HAL-BRGM, les publications scientifiques en libre accès du BRGM
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Next level for QGIS: 3D representation and modelisation
No full textInternational audienceBRGM, the French geological survey, has been working for many years on the development of 3D modeling software to make it available, in particular, for modeling geologists. 3D modeling requires georeferenced data from a variety of sources: acquisition of contact and dip point data in the field, interpretation of geophysical data such as seismic data, electrical profile, gravimetry, magnetism, etc., and of course modeling algorithms. The open-source community is very active in many fields. The QGIS open-source GIS software community has become very active and innovative. The growing popularity of QGIS, and its use by a large community of geoscientists as a data integration platform, motivated BRGM to integrate its 3D modeling tools. Software bricks have been redeveloped as plugins in QGIS. They not only provide an operational alternative to historical tools, but also offer a modular, scalable environment that can be recomposed according to production needs, adapted to modern computing resources and serving as an incubator for research projects. Nevertheless, GIS software has been designed primarily for 2D, and the move towards 3D representation and use is driven by the emergence of new needs, such as BIM (Building Information Modeling) or the use of large point clouds like Lidar. This evolution now seems necessary. It calls for reflection and development supported by a 3D modeling community. Developments for 3D in QGIS are underway, such as 3D visualization, the representation of borehole data, or the development of functionalities in the elevation profile. Integrating 3D into QGIS requires all 2D and 3D communities to exchange ideas and make joint developments. To this end, BRGM has financed the development of functionalities in the elevation profile, which have been included in a QEP proposal. However, many other options, such as the ability to move around in 3D space, are developments that seem necessary if QGIS is to continue its evolution towards 3D. Collaborative efforts to invest in the development of 3D-related functionalities in the heart of QGIS would enable the rapid evolution of 3D functionalities useful to the whole community
Dynamics of tephra fallout and column collapse during Plinian eruptions of Mt. Pelée volcano (Lesser Antilles): The 2.5 ka cal BCE P5 eruption
No full textInternational audienceMt. Pelée volcano in Martinique (Lesser Antilles) has undergone volcanic unrest since 2019 and an increase of alert level to yellow (vigilance) in 2020. Historical records and field studies show that this volcano has produced numerous dome-forming and sub-Plinian to Plinian eruptions over the last 25 kyr. The 2.5 ka cal BCE P5 eruption dispersed lapilli and ash across the northern part of Martinique. Here, we present field data on tephra dispersal and grain-size distribution to estimate the eruption source parameters and reconstruct the dynamical evolution of the eruption. Our results show that after a short opening phase, the P5 Plinian eruption formed a 26 km-high stable column with a northeastern dispersal axis. The total volume of the eruption is estimated to be 0.39 ± 0.03 km^3 DRE making P5 a VEI 4 event. The mass eruption rate increased from ≈ 1.1 × 10^8 kg s-1 to ≈ 4.8 × 10^8 kg s-1 leading to column collapse and the formation of pyroclastic density currents on the flanks of the volcano. The transition from stable column to collapse occurred at conditions well predicted by a 1D model of volcanic plumes and confirms that this eruptive scenario is recurrent at Mt. Pelée volcano over the last 5 kyr. The tephra dispersal of the P5 eruptive products reinforces the accuracy of the current hazard map and provides volcanological constraints to better assess volcanic hazards in the Lesser Antilles
Le programme de subvention des intrants agricoles au Sénégal: une analyse des impacts sur les ménages agricoles
No full textDepuis maintenant une quinzaine d’années, le Sénégal a mis en place un programme de subvention d’intrants agricoles à destination de ses producteurs agricoles visant à leur fournir engrais, semences et matériel agricole. Cependant, très peu d’études ont été réalisées pour en évaluer les impacts. Ce rapport présente les résultats détaillés d’une étude visant à évaluer les effets de ce programme dans deux régions, la vallée du fleuve Sénégal et le bassin arachidier, au travers de plusieurs dispositifs de collecte de données, et notamment d’une enquête auprès des producteurs agricoles. Les résultats mettent en lumière plusieurs faiblesses du programme. La mise en oeuvre du programme diffère d’une région à l’autre et les critères de sélection des bénéficiaires souffrent d’un certain flou. Les bénéficiaires du programme tendent à être sélectionnés parmi les producteurs les plus grands, mieux éduqués et bénéficiant d’un capital social plus important. L’estimation des effets du programme en prenant en compte ce biais de sélection montre bien un effet positif sur l’utilisation d’engrais inorganique, et dans le cas du bassin arachidier, également sur le recours aux semence certifiées. Cependant, aucun effet ni sur les rendements des principales cultures ni sur les performances économiques des exploitations bénéficiaires n’est observé dans les deux régions de l’étude. Cela s’explique notamment par les dysfonctionnements du programme, tels que les retards de livraison des intrants ou leur mauvaise qualité, mais aussi par un effet d’éviction sur les achats d’engrais commerciaux, notamment dans la vallée du fleuve Sénégal, qui résulte d’un ciblage inefficace des bénéficiaires. Au final, le programme de subvention des intrants agricoles ne permet pas d’augmenter la production ou le revenu des producteurs bénéficiaires. Le rapport conclut sur des recommandations pour améliorer l’efficacité de cet outil essentiel de la politique agricole Sénégalaise
