of a former coal refinery plant in France to determine the spatial extent of a DNAPL contaminant plume and to optimize the implantation of further boreholes. The site, which is located in the north of France, is the subject of an extensive conventional site investigation including the characterization of the contaminant source and natural attenuation monitoring of the plume. Previous geochemical and geophysical studies including 2D electrical resistivity profiles have shown a possible migration of the contaminant plume toward a housing estate, where no geochemical boreholes have been drilled. The purpose of this study was to achieve a real 3D imaging of the subsoil to locate the extent of the contaminant plume above the housing estate and determine the location of new boreholes. As geophysical measurements are difficult to perform in such an urban environment, a new electrical campaign was realized with innovative acquisition geometries and arrangement of surface electrodes in an L-shape, disposing electrodes in lines around the housing estate. Data were inverted in 3D with ERTLab software from Multi-Phase Technologies and Geostudi Astier. Results show a conductive plume emanating from old tar ponds and a slag heap that spreads through the housing estate. Based on these results, new boreholes were drilled in the housing estate, which confirm the extent of the contaminant plume estimated from the 3D electrical resistivity survey.

Detection of landfill cover damage using geophysical methods

On closed landfills, impermeable covers are capping the waste in order to minimize water infiltration and accumulation of leachate inside the waste. In France, the cover composition depends notably on the kind of stored waste. In cases of hazardous waste, the cover must be composed of a drainage layer under the top soil and a geomembrane or a Geosynthetic Clay Liner (GCL) associated with an underlying 1.0 m thick low permeability material to ensure its tightness. However, this protection cover is sometimes damaged leading to an escape of landfill gazes and an unusual increase of leachate within the waste after rainy events. As leachate treatment is very expensive, it appears necessary to locate the weakness zones of the cover and assess their sizes in order to limit maintenance cost on landfills. In order to detect damages in the cover, the 2 following geophysical methods have been carried out on a French landfill: cartography with an Automatic Resistivity Profiling (ARP©), the Electrical Resistivity Tomography (ERT) and the Self Potential method (SP). The joint use of these methods has given us the opportunity to determine the contribution of each of them for covers characterization. The ARP survey has put in evidence the lateral heterogeneity of cover materials on the whole landfill. The ERT has confirmed the variability of the cover composition but also provided information about the cover thickness and the damaging of the GCL. SP measurements have revealed a negative anomaly at the top of the landfill, possibly linked with a thin and damaged cover and therefore a greater proximity of waste. Finally, manual augers holes have enabled to associate electrical resistivity properties with different materials used in the cover and the damaged GCL. This study on a hazardous waste landfill shows that geophysical methods associated with manual auger holes have allowed to improve the knowledge of the cover. Thus, the damaged areas detected thanks to measurements performed on site may be useful for the landfill manager who can optimize the drilling survey necessary to check the nature of defects and then choose a suited remediation of the cover.

Geophysical methods applied to characterize landfill covers with geocomposite

We attempt to characterize with geophysical methods the state of landfill covers to detect damages that can induce preferential water pathways and unusual increase of leachate within the waste mass. The geophysical methods used were the Electrical Resistivity Tomography (ERT), cartography with an Automatic Resistivity Profiling (ARP©), and the Self Potential method (SP). We worked on experimental parcels reproducing common defaults on landfill covers (clay material and geocomposite) and on a larger scale on a french landfill cover. The joint use of these methods gives us the opportunity to test their ability to detect defects. Results on the parcels have shown a good detection of the larger cracks (0.10 m) on the compacted clay cover but a less easy detection of defaults on the geocomposite. Results on the landfill have shown conductive zones correlated with important SP variations that could indicate a preferential infiltration zone in the cover.

3D Electrical Resistivity Tomography to locate DNAPL contamination in an urban environment

This study presents results from electrical resistivity campaigns performed around a housing estate located downstream to an old coke tar site in France. This coke tar has been previously studied with geochemical analyses and geophysical prospections. The previous results have shown a possible migration of the contaminant plume toward the housing estate. As geophysical measurements are difficult to perform in such an urban environment, the electrical array has been deployed all around the housing estate with an innovative arrangement of surface electrodes in C-shape and acquisition geometries in order to achieve a real 3D imaging of the subsoil and locate the contaminant plume above the housing estate. The electrical resistivity data were inverted with the ERTLabTM 3D inversion software developed by Multi-Phase Technologies and Geostudi Astier. Results show, at the depth of the aquifer, a very conductive plume emanating from old tar ponds and a slag heap and spreading through the housing estate.

On the Geoelectrical Characterization of an Old Landfill Cover

Two geoelectrical methods were used to characterize the state of an old French landfill cover. The objectives were to locate different materials used for the covering such as clay material or geomembrane and to identify heterogeneities that could be linked to possible defects in the cover due to fractures or cracks. These damages can induce preferential water pathways and unusual increase of leachate within the waste mass. The geoelectrical methods used were the electrical resistivity cartography with an Automatic Resistivity Profiling (ARP©) and the Self Potential method (SP). Results have put in evidence three distinct zones with different geoelectrical signatures that are correlated with three different phases of landfill covering. ARP also seems to have detected the presence of geomembrane installed all around the old landfill to improve its stability and geodrains over two closed alveoli. Local differences in apparent resistivity and self-potential signals have also been identified and could result in default in the clay cover due to thickness variations but also in different lithology, compaction and water content.