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International audience; A bridge project across the Loire River, east of Orléans, is located on covered karst terrains prone to sinkholes and prone to water resource disturbance. Conducting an overview of past sinkhole collapses and various field surveys, the French Geological Survey precised the past analysis to help the project planner to improve the hazard management.; Le projet d'un nouveau pont sur la Loire à Jargeau, en amont d'Orléans, s'inscrit dans un contexte karstique qui non seulement présente un aléa de mouvements de terrain mais peut également interpeller quant à la préservation de la ressource en eau. Grâce à une série d'investigations, le BRGM a précisé les analyses préexistantes afin d'améliorer la gestion de ces contraintes géologiques par le maître d'ouvrage.

Exploring for geothermal resources using land-based electromagnetic methods can be very challenging due to the presence of strong human-generated and "geological" noise. In such context, some passive EM techniques like the Magneto-Telluric method may not be applicable at all and a dedicated toolbox of EM techniques capable of dealing with these issues is required. In this paper, we will focus on the challenges encountered while exploring for geothermal resources in volcanic islands, namely a highly heterogeneous near-surface creating strong static-shifts, the presence of a sea/land transition zone and a high degree of urbanization over the area of interest. We will show that the combination of airborne with land-based controlled-source EM (CSEM) and MT surveying provide a robust approach and illustrate these aspects with two EM datasets acquired over the Martinique Island, in the French Lesser Antilles. The first study extends over the Lamentin bay area located near Fort de France, the main city of the Martinique Island. Here, exploratory drilling since the 1980's highlighted a polyphased hydrothermal system with fossilized hydrothermal features and a recent hydrothermal activity. The geothermal system is poorly understood and has prevented any exploitation of the geothermal resource to date. In an attempt to reduce the geological risk for future geothermal development, a joint 3D airborne and CSEM survey was performed. Here, the presence of anthropic EM noise made MT data hardly usable. Processing and inversion of the EM dataset allowed to map the geometry of the hydrothermal alteration "caprock" as well as underlying volcanic substratum and potential geothermal reservoir. The second study extends over the Petite-Anse area located in the SouthWest of the Martinique island. Here, evidences for high-temperature geothermal system (>200°C) are present and the design of exploratory boreholes requires to map accurately the geometry of the "caprock" and potential geothermal reservoir. To do so, a high-resolution 3D CSEM and MT survey was conducted in Spring 2019. Similarly to the Lamentin area, MT data suffered from strong cultural noise. CSEM data quality is sufficiently good to map the extent of the area of geothermal interest and areas to avoid drilling into (e.g. dykes).

International audience; The transition zone between the basement and the sedimentary cover is becoming an increasingly attractive target for the development of geothermal energy in deep sedimentary basin as encountered in the Upper Rhine Graben. Several geothermal power plants already exploit this target but the transition zone is however still poorly known with the presence of large heterogeneities. Studies need to be carried out in order to develop conceptual models on how it is formed and how it can be exploited to produce heat. In this study, we evaluate the ability of resistivity imaging by Controlled-Source Electromagnetic (CSEM) method, to identify favorable areas for the development of Enhanced Geothermal System (EGS) heat exchangers. We performed a land-CSEM survey on an analogue of the transition zone in a well-known catchment basin at Ringelbach, Vosges to assess the relevance of such data. Gathered data consist in a 3D-grid of 48 reception sites uniformly distributed over the whole basin, using a single transmitter. We performed 2.5D inversions of this dataset with the parallel adaptive finite-element code MARE2DEM to image the resistivity structure through a profile of interest and compared the result to a former Electrical Re-sistivity Tomography (ERT) inversion. CSEM inversion extends the shallow ERT image in depth and allowed to obtain a resistivity image of the transition zone. The integration of these results with existing geological and geophysical knowledge allowed identifying and mapping a fault zone as well as the fractured zone at the top of the unaltered granite basement. Results of this study demonstrate the importance of acquiring resistivity data at the target depth before drilling to maximize the success rate of a deep EGS project and point out the interest of pursuing the study with the 3D inversion of the whole set of data.