Unconformity-related mineralization and fluid transfers in the northern Aquitaine Basin (France) revealed by fluid inclusions and S-Sr isotopes studies
No full textInternational audienceIn sedimentary basins, unconformity between basement and sediments is the ideal site where fluids from different sources can flow and mix, initiating the formation of ore deposits. In western Europe there are numerous F-Pb-Zn-Ba (±Ag, Ge) basin-hosted deposits located near the unconformity between Mesozoic Basins and the Variscan basement as for example the deposits of the Vendée Coast (France) containing fluorite, baryte, pyrite and quartz. Here, microthermometric data on the primary fluid inclusions of these minerals indicate salinity ranging from 1 to 20 wt% eq. NaCl and homogenization temperatures between 100 and 390 °C. We interpret these data as resulting of a fluids circulation in the ore deposit zone, with an early incursion of basin brines expelled by the leaching of Hettangian evaporites buried several tens of kilometers away, followed by an ascent of basement-derived fluids and, finally, a recharge of seawater. In other French deposits, the δ34S isotopes of baryte are also consistent with a source of fluids from buried evaporites. The 87Sr/86Sr ratio of baryte demonstrates a crustal source of elements associated with brines-leached base metals and Fsingle bondBa. The process of buried-derived evaporites brines altering the basement along the unconformity is ubiquitous in all unconformity deposits in France.With our results, we confirm that the basin deposits in the Southeastern Massif Central occur along structures formed during Tethys rifting at around 200 Ma, whereas in the Western part they form at ca. 145 Ma in link with the opening of the Bay of Biscay. This highlights that these basin-hosted deposits are preferentially formed during extensional activity in rifting settings, rather than in compressive settings
GEOMAPLEARN 1.2: detecting structures from geological maps with machine learning – the case of geological folds
No full textInternational audienceThe increasing availability of large geological datasets and modern methods of data analysis facilitate a data science approach to geology in which inferences are drawn from geological data using automated methods based on statistics and machine learning. Such methods offer the potential for faster and less subjective interpretations of geological data than are possible from a human interpreter, but translating the understanding of a trained geologist to an algorithm is not straightforward. In this paper, we present automated workflows for detecting geological folds from map data using both unsupervised and supervised machine learning. For the unsupervised case, we use regular expression matching to identify map patterns suggestive of folds along lines crossing the map. We then use the HDBSCAN clustering algorithm to cluster these possible fold identifications into a smaller number of distinct folds. This clustering algorithm is chosen because it does not require the number of clusters to be known a priori. For the supervised learning case, we use synthetic models of folds to train a convolutional neural network to identify folds using map and topographic data. We test both methods on synthetic and real datasets, where they both prove capable of identifying folds. We also find that distinguishing folds from similar map patterns produced by topography is a major issue that must be accounted for with both methods. The unsupervised method has advantages, including the explainability of its results, and provides clearly better results in one of the two real-world test datasets, while the supervised learning method is more fully automated and likely more easily extensible to other structures. Both methods demonstrate the ability of machine learning to interpret folds on geological maps and have potential for further development targeting a wider range of structures and datasets
Multi-isotopic (Fe-Cu-Zn) constraints on the magmato-hydrothermal history during mantle exhumation at slow-spreading centers
No full textInternational audienceAt slow to ultraslow-spreading ridges, tectonic mantle exhumation and magmatic processes accounts for heterogeneity in the lithosphere and drives deep hydrothermal circulation and fluids venting at the seafloor. However, the spatio-temporal evolution and the interplay between magmatic and hydrothermal processes during mantle exhumation, as well as their consequences for chemical exchange at mid-ocean ridges are poorly constrained.We carried out a Fe, Cu and Zn isotope study of mantle rocks drilled at the Mid-Atlantic Ridge Kane (MARK) area (23'30°N) to decipher the consequences of magmatic versus hydrothermal chemical exchange on lithospheric mantle composition. At MARK, mantle rocks undergo complex melt-rock interaction during melt percolation overprinted by high temperature (HT, >350°C) hydrothermal circulation that leads to the formation of secondary mineral assemblages (e.g., amphibole, chlorite, ilvaite, hydro-andradite, clinopyroxene, talc, serpentine). Serpentinized peridotites