Polarisation provoquée : expérimentation, modélisation et applications géophysiques

The physical mechanisms responsible for the induced polarization response over the frequency range (from 1 mHz to 20 kHz) are not completely understood. In particular, within the framework of this thesis, two subjects have been addressed. The origin of the signal observed at high frequency (HF) (>1 kHz) was analyzed by carrying out Spectral IP measurements on tap water samples. A phase deviation from the expected response has been observed at HF. The resulted deviation in phase appears to be dependent on the measuring electrode type (potential electrodes) and conductivity of the medium. A model based on an equivalent electrical circuit and designed to represent HF response, has been proposed to correct these effects. The mechanism responsible for the polarization in a medium containing semi-conductor grains has been investigated by analyzing the dependence of the relaxation time. We carried out experimental measurements on a sand medium containing different types of semi-conductors. The spectral response was studied by varying the concentration and type of the electrolyte, the size and content of semi-conductor grains. By using the finite element method, a 2D numerical simulation based on Poisson-Nernst-Planck equations was performed in time and frequency domains. The experimental results are qualitatively in accordance with numerical simulation. It showed a comparable decrease in the relaxation time when increasing the electrolyte concentration. Finally, field measurements on a paleo-mining site containing semi-conductor grains have been acquired. Time-domain IP measurements allowed us to define the zones of slag in the site and led to estimate the slag volume.Les mécanismes produisant les signaux observés par la méthode de Polarisation Provoquée (PP) sur une large gamme de fréquences (entre 1 mHz et 20 kHz) ne sont pas encore complètement identifiés. Deux sujets particuliers ont été abordés dans cette thèse. L'origine du signal observé à haute fréquence (>1 kHz) a été analysé en effectuant des mesures de la résistivité complexe sur de l'eau à différentes salinités. Les résultats montrent des écarts sur la phase par rapport à la réponse attendue à haute fréquence. Ils dépendent du type d'électrode de mesures et de la conductivité du milieu. Un modèle basé sur un circuit électrique équivalent a été proposé pour modéliser ces effets. Nous avons aussi exploré le mécanisme responsable de la polarisation en présence de grains semi-conducteurs en analysant la dépendance de temps de relaxation. La réponse spectrale d'un milieu sableux saturé a été étudiée en variant la concentration et le type de l'électrolyte, la taille, le type et la quantité de grains semi-conducteurs insérés. En utilisant la méthode des éléments finis, nous avons entrepris une simulation numérique 2D basée sur la solution des équations de Poisson-Nernst-Planck dans le domaine temporel et spectral. Les résultats expérimentaux sont conformes à ceux issus de la simulation numérique et montrent une décroissance comparable du temps de relaxation avec l'augmentation de la concentration de l'électrolyte. Finalement, une campagne géophysique de terrain sur un site paléo-miniers contenant des grains semi-conducteurs complète l'approche de laboratoire. Des mesures de PP temporelles permettent de délimiter les zones de scories sur le site et d'en estimer le volume

Induced polarization : experimentation, simulation and geophysical applications

Les mécanismes produisant les signaux observés par la méthode de Polarisation Provoquée (PP) sur une large gamme de fréquences (entre 1 mHz et 20 kHz) ne sont pas encore complètement identifiés. Deux sujets particuliers ont été abordés dans cette thèse. L'origine du signal observé à haute fréquence (>1 kHz) a été analysé en effectuant des mesures de la résistivité complexe sur de l'eau à différentes salinités. Les résultats montrent des écarts sur la phase par rapport à la réponse attendue à haute fréquence. Ils dépendent du type d'électrode de mesures et de la conductivité du milieu. Un modèle basé sur un circuit électrique équivalent a été proposé pour modéliser ces effets. Nous avons aussi exploré le mécanisme responsable de la polarisation en présence de grains semi-conducteurs en analysant la dépendance de temps de relaxation. La réponse spectrale d'un milieu sableux saturé a été étudiée en variant la concentration et le type de l'électrolyte, la taille, le type et la quantité de grains semi-conducteurs insérés. En utilisant la méthode des éléments finis, nous avons entrepris une simulation numérique 2D basée sur la solution des équations de Poisson-Nernst-Planck dans le domaine temporel et spectral. Les résultats expérimentaux sont conformes à ceux issus de la simulation numérique et montrent une décroissance comparable du temps de relaxation avec l'augmentation de la concentration de l'électrolyte. Finalement, une campagne géophysique de terrain sur un site paléo-miniers contenant des grains semi-conducteurs complète l'approche de laboratoire. Des mesures de PP temporelles permettent de délimiter les zones de scories sur le site et d'en estimer le volume.The physical mechanisms responsible for the induced polarization response over the frequency range (from 1 mHz to 20 kHz) are not completely understood. In particular, within the framework of this thesis, two subjects have been addressed. The origin of the signal observed at high frequency (HF) (>1 kHz) was analyzed by carrying out Spectral IP measurements on tap water samples. A phase deviation from the expected response has been observed at HF. The resulted deviation in phase appears to be dependent on the measuring electrode type (potential electrodes) and conductivity of the medium. A model based on an equivalent electrical circuit and designed to represent HF response, has been proposed to correct these effects. The mechanism responsible for the polarization in a medium containing semi-conductor grains has been investigated by analyzing the dependence of the relaxation time. We carried out experimental measurements on a sand medium containing different types of semi-conductors. The spectral response was studied by varying the concentration and type of the electrolyte, the size and content of semi-conductor grains. By using the finite element method, a 2D numerical simulation based on Poisson-Nernst-Planck equations was performed in time and frequency domains. The experimental results are qualitatively in accordance with numerical simulation. It showed a comparable decrease in the relaxation time when increasing the electrolyte concentration. Finally, field measurements on a paleo-mining site containing semi-conductor grains have been acquired. Time-domain IP measurements allowed us to define the zones of slag in the site and led to estimate the slag volume

