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Décrypter l'histoire géodynamique passée grâce à la datation et à l'analyse chimique des produits volcaniques à terre et en mer.
No full textInternational audienceLes produits volcaniques constituent des archives précieuses pour étudier l'histoire géologique d'une région : leurs compositions chimiques et minéralogiques sont liées aux processus de genèse et de transport des magmas, tandis que la fréquence des éruptions reflète des changements long terme des processus magmatiques. Nous présentons une synthèse des travaux réalisés ces dernières années dans les Andes du Nord qui améliorent la compréhension des relations entre le volcanisme, la tectonique et le contexte géodynamique régional.Dans la cordillère équatorienne, nous avons daté (K-Ar sur des laves et des ponces) les unités clés d'une trentaine de volcans associés à l’arc actuel. Lors des éruptions majeures, les cendres volcaniques sont transportées par les vents vers l'océan Pacifique. De nombreux niveaux de cendres sont ainsi préservés dans les séquences sédimentaires marines. Ces archives couvrent une période de temps de ~10 Ma nous permettant d’affiner le catalogue des éruptions majeures régionales et d’étudier l’évolution de la signature chimique des magmas depuis le Miocène.Nous montrons que (1) le volcanisme du nord Pérou a perduré au moins jusqu'à 4.7 Ma avant de s’éteindre en raison de l’horizontalisation du slab, (2) plusieurs périodes d’activité explosive aux Galápagos témoignent d’interactions point chaud-dorsale océanique, (3) la faille de Pallatanga est active depuis plus de 350 ka et présente une vitesse de cisaillement de 3,3 à 10 mm/an au cours des derniers 6 ka, et (4) l'activité de l’arc nord-andin actuel a commencé dès 4,6 Ma, s’est intensifiée avec la construction des stratovolcans quaternaires, dont le nombre augmente significativement depuis ∼600 ka.Nous montrons que l’activité volcanique nord andine et les changements géodynamiques régionaux (subduction d’un plateau basaltique depuis le début du Pliocène, anomalie thermique et changement géométrique de la plaque en subduction qu'il engendre, activation des failles crustales régionales, etc.) sont synchrones. La répartition des volcans est liée en premier lieu à la géométrie du slab, mais semble également être guidée par des mécanismes profonds (nature des fluides, hétérogénéités mantelliques, etc.) et d’anciennes structures tectoniques crustales, tandis que la néotectonique aurait un rôle secondaire, et influencerait principalement la morphologie des édifices (dissymétries, effondrements de flancs, etc)
Fifteen years of Gamma-Ray Burst observations at Very High Energies with H.E.S.S
No full textInternational audienceWe present results from the High Energy Stereoscopic System (H.E.S.S.) follow-up observations of Gamma-ray Bursts (GRBs) between 2004 and 2019. We are focusing on non-detections and providing the most extensive set of very-high-energy (VHE, >100 GeV) upper limits to date. We use this catalogue to constrain the properties of VHE-detected GRBs and compare them to those detected at VHE. Our study finds that VHE-detected GRBs are not a distinct population but are instead associated with bright X-ray afterglows and low redshifts. In addition, we model the multi-wavelength emission of a few of the observed GRBs and discuss the results in the context of their obtained microphysical parameters. The results from this work help put current VHE observations into perspective and highlight the capabilities of next-generation instruments, in detecting fainter and more distant GRBs at VHE
Le champ d'onde sismique et ses gradients dans des milieux complexes : Simulations numériques avec application à la détection d'hétérogénéités superficielles et à la caractérisation in situ d'une structure magmatique volcanique
No full textSeismic wave analysis is a fundamental tool in geophysics, enabling a deeper understanding of the Earth's interior and its complex structures. As seismic waves propagate through the Earth, they interact with the surrounding medium, encountering diverse geological formations. These interactions can cause changes in wave velocity, direction, phase, and, more broadly, lead to scattering phenomena. This scattering process significantly shapes the observed seismic signals, especially in heterogeneous media characterized by rapid variations in material properties. A thorough understanding of seismic wave scattering is crucial for gaining insights into both the properties of the medium through which the waves propagate and the seismic sources that generate them.In this context, the present study employs advanced numerical simulations to model seismic wave propagation in complex media. This research explores not only traditional seismological observables, such as displacements or velocities, but also emerging observables in the form of field gradients, such as rotations and linear strain. The growing interest in these new observables is particularly driven by recent advances in Distributed Acoustic Sensing (DAS) technology (or distributed dynamic strain sensing) for strain measurements, whose widespread use has gained significant importance over the past decade. Numerous studies have shown that these new observables offer distinct sensitivities compared to traditional measurements.This research investigates wave scattering across various environments and scattering regimes, from single to multiple scattering.In the single scattering regime, we investigated the sensitivity of the new observables compared to traditional ones in terms of phase shift and amplitude changes for detecting subsurface heterogeneities. The new observables showed enhanced sensitivity