International audience; The transition zone between the basement and the sedimentary cover is becoming an increasingly attractive target for the development of geothermal energy in deep sedimentary basin as encountered in the Upper Rhine Graben. Several geothermal power plants already exploit this target but the transition zone is however still poorly known, with presence of large permeability heterogeneities. Studies in the framework of the CANTARE-Alsace project are currently on going in order to develop conceptual models on how it is formed and how heat can be exploited. In this study, we evaluate the ability of resistivity imaging by Controlled-Source Electromagnetic (CSEM) method in frequency domain, to identify favorable areas for the development of Enhanced Geothermal System (EGS). We performed a land-CSEM survey on a shallow analogue of the transition zone in a well-known catchment basin at Ringelbach (Vosges), to assess the relevance of such data. The catchment basin of Ringelbach is a well-known hydrogeological site, analogous in surface to our target in depth. According to previous geophysical studies (Two well logs, ERT…), the site is composed of resistive sandstone (>1600 Ω.m) superposed over two granitic facies with a NE-SW fault separating the site in two blocks. The first one is identified as a weathered granite being conductive (250-1600 Ω.m), whereas the second one appears as a block of fresh granite with a strong resistive signature (>1600 Ω.m). We know from former laboratory measurements (Belghoul, 2007) that conductivity of Ringelbach granites is related to its alteration degree and porosity. Furthermore, weathered granites show signs of former hydrothermal alteration succeeding the granite burial (Wyns, 2012). Fresh granitic basement is not reached by wells (150 meters depth) or former geophysical studies (Limited around 50 meters depth) The 3D Land-CSEM survey consists in a 3D-grid of 48 reception sites uniformly distributed over the whole basin and using a single transmitter located at 1.5 km north from the grid. We performed 2.5D inversions of a data subset with the 2.5D inversion code MARE2DEM (Key, 2016) to image resistivity structures through a profile of interest (Figure 1) and compared the result to a former Electrical Resistivity Tomography (ERT) inversion. 3D inversion of the entire dataset are currently ongoing using the BRGM proprietary software POLYEM3D for 3D CSEM and MT data inversion. Preliminary 3D results are consistent with the 2.5D resistivity image. CSEM inversion results extended shallow ERT images in depth and allowed to obtain a resistivity image of the transition zone analogue. Integration of these results with existing geological and geophysical knowledge allowed identifying and mapping the different resistivity blocks: the resistive sandstone; the altered granite area more conductive, altered and potentially more permeable; the resistive fresh granites with the "Bunker" block (Right on Figure 1) as well as the fresh granitic basement, less porous thus less favorable for a geothermal reservoir. Results of this analogue study demonstrate the importance of obtaining resistivity data at the target depth before drilling to maximize the success rate of a deep EGS project. Upscaling work will be part of the last work package of the CANTARE-Alsace project.

