Singular value decomposition as a denoising tool for airborne time domain electromagnetic data

International audienceAirborne Time Domain Electromagnetic (TDEM) surveys are increasingly carried out in anthropized areas as part of environmental studies. In such areas, noise arises mainly from either natural sources, such as spherics, or cultural sources, such as couplings with man-made installations. This results in various distortions on the measured decays, which make the EM noise spectrum complex and may lead to erroneous inversion and subsequent misinterpretations. Thresholding and stacking standard techniques, commonly used to filter TDEM data, are less efficient in such environment, requiring a time-consuming and subjective manual editing. The aim of this study was therefore to propose an alternative fast and efficient user-assisted filtering approach. This was achieved using the Singular Value Decomposition (SVD). The SVD method uses the principal component analysis to extract into components the dominant shapes from a series of raw input curves. EM decays can then be reconstructed with particular components only. To do so, we had to adapt and implement the SVD, firstly, to separate clearly and so identify easily the components containing the geological signal, and then to denoise properly TDEM data. The reconstructed decays were used to detect noisy gates on their corresponding measured decays. This denoising step allowed rejecting efficiently mainly spikes and oscillations. Then, we focused on couplings with man-made installations, which may result in artifacts on the inverted models. An analysis of the map of weights of the selected "noisy components" highlighted high correlations with man-made installations localized by the flight video. We had therefore a tool to cull most likely decays biased by capacitive coupling noises. Finally, rejection of decays affected by galvanic coupling noises was also possible locating them through the analysis of specific SVD components. This SVD procedure was applied on airborne TDEM data surveyed by SkyTEM Aps. over an anthropized area, on behalf of the French geological survey (BRGM), near Courtenay in Région Centre, France. The established denoising procedure provides accurate denoising tools and makes, at least, the manual cleaning less time consuming and less subjective

Combined use of geophysical methods and remote techniques for characterizing the fracture network of a potential unstable cliff site (the “Roche du Midi”, Vercors massif, France)

International audienceStability assessment of a cliff strongly depends on the fracture pattern and the face topography. Geological observations as well as classical geodetic measurements are difficult to perform on high nearly vertical cliffs like the ones surrounding the town of Grenoble (French Alps). In this study we combine remote and ground imaging techniques for characterizing the geometry and the fracture pattern of potential unstable cliff sites. A Dense Digital Surface Model (DDSM) of the rock face can now be obtained from laser scanning (Lidar) or photogrammetry. These techniques are safer and quicker than direct measurements. They offer the possibility to collect structural data and to sample the shape of the outcrop at a centimetric resolution. We applied these two techniques to a potential unstable site (the “Roche du Midi”, Vercors massif) for determining the main fracture families affecting the rock mass and we obtained results similar to direct measurements performed on the nearby outcrops and on the cliff face itself. The laser scanning data suffers a bias in the illumination of the site. Geophysical experiments were also conducted on the plateau and on the cliff face in order to delineate the fracture pattern inside the rock mass. ERT (Electrical Resistivity Tomography) and GPR (Ground Penetrating Radar) profiles were performed on the plateau and allowed near-vertical open fractures to be located in the vicinity of the surface. Best geophysical results in terms of penetration and resolution were however obtained from GPR profiles conducted directly on the cliff face. Laser scanning data were combined with GPR data in order to take into account the shape of the sampled profiles. The combination of vertical and short horizontal profiles allowed the strike and dip of the discontinuities to be determined. The two main families were imaged, as well as a major continuous inward dipping reflector which was not shown during the initial reconnaissance. Further investigation inside the mass effectively showed the existence of this fracture. These results highlight the power of the GPR technique in characterizing the discontinuity pattern inside rock mass for improving the model in view of hazard assessment

Dengue Spatial and Temporal Patterns, French Guiana, 2001

To study a 2001 dengue fever outbreak in Iracoubo, French Guiana, we recorded the location of all patients’ homes and the date when symptoms were first observed. A geographic information system was used to integrate the patient-related information. The Knox test, a classic space-time analysis technique, was used to detect spatiotemporal clustering. Analysis of the relative-risk (RR) variations when space and time distances vary, highlighted the maximum space and time extent of a dengue transmission focus. The results show that heterogeneity in the RR variations in space and time corresponds to known entomologic and epidemiologic factors, such as the mosquito feeding cycle and host-seeking behavior. This finding demonstrates the relevance and potential of the use of GIS and spatial statistics for elaborating a dengue fever surveillance strategy

Assessment of CO2 health risk in indoor air following a leakage reaching unsaturated zone: results from the first representative scale experiment

