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International audience; It has been previously established that Western Europe underwent an important episode of lateritic weathering during early cretaceous times. In this context, the red clays and ferrugineous pisoids deposits occurring in the graben of Saint-Maixent-l'Ecole (Poitou saddle) may represent remnants of lateritic formations associated with this paleoweathering episode. However, their exact age is unknown, and since they occur on top of the Oligocene limestones filling the graben, it cannot be ruled out that these lateritic soils formed more recently. In order to better constrain the age of these lateritic formations, goethite pisoids were sampled in the Fouilloux forest and investigated for Rb-Sr, U-Th and delta O-delta D isotopic systems. Rb and Sr have been measured on bulk pisoids, and also on leachates and residues from leaching experiments carried out in the attempt to isolate a radiogenic component. No reliable Rb-Sr isochron age could be derived, although the isotopic data would favor an early Cretaceous age for these lateritic formations. delta O-18-delta D measurements have been done on bulk goethite pisoids, and also on sub-fragments. The results indicate that the pisoids likely formed under a warm and/or arid climate. No argument for a late of post - oligocene age can be found from the oxygen and hydrogen isotope data. Finally, U-Th analyses have been carried on bulk samples of goethites, and on sub-fragments as well. U-234/U-238 activity ratios are typically slightly higher than the equilibrium value, which indicates that goethites have recently evolved as open systems (if not still open). Hence, pedogenesis processes are still active from a geochemical point of view in these lateritic soils. This study has not allowed to derive a reliable age for these lateritic soils. However, when the results from the different isotopic systems investigated here are considered together, the best interpretation is that these lateritic soils developed most probably over the kimmeridgian limestones along the edge of the Saint-Maixent-l'Ecole graben, and were reworked and redeposited at the bottom of the graben later on.; -Le début du Crétacé a été marqué en Europe occidentale par le développement de paléoaltérations de type latéritique. Les argiles rouges à cuirasse ferrugineuse pisolitique du graben de Saint-Maixent-l'École (seuil du Poitou) peuvent constituer un exemple-témoin de ce type d'altération. Toutefois, leur âge reste inconnu et leur position stratigraphique, au-dessus des calcaires oligocènes du fond du graben, ne permet pas d'exclure un âge de formation beaucoup plus récent. Des pisolites de goethite provenant de ces sols latéritiques ont été échantillonnés dans la forêt de Fouilloux et ont fait l'objet d'analyses isotopiques Rb-Sr, U-Th et d 18 OdD , afin de tenter de lever l'indétermination sur l'âge de leur formation. Pour Rb-Sr, outre les analyses sur pisolites totaux, des expériences de lessivages ménagés ont été effectuées afin d'essayer d'isoler un composant radiogénique. Les résultats obtenus n'ont pas permis de donner un âge fiable par la méthode des isochrones, même si les données tendraient plutôt à faire remonter l'épisode d'altération au début du Crétacé. Les analyses d 18 OdD ont été effectuées sur des pisolites de goethite totaux, ainsi que sur des fragments sous-échantillonnés. Les résultats plaident en faveur de conditions de formation en zone climatique chaude et/ou aride. Aucun argument en faveur d'un âge fini ou post-oligocène ne peut être tiré de l'étude des isotopes de l'oxygène et de l'hydrogène. Enfin, les analyses U-Th ont été effectuées sur des goethites totales ainsi que sur des fragments sous-échantillonnés. Les rapports d'activités 234 U/ 238 U sont tous légèrement supérieurs à l'équilibre séculaire, indiquant que les goethites évoluent actuellement et/ou ont évolué récemment en système ouvert. Ainsi, les sols latéritiques apparaissent toujours comme géochimiquement actifs. Si cette étude n'a pas permis d'apporter une réponse claire sur l'âge des sols latéritiques, l'hypothèse la plus probable au vu des données isotopiques est que ces sols latéritiques se sont développés au-dessus des calcaires kimméridgiens sur les bordures du graben de Saint-Maixent-l'École, puis remaniés et finalement resédimentés au fond du graben.