cut by hydrothermally overprinted magmatic veins have increased isotopic heterogeneity to both lighter and heavier isotope compositions (δ 56 Fe from -0.44 to 0.07 ± 0.03‰; δ 66 Zn from -0.24 to 0.32 ± 0.04‰), expending the predictive unaltered composition of the primitive mantle (δ 56 Fe = 0.025 ± 0.025‰ and δ 66 Zn = 0.16 ± 0.06‰). Such variability is ascribed to diffusion-related kinetic isotope fractionation during the percolation of Fe-and Zn-rich melt in mantle rocks. Low isotopic values are due to preferential diffusion of lighter isotope in mantle rocks, while high values may involve mixing of serpentinized peridotites with isotopically heavy magmatic veins. The lower Cu content (0.5 to 23.9 ppm) and either lower or higher δ 65 Cu (-0.11 to 0.32 ± 0.04‰) of abyssal peridotites, compared to the primitive mantle (30 ppm Cu, δ 65 Cu = 0.07 ± 0.1‰), can be explained through Cu leaching during hydrothermal alteration of sulfide, and possibly oxide, at high temperature (~450-600°C). Hydrothermal veins in serpentinites formed at decreasing temperature (~300°C) from a metal-and sulfur-rich fluid interacting with serpentinized peridotites. Iron, Cu and Zn isotopes record the inventory of magmatohydrothermal processes during mantle exhumation at (ultra-)slow spreading centers, from HT melt-rock interaction to late low-temperature (LT) fluid-rock interaction.</div
Logiciel Cassiopée version 4.19.0 - Documentation Utilisateur
No full textInternational audienceThe Cassiopée documentation, version 4.19.0, introduces software dedicated to river hydraulics, particularly useful for dimensioning fish passes, agricultural hydraulics, and open-channel hydraulics. The software is structured into independent calculation modules, allowing for the resolution of specific problems and the execution of complex calculation sequences. Accessible online via a modern browser or installable on various platforms (Windows, Linux, macOS, Android), Cassiopée requires no prior installation.The software offers advanced features for managing calculation parameters, allowing users to fix, vary, or calculate specific parameters. Results can be visualized as graphs and tables, with download options in XLS and PNG formats. The modules cover various aspects of hydraulics, including head loss, the Lechapt and Calmon formulas, Strickler, and distribution pipes.Cassiopée also includes tools for open-channel hydraulics, enabling the calculation of uniform flow, backwater curves, and upstream/downstream water levels in a reach. The software supports different section types (rectangular, circular, trapezoidal, parabolic) and uses formulas like Manning-Strickler for hydraulic calculations.Additionally, Cassiopée provides modules for simulating structure laws, including unsubmerged weirs, partitions, and specific structure equations. The software is equipped with tools for verifying fish pass crossing criteria and for calculating downstream migration and jet impact.The documentation provides detailed information on installation, module usage, and numerical resolution methods. It also includes references and tables for coefficients used in various formulas. In summary, Cassiopée is a comprehensive and versatile tool for engineers and hydraulics professionals, offering an integrated solution for modeling and simulating hydraulic systems.La documentation de Cassiopée, version 4.19.0, présente un logiciel dédié à l'hydraulique des rivières, particulièrement utile pour le dimensionnement des passes à poissons, l'hydraulique agricole et l'hydraulique à surface libre. Le logiciel est structuré en modules de calcul indépendants, permettant de résoudre des problèmes spécifiques et de réaliser des enchaînements de calculs complexes. Accessible en ligne via un navigateur récent ou installable sur diverses plateformes (Windows, Linux, macOS, Android), Cassiopée ne nécessite aucune installation préalable.Le logiciel offre des fonctionnalités avancées pour la gestion des paramètres de calcul, permettant de fixer, varier ou calculer des paramètres spécifiques. Les résultats peuvent être visualisés sous forme de graphiques et de tableaux, avec des options de téléchargement en formats XLS et PNG. Les modules couvrent divers aspects de l'hydraulique, incluant la perte de charge, les formules de Lechapt et Calmon, Strickler, et les conduites distributrices.Cassiopée intègre également des outils pour l'hydraulique à surface libre, permettant le calcul du régime uniforme, des courbes de remous, et des cotes amont/aval d'un bief. Le logiciel prend en charge différents types de sections (rectangulaire, circulaire, trapézoïdale, parabolique) et utilise des formules comme celle de Manning-Strickler pour les calculs hydrauliques.En outre, Cassiopée propose des modules pour la simulation des lois d'ouvrages, incluant des déversoirs dénoyés, des cloisons, et des équations d'ouvrages spécifiques. Le logiciel est également équipé d'outils pour la vérification des critères de franchissement pour les passes à poissons, ainsi que pour le calcul de la dévalaison et l'impact de jet.La documentation fournit des informations détaillées sur l'installation, l'utilisation des modules, et les méthodes numériques de résolution. Elle inclut également des références et des tableaux pour les coefficients utilisés dans les différentes formules. En somme, Cassiopée est un outil complet et polyvalent pour les ingénieurs et les professionnels de l'hydraulique, offrant une solution intégrée pour la modélisation et la simulation des systèmes hydrauliques
Mapping seasonal flood-recession cropland extent in the Senegal River Valley
