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

Simulation des variations de stock d'eau et mesures de gravimétrie absolue à Nalohou, Bénin

Dans le cadre du projet ANR GHYRAF (Gravimétrie et HYdRologie en Afrique), trois années de mesures de gravimétrie absolue ont été réalisées en zone de mousson ouest africaine au Bénin. Ces données sont comparées aux variations de stocks d'eau simulées grâce à un modèle numérique contraint par un suivi hydrologique (Sonde à neutrons, piézométrie) et dont la structure est basée sur des données géophysiques (résistivité EM et OC, RMP). La bonne concordance obtenue permet d'évaluer la zone de sensibilité du gravimètre, et d'estimer les contributions relatives de la zone non saturée et de la nappe phréatique dans la variation totale de stock. On montre que la gravimétrie absolue est une source de données indépendantes permettant de contraindre le modèle hydrologique. (Résumé d'auteur

Three-dimensional magnetic resonance imaging for groundwater

International audienceThe surface nuclear magnetic resonance method (SNMR) is an established geophysical tool routinely used for investigating one-dimensional (1D) and sometimes 2D subsurface water-saturated formations. We have expanded the tool by developing a 3D application. 3D-SNMR is a large-scale method that allows magnetic resonance imaging of groundwater down to about 80 m. Similar to most surface geophysical methods, 3D-SNMR has limited resolution, but it is effective for investigating water-saturated geological formations larger than several tens of meters. Because the performance of the method depends on variable survey conditions, we cannot estimate it in general. For demonstration purposes, we present an example of numerical modeling under fixed conditions. Results show that under certain conditions it is possible to detect a water volume as small as 500 m(3) and the detection threshold depends on the ambient electromagnetic noise magnitude and on the location of the target volume relative to the SNMR loops. The 3D-SNMR method was used to investigate accumulated water within the Tete Rousse glacier (French Alps). Inversion of the field measurements made it possible to locate the principal reservoir in the central part of the glacier and estimate the volume of accumulated water. These results were verified by 20 boreholes installed after the 3D-SNMR results were obtained and by pumping water out of the glacier. Very good correspondence between the 3D-SNMR and borehole results was observed

L'expérience Ghyraf au Bénin : première comparaison entre suivi gravimétrique absolu et variation de stock hydrique

Sur le site soudanien du SO Amma-Catch, le projet ANR GHYRAF effectue des mesures trimestrielles de gravimétrie absolue depuis 2008. Ces mesures sont comparées à des variations gravimétriques simulées à partir des données hydrologiques (piézométrie, sonde à neutrons) appliquées à un modèle homogène du milieu souterrain. Les mesures gravimétriques sont cohérentes avec les observations hydrologiques et suffisamment précises pour assurer un suivi de la variabilité interannuelle des stocks ou une estimation de l'évapotranspiration. (Résumé d'auteur

Interactive comment on “Monitoring water accumulation in a glacier using magnetic resonance imaging” by A. Legchenko et al.

Tête Rousse is a small polythermal glacier located in the Mont Blanc area (French Alps) at an altitude of 3100 to 3300 m. In 1892, an outburst flood from this glacier released about 200 000 m3 of water mixed with ice, causing much damage. A new accumulation of melt water in the glacier was not excluded. The uncertainty related to such glacier conditions initiated an extensive geophysical study for evaluating the hazard. Using three-dimensional surface nuclear magnetic resonance imaging (3-D-SNMR), we showed that the temperate part of the Tête Rousse glacier contains two separate water-filled caverns (central and upper caverns). In 2009, the central cavern contained about 55 000 m3 of water. Since 2010, the cavern is drained every year. We monitored the changes caused by this pumping in the water distribution within the glacier body. Twice a year, we carried out magnetic resonance imaging of the entire glacier and estimated the volume of water accumulated in the central cavern. Our results show changes in cavern geometry and recharge rate: in two years, the central cavern lost about 73% of its initial volume, but 65% was lost in one year after the first pumping. We also observed that, after being drained, the cavern was recharged at an average rate of 20 to 25 m3 d−1 during the winter months and 120 to 180 m3 d−1 in summer. These observations illustrate how ice, water and air may refill englacial volume being emptied by artificial draining. Comparison of the 3-D-SNMR results with those obtained by drilling and pumping showed a very good correspondence, confirming the high reliability of 3-D-SNMR imaging

Aerosol retrieval experiments in the ESA Aerosol_cci project

Within the ESA Climate Change Initiative (CCI) project Aerosol_cci (2010–2013), algorithms for the production of long-term total column aerosol optical depth (AOD) datasets from European Earth Observation sensors are developed. Starting with eight existing pre-cursor algorithms three analysis steps are conducted to improve and qualify the algorithms: (1) a series of experiments applied to one month of global data to understand several major sensitivities to assumptions needed due to the ill-posed nature of the underlying inversion problem, (2) a round robin exercise of "best" versions of each of these algorithms (defined using the step 1 outcome) applied to four months of global data to identify mature algorithms, and (3) a comprehensive validation exercise applied to one complete year of global data produced by the algorithms selected as mature based on the round robin exercise. The algorithms tested included four using AATSR, three using MERIS and one using PARASOL. This paper summarizes the first step. Three experiments were conducted to assess the potential impact of major assumptions in the various aerosol retrieval algorithms. In the first experiment a common set of four aerosol components was used to provide all algorithms with the same assumptions. The second experiment introduced an aerosol property climatology, derived from a combination of model and sun photometer observations, as a priori information in the retrievals on the occurrence of the common aerosol components. The third experiment assessed the impact of using a common nadir cloud mask for AATSR and MERIS algorithms in order to characterize the sensitivity to remaining cloud contamination in the retrievals against the baseline dataset versions. The impact of the algorithm changes was assessed for one month (September 2008) of data: qualitatively by inspection of monthly mean AOD maps and quantitatively by comparing daily gridded satellite data against daily averaged AERONET sun photometer observations for the different versions of each algorithm globally (land and coastal) and for three regions with different aerosol regimes. The analysis allowed for an assessment of sensitivities of all algorithms, which helped define the best algorithm versions for the subsequent round robin exercise; all algorithms (except for MERIS) showed some, in parts significant, improvement. In particular, using common aerosol components and partly also a priori aerosol-type climatology is beneficial. On the other hand the use of an AATSR-based common cloud mask meant a clear improvement (though with significant reduction of coverage) for the MERIS standard product, but not for the algorithms using AATSR. It is noted that all these observations are mostly consistent for all five analyses (global land, global coastal, three regional), which can be understood well, since the set of aerosol components defined in Sect. 3.1 was explicitly designed to cover different global aerosol regimes (with low and high absorption fine mode, sea salt and dust)