Streaming potential dependence on water-content in Fontainebleau sand

The final version is available on www.blackwell-synergy.comInternational audienceThe electrokinetic potential results from the coupling between the water flow and the electrical current because of the presence of ions within water. The electrokinetic coupling is well described in fluid-saturated media, however its behaviour under unsaturated flow conditions is still discussed. We propose here an experimental approach to investigate streaming potential variations in sand at unsaturated conditions. We present for the first time continuous records of the electrokinetic coefficient as a function of water content. Two drainage experiments have been performed within a column filled with a clean sand. Streaming potential measurements are combined with water pressure and water content measurements every 10 centimeters along the column. In order to model hydrodymanics during the experiments, we solve Richards equation coupled with an inverse problem to estimate the hydraulic parameters of the constitutive relations between hydraulic conductivity, water pressure and water content. The electrokinetic coefficient $C$ shows a more complex behaviour for unsaturated conditions than it was previously reported and cannot be fitted by the existing models. The electrokinetic coefficient increases first when water saturation decreases from 100\% to about 65\% - 80\%, and then decreases as the water saturation decreases, whereas all previous works described a monotone decrease of the normalized electrokinetic coupling as water saturation decreases. We delimited two water saturation domains, and deduced two different empirical laws describing the evolution of the electrokinetic coupling for unsaturated conditions. Moreover we introduce the concept of the electrokinetic residual saturation $S_w^{r,ek}$, which allows us to propose a new model derived from the approach of the relative permeability used in hydrodynamics

Reply to comment by A. Revil and N. Linde on 'Streaming potential dependence on water-content in Fontainebleau sand'

International audienceRevil and Linde recently commented our paper concerning streaming potential (SP) measurements in unsaturated sand during drainage experiments. This comment suggests that the approach used to infer SP coefficients was inappropriate for unsaturated conditions, and therefore yielded wrong conclusions and 'unphysical' results regarding the behaviour of the relative SP coefficient. This reply argues that even if in Allègre et al. we neglected some secondary electrokinetic sources, the resulting conclusions are still representative of the behaviour of the true SP coefficient, and that the remarks of Revil & Linde arose from a misunderstanding of the drainage experiment conditions. We also find support for our results from a comparison between our observations and previous experimental studies

Saturation dependence of the streaming potential coefficient

Observations of streaming potential for unsaturated conditions do not always show the same trend de10 pending on the hydrodynamic conditions and because of a lake of coherency between the data processing procedures. We combine the data from three studies published in the literature, acquired during non-steady state drainage experiments, and apply the same processing steps. We model the hydrodynamic behaviour of these experiments to confirm that they experienced different flow dynamics. We argue that the raw SP data should not be corrected unless a clear drift of the electrodes stability is observed. The combined hydrody namic behaviour and the streaming potential response show that (a) the observations of one of the experiment (exp #1) are associated to a limited range of water saturation (0.85-1). The corresponding signals could therefore be fairly modelled assuming no saturation dependence of the SPC whatsoever; (b) the observations of exp #3 led to a SPC that can be larger than its value at saturation; (c) the observations of the exp #2 show a non-monotonous behaviour of the SPC as saturation decreases. The underlying physics of a non-monotonous SPC is related to water/air interfaces as shown by the results of the lattice Boltzmann numerical simulations. The main contribution to the SPC behaviour comes from the charged water/air interfaces and depends on the dynamic state of moving or entrapped bubbles. We finally describe the consequences of such a behaviour on the seismoelectric conversions for unsaturated conditions.Facultad de Ciencias Astronómicas y Geofísica

