Cartographie AMT du biseau salé sur le flanc sud du Piton de la Fournaise (Ile de la Réunion)

Dans le secteur du Baril, situé sur la planèze sud du volcan de la Fournaise, l'interprétation unidimensionnelle (1-D) de 34 sondages AMT montre une séquence à trois terrains, avec disparition du deuxième dans la zone côtière. Cette séquence comprend un horizon supérieur résistant (> 1000 Ohm.m), un horizon intermédiaire plus conducteur (100 à 600 Ohm.m), et un substratum très conducteur (< 10 Ohm.m). Les résistivités obtenues permettent d'assimiler ce substratum au biseau salé. Les variations importantes dans la topographie du toit du biseau salé apportent des données nouvelles concernant les phénomènes de pénétration saline dans les aquifères volcaniques et, indirectement, une meilleure connaissance de la géométrie et du fonctionnement de l'aquifère sus-jacent. (Résumé d'auteur

Dynamic recycling of gaseous elemental mercury in the boundary layer of the Antarctic Plateau

International audienceGaseous elemental mercury (Hg0) was investigated in the troposphere and in the interstitial air extracted from the snow at Dome Concordia station (alt. 3320 m) on the Antarctic Plateau during January 2009. Measurements and modeling studies showed evidence of a very dynamic and daily cycling of Hg0 inside the mixing layer with a range of values from 0.2 ng m−3 up to 2.3 ng m−3. During low solar irradiation periods, fast Hg0 oxidation processes in a confined layer were suspected. Unexpectedly high Hg0 concentrations for such a remote place were measured under higher solar irradiation due to snow photochemistry. We suggest that a daily cycling of reemission/oxidation occurs during summer within the mixing layer at Dome Concordia. Hg0 concentrations showed a negative correlation with ozone mixing ratios, which contrasts with atmospheric mer- cury depletion events observed during the Arctic spring. Un- like previous Antarctic studies, we think that atmospheric Hg0 removal may not be the result of advection processes. The daily and dramatic Hg0 losses could be a consequence of surface or snow induced oxidation pathways. It remains however unclear whether halogens are involved. The cycling of other oxidants should be investigated together with Hg species in order to clarify the complex reactivity on the Antarctic plateau

Debris avalanches on the western flank of Piton des Neiges shield volcano (Reunion Island)

Les faciès bréchiques reconnus à terre dans la région littorale de Saint-Gilles, sur le flanc ouest du volcan Piton des Neiges, sont clairement identifiés comme des dépôts d’avalanche de débris. Leur étude pétrographique, texturale et structurale permet de montrer, grâce à la présence de coulées autochtones interstratifiées, qu’au moins quatre glissements successifs se sont produits. Le premier est de grande ampleur puisqu’il affecte la zone centrale hydrothermalisée du volcan. Il est proposé que ce premier glissement, favorisé par la présence de matériaux ayant subi une importante altération hydrothermale, crée une zone d’instabilité qui influe sur le comportement ultérieur du Piton des Neiges. Dans ce même secteur, les glissements postérieurs concernent uniquement des domaines plus superficiels, mettant en jeu des matériaux non hydrothermalisés et vacuolaires. La bréchification des formations, et en particulier la désagrégation progressive des “blocs” durant le transport de l’avalanche de débris, implique une vitesse de progression élevée. Dans la zone du Cap La Houssaye, l’abrasion et la striation des formations laviques situées sous les unités bréchiques, ainsi que les phénomènes de tassement observés, sont interprétés comme des figures d’arrêt de l’avalanche

Electromagnetic investigations of aquifers in the Grand Brulé coastal area of Piton de la Fournaise volcano, Reunion island

A geophysical survey was carried out on the eastern flank of Piton de la Fournaise volcano in the Grand Brulé area using combined audiomagnetotelluric (AMT) and time domain electromagnetic (TDEM) techniques. The aims of the present study were firstly to define the shallow geoelectrical structure, and subsequently to locate large bodies of ground water with anomalous electrical properties. Most of the AMT sites are isotropic across the entire observational bandwidth (7500-10 Hz), except for a few sites where static-shift effects are present. Joint one-dimensional interpretation of the TDEM and AMT data indicates basically three-layered structures with the following resistivity zones, progressing from the surface downward : a resistivity unsatured surface layer of subaerial basaltic flows with a 2000 to 10,000 ohm.m resistivity, its base being approximately at mean sea leval, an intermediate-resistivity layer (100-250 ohm.m) 50-300 m thick, interpreted as being fresh-water-saturated basaltic formations, and a low-resistivity layer of less than 15 ohm.m probably due to more saline water at depth and/or higher clay content in crushed materials. In the center of the study area, the results have also shown the presence of a prominent boundary dividing the area into a southern zone where the aquifer sequence is thin (60-100 m) and a northern zone corresponding to a relatively thick aquifer (> 150 m). Geoelectrical models are compared with the available geological information given by two geothermic exploration drillings. The coastal drill hole gives a crucial information, i.e. the thick intermediate-resistivity layer in the northern part of Grand Brulé can be related to water-saturated alluvium deposits down to 200 m below sea level. AMT and TDEM soundings have revealed a major paleoalluvial drainage system buried under recent lava flows... (D'après résumé d'auteur

CSAMT mapping of a deep hydrogeological structure in Mauritius island

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Electromagnetic mapping of subsurface formations in the lower northeast Rift zone of Piton de la Fournaise volcano : geological and hydrogeological implications

Time-domain electromagnetic (TDEM) and audiomagnetotelluric (AMT) surveys were conducted on the lower northeast rift zone of Piton de la Fournaise volcano, as a part of a groundwater exploration project. The objective of the study was twofold : to evaluate the possibility of mapping of volcanic medium in areas where little is known about the subsurface geology and to infer shallow geological structure from the electrical interpretation, and to identigy formations that may present good aquifers, and subsequently to estimae the relationship between groundwater resources and geological structures. Data collecte at 35 locations were interpreted with one-dimensional layered models, which were then pieced together along survey lines to make electrical cross sections. From the surface down, the cross sections have revealed taht the rift zone consists of subhorizontal layers having resistivities ranging from greater than several hundred ohm meters to less than 8 ohm-m at moderate depths. The study suggests that the investigated area can be broadly divided into two hydrogeological sections. The southern section, bounded by the Bois Blanc cliff, is consistent with a simple coastal groundwater system, where the freshwater lens rests on the saltwater. In the northern section, the saltwater wedge appears to be disturbed by the presence of clayey, poorly permeable basement at depths close to sea level. The aquifer boundaries appear to be well defined and delineated. The hydrogeological features of the study area are found to be mainly dependent on the tectonic evolution of the rift zone. (Résumé d'auteur

TDEM survey in heterogeneous volcanic aquifers: correlation between basic one dimensional models and hydrogeological data at 15 borehole test sites of Reunion Island

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