in terms of amplitude, proving particularly effective at identifying shallow heterogeneities near the observation point. After theoretically characterizing this sensitivity, the concluding part of the study proposed a new method for identifying localized velocity variations in the shallow subsurface, which was then applied to a real-field experiment using distributed acoustic sensing.In the multiple scattering regime, two studies were conducted. The first study, after confirming the reliability of SEM (Spectral Element Method) simulations in producing results consistent with those predicted by the Radiative Transfer Equation, focused on analyzing the energy evolution of the standard and new observables with different source polarizations. The difference in the dynamic of the energy evolution observed in rotations suggests that these observables can provide valuable constraints for better characterizing the mechanism and polarization of the seismic source.In the second study, we analyzed signals generated by sources densely distributed within a volcanic structure, correlating them to the scattering properties of the source area. This study was motivated by the stability of frequency peaks observed in volcanic tremor data over time, whose origin remains not fully clear. Our findings suggest that while this stability may result from resonances in dykes and sills, that is a source effect following the main current interpretation, it is also possible to generate resonances across much larger areas, where specific wavelengths become confined due to spatial variations at the scale of the medium correlation length.Finally, in a third study using the same dataset, we applied machine learning techniques to cluster the signals generated in different areas characterized by different elastic and scattering properties, offering a new opportunity for in-situ characterization of the medium's complexity.Overall, the findings highlight the importance of wave scattering and provide methods and insights for characterizing the Earth's subsurface and improving seismic data interpretation.L'analyse des ondes sismiques est un outil fondamental en géophysique, permettant de mieux comprendre l'intérieur de la Terre et ses structures complexes. Les milieux hétérogènes, caractérisés par des variations rapides des propriétés des matériaux, peuvent donner lieu à des phénomènes de diffraction qui façonne de manière significative les signaux sismiques observés. Une compréhension approfondie de la diffraction des ondes sismiques est cruciale pour obtenir des informations sur les propriétés du milieu de propagation ainsi que sur les sources sismiques qui les génèrent. La présente étude utilise des simulations numériques pour modéliser la propagation des ondes sismiques dans des milieux complexes, explorant non seulement les observables sismologiques traditionnelles, telles que le déplacement ou la vitesse du sol, mais aussi des observables émergentes sous forme de gradients de champ, tels que les rotations et la déformation normales. L'intérêt croissant pour ces nouvelles observables est motivé par les récents progrès de la technologie de détection acoustique distribuée (DAS) pour les mesures de déformation. De nombreuses études ont montré que ces nouvelles observables offrent des sensibilités distinctes par rapport aux mesures traditionnelles. Ce travail étudie la diffraction des ondes dans divers environnements et régimes de diffraction. Dans le régime de diffraction simple, nous avons étudié la sensibilité des nouvelles observables par rapport aux observables traditionnelles pour détecter les hétérogénéités proches de la surface, en termes de changement de phase et d'amplitude, ce dernier se révélant particulièrement efficace pour détecter les hétérogénéités superficielles proches du point d'observation. Après avoir théoriquement caractérisé cette sensibilité, la partie finale de l'étude propose une nouvelle méthode pour identifier les variations localisées de vitesse dans le sub-surface. La méthode a ensuite été appliquée à des données DAS de terrain. Dans le régime de diffraction multiple, deux études ont été réalisées. Après avoir confirmé la fiabilité des simulations numériques par Éléments Spectraux par une comparaison avec l'équation du transfert radiatif, la première étude s'est concentrée sur l'analyse de l'évolution énergétique des observables classiques et nouvelles avec différentes polarisations de source. La différence dans la dynamique de l'évolution énergétique observée dans les rotations suggère que ces observables peuvent apporter des contraintes précieuses pour mieux caractériser le mécanisme et la polarisation de la source sismique. Dans la deuxième étude, nous avons analysé les signaux générés numériquement par des sources densément distribuées au sein d'une structure volcanique, en les corrélant aux propriétés de diffusion de la zone de source. Cette étude a été motivée par la stabilité des pics de fréquences observés dans les données de tremblements volcaniques au fil du temps, et dont l’origine a été proposé être le résultat de résonances dans les dykes et les sills. Nous démontrons que ces résonances peuvent également être créés au sein des zones beaucoup plus vastes, où des ondes de certaines longueurs d'onde deviennent confinées en raison de la longueur de corrélation des variations spatiales du milieu. Enfin, dans une troisième étude utilisant le même ensemble de données synthétiques, nous avons appliqué des techniques d'apprentissage automatique pour regrouper les signaux générés dans différentes zones caractérisées par des propriétés élastiques et de diffusion distinctes, offrant ainsi une nouvelle opportunité pour la caractérisation in situ de la complexité du milieu. Dans l'ensemble, les résultats soulignent l'importance de la diffusion des ondes et fournissent des méthodes et des perspectives pour caractériser le sous-sol terrestre et améliorer l'interprétation des données sismiques