Couplages électrocinétiques en milieux poreux non-saturés

Cette thèse a pour objet l étude des couplages électrocinétiques en milieux non-saturés : d une part les phénomènes d électrofiltration liés à l écoulement de l eau, et d autre part les couplages sismo-électriques, c est à dire le champ électrique associé à la propagation d une onde sismique sur le terrain. Ces couplages existent en raison du mouvement de particules chargées présentes dans l eau. Le premier axe développé est une approche expérimentale. Des différences de potentiels d électrofiltration ont été mesurées lors d expériences de drainage réalisées dans une colonne de sable. Ces mesures combinées aux mesures des conditions hydrodynamiques ont mené aux premiers enregistrements continus de coefficients d électrofiltration C en fonction de la saturation Sw. Les valeurs de coefficients normalisées ne varient pas de fa con monotone avec la saturation Sw. En effet, C augmente lorsque Sw diminue jusqu à atteindre un maximum pour Sw égale à 65-85%, puis diminue avec la saturation. En outre, les valeurs expérimentales semblent pouvoir dépasser d un facteur 100 la valeur mesurée à saturation. Ce comportement a été observé de façon similaire lors de trois expériences de drainage. La modélisation de ces signaux constitue le deuxième axe de travail. Une nouvelle expression, intégrant une contribution dynamique liée aux variations temporelles de pression, est proposée. Les résultats illustrent la nécessité d intégrer cette contribution dynamique pour modéliser les signaux d une des expériences, particulièrement lorsque C augmente pour Sw =[0.65-0.85 ; 1]. La dernière expérience tend à montrer que les conditions d écoulement, et plus précisément le flux hydrique, pourraient jouer un rôle important dans le comportement de C. Le dernier axe développé est une approche de terrain. Les amplitudes de signaux sismo-électriques ont été étudiées en non-saturé, par la mesure d un champ électrique Ex et d un déplacement sismique ux, et comparées au calcul de fonctions de transfert Ex/üx. Une image de saturation du sol a été déduite de l interprétation conjointe de données radar et de résistivité électrique. Les amplitudes sismo-électriques liées aux arrivées des ondes P et des ondes de surface, traitées indépendamment, ont pu être comparées à la teneur en eau, puis à des mesures de résistances électriques. Les résultats suggèrent que ces amplitudes sont plus faibles en milieu non-saturé. De plus, le traitement des ondes de volume et des ondes de surface a montré que ces dernières doivent être considérées indépendamment.The aim of this PhD thesis is to study electrokinetics for unsaturated conditions, both through streaming potential (SP) and seismo-electrics conversion. Electrokinetic coupling results from the motion of charges inside the water. Using an experimental approach, (SP) have been measured during drainage experiments in sand. These measurements were combined with hydrodynamics measurements and led to the first continuous recordings of streaming potential coefficient C as a function of water saturation Sw. The normalized streaming potential coefficient was found to be not monotoneously dependent on water saturation. Values of C increases first when Swdecreases until a maximum value for Sw around 65-85 %, and then decreases for decreasing saturation. In addition, experimental electrokinetic coefficient seems to be larger up to two orders of magnitude than classical values measured for saturated conditions. A similar behaviour was observed for three drainage experiments. The second part of this manuscript deals with the modelling of this unexpected behaviour. A new model, including a dynamic component linked to pressure temporal variations, is proposed. Modelling results show that this component is necessary to explain electrokinetic coefficient data of the first experiment, particularly for Sw =[0.65-0.85 ; 1]. Processing of the last experiment suggests that hydrodynamic conditions, and particularly the instant fluid flow magnitude, could be involved in the behaviour of C. The third part of this work deals with a field approach using seismoelectrics. Seismoelectric amplitudes have been studied for unsaturated conditions computing transfert functions defined as Ex/üx. First, the soil water saturation map has been deduced from the joint processing of GPR and ERT data. Thus, seismoelectric amplitudes deduced from P wave and surface waves independently, have been compared to water-content and measured electrical resitance. Results suggest that these amplitudes are smaller in unsaturated soils than for saturated conditions. In addition, the independent processing of P wave and surface waves shows that surface waves creates some artefacts in the computed transfert functions, and consequently have to be considered apart.STRASBOURG-EOST (674822249) / SudocSudocFranceF

