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La préhistoire, champ d'étude pluridisciplinaire
No full textLe récent Colloque « Climats, Cultures et Sociétés aux Temps Préhistoriques» (de l'apparition des hominidés jusqu 'au Néolithique) a montré comment les études préhistoriques se sont complètement transformées au cours des vingt dernières années par l'utilisation de toutes sortes de technologies modernes empruntées à d'autres disciplines scientifiques. Le métier de préhistorienest aujourd'hui un métier très différent de ce qu'il était il y a quelques décennies.Autrefois les préhistoriens ramassaient de beaux objets, des objets qui étaient des témoignages des hommes fossiles, on disait même à une époque des hommes antédiluviens. Les notions scientifiques sont apparues avec