Auscultation microsismique appliquée à la détection des éboulements des falaises crayeuses

National audienceThe microseismic monitoring carried out on an experimental site showed that the chalk rock ruptures are recordable by a seismic network composed of geophones and accelerometers. The microseismic method also showed the capacity to detect the precursory signs of a cliff collapse. This study enabled to dimension the essential parameters for the implementation of an operational microseismic system to monitor unstable cliffs zones and predict rock-falls hazard.L'auscultation microsismique menée sur une falaise maritime en tant que site expérimental a démontré que les microruptures d'un massif de craie sont détectables par un dispositif de capteurs de type géophone et accéléromètre. Elle a aussi montré les capacités de la méthode microsismique à détecter les signes précurseurs d'un éboulement. L'étude a permis d'établir les paramètres pour le dimensionnement d'un système de surveillance micro-sismique opérationnel d'un risque d'éboulement des falaises crayeuses

Apports du radar géologique à l'étude de la dynamique sédimentaire à l'Holocène récent dans la plaine interdunaire de Merlimont (Pas-de-Calais).

International audienceA Ground Penetrating Radar (GPR) survey was carried out on the Pas-de-Calais coast in arder la quantify the evolution of sand dunes during the holocene period. 15 km of GPR profiles were recorded with 100 and 500 MHz antennae. Some boreholes were drilled to control the GPR results. Correlation between GPR and boreholes is better than 35 cm. At last, 7 peat layers have been found wilh a north-east dip. It reveals the existence of changes in hydrogeological conditions (variations of water table) during the last 900 years (mast ancient peat in dunes peat layers). Moreover, some ancient dunes features have been outlined between flat peat layers. GPR is a robust tool for sedimentological interpretation in sandy context

Monitoring of the biodegradation of toluene-contaminated sand in columns by SIP measurements, CO2 content and its 13C/12C isotopic signature.

Hydrocarbon contaminated soils represent an environmental issue as it impacts on ecosystems and aquifers. Bioremediation uses the ability of bacteria naturally present in the ground to degrade hydrocarbons. It represents an effective solution to fight the pollution but in situ monitoring before and during soil treatment is difficult and challenging. Indeed, where significant subsurface heterogeneity exists, conventional intrusive groundwater sampling can be insufficient to obtain a robust monitoring as the information they provide is restricted to vertical profiles at discrete locations, with no information between sampling points. In order to obtain wider information, complementary methods can be used like geo-electrical techniques. Induced polarization (IP) seems to be the more promising to study the effects of biodegradation processes. Indeed, laboratory and field experiments have shown an enhancement of real and imaginary parts of electrical conductivity while bacterial treatment is progressing (Abdel Aal et al., 2006 ; Atekwana et Atekwana, 2010). Moreover, microbial activity induced CO2 production and isotopic deviation of carbon (Aggarwal and Hinchee, 1991). The ratio δ13C(CO2) will come closer to δ13C(hydrocarbon). From these findings, the French project BIOPHY, supported by the French National Research Agency (ANR), proposes to use electrical methods and gas analyses to develop a non-destructive method for monitoring in situ biodegradation of hydrocarbons in order to optimize soil treatment. Laboratory experiments in columns are carried out to demonstrate its feasibility. Our objectives were to monitor aerobic microbial activity in toluene-contaminated sand columns using complex electrical resistivity measurements (SIP, Spectral Induced polarization and GEIS, Galvanostatic Electrochemical Impedance Spectroscopy) and measuring concentration and δ13C isotopic ratio of produced CO2

Developing tools for the prediction of catastrophic coastal cliff collapse

International audienceThe erosion of coastal cliffs is inevitable. The cliff collapses are a hazard, a problem for coastal land use planners and limit the use of the coastline as an amenity. Techniques to increase our knowledge of coastal cliff failure and provide pre-cursors to impending collapse are presented. The rock mass cracks before a collapse and the cracking generates high frequency seismic signais. An increase in emitted seismic energy has been recorded on accelerometers, in boreholes within a cliff, during the fifteen hours before a cliff fall. Comparison of the seismic signals with those generated in the laboratory has identified a number of collapse mechanisms that occur during different phases of the fall. A second technique assumes that in cliffs composed of a highly fractured rock, any sub-vertical fractures will gradually dilate with time before a collapse. The electrical resistance of the rock varies with azimuth reflecting the dominant fracture orientation. A factor of anisotropy, calculated from the measurements, was found to change dramatically due to a cliff fall. Large values of anisotropy prior to the fall were interpreted as evidence of dilating fractures. In addition, the research has demonstrated the existence of a cliff parallel fracture adjacent to the cliff edge. It is likely that cracking within this fracture set is responsible for the seismicity and the variations in the measured anisotropy. Laboratory analysis has identified a significant weakening of the rock due to sait crystals within the matrix. The integration and implications of these results are discussed

Improved monitoring of the reductive dechlorination of PCE in polluted soils by using geophysical and electrochemical measurements carried out in columns