International audience; Resistivity imaging is a key parameter in most geothermal exploration programs, and particularly in volcanic environment. It is assumed that resistivity variations allow imaging the caprock, but also hydrothermal weathering or preferential water flow. The geothermal fluid is generally brine water which drastically decreases the electrical resistivity and the temperature increase also decreases resistivity. The resistivity signatures for several conceptual geothermal reservoirs can be found in the literature. Magneto-telluric (MT) is generally used to image resistivity variations down-to a relevant investigation depth of several kilometers. Based on natural source signal, MT prospection efficiency depends on the Sun activity during the survey but moreover depends on the noise conditions. In many cases, because industrial activities have been developed close to the geothermal resource, the noise conditions make it difficult to obtain reliable MT tensors and hence a good resistivity image. Generally, the only way to counter is recording longer time series, to apply robust noise filtering and combining with a remote filtering. The noise issue in MT prospection is even more drastic in Island context. We investigated the substitution of controlled-source EM instead of MT measurements close to urbanized area. The distance constraints to respect the far field conditions are so challenging that they are often logistically not possible. Indeed, when it is possible to measure the receiver stations far enough from the source, the distortion due to the controlled-source disappear. But when the conditions for applying standard processing of CSAMT are not fulfilled, it is necessary to consider the source effect to interpret properly the EM response. We have studied and performed EM imaging in the near field, using a pair of pre-existing boreholes to inject high current into the ground through metallic casings. Based on numerical modeling, we propose a methodology to obtain maps and section of apparent resistivity. The results obtained were confronted to previous geophysical campaign results performed in the 1980's and recent shallow prospection. This measuring protocol was applied to prospect the Lamentin area (Martinique, Lesser Antilles, France). This survey was performed in 2013 within the framework of a large exploration program founded by the FEDER, ADEME, Regional Council and SMEM designed to explore the geothermal potential of Martinique.

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

International audience; The usefulness and reliability of magnetic resonance information to characterize water bearing geological structures has been widely demonstrated these last two decades all over the world and many future applications just begin. The main limitation of MRS applicability is its sensitivity to the electromagnetic noise which results in a long and site dependent measuring duration, and generally prove to be impossible in urban conditions. Many improvements have been performed all along the development of MRS technology. Nowadays, numerous mono and multi-channel processing schemes have been published, but efficiency remains site and time dependent. We have reviewed data from various contexts and compared the noise removal efficiency and impact of the filtering on synthetic signal added to real noise data. We also used methods derived from magneto-telluric to study the structure of the noise and present a continuous EM field monitoring during a storm event in mountain where we performed a MRS survey. We observed that the reconstruction of natural noise is a percentage of the ambient noise, the ratio is almost stable. Despite this observation of stable removal performance, it means that when the level of noise is multiplied by 10 to 100 and more… it is better to stop measuring MRS and wait for a quiet period of time.

International audience; A 2012-2013 additional and combined – geological, hydrogeological, geochemical, geophysical -surface exploration results in a new insight of the Martinique geothermal systems, including Petite Anse – Diamant area. The Petite Anse – Diamanthot spring is located at the foot of a small volcano -Morne Jacqueline – part of a NW-SE volcano-tectonic corridor. The Southern flank of this volcano collapsed leading to the outcrop: 1) A previous neutral, high temperature hydrothermal activity corresponding to a MT conductive layer interpreted as a clay cap of the present geothermal system. The bottom of the conductive layer displays also a doming shape, characterizing the top of geothermal system; 2) A dyke of diorite coinciding with the top of a huge MT resistive body, following the volcanic corridor, and interpreted as a magma intrusion; it generates several on and off-shore spots of magmatic degassing; this intrusive gas –rock may act as a heat source and interact with the geothermal fluid. The Petite Anse spring water is of Na-Cl type due to a seawater contribution. The associated geothermal reservoir temperature is estimated at 190-210°C. The recharge of the system is likely to be low, because of both a bad permeability resulting from the extensive acidic alteration related to previous fumarolic expression, and limited rainfalls. An exploration drilling area is proposed at the North of the Petite Anse village.