Effect of capillary rise on mechanical behaviour of unstabilized rammed earth

International audienceMoisture ingress through capillarity has a great effect on the mechanical properties of unstabilized rammed earth (URE). Limited experimental studies exist to quantify this effect. In this paper, an experimental setup for the capillary test was designed. Rammed earth samples of 15cm x 15cm x 45cm equipped with humidity capacitive sensors were placed on a saturated layer of sand which imposes controlled matric suction at the base of the sample. The kinetics of the capillary rise was determined by continuous measurement of the mass of the sample, height of moisture, and relative humidity. Subsequently, unconfined compressive strength (UCS) tests were performed before and after capillary rise. Samples tested after capillary rise show a decrease in mechanical resistance of 95% (a usual compression strength of 1.4 MPa decreases until 0,07 MPa). Other samples, dried after capillarity, show under uniaxial compression tests a restoration in compressive strength by 80% of the original dried samples. In consequence, a loss of strength capacity is observed under the capillary rise cycle. This contributes to explain many pathologies encountered on rammed earth structures

Hydro-Mechanics Coupling on Rammed Earth Material: Drying Experiment at Structural Scale

International audienceRammed earth structures are very sensitive to hydric conditions. Experimental studies have been undertaken to understand the link between liquid water transfer and mechanical behavior at structural scale. This study was done on a prismatic rammed earth sample of 15cm x 15cm x 45cm, structured as a wall element with several layers. Samples were subjected to one dimensional drying in an indoor environment. Humidity and temperature sensors were placed on each layer inside the sample. The kinetic of drying was monitored by continuous weighing the sample and humidity measurement at a regular interval. Results of water content evolution suggest that samples dry in two stages; the first stage is associated with relatively high evaporation flux of 13.88 g m -2 h -1 while the second stage has very low flux of moisture evaporation. Unconfined compressive strength was performed in drying samples after 0, 2, 6 and 8 weeks of drying. In parallel, digital image correlation was used to determine the stiffness of samples. Results show an increase in compressive strength by the rate of 98 kPa per week in the first two weeks, then this rate reduces to 23 KPa per weeks after 8 weeks. These experimental results will allow to enhance the 3D hydro mechanical numerical model developed in the laboratory

Long-Term Effectiveness, Safety and Tolerability of Fingolimod in Patients with Multiple Sclerosis in Real-World Treatment Settings in France: The VIRGILE Study

Online ahead of printInternational audienceIntroduction: It is important to confirm the effectiveness and tolerability of disease-modifying treatments for relapsing-remitting multiple sclerosis (RRMS) in real-world treatment settings. This prospective observational cohort study (VIRGILE) was performed at the request of the French health authorities. The primary objective was to evaluate the effectiveness of fingolimod 0.5 mg in reducing the annualised relapse rate (ARR) in patients with RRMS.Methods: Participating neurologists enrolled all adult patients with RRMS starting fingolimod treatment between 2014 and 2016, who were followed for 3 years. Follow-up consultations took place at the investigator's discretion. The primary outcome measure was the change in ARR at month 24 after fingolimod initiation. Relapses and adverse events were documented at each consultation; disability assessment (EDSS) and magnetic resonance imagery were performed at the investigator's discretion.Results: Of 1055 eligible patients, 633 patients were assessable at month 36; 405 (64.0%) were treated continuously with fingolimod for 3 years. The ARR decreased from 0.92 ± 0.92 at inclusion to 0.31 ± 0.51 at month 24, a significant reduction of 0.58 [95% CI - 0.51 to - 0.65] relapses/year (p < 0.001). Since starting fingolimod, 461 patients (60.9%) remained relapse-free at month 24 and 366 patients (55.5%) at month 36. In multivariate analysis, no previous disease-modifying treatment, number of relapses in the previous year and lower EDSS score at inclusion were associated with a greater on-treatment reduction in ARR. The mean EDSS score remained stable over the course of the study. Sixty-one out of 289 (21.1%) patients presented new radiological signs of disease activity. Treatment-related serious adverse events were lymphopenia (N = 21), bradycardia (N = 19), elevated transaminases (N = 9) and macular oedema (N = 9).Conclusions: The effectiveness and tolerability of fingolimod in everyday clinical practice are consistent with findings of previous phase III studies. Our study highlights the utility of fingolimod for the long-term management of patients with multiple sclerosis