International audience; Lateritic planation surfaces cover a predominant part of the West African landscape. The geomorphological framework and its evolution since Late Jurassic has been described as a succession of 3 bauxitic surfaces followed by an alumino-ferruginous surface, followed itself by 3 ferruginous duricrustsurfaces attributed to Tertiary age. There is little time constraint as conventional geological or geochemical methods cannot be applied to lateritic weathering profiles. This paper presents the first ever relative paleomagnetic dating results obtained from a weathering profile in Africa. The studied profile was sampled at the Léro gold mine in the North of Guinea. A full 100 m thick weathering profile has been sampled from the weathering front up to the surface. The saprolite in Léro recorded good quality paleosignals carried mostly by hematite. Our results confirm the vertical model oflateritic weathering profile development, with the oldest levels in the upper part of the profile and the most recent ones at the bottom. Surprisingly, the bottom part of the profile close to the weathering front, did not recorded any recent signal, suggesting that the deepening of the weathering front has greatly slowed down or was stopped for a long period. The bottom part that represents a layer of about 50 m thick shows likely a Miocene age with a mean pole falling between the 10 to 20 Ma poles of the African APWP. The upper part of the saprolite shows a Late Cretaceous age of 70 Ma. Finally, the top part of the saprolite, close to the ferruginous duricrust, indicates a recent magnetization phase that could be related to an ongoing duricrust forming process. Based on the landsurface correlation, the predominant alumino-ferruginous duricrust surface in the region may probably be attributed to a Late Cretaceous paleosurface rather than Oligocene as supposed from the literature. This leads to reconsider the general geomorphological framework of the West African region.

International audience; The Pyrenean Range results from the collision between Eurasia and Iberia plates that started during the Santonian. The Pyrenean orogeny is considered to end in Early Miocene times, since from that period onwards, Iberia shows no more differential motion with respect to the rest of Europe and becomes part of the Eurasian plate. This coincided with the slowing down of African-Eurasian convergence. Nevertheless, the Pyrenean relief still displays an unexpected strong relief and a combination of various geological methods highlights the occurrence of local Upper Miocene to present-day uplifts in the Pyrenees (low temperature thermochronological dating, sedimentology, karstology, palynology, palaeoaltimetry). While it has already been studied extensively in the Eastern Pyrenees, fewer works refer to the Western Pyrenean uplift. Depending on the method considered, the timing and the vertical amplitude of this uplift differs.We are initiating a geomorphological study of the incisions in the Western Pyrenees in order to constrain this post-shortening uplift. We focused on the Aspe Valley, where Miocene highly elevated – low relief erosion paleosurfaces observed as relicts on top of the interfluves have been strongly dissected. This dissection was achieved in several steps, including a low erosion episode, followed by an acceleration of erosion which could be attributed to the Late Miocene to actual Western Pyrenean uplift event. The sharp incisions are highlighted by erosion triangular facets which cannot be attributed to glacial erosion as they occur beyond the last glacial deposits. We attempt to reconstitute the landscape prior to this late incision in order to quantify the subsequent erosion and to try to link it with foreland deposits.Evidence of a Late Miocene to present-day uplift is well described in the Eastern Pyrenees and at a larger scale, in the Central Massif, the peri-alpine area and in northern and central Iberia thus suggesting an uplift at a lithospheric scale. However, discrepancies in the uplift rates indicated by already published data are found at a kilometric scale in the Western Pyrenees, which leads us to consider a possible contribution of both local and regional processes to explain the recent incisions in the Aspe Valley.This work is funded and carried out in the framework of the BRGM-TOTAL project Source-to- Sink.