2p; Within the framework of a global French program towards development of renewable energies, Martinique Island (Lesser Antilles, France) has been extensively investigated (from 2012 to 2013) through an integrated multi-disciplinary approach, with the aim to identify precisely the potential geothermal resources previously highlighted (Gadalia et al., 2014). Among the investigation methods deployed (geological, geochemical and hydrogeological), we carried out three magnetotelluric (MT) surve ys at three of the four most promising areas of Martinique, namely the Anses d'Arlet, the Montagne Pelée and the Pitons du Carbet prospects. A total of 32 MT tensors were collected in the Anses d'Arlet area in the frequency range 1000 Hz to 10-2-10-3Hz togetherwith TEM soundings for potential static shift correction. A 3-D MT inversion of the full tensor was carried out including the coast effect. The 3-D resistivity model reveals a major resistive body elongated in the NNO -SSE direction, corresponding to the main structural volcanic axis of the area, and interpreted as a deep intrusion almost reaching the surface. Based on geological observations -an alteration zone located between Anses d'Arlet and Petite Anse-the shallow conductive layer identified eastward is interpreted as the remaining of an old cap-rock partly eroded that becomes thicker southwards. The latter could be related with the altered core of the Morne Larcher. Other studies allowed the reconstructing of the geothermal system evolution to its present and moderate apparent activi ty mainly located south of the resistive anomaly. This intrusion could act as heat source for the geothermal system. This sector is identified as the most interesting for further exploration wells. In order to better understand structures highlighted by the real data distribution and test the strategy to integrate correction coefficients for the coast effect (modeled separately) during the inversion, we designed a forward model using the s ame bathymetry, topography and MT sounding distribution of the survey. Impedance tensors were calculated for the 32 sites. The geometry of the 3-D structure has been designed quasi-independently of the data distribution to assess the impact of un-appropriate acquisition grid. The 3-D inversion was run with these synthetic data (Hautot et al, 2000, 2007). The results indicate that the general structure is recovered for the first 1000 m. Deeper, and southward, the absence of MT soundings surrounding the resistive body prevented a good lateral constraint, causing its disappearance. To the north, the conductive body "pseudo caprock" is well defined, especially on layer 5 (Figure 2), were the apex of a supposed geothermal reservoir could be identified (sites 3 and 6). Deeper the eastern border remains very well constrained by the eastern MT synthetic soundings.

Geothermal exploration of the Martinique Island (Lesser Antilles) focused, at first (1960-1980 years), on the Lamentin lowlands and on the Southern flank of the Mount Pelée volcano. In 2003, a new step was done with the identification of two areas of interest: the SW side of Mount Pelée dome and the surroundings of the Petite Anse - Diamant hot springs (Sanjuan et al., 2003). The 2012-2013 additional and combined - geological, hydrogeological, geochemical, geophysical - surface exploration results in a new insight of the Martinique geothermal systems, including Mount Pelée volcano, Petite Anse - Diamant area, Pitons du Carbet domes and Lamentin lowlands. High temperature geothermal systems are supposed to be active at both the former sites whereas only signs of medium to low temperature were detected at the Lamentin and Pitons du Carbet areas.

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.