International audienceLeakage of CO2 from geological reservoirs is one of the most fearsome unexpected scenarios for CO2 storage activities. If a leakage reaches the ground level, exposure to high CO2 concentrations is more likely to occur in low ventilated spaces (pit dug in the ground, basement, building) where CO2 could accumulate to high concentrations. Significant literature and models about indoor exposure resulting from intrusion of soils gases in building are available in several domains (e.g., contaminated soils, radon, etc.). However, there is no guarantee that those approaches are appropriate for the assessment of consequences of CO2 leakage due the specificity of CO2 and due to the singularities of the source in case of leakage from anthropic reservoirs. Furthermore, another singularity compared to conventional approaches is that the risk due to CO2 exposure should be evaluated considering acute concentrations rather than long term exposure to low concentrations. Thus, a specific approach is needed to enable a quantitative assessment of the risk for health and living in indoor environment in case of leakage from a reservoir reaching the unsaturated zone below the buildings. We present the results of the IMPACT-CO2 project that aims at understanding the possible migration of CO2 to indoor environment and to develop an approach to evaluate the risks. The approach is based on modelling and experiments at laboratory scale and at field representative scale. The aim of the experiment is to capture the main phenomena that control the migration of CO2 through unsaturated zone, and its intrusion and accumulation in buildings. The experimental results will also enable numerical confrontation with tools used for risk assessment. Experiments at representative scale (Figure 1) are performed on the PISCO2 platform (Ponferrada, Spain) specifically instrumented and designed for understanding the impacts of CO2 migration towards the soil surface. The experiment is composed of a 2.2 m deep basin filled with sand upon which a specifically designed cylindrical device representing the indoor condition of a building (with controlled depressurization and ventilation) is set up. The device includes a calibrated interface that represents a cracked slab of a building. The injection of CO2 is performed at the bottom of the basin with a flow rate in the range of hundreds of g/d/m². The first results show that the presence of a building influences significantly the transport of CO2 in the surrounding soil leading to two competing phenomena: 1) seepage in the atmosphere mainly controlled by diffusion gradient and 2) advective/diffusive flux entering the building due to the depressurization. Models have been established to quantitatively assess the proportion of CO

Effects of thermal and chemical enhancements on the recovery of heavy chlorinated compounds in saturated porous media: Treatability tests and modeling

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Value of syndromic surveillance within the Armed Forces for early warning during a dengue fever outbreak in French Guiana in 2006

<p>Abstract</p> <p>Background</p> <p>A dengue fever outbreak occured in French Guiana in 2006. The objectives were to study the value of a syndromic surveillance system set up within the armed forces, compared to the traditional clinical surveillance system during this outbreak, to highlight issues involved in comparing military and civilian surveillance systems and to discuss the interest of syndromic surveillance for public health response.</p> <p>Methods</p> <p>Military syndromic surveillance allows the surveillance of suspected dengue fever cases among the 3,000 armed forces personnel. Within the same population, clinical surveillance uses several definition criteria for dengue fever cases, depending on the epidemiological situation. Civilian laboratory surveillance allows the surveillance of biologically confirmed cases, within the 200,000 inhabitants.</p> <p>Results</p> <p>It was shown that syndromic surveillance detected the dengue fever outbreak several weeks before clinical surveillance, allowing quick and effective enhancement of vector control within the armed forces. Syndromic surveillance was also found to have detected the outbreak before civilian laboratory surveillance.</p> <p>Conclusion</p> <p>Military syndromic surveillance allowed an early warning for this outbreak to be issued, enabling a quicker public health response by the armed forces. Civilian surveillance system has since introduced syndromic surveillance as part of its surveillance strategy. This should enable quicker public health responses in the future.</p