International audience; The interest in surface formations or "regoliths" is generating a growing demand, not only in terms of geological knowledge (cartography, lithology, origin, properties, geological connections) but also in terms of issues for the territories (planning, risks, resources management) and therefore economic interests. In 2009, the BRGM carried out a first survey of the knowledge of the Regolith in France, at a scale of 1: 1 000 000, both for allochthonous deposits (continental deposits linked to the transport process) and for land. autochthonous deposits (weathering patterns). This work, continued since 2013, highlighted a strong heterogeneity in the quality of available geological information and targets the work to be done, especially in the field. Geometric modeling of the geological units constituting the Regolith is an essential tool to advance the knowledge of the spatial distribution of formations. The application of this tool, closely associated with known geological knowledge, constitutes a first approach and a guide for the development of a potential acquisition strategy on the field. The methodology implemented in the framework of the modeling of allochthonous and autochthonous regolith formations, carried out under GDM-Multilayer, at the level of each department area, then assembled at the scale of a region (from available geological maps data and drilling data from the Underground Data Bank, BRGM). This work was done initially at the scale of the Brittany region and it aims to expand to the entire Armorican basement (Fig. 1). Interpolated data maps (roof and thickness) also highlight over-or under-valued results in certain geographic areas, which raises questions about both the relevance of the model input data and the need for local control in the field. The results make it possible to propose new products in terms of tracks for predictive geosciences, such as lithological map and to offer support for over fields of geosciences (hydrogeological units for example). http://www.brgm.fr/content/regolithe-geologie-tres-proche-sous-sol-amenagement

International audience; The characterization of the critical zone along depth profiles remains a major scientific issue for understandingand modelling the response of continental surfaces to climatic, tectonic and anthropogenic forcings. Besides char-acterization it requires the modelling of the water circulations within the substratum of the critical zone. A seriesof boreholes drilled along the north and the south slopes of the Strengbach watershed makes it possible to charac-terize the critical zone to depths of100 to 150 m within this critical zone observatory. In this study we attemptto combine mineralogical and petrological observations of the cores recovered through the drilling with chemicaldata of waters collected in each of these wells and hydro-geophysical data in order to characterize processes ofwater-rock interactions, visualize the water arrivals within the boreholes and bring new information on the deepwater circulations within the watershed.Mineralogical, petrological and hydrogeophysical data suggest that deepwater circulation in the watershed likelyoccurs along fractures, concentrated in relatively narrow areas, several centimeters wide, interspersed with areaswhere the granite is much less fractured. This points to the occurrence of deep waters circulating in a networkof more or less independent conduits, which could extend over several tens to hundreds of meters deep. The hy-drochemical data from the boreholes, show contrasting characteristics for surface waters collected at 10 to 15 mdepth and the deeper waters collected between 50 to 80m depth; the surface waters are very similar to those of thespring waters collected in the watershed (Pierret et al., 2014), and the deeper waters collected between 50 to 80mdepth. The residence times of the circulating waters are also very variable, with ages of up to a few months forsurface and subsurface waters and ages exceeding several decades for the deep waters. These differences suggestthat the subsurface circulation systems are quite different from the deeper circulation ones. They also point to theimportance to focus future studies on deep-water circulations in order to properly characterize the functioning ofthe critical zone in watersheds, especially in mountainous areas, such as the Strengbach watershed