Bouillante geothermal field (Basse-Terre, Guadeloupe, French West Indies) is located near the Western coast of Guadeloupe Island, about 15 km from the presently active Soufrière volcano. We have studied the subsurface structure of a zone encompassing the geothermal area using geophysical data. The need to understand the geological context of the Bouillante geothermal Province has led to numerous onshore and offshore geophysical investigations. This work presents a compilation of all available, subaerial and marine, gravity and magnetic, electric and magnetotelluric data acquired during the last 30 years. To the first order, this synthesis could act as a guide in the implantation of upcoming surveys in order to better constrain the Bouillante geothermal Province, and to improve the knowledge the island evolution as a whole. Oldest electric and magnetotelluric data were digitalized from manuscripts. Although we faced different problems related to missing acquisition parameters, we partially reconstruct MT tensors to provide a qualitative interpretation of resistivity distribution at depth. Geophysical models have been initiated based on geological constraints and previous geophysical interpretations. We are able to differentiate structures pre- and post-dating the different magnetic inversions using the magnetic measurements according to the polarity of the anomalies. The gravity measurements are used to detect and characterise the dense complexes from the low dense areas, allowing us to complete the magnetic measurements in regard of the nature of the Bouillante Chain formations. The electromagnetic surveys allow us to determine the distribution of electrical resistivities which we interpret in terms of water saturation of rocks, hydrothermal alteration and the presence of hydrated mineral. The integration of the geophysical results has allowed us to build up an updated model of the geothermal system based on geological constraints and previous geophysical interpretations. At the scale of the Province of geothermal Bouillante, the previously proposed conceptual model is clarified through a formations layering associated with petrophysical parameters (density, magnetization, resistivity), in agreement with the classical scheme of geothermal reservoirs. The distribution of resistivities in depth highlights the succession of resistive-conductive-resistive layers, classic in volcanic field. The shallow resistant is relatively well explain in terms of massive formations, dense (not affected by hydrothermal alteration) and recent, associated with the Axial Chain or Chain Bouillante. The conductive intermediate layer marks the intermediate clay cover (cap rock) of the geothermal system, associated with low dense and demagnetized rocks altered by hydrothermal action (at least in the productive areas recognized on the surface). The deep resistive layer is associated with dense formations, older according to depth (oldest formations of the Axial Chain or lava flows from the northern chain). This latter could correspond to the deep geothermal reservoir whose morphology and extension are discussed. Based on our geophysical data and modeling, the geothermal activity could be more developed toward the north compared to the previously proposed extension. Moreover, a major volcano-structural control is evident on various configurations of the geothermal field. The various faults recognized in this key sector seem to affect significantly the internal structure of the Province of Bouillante, and consequently the development of geothermal activity. The horst and graben structure in conditions undoubtedly the morphology of the clay cover and operation of the field. This post-doctoral study was carried out in the framework of a "Carnot institute" labeled project.

International audience; In November 2011 and July 2012 were conducted geophysical prospections in the southern part of New Caledonia. Both electrical tomography (ERT) and Magnetic Resonance Sounding (MRS) were performed to obtain a better understanding of the water storage and circulation. The geological context is the weathering profile of peridotite rock (Maurizot and Vendé-Leclerc, 2009). We present a review of the various ERT and MRS responses observed in this context.

International audience; The need to understand the geological context of the Bouillante geothermal Province (Basse-Terre, Guadeloupe, French West Indies) has led to numerous onshore and offshore geophysical investigations. This work presents a compilation of all available, subaerial and marine, gravity and magnetic, electric and magnetotelluric data acquired during the last 30 years. Oldest electric and magnetotelluric data were digitalized from manuscripts (area of interest from Mahaut to Basse-Terre). Although we faced different problems related to missing acquisition parameters, we partially reconstruct MT tensors to provide a qualitative interpretation of resistivity distribution at depth. The previous interpretations have been improved and evidenced well developed conductive anomalies overlying more resistive ones at the location of, but also beyond, the geothermal field area. The large-scale gravity signal (from Montserrat to La Dominique) is used to explore existence and geometry of deep dense intrusive complexes beneath La Soufrière and the Bouillante Chain (if existing) volcanoes which could be interpreted as heat sources for the geothermal systems. Modelling has been initiated to constrain the geometry of such structures together with the distribution of more or less dense products both onshore and offshore. Because Guadeloupe volcanism spars the Brunhes-Matuyama geomagnetic reversal (0.78 Ma), analysis of magnetic anomalies highlight areas predominantly composed of formations either younger or older than the reversal. Modeling is used to reconstruct a generalized topography of the island at 0.78 Ma. The detailed analysis of the offshore magnetic lineaments will be tentatively used to understand the inheritance of the lithosphere structure in the island evolution. An updated large-scale model of the geothermal system will be proposed based on geological constraints and previous geophysical interpretations. This synthesis could act as a guide in the implantation of upcoming surveys in order to better constrain the Bouillante geothermal Province, and to improve the knowledge the island evolution as a whole. This post-doctoral study is carried out in the framework of a "Carnot institute" labeled project.