No full textInternational audienceFlood-recession agriculture (FRA) represents a crucial source of livelihood for numerous communities across Africa who reside near expansive floodplains and wetlands. However, it is currently insufficiently monitored. In this study, we present a methodology for mapping FRA harvested areas in the Senegal River Valley that is both reproducible and scalable. Our methodology entails the integration of optical and radar data from Sentinel platforms, conducted through a multitemporal analysis with a seasonal focus, and the application of the Random Forest algorithm. The results, supported by a kappa coefficient of 91.9%, demonstrate the first comprehensive mapping of FRA in the Senegal River valley, conducted between 2019 and 2023. This mapping facilitates the identification of the hydrological factors that influence FRA harvesting. The results of the analyses have demonstrated the importance of interannual variability in the cultivated areas of FRA, which range from 14,000 to 75,000 ha depending on the intensity of the annual flood. The duration and flooded extension are the primary factors that regulate the cropping pattern of FRA over the floodplain. The flood duration must be around 35 days to permit the cultivation, with growth generally starting between 10 and 30 November. In consideration of these findings, we recommend that future water management strategies and rural development initiatives give due consideration to FRA, to enhance the visibility of farmers
An open-source toolbox for 3D geological modelling in QGIS
No full textInternational audience3D modeling is a major asset for the understanding and quantitative characterization of subsurface geology. Geological survey organizations have produced 3D models for decades and are nowadays facing new kinds of demands for increasingly complex accurate representations of the subsurface. New challenges include "usual" difficulties such as integrating large sources of heterogeneous data, handling a wide range of possible model scales (from urban to national), but also new requirements on model uses. As an example, models should be easily (if not automatically) updatable and computable on various environments (not only in desktop software, but also in web / platform environments), models should also serve for multiple purposes and applications (which requires generating various kinds of representations of a single model), etc. In this talk, we present the toolbox we have been developing at the French Geological Survey over the last years to progressively replace our two historical and homemade solutions for 3D geological modeling (GDM and GeoModeller software). The toolbox contains two parts. The first one is a set of python and C++ libraries that provide data structures and computational capabilities. These libraries can run on a wide range of software environments. The second one is a set of QGIS plugins. They provide access to 3D modelling capabilities to the geologists directly in the GIS environment without requiring them to have development skills.Throughout the presentation, we illustrate how the different design choices we made helped us achieve our main objectives: extensibility of the toolbox capacities, reusability of the software components and performance. One of the key features to achieve these objectives is the design of modular and open software components. It enables models and workflows to be easily adapted to fit a wide range of production needs. The integration into the open-source ecosystem also provides numerous benefits, and we illustrate how we had to support the development of core QGIS functionalities to better manage 3D geological objects
H2 storage with CO2 cushion gas in aquifers: Numerical simulations and performance influences in a realistic reservoir model
No full textInternational audienceThis paper presents the results of comprehensive numerical simulations utilizing the GEM modelling tool from Computer Modelling Group (CMG) to investigate the feasibility of seasonal Underground Hydrogen Storage (UHS) using CO2 as the cushion gas in a realistic aquifer with a well-defined geological structure. The primary focus of the study is to address the challenge of efficiently recovering pure hydrogen (H2 ) in UHS, as the contamination and recovery of the cushion gas significantly impact the economic viability of the project. The research aims to analyse and understand how various parameters, including operational, geological, and reservoir-related factors (e.g., injection and production rates, heterogeneity, gas solubility, etc.), influence the performance of the H 2 storage system, with the ultimate goal of identifying optimal storage scenarios.The study evaluates the performance of a single injection and withdrawal well positioned at the summit of the anticline. The results highlight that the amount of recovered H 2 progressively increases with successive withdrawal cycles. Several parameters are found to influence the quality and quantity of the gas produced at this site, with geological heterogeneities playing a crucial role in attenuating up-coning effects. Notably, short-period cycles lead to an enhanced Hydrogen Productivity Index, while the production rate significantly impacts the quality of the produced gas and the mixing zone extent and location. The findings of this work demonstrate that H 2 storage can be achieved with reasonable gas recovery parameters. However, it is important to note that no geochemical considerations have been incorporated into this analysis