International audience; Perched paleosurfaces of the Central and Eastern Pyrenees have been first described bygeomorphologists for a long time (Birot, 1937; Lagasquie, 1982; Calvet, 1996) and more recentlyby geologists (Bosch et al., 2015; Monod et al., 2015; Bosch, in progress). These studiesare of peculiar interest since they provide robust constraints for the timing and mechanisms ofrelief creation in a collision belt.Pyrenean paleosurfaces have been recognized over large areas in the Axial Zone betweenthe Aure valley to the West and the Cerdagne and Capcir grabens to the East. The actual elevationof theses surfaces varies between 1000 and 2500 m above sea level. In Cerdagne, the grabenfilling has been dated of Vallesian age (Middle to Late Miocene). Based on this constraint,the entire Pyrenean perched paleosurfaces have been considered of Miocene age. Since thepalynofacies of the Vallesian sediments is interpreted as a near-sea level deposit, the presentelevation of the paleosurfaces should have been acquired after the Vallesian assuming thatall paleosurfaces have nearly a Miocene age. In the vicinity of Cerdagne and Capcir grabens,the central Pyrenees paleosurface appears connected to the uppermost terraced paleosurface,suggesting an age prior to the Vallesian times.A common feature of the Pyrenean paleosurfaces is the preservation of thick lateriticweathering profiles. In the Eastern Pyrenees, the paleosurfaces display a terraced pattern withthree or more levels, the highest being likely the oldest. In the Central Pyrenees (from the AstonMassif to the East to the Neouvielle Massif to the West), The single paleosurface is apparentlyoffset along numerous faults. Field work will be conducted in order to collect structural dataallowing a detailed tectonic evolution to be reconstructed.In the Louron valley (close to the Aure valley), the central Pyrenean paleosurface is apparentlydownthrown along N160 trending faults facing the valley that define a graben-like featurewith an overall NS trend. Field work will be conducted to check the structural relationshipsof such possible features with the reverse faults and thrusts linked to the Pyrenean compression.Preliminary investigations show that fault planes here bear shallow dipping slickenslidesindicating a probable transtensional mechanism. A second set of normal faults trending N100to N130, parallel to the belt, seems to offset the central Pyrenean paleosurface. These faults areobserved in the Massif of Aston (SW of Ax les Thermes) where a detailed tectonic analysis willbe conducted.Paleosurfaces will be followed from the Aure valley (central Pyrenees) to the East in orderto decipher their origin and evolution: are they terraced surfaces or a single surface offsetalong recent faults having a general N160 and N100-130 orientation? Special attention willbe payed to the occurrence of the preserved lateritic weathering profiles that allows reconstructingthe lateral correlation of the paleosurface remnants. Finally, we plan to map thesepaleosurface remnants from the central Pyrenees to the Cantabric belt with a special attentionto possible faults that might offset some portions. An attempt to date the weathering profilesusing syn-weathering minerals (adulaires) will be conducted in the meantime. These data willcollectively help better understanding the significance of paleosurfaces in collision belts.This work is part of the project “Source to Sink”, we thank Total and BRGM

International audience; Le projet CROCO-DYL est dédié à l'évolution du trait de côte rocheux { l’échelle du Quaternaire, et en particulier à l’évolution au cours du temps du positionnement de la limite falaise/plateforme littorale rocheuse. Deux sites littoraux à caractéristiques géologiques et océanographiques différentes ont été sélectionnés sur la façade maritime française, pour permettre un comparatif de situations. Il s’agit d’une côte jeune (Manche orientale, Normandie) à hautes falaises verticales creusées dans la craie du Bassin de Paris et développant des plateformes littorales de type A (site de Mesnil-Val, Criel-Sur-Mer) et d’une côte ancienne (Atlantique, Bretagne, sud Finistère) à côte rocheuse basse, développée aux dépens de granites hercyniens du Massif Armoricain et associées à des plateformes littorales de type B (site de Penmarc’h).La méthodologie mise en oeuvre est basée sur la constitution de MNT Terre-Mer à haute définition fusionnant des données topographiques de LiDAR aérien (RGE Alti, Litto3D) à des données acoustiques sous-marines petits fonds (écho-sondeur interférométrique et sondeur multi-faisceaux) issues de plusieurs campagnes océanographiques côtières. L’analyse géomorphologique de ces MNT haute-résolution révèle une grande diversité de plateformes littorales dont l’origine est attribuée à la variabilité des processus érosifs, eux-mêmes fonction du contexte géologique et structural des sites étudiés.Le processus d’érosion à l’origine des côtes de craie de Normandie est très actif en terme de recul dans le plan horizontal et remarquable à l’échelle contemporaine du fait de grandes vitesses d’érosion (plusieurs dm/an).Les morphologies repérées sur la plateforme littorale apparaissent comme étant héritées du dernier interglaciaire, mais des datations complémentaires nécessitent d’être encore produites.