The Bouillante geothermal field, located on the west coast of Guadeloupe (Lesser Antilles), was explored in the 1970s, developed in the 1980s, brought into production in 1986, and expanded in 2005 to currently produce up to 15 MWe. In addition to its electricity-producing role, the Bouillante field is an outstanding research laboratory for improving our knowledge of a reference high temperature (260°C) geothermal system in island-arc environment. A great deal of scientific research and exploration investigation have been carried out on the Bouillante geothermal field for 10 years, including geological investigation (volcanism), structural analysis of the fluid conduits, geophysical investigations both offshore (magnetics and high-resolution shallow seismics, see Calcagno et al.) and onshore (gravimetry, electrical resistivity tomography profile and passive seismic, see Gailler et al.), characterization of the surface and deep hydrothermal manifestations, numeric geological modelling of the developed field, fluid geochemistry and tracer tests (Sanjuan et al.) and hydrogeological modelling. This large range of multidisciplinary data has made it possible to develop an innovative conceptual model of this type of high-temperature geothermal system. The model highlights the major influence of structural control on the development of the geothermal activity at different scales and especially on the geometry of the reservoir. It also explores new ideas concerning the geothermal system in terms of heat source, fluid circulation and spatio-temporal scenario. This scenario takes into account all events from the early magmatic activity of the Bouillante volcanic Chain (~500,000 y) up to the beginning of the geothermal field some 250,000 years later, having probably started with a phreatic explosion in the bay of Bouillante (see Patrier-Mas et al., Verati et al.). This 2D geothermal resource conceptual model will be used as an exploration tool to reduce geological risks for the next exploration drilling campaign. This work is supported by the French Agency for Energy and Environment (ADEME).

International audience; The Bouillante geothermal field is located on the west coast of the French West Indies island of Guadeloupe (France, Lesser Antilles). The large range of scientific (geology, geochemistry, geophysics, hydrogeology) results obtained over the last 10 years on this field have made it possible to develop an innovative conceptual model of this hightemperature geothermal system; a reference for island-arc environments. The model highlights the major influence of structural control on the development of the recent volcanism and subsequent geothermal activity at different scales, and also explores new ideas concerning the Bouillante system in terms of heat source, deep fluid flow and reservoir geometry. In addition, a spatio-temporal scenario takes into account all events from the early magmatic activity of the Bouillante volcanic Chain complex (estimated at ~0.5 Ma) up to the development of the geothermal field some 200,000 years later.

International audience; Nowadays, MRS contribution to characterize aquifers in common conditions down to about 100 meters deep is highly valuable in rocks that exhibit hydraulic behaviour of non-consolidated aquifer at the sounding scale (e.g. sediments, weathered and fissured hard-rocks, densely fissured or highly interstitial porous carbonates). In rocks that exhibit behaviour of fractured aquifer (e.g. low density fractured crystalline basements and limestone, karsts) MRS is a useful complementary method but is not always effective for common engineering studies. In magnetic rocks MRS measurements are often impossible. On the one hand, field experiences reveal that MRS is useful to characterize aquifers. (1) The geometry of saturated aquifer can be estimated in 1D case. Interpolation in-between 1D soundings also reveals 2D geometry if the size of MRS loops is smaller than about half the heterogeneity size. (2) Links between MRS water content and aquifer total porosity and storativity have been found. (3) The transmissivity is accurately estimated from MRS parameters in several geological contexts, using the appropriate conversion equation. (4) Thanks to the integrative property of the sounding, the spatial scale of measurement is considered by hydrogeologists as appropriate for aquifer characterization and modelling. (5) The aquifer characterization is improved when MRS is used in the framework of a hydrogeological methodology and jointly with complementary geophysical methods. On the other hand, field experiences reveal the main limitations encountered in the use of MRS with the actual instrumentation. (1) MRS is not yet self-sufficient to characterize aquifers as the MRS output parameters still need to be compared to hydrogeological properties to achieve quantitative estimation of transmissivity. Storage related parameters are not yet quantitatively accessible from MRS. (2) The electromagnetic noise lowers the signal to noise ratio and makes urban areas but also low interstitial porosity and poorly fractured rocks difficult to survey. (3) Heterogeneity of the magnetic properties of rocks makes measurements impossible or interpretations wrong. (4) Electrically conductive layers reduce the investigation depth of MRS in salty water and clayey environments.