Streaming potential in volcanic rocks from Mount Pelée

International audienceStreaming potential and electric conductivity have been measured in a laboratory on 11 consolidated samples coming from five deposits of the different evolutionary stages of Mount Pelde volcano. The streaming potential coupling coefficient ranges from-35 to-4905 mV MPa-1 and increases with increasing permeability. This increase is mainly due to the dependency of rock effective conductivity with permeability. The permeability of the samples varies from 0.146x10-12 to 34x10-12 m 2. The zeta potential, at pH-7 and water conductivity of 2.1x10-4 S m-l• is relatively small for the majority of the samples. It ranges from-4 to-19 reV. According to water conductivity analysis on Mount Pel•e, streaming potential coupling coetficients of-25 to-406 mV MPa-1 can be expected for this volcano

Clinical efficacy and cost-effectiveness of endobronchial ultrasound-guided transbronchial needle aspiration for preoperative staging of non-small–cell lung cancer: Results of a French prospective multicenter trial (EVIEPEB)

International audienceThis two-step study evaluated the cost-effectiveness of endobronchial ultrasound-guided transbronchial needle aspiration (EBUS-TBNA) for presurgery staging of non-small cell lung cancer (NSCLC) in France (EVIEPEB; ClinicalTrial.gov identifier NCT00960271). Step 1 consisted of a high-benchmark EBUS-TBNA-training program in participating hospital centers. Step 2 was a prospective, national, multicenter study on patients with confirmed or suspected NSCLC and an indication for mediastinal staging with at least one lymph node > 1 cm in diameter. Patients with negative or uninformative EBUS-TBNA and positron-emission tomography-positive or -negative nodes, respectively, underwent either mediastinoscopy or surgery. Direct costs related to final diagnosis of node status were prospectively recorded. Sixteen of 22 participating centers were certified by the EBUS-TBNA-training program and enrolled 163 patients in Step 2. EBUS-TBNA was informative for 149 (91%) patients (75 malignant, 74 non-malignant) and uninformative for 14 (9%). Mediastinoscopy was avoided for 80% of the patients. With a 52% malignant-node rate, EBUS-TBNA positive- and negative-predictive values, respectively, were 100% and 90%. EBUS-TBNA was cost-effective, with expected savings of €1,450 per patient, and would have remained cost-effective even if all EBUS-TBNAs had been performed under general anesthesia or the cost of the procedure had been 30% higher (expected cost-saving of €994 and €1,427 per patient, respectively). After EBUS-TBNA training and certification of participating centers, the results of this prospective multicenter study confirmed EBUS-TBNA cost-effectiveness for NSCLC staging

Transport properties of pyroclastic rocks from Montagne Pelée volcano (Martinique, Lesser Antilles)

International audienceThe hydraulic and electrical properties of pyroclastic rocks have been investigated in laboratory on a representative sampling of Montagne Pelée (Martinique, France) deposits with renewed interest in geophysical applications. This sampling covers all the lithologic units of this volcano: lava dome and lava flows, pumices from ash-and-pumice fall and flow deposits, lava blocks from block-and-ash flow and Peléean '' nuées ardentes'' deposits, scoriae from scoria flow deposits. The connected porosity varies over a wide range from 3 to 62%. The unconnected porosity is important only on pumices where it can reach 15%. The permeability covers more than 5 orders of magnitude, ranging from 10 À16 to 35 Â 10 À12 m 2. The higher values are obtained on lava blocks and the scoriae, even if these rocks are less porous than the pumices. The formation factor ranges from 7 to 1139. The transport properties of these rocks are slightly correlated with porosity. This indicates that these properties are not only controlled by the connected porosity. To connect the transport properties to the textural characteristics of the pore network of pyroclastic rocks, different models, based on geometrical considerations or percolation theory, were tested. The pore access radius distribution and the tortuosity control the transport properties of pyroclastic rocks. Consequently, the models (electric and hydraulic) based on the concept of percolation (e.g., the models of Katz and Thompson), apply better than the equivalent channel model of Kozeny-Carman. In addition, the difference in transport properties observed on lava blocks and pumices confirms that the mechanisms of degassing and vesiculation are different for these two types of rock