International audienceToday, reductive dehalogenation (RDH), using strong reducers, is one of the most important emerging remediation techniques for halogenated hydrocarbons (HHC). The use of nanomaterials, all alone or coupled with strong reducers, for RDH is emerging too as a good alternative and promising technological approach to clean up polluted environments by HHC. The purpose of the present study was to set up an enhanced monitoring of the reductive dechlorination (RDC) of Perchlorethylene (PCE) in soils, based on geophysical and electrochemical measurements and practiced, at this stage, only in columns. This study is accomplished within the framework of DECHLORED, a project that aims to clean up aquifers by using strong reducers with or not the combined effect of Nano-iron particles (NIP), reaching the RDC of chlorinated hydrocarbons (CHC) and illuminating the field under RDC process. For this purpose, columns (height 50cm; diameter 20cm) are designed and made in Kynar® and equipped with sophisticated systems for non-destructive monitoring of time changing physical parameters. This monitoring was complemented by physicochemical probes (temperature, pH, redox, conductivity, oxygen and chloride) and Gas Chromatography for PCE and its degradation products (DCE, ethene and ethane) and Ion Chromatography for Cl- and SO42- in water at the column output to correlate RDC, NP migration and electrical properties. The entire system was placed in air-conditioned cupboard. Columns are implemented with two rings of Hasteloy® as electrodes for alternative sinusoidal current (AC) injection and six ports equipped with Ag-AgCl electrodes for potential measurement. Two methods are used: i) frequency-domain induced polarization (IP) measurements (which consist of imposing an AC current at a given frequency and measuring the resulting electrical potential difference between two other non-polarizing electrodes), and ii) galvanostatic electrochemical impedance spectroscopy (GEIS). Both by measuring complex electrical conductivity in the mHz to kHz range, are sensitive to electrolytic conductivity, grain surface conductivity and metallic conductivity. Under time varying electric field, pore fluid/solid polarization effects could be measured from surface using IP impedance analyzer or time-domain resistivimeter. Indeed, IP can map real and complex resistivities of the media at any time and particularly, before, during and after the RDC process application on the column. Two different soils, polluted or not by PCE, different solutions/suspensions of reactants (reducers or NIPs separately or reducers and NIPs together) at two different temperatures (12 and 25°C) are tried. Experiments in columns were carried out to test reactants behavior, in the absence and in the presence of PCE, and allow us to: * estimate how reactants and PCE chemical degradation products transform electrical properties * assess reactant dispersion, distribution, transport and reactivity on PCE degradation, inside the columns (over all its length) and as a function of time and for different soils * Improve understanding of reactant efficacy, their transformation and persistence, and also their potential negative effects * model these flow-through systems (1D or 2D column set up) by modeling softwares (PHREEQC® and MARTHE® reactive transport simulations). Our results claim for combining geophysical with electroanalytical measurements to monitor RDC processes and will allow us to give an advantageous method for monitoring and checking in situ remediation

Projet ATTENA : Développement de mesures de terrain permettant la mise en évidence de l'atténuation naturelle des polluants organiques

National audienceDe nombreux acteurs se trouvent actuellement confrontés au problème de la gestion de sites contaminés et à leur réhabilitation. La mise en place de méthodes de traitement dites classiques peut engendrer des investissements importants et se révéler difficile d'un point de vue technique, voire inefficace, notamment dans le cas de pollutions d'aquifères ou de contamination étendue des sols. A partir de ce constat, la gestion des sites pollués peut être abordée différemment en intégrant une approche alternative privilégiant les mécanismes naturels de dégradation et communément appelée atténuation naturelle surveillée (ANS)

Microseismic monitoring applied to prediction of chalk cliffs collapses and contribution of numerical modelling

International audienc

Combining Geoelectrical Measurements and CO2 Analyses to Monitor the Enhanced Bioremediation of Hydrocarbon-Contaminated Soils: A Field Implementation

Hydrocarbon-contaminated aquifers can be successfully remediated through enhanced biodegradation. However, in situ monitoring of the treatment by piezometers is expensive and invasive and might be insufficient as the information provided is restricted to vertical profiles at discrete locations. An alternative method was tested in order to improve the robustness of the monitoring. Geophysical methods, electrical resistivity (ER) and induced polarization (IP), were combined with gas analyses, CO2 concentration, and its carbon isotopic ratio, to develop a less invasive methodology for monitoring enhanced biodegradation of hydrocarbons. The field implementation of this monitoring methodology, which lasted from February 2014 until June 2015, was carried out at a BTEX-polluted site under aerobic biotreatment. Geophysical monitoring shows a more conductive and chargeable area which corresponds to the contaminated zone. In this area, high CO2 emissions have been measured with an isotopic signature demonstrating that the main source of CO2 on this site is the biodegradation of hydrocarbon fuels. Besides, the evolution of geochemical and geophysical data over a year seems to show the seasonal variation of bacterial activity. Combining geophysics with gas analyses is thus promising to provide a new methodology for in situ monitoring

Structural study of the Ballandaz landslide (French Alps) using geophysical imagery

International audienceThis study of the Ballandaz landslide (Savoie, French Alps) was carried out as part of the ANR/PGCU-SIGMA research project. Its aim was to characterize the slope by different methods of geophysical imagery, and then use this characterization to provide a combined interpretation of the morpho-structures in order to simulate the geomechanical behavior of the sliding mass. First, electromagnetic mapping was used to identify the variations of the surface lithology and moisture from which one can more precisely locate the active zone of the slope. Then, electrical and seismic 2D imagery methods were used along several transverse and longitudinal profiles in order to produce electrical resistivity and seismic-velocity depth sections showing up the slope's structures. The H/V method was also tested locally to determine the depth to basement so as to complement the profiles: it reveals potential seismic site effects from the deeper structures. Helped by drilling and inclinometer surveys, the geophysical interpretations have revealed the various units structuring the landslide: (i) the active, very heterogeneous shallow level overlying (ii) a more rigid, less porous, and probably stable shallow bedrock, (iii) boulders, and (iv) the sound geological basement of quartzite and gypsiferous facies. The study has shown the usefulness of combining such different sounding techniques for studying complex environments like landslides; it has also revealed the limitations of each method when used for studying very heterogeneous environments

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