Etude des éboulements rocheux par méthodes géophysiques

Rock falls pose critical problems to risk management, due to the suddenness of the phenomena and the lack of precursors. Detection of unstable rock mass, evaluation of stability and comprehension of dynamic phenomenon are major elements in order to evaluate the rock falls potential hazard. In first step, the Ground Penetrating Radar allowed the 3D fractures network to be determined. An inversion strategy for fractures characterization (aperture and filling) is proposed by using the frequencial and angular sensitivity of the reflectivity of the electromagnetic waves with a thin bed approach. This methodology was applied successfully on two different study sites. In second part, a new seismic magnitude scale was defined, which allowed us to compare and classify ground-motion vibrations. No relation was found between rock-fall parameters (fall height, runout distance, volume, potential energy) and rock-fall seismic magnitudes. On the other hand, the signal duration shows a rough correlation with the potential energy and the runout distance, highlighting the control of the propagation phase on the signal length. The signal analysis suggests the existence of at least two distinct seismic sources: one corresponding to the initial rupture associated with an elastic rebound during the detachment and the other one generated by the rock impact on the slope, whose frequency shape was confirmed by a 2D finite-element simulation.Les éboulements de terrain posent des problèmes importants pour la gestion des risques, à cause de leur soudaineté et de l'absence de signe précurseur. La détection d'un compartiment instable, l'évaluation de sa stabilité ainsi que la compréhension de la dynamique de sa propagation sont des éléments majeurs dans l'évaluation de l'aléa d'éboulement. En premier lieu, le géoradar a permis de mieux contraindre la géométrie 3D du réseau de fracture. Une stratégie d'inversion des caractéristiques des fractures (épaisseur et remplissage), basée sur la sensibilité fréquentielle et angulaire des réflexions des ondes électromagnétiques avec une approche de couche mince a été développé et appliqué de façon convaincante sur des données réelles acquises sur deux sites différents.La seconde partie est consacrée à l'exploitation des signaux sismiques enregistrés lors de douze éboulements différents. Une nouvelle échelle de magnitude a été définie, qui a permis de classer les différents événements. Aucune relation n'a été trouvée entre les paramètres géométriques (et l'énergie potentielle dérivée) et la magnitude. Par contre la durée du signal montre une bonne corrélation avec l'énergie potentielle. L'analyse détaillée des signaux suggère l'existence d'au moins deux sources sismiques, une correspondant à la rupture associée au rebond élastique induit par le détachement de la masse rocheuse et une autre générée par l'impact de la masse sur la pente, dont l'aspect fréquentiel a été confirmé par une analyse numérique 2D par la méthode des éléments finis

Inversion Methodology of Dispersive Amplitude and Phase versus Offset of GPR Curves (DAPVO) for Thin Beds

The presence of a thin layer embedded in any formation creates complex reflection patterns because of interferences within the thin bed. Amplitude variation with offset (AVO) is used increasingly in seismic interpretation and has been tested more recently on ground-penetrating radar (GPR) data to characterize nonaqueous-phase liquid contaminants. In those analyses, phase and dispersion properties of the reflected signals generally are omitted, although they contain useful information. An inversion methodology to examine thin-bed properties -- dispersive amplitude and phase versus offset (DAPVO) -- combines all reflectivity properties (amplitude, phase, and dispersion) of the reflected GPR signal generated by a thin bed embedded within a homogeneous material. A brief description of electromagnetic (EM) phenomena is presented. The dispersive properties of the dielectric permittivity of investigated materials can be described using a Jonscher parameterization, which allows the study of the dependency of amplitude and phase versus offset (APVO) curves on the frequency of thin-bed properties (filling nature, aperture). Simplifying assumptions and using careful corrections are necessary to convert raw common-midpoint (CMP) reflected data into DAPVO curves and to study the propagation and radiation-pattern corrections. The inversion methodology is explained and validated to a synthetic set of CMP GPR data and can be illustrated with a real CMP data set acquired along a vertical cliff. This allows for extraction of the characteristics of a subvertical fracture while simultaneously satisfying resolution and confidence. Such a study motivates interest in combining the dispersion dependency of the reflection-coefficient variations with classical AVO analysis for thin-bed characterization

On the use of dispersive APVO GPR curves for thin-bed properties estimation: Theory and application to fracture characterization

International audienceThe presence of a thin layer embedded in any formation creates complex reflection patterns caused by interferences within the thin bed. The generated reflectivity amplitude variations with offset have been increasingly used in seismic interpretation and more recently tested on ground-penetrating radar (GPR) data to characterize nonaqueous-phase liquid contaminants. Phase and frequency sensitivities of the reflected signals are generally not used, although they contain useful information. The present study aims to evaluate the potential of these combined properties to characterize a thin bed using GPR data acquired along a common-midpoint (CMP) survey, carried out to assess velocity variations in the ground. It has been restricted to the simple case of a thin bed embedded within a homogeneous formation, a situation often encountered in fractured media. Dispersive properties ofthe dielectric permittivity of investigated materials (homogeneous formation, thin bed) are described using a Jonscher parameterization, which permitted study of the dependency of amplitude and phase variation with offset (APVO) curves on frequency and thin-bed properties (filling nature, aperture). In the second part, we discuss and illustrate the validity of the thin-bed approximation as well as simplify assumptions and make necessary careful corrections to convert raw CMP data into dispersive APVO curves. Two different strategies are discussed to correct the data from propagation effects: a classical normal-moveout approach and an inverse method. Finally, the proposed methodology is applied to a CMP GPR data set acquired along a vertical cliff. It allowed us to extract the characteristics of a subvertical fracture with satisfying resolution and confidence. The study motivates interest to use dispersion dependency of the reflection coefficient variations for thin-bed characterization