Challenges of Governance and Research on Second Homes in the Alps: Trends in Spatial Development and Housing Uses of Second Homes in the Alps: Regulatory Challenges and Policies
No full textInternational audienceSecond homes are a widespread phenomenon in many parts of the Alpine region. In some places, they are welcomed as second home owners bring capital and life to the villages. Elsewhere, they are criticised because they lead to increasing land consumption as well as housing prices, and the owners tend to remain anonymous. This complex phenomenon does not allow for simple answers and is a matter for both scientific research and spatial planning governance. The ARL European Working Group AlpPlan (alpine spatial planning network) aims to explore this topic by gathering two renowned experts for an ARL Lunch talk to discuss current and future challenges in this field
New methodology for the mechanical characterization of interfaces in all-solid-state lithium batteries
No full textInternational audienceResearch on all-solid-state lithium batteries (LiSSBs) is essential to develop an alternative to conventional lithium-ion batteries. The aim is to enhance the efficiency and the safety of the battery, while using more sustainable and recyclable materials. In order to have a better understanding of the operating behavior of the all-solid-state battery, the materials should be studied from a physico-chemical and a mechanical perspective. Besides, it is necessary to carry these studies on the intrinsic properties of the materials and on their interactions at the interfaces between them (Hao et al., 2024).One critical issue in LiSSBs is the contact failure of the solid/solid interface. It has a significant impact on the electrochemical efficiency and its ability to operate successfully. One of the key interfaces within an all-solid-state lithium metal battery is between the anode, made of thin lithium metal, and the solid-hybrid-electrolyte, made of both organic and inorganic components (Chen et al., 2021; Kalnaus et al., 2023) . In this context, this work focuses on analyzing the behavior of this interface through mechanical measurements of the adhesion between the two components, as well as their individual behavior.The first focus is made on the intrinsic properties of the materials. As lithium metal is the anode, the analysis of its mechanical properties is needed to guarantee its mechanical integrity and stability. Besides, there are a few studies on the mechanical characterizations of lithium metal and fewer in the all solid-state batteries field on lithium metal alone (LePage et al., 2019; Masias et al., 2019; Wang et al., 2024). Thereby some tensile tests were carried out on thin lithium metal and on the solid-electrolyte to determine their properties such as the Young’s modulus and the yield strength.In a second phase, a new and innovative way of characterizing the adhesion of the lithium metal / solid electrolyte interface has been developed. This methodology enables repeatable measurements, of the peel strength and the fracture energy inside an argon glovebox by using a 180° angle peel test principle(Jiang et al., 2020; Bartlett et al., 2023). This data on the level of adhesion makes it possible to assess whether contact can be guaranteed between the lithium metal and the solid electrolyte.For instance, the development of this methodology has enabled us to study the influence of time at 80°C under a 1 bar pressure on the lithium metal / solid-electrolyte interface. This work is completed by analyses of the surfaces and interfaces studied, such as SEM, XPS and impedance with the aim of correlating mechanical behaviors and electro-chemical ones
Fight for Life or Mutual Aid of the Living: A Questioning of the Foundation of Land Ethic
No full textInternational audienceTo defend the thesis of land ethic, John Baird Callicott inspired by Aldo Leopold based the notions of holistic ethics, land community and natural morality on Darwinism. However, how could the theory of evolution, which implies, according to its author, a struggle for life, serve as a foundation for an environmental ethical solidarity? Of course, Darwin does not reduce the behavior of the living to selfishness, and he actually considers solidarity to be a benefit of civilization. Nevertheless, is it not rather natural aggressiveness, which illustrates a war of all against all among the living, which explains human ferocity at the expense of its environment and even its own? In contrast to Darwin’s struggle for life, we will study the competing thesis of mutual aid that Élisée Reclus (French geographer, precursor of ecology, supporter of a solidarist reading of the theory of evolution and anarchist), supported as an ethic of intra- but also inter-species solidarity, the foundation of a legitimate environmental ethic. However, how far can this solidarity go? If there is not only interaction, but even consubstantiality between human and non-human living beings, between living beings and their biotope, is there still a wild nature to be preserved from human impact? We will examine the question of harmony between man, animal and nature and also of what we call today ethnoecology, already implicitly posed by the thought of Reclus (« Histoire d’un ruisseau », 1869 ; « Histoire d’une montagne », 1880)
Local explanations of aggregation results by fuzzy linguistic rules: application to avalanche risk