International audience; Alpine Corsica is located in the northeastern part of the Corsica Island (France). It consists of a series of metamorphosed units including (1) the meta-ophiolite unit which mainly consists in metabasalt, metagabbro and serpentinite and (2) the “Schistes Lustrés” complex formed of metasediments such as calcschists or marbles.The Alpine Corsica groundwater body is a key water source for drinking supply that represents nearly 25% of the groundwater withdrawal in Corsica. However, the geological and hydrogeological properties of these formations are poorly known and differ from those of granite. A conceptual hydrogeological model of these hard-rock aquifers based on the results of a multidisciplinary approach performed at several spatial scales, from the lithological contact (meters) to the catchment (kilometers), was developed in order to improve the knowledge of these complex aquifers.Lithology and tectonic determine the groundwater circulations inside the Alpine Corsica massif. A statistical analysis based on air-lift flow measurements performed during the drilling of sixty boreholes has been conducted. It provides a qualitative characterization of the vertical distribution of permeable zones which are, statistically, mainly located within the first 40 m from the ground surface but may reaches up to 70 – 80 m deep in schists and marbles. Distribution and location of springs, and the measurements of differential stream-flow during base-flow periods allowed identifying aquifers and estimating their hydrodynamic properties. Hydraulic conductivity of the meta-ophiolite unit is found homogeneous and estimated between 1.10-8 and 2.10-8 m/s (minimum estimates of K). For this unit the hydrogeological model of these formations is similar to the “classical” granite model. On the contrary, the hydraulic conductivities of schists and marbles show a wide variability -from 1,4.10-9 to 4,2.10-8 m/s, which reflects the heterogeneous and compartmentalized characters of these aquifer systems because of local dissolution of marbles.

International audience; Mineralization processes in ultramafic laterites are partly controlled by fluid flow regimes in these porous and permeable systems. During weathering of ultramafic rocks, exothermic chemical reactions, such as hydration of olivine, may generate shallow subsurface (< 200 m) temperatures of several tens of °C. The reaction-induced fracturing leads to further reactions and increases the permeability. For more than 20 Myr, the peridotite massifs of New Caledonia have undergone intense weathering, and the observable undulations along the weathering front, attest to a corrugated bedrock topography. Combined together, the excess heat (up to 70-90°C) and high permeability (10-14 to 9  10-13 m²) might have potentially triggered hydrothermal convection, as confirmed by Rayleigh number estimates. This was numerically modeled by accounting for temperature-dependent fluid properties, and for time-dependent and spatially varying parameters simulating the deepening of the weathering front. Modeling the transient evolution of the thermal and flow velocity fields over 10 Myr reveals that hydrothermal convection can develop in the New Caledonian laterites, even on sloped surfaces where topography-driven flow prevails. Convective cells develop above the weathering front, and the models reveal two-dimensional corrugations below which weathering is no longer efficient.

International audience; Une des étapes du projet « SILURES Bretagne » (Système d’Information pour la Localisation et l’Utilisation des Ressources en Eau Souterraine), mené de 2002 à 2008, a consisté à modéliser les hydrogrammes des rivières bretonnes afin de connaître la participation des eaux souterraines au débit de ces rivières. En s’appuyant sur le schéma conceptuel des aquifères de socle [1], 70 modélisations globales pluie-débit ont été réalisées au pas de temps journalier avec le logiciel BRGM Gardénia [2].Sur 12 des 70 bassins versants étudiés en zone de socle (l’Elorn, l’Aulne, le Yar, l’Horn, le Coët-Dan, le Dourduff, la Maudouve, la Noë sèche, l’Oust, l’Yvel, le Nançon et l’Aron), les volumes d’eau souterraine ont été calculés sur les 50 premiers mètres du sous-sol (dans le cadre de 5 études menées entre 2003 et 2014). A partir d’une modélisation des épaisseurs des aquifères (altérites et milieu fissuré) dans les roches constituant le sous-sol du bassin versant, et des teneurs en eau libre mesurées grâce à des sondages de Résonance Magnétique Protonique, le volume d’eau souterraine est approché [1].Connaissant le débit souterrain moyen annuel sortant d’un bassin versant et le volume d’eau souterraine, ainsi que la vitesse moyenne que met une goutte d’eau à passer du sol à la nappe [3], des temps moyens de résidence des eaux souterraines ont été calculés (temps moyen que met une goutte d’eau à s’infiltrer du sol à la nappe, puis de la nappe à la rivière). Ces temps sont très variables (de 2 à 8 ans) selon la géologie du bassin et des pluies efficaces l’alimentant.Ces temps de résidence ne correspondent pas à des temps de reconquête de la qualité de l’eau en raison des phénomènes de mélange qui se déroulent entre le sol et la nappe, dans les altérites et dans le milieu fissuré. Par contre, ce sont des temps minimums en deçà desquels il est utopique d’attendre des résultats tangibles sur la qualité des nappes et ensuite sur les cours d’eau, en réponse à une action de surface.

International audience; Le BRGM réalise et supervise des explorations géophysiques des ressources en eau en domaine de socle, et participe à des projets de recherche visant à la caractérisation de ces milieux sur des terrains d'application variés à travers le monde. Depuis les années 1990, les méthodes classiques du traîné et du sondage électrique ont été remplacées par la tomographie électrique pour une résolution optimisée des variations latérales de résistivités qui rende mieux compte des géométries discontinues des aquifères de socle. L’application des tomographies électriques utilise des équipements et des configurations de mesure, des procédés de traitement et d’inversion des données spécifiquement adaptés à ces environnements particuliers.Ces procédures de travail, leurs prérequis, leurs limites et les résultats obtenus sont illustrés par des exemples d’application sur différents sites bien documentés par des données de forage et diagraphie dans différents environnements (granite, cornéennes, méta-sédiment) en France, en Inde, en Guyane, en Uruguay et en Chine.Une fois les données acquises, analysées, qualifiées, traitées et inversées au moyen de méthodes adaptées, les résultats de la tomographie électrique de résistivité sont globalement la définition d’une succession résistant-conducteur-résistant rendant compte du profil d’altération stratiforme et des couches aquifères associés, tels que définies par Wyns et al. (2004), ainsi que l’épaississement localisé, voire l’enracinement de la couche conductrice intercalée, traduisant généralement la présence de discontinuités subverticales (fractures, filons, contacts lithologiques, …). Les performances et limites de la méthode pour distinguer, dans la couche conductrice intercalée, plusieurs niveaux imageant les parties capacitive et transmissive de l’aquifère ou le passage de zone non saturée à zone saturée sont discutées. Sur la base de ces résultats, une procédure générale d’exploration en vue de forages hydrogéologiques se dessine, ciblant d’une part les profils d’altération les plus épais pour une épaisseur maximale de l’aquifère et de la zone fissurée productive à sa base, et d’autre part les discontinuités subverticales pour bénéficier de la productivité des failles, filons et contacts ; ces dernières constituent des cibles de forage de choix, notamment lorsque l’horizon fissuré du profil d’altération stratiforme est réduit. Parallèlement, l'identification des zones les plus conductrices, permet d'éviter leur forage quand elles correspondent à des formations argilisées peu perméables. Wyns R., Baltassat J.M. , Lachassagne P., Legtchenko A., Vairon J. (2004).- Application of proton magnetic resonance soundings to groundwater reserve mapping in weathered basement rocks (Brittany, France). Bull. Soc. Géol. Fr., 175(1), 21-34.

International audience; During shallow subsurface (< 200 m depth) weathering processes, temperatures may reach several tens of °C as a result of exothermic chemical reactions, such as hydration of olivine in ultramafic rocks or chloritization of biotite in granitic rocks. These mineralogical transformations enhance mineral fracturing, and the growth of fracture networks leads to further reactions and increases the permeability. The subsequent deepening of the weathering front creates new reactions, thus self-maintaining the weathering process over several million years (Myr). For more than 20 Myr, the peridotite massifs of New Caledonia have undergone intense weathering that has produced thick lateritic weathering mantles. The observable undulations of the weathering front and the protrusions of unweathered peridotite, from several meters to several tens of meters high, attest to a corrugated bedrock topography, which may result from inhomogeneous fluid circulation patterns within the coarse, permeable and porous (30-50%) saprolite layer. Combined together, the excess heat (up to ≈ 100°C) and high permeability (10-14 to 2  10-13 m²) within lateritic weathering mantles could potentially trigger hydrothermal convection (buoyancy-driven flow). This was numerically modeled by accounting for temperature-dependent fluid density and viscosity, and for time-dependent and spatially varying parameters simulating the deepening of the weathering front. Modeling the transient evolution of the thermal and flow velocity fields over 10 Myr reveals that hydrothermal convection can be triggered in the weathering lateritic mantles of New Caledonia, even on sloped surfaces where topography-driven flow prevails. Convective cells develop above the weathering front, and the amplitudes of thermal undulations are enhanced when feedback mechanisms between permeability and temperature are accounted for. The models also allow definition of the most probable zones of mineralization and reveal two-dimensional corrugations below which weathering is no longer efficient.

National audience; Le régolithe, espace de transitionentre roche (domaine d’étude desgéologues) et sol (domaine d’étudedes pédologues), comprendgénéralement un profil d’altération(« régolithe autochtone »)éventuellement surmonté desédiments continentaux (« régolitheallochtone » : terrains alluviaux,éoliens, colluviaux, glaciaires,lacustres…). Sol et profil d’altérationsont régis par des dynamiquespropres ayant des constantes detemps différentes. Ils sont le siègede nombreuses ressourcesessentielles à la vie et doivent êtremieux compris et protégés.

International audience; « SILURES Bretagne » is a scientific program (Information system for the Localization and the Use of groundwater resources), which aims to realize decision-making tools for a policy of management of the water (better consideration of groundwater, definition of priorities in the actions of restoration quality of Breton superficial waters). One of the stages of this project consisted in modelling the hydrograms of rivers to know the contribution that groundwater makes to the flows of Breton rivers and estimate the slowness of groundwater physical environment. By basing on the conceptual diagram of hard-rocks aquifer platforms (R. Wyns and al. 2004, B. Dewandel and al. 2006), 70 global modellings rains/river flow were realized in the step of daily time with the software BRGM Gardenia.On 11 of 70 catchments studied (Elorn, Aulne, Yar, Horn, Coët-Dan, Dourduff, Maudouve, Noë sèche, Oust, Yvel and Nançon), the volumes of groundwater in the first 50 metres of basement were calculated. By modelling the thickness of the aquifer layers (regolith and fissured horizon) in the rocks that form the bed of a catchment, and the content of free water in rocks measure with Protonic Magnetic Resonance system, the volume of groundwater present in the zone can be estimated (R. Wyns and al. 2004).Knowing the amount of groundwater flow exiting a river catchment, as well as the average speed a water droplet takes to reach the water table from the ground, (according to the bibliography, this speed varies between 1.75 and 2.5 m/yr.), response times of groundwater can be estimated (the time taken for a drop of water to seep from the ground to the water table and then from the water table to the river). These times are very varied (between 2 and 8 years), depends very much on the geology of the catchment and also on the amount of rainfall they have each experienced.These times do not correspond to the amount of time taken for water quality to be recovered. This is for a mixture of reasons based around the ground, the water table, the regolith and the fissured horizon. However, these are minimum times and it is imperative to obtain tangible results on the quality of the water tables and consequently the water system, which can then be followed by action on the surface.

International audience; The hydrogeology of superficial (∼0-100 m b.g.l.) Hard Rock Aquifers (HRA; i.e. plutonic and metamorphic rocks) has so far been dominated by a few concepts considered to be relevant by a large majority of the HRA community. One of the most fundamental of these concepts is that their (secondary, fissure/fracture) permeability is either of tectonic origin or related to unloading processes. We will show that these genetic concepts are erroneous. We will demonstrate how the hydraulic conductivity of HRAs is a consequence of the (palaeo) weathering processes, with a stratiform fissured layer located immediately below the unconsolidated saprolite and, to a lesser extent, a verticalized fissured layer at the periphery of (or within) pre-existing discontinuities (veins, joints, ancient faults, lithological contacts, etc.). This result opens up large perspectives in terms of applied hydrogeology and applied geology. A specifically dedicated methodological toolkit well adapted to the operational survey, management and protection of HRAs is briefly presented.

http://home.agh.edu.pl/~iah2010/extab/ext-abstract/460-iah2010_wyns.pdf; International audience