International audience; Karstic conduits play a crucial role for water supply in many parts of the world. However, the imaging of such targets is generally a difficult task for most geophysical methods. Magnetic Resonance Sounding (MRS) is a geophysical method designed for imaging of water bearing structures. Initially, MRS was developed for characterizing horizontally stratified aquifers. However, when applying a 1D MRS measuring setup to the imaging of 2D–3D targets, the size of which may be much smaller than the loop, the accuracy and the lateral resolution may not be sufficient. We have studied the possibility of simultaneously processing several MRS aligned along a profile to perform a Magnetic Resonance Tomography (MRT). This work emphasizes the gain of resolution for 2D–3D imagery of MRT versus the interpolation of 1D inversion results of MRS along the same profile. Numerical modelling results show that the MRT response is sensitive to the size and location of the 2D target in the subsurface. Sensitivity studies reveal that by using the coincident transmitting/receiving (TX/RX) setup and shifting the loop around the anomaly area, the depth, section and position of a single karstic conduit with a size smaller than the MRS loop size can be resolved. The accuracy of the results depends on the noise level and signal level, the latter parameter being linked to the depth and volume of the karstic conduit and the water content in the limestone matrix. It was shown that when applying MRT to the localization of 2D anomalies such as karstic conduits, the inclination of the geomagnetic field, the orientation of the MRT profile and the angle of crossover of the conduit by the MRT profile must be taken into account. Otherwise additional errors in interpretation should be expected. A 2D inversion scheme was developed and tested. Both numerical and experimental results confirm the efficiency of the developed approach.

International audience; The increasing demand for energy in the Caribbean islands and the aim to enlarge energy autonomy have resulted in starting again geothermal research on the Martinique island, in the Lesser Antilles. The first works, which were the drilling of three deep wells (1000 m) in the Lamentin plain in 2001, did not show the existence of high temperature geothermal resources. Consequently, new studies of geothermal exploration were carried out between 2001 and 2003 in two other areas which had already been noticed for their geothermal interest: Mount Pelée volcano-Morne Rouge and Diamant. These areas belong to the recent arc of post-miocene age. All the data obtained in water geochemistry during these studies have led to a better comprehension of the geothermal systems associated to the thermal springs known in these areas (less than 15). Sixteen soil gas profiles (CO2, CH4, O2, He) could be carried out in July 2002 in the studied areas, which represents approximately 260 analyzed gas samples. Among these samples, 11 were selected in order to perform additional chemical and isotopic (13C, 3He/4He) analyses. The geochemical results obtained during these studies made it possible to select three areas of high temperature geothermal interest: 1) the area of the thermal spring of Petite Anse du Diamant, whose the relatively saline water (TDS ≈ 20 g/l) and the CO2 gas emanations indicated high temperature conditions (around 180 °C) at depth, despite a low flow rate and temperature (35°C) at the emergence. 2) the area of the upper Chaude river, where the chemical and isotopic compositions of the waters collected from the thermal springs (TDS from 1.0 to 1.5 g/l, the hottest temperatures at the emergence in the island: from 38 to 65°C) suggested the existence of a deep geothermal reservoir at about 200 °C and the presence of primarily magmatic CO2 gas. The deep hot water is after diluted and cooled by superficial fresh waters, during its rise. 3) the area of the Desgrottes drill-hole, where the gas emanations testified of a magmatic origin but where no index of deep hot water was observed at the surface. For these three areas, results obtained from geological and geophysical studies supported the geochemical data. However, only the drilling of several deep exploration wells will be able to confirm the existence of workable high temperature geothermal resources in these areas.