No full textInternational audienceExplainable Artificial Intelligence (XAI) is relatively new in offering the possibility for intelligent systems to give robust motivations for their decisions and behaviours. Since XAI is human-centred, it has strong connections with fuzzy systems. In this paper, we consider the local explanation of the output of a set of fuzzy linguistic rules from the contributions of the inputs to the output. The proposed contributions are defined from the average of the gradients on the line linking the start point to the end point. This approach ensures that the variation in the output is equal to the sum of the contributions of each input variable to the output. The considered application is based on avalanche expert knowledge expressed by a set of fuzzy rules that combines different physical variables to build an ordinal fuzzy scale of avalanche vigilance ("Relaxed", "Suspicious", "Alert", "Gamble"). The level of vigilance to be applied locally, along a mountain ski route, is explained in a way that is close to expert reasoning in order to decide modifications of the route
On the growth mechanisms of multifunctional metal-oxide nanoparticles based on LiNbO3 from the solvothermal aqueous alkoxide route
No full textInternational audienceNon-centrosymmetric metal oxide nanoparticles, also referred as harmonic nanoparticles, are known to exhibit functional properties that are similar to their bulk counterpart when their size is typically above 20 nm. Among them, lithium niobate (LiNbO3) nanocrystals have for instance gained significant attention as promising candidates for advanced applications in biomedical imaging, nonlinear optics, and sensing owing to their high nonlinear optical coefficients [1,2]. Development of multimodal agents capable of generating diverse contrasts through various physical mechanisms, encompassing MRI [3], computed tomography (CT), photoluminescence from rare-earth ions, and nonlinear optical processes from the same core nanomaterial, is also of current interest. A strict control over the nanoparticle morphology, dispersion, chemical composition and surface chemistry is then needed and, this can be usually achieved if their entire reaction pathways and formation mechanisms are understood. Our poster presentation aims at describing how a non-classical nucleation scheme, featuring octanuclear complexes with a {Li4Nb4O10} core, has first been evidenced when a simple aqueous sol-gel route is applied for the preparation of LiNbO3 nanocrystals from alkoxide precursors. Heat treatment of the as-obtained colloidal gel at room-temperature then demonstrates a non-classical, aggregation-mediated crystallization process [4]. Interestingly, this sol-gel route can be easily extended to the preparation of i) highly-doped LiNbO3 nanocrystals with, for example, optimal concentrations in lanthanide ions such as Er and Yb [5], ii) lithium tantalate (LiTaO3) nanocrystals and iii) non-stoichiometric LiNbO3 nanocrystals with exalted second harmonic properties for a given range of compositions. These latter are quantitatively assessed from a state-of-the art multiphoton platform featuring microscopic measurement at the nanocrystal scale and a second-harmonic-scattering spectroscopic configuration for nanoparticle suspensions [6]
Preschool teachers provide fewer participation opportunities to working-class students than those from more privileged backgrounds
No full textInternational audienceSocial class disparities exist from the earliest stages of education. Research has suggested that class-based differences in factors such as socialization practices and access to resources partly explain this phenomenon, but less work has explored whether teachers’ practices also exacerbate these inequalities. Using whole-class observations of 63 preschool classroom discussions (N = students, 10 teachers), we coded 7, student participation attempts and subsequent responses from teachers. Mixed-effects Bayesian logistic regressions showed that whether students played by the rules by raising their hands or broke the rules by calling out, they were less likely to have their participation attempts accepted if they came from a working-class background, even when their perceived language skills were matched to their middle- and upper-class peers. These results suggest that early schooling experiences may serve to exacerbate inequalities rather than level the playing field
Power Electronics Solar Inverter on a Biosourced and Biodegradable Substrate -Thermal Study
No full textInternational audienceThis paper presents a comparative study between traditional and sustainable approaches on a power electronics inverter; it focuses on the thermal management of the DC-AC bridge thermal losses. The system is inspired from state-of-theart designs of photovoltaic inverter DC-AC stages. We realized a design on a traditional printed circuit board Flame Retardant 4 (FR4) substrate, made of 4 copper layers (further called "reference design"). An adaptation of this design was prepared to implement it on a novel biobased and biodegradable substrate based on polylactic acid (PLA), flax fibers and biosourced flame retardant, with only 2 copper layers. Because this composite has a reduced glass transition temperature compared to FR4, it makes it more sensitive to intense thermal dissipation. In this system, the thermal dissipation is achieved through the PCB with thermal vias. An analytical modeling was performed to evaluate the thermal resistance of the vias in both designs and the resulting increase of temperature of the substrate. Thermal infrared measurements were then conducted on both designs, at selected operating points to evaluate the possibilities and limitations of the bio-substrate on such application