International audience; Recent numerical experiments based on tectonically realistic formulation for the lithosphere and representative mantle rheology, have shown that plume-induced undulations exhibit temporal successions of uplift and subsidence at various wavelengths (A). From spectral (Fourier) analyses of the undulations would appear that two groups of wavelengths (200-400 km and 60-100 km) predominate. Interestingly, a spectral analysis of Europe's topography also reveals two dominant groups (B). To discriminate between tectonically-induced undulations (uni-directional deformation) and plume-induced undulations (omni-directional deformation) (C), we use a spectral analysis with a wavelet formulation (D). The European lithosphere is well-suited for this approach since it has been suggested that two mantle plumes (the Massif Central and the Eifel area) underlie Western Europe (E), where Alpine compression has folded the lithosphere over several hundreds of kms. The wavelet analysis of Europe's surface topography confirms that energy distribution of the topographic undulations outside the two main volcanic provinces is homogeneous (F), thus contrasting with the large-scale and medium-scale high-energy features that are obtained for the Massif Central and Eifel areas (G). Similar signatures are also found beneath the northern Sudetes area (H).

International audience; Pumping tests carried out in the fissured layer of a granitic hard-rock aquifer, interpreted at the observation wells by means of the analytical solution of Neuman and at the pumping wells using that one of Gringarten show the vertical anisotropy of this layer of the aquifer; the horizontal permeability being clearly and systematically higher than the vertical permeability. These results perfectly agree with the geological observations, the fissured layer of the weathered granite profile showing the existence of many sub-horizontal fractures. It confirms dominating role, within the fissured layer, of the permeability of fractures due to the weathering process over that one of fissures with a tectonic origin.

International audience; Pumping tests carried out in the fissured layer of a granitic hard-rock aquifer, interpreted at the observation wells by means of the analytical solution of Neuman and at the pumping wells with that of Gringarten show the existence of a strong vertical anisotropy of this layer of the aquifer; the horizontal permeability is clearly and systematically higher than the vertical permeability. These results agree perfectly with the geological observations, the fissured layer of the weathered granite profile showing the existence of many sub-horizontal fissures. It confirms that, within the fissured layer, the permeability of sub-horizontal fissures due to the weathering process dominates over that of sub-vertical fissures of tectonic origin.

International audience; First arrival times of the deep seismic profile Armor AR2, recorded in 1995 in the northern part of the Armorican Belt. are inverted in order to obtain the superficial velocity field. Initially, first breaks from the unit shots are taken as the observed traveltimes data set. Then. they are compared to ones computed by ray-tracing using an initial velocity model. An iterative algorithm, based on damped least-square method, is used to invert the data, until the velocity model converges to an acceptable solution. This is later discussed in terms of resolution and uncertainties before being compared with geological observations. The results allow us to explain the upper seismic velocity layer as a shallow weathered zone probably corresponding to a Tertiary palaeosurface. The weak velocity zone, observed at depth in the central part of the profile, underlines the importance of deformation and fracturation on weathering development linked to the North Armorican Shear Zone. Les données du profil sismique écoute longue Armor AR2, acquis en 1995 dans le Nord du Massif armoricain, sont inversées, de façon à obtenir le champ de vitesse sismique superficiel. Les premières arrivées sismiques provenant des tirs unitaires sont tout d'abord pointées, afin de dégager les temps de trajet observés, qui sont comparés aux temps de trajet calculés par tracé de rai à partir d'un modèle de vitesse initial. Un algorithme itératif, basé sur les moindres carrés amortis, permet d'inverser ces données, de façon à converger vers un modèle de vitesse acceptable. Celui-ci est ensuite discuté en terme de résolution et d'incertitudes, avant d'être comparé à la géologie de surface. Il apparaît alors que la première couche sismique correspond à une zone superficielle altérée, associée à une paléosurface tertiaire. La faible vitesse observée en profondeur dans la partie centrale du profil souligne l'importance de la fracturation et de la déformation sur le développement de l'altération, au niveau du Cisaillement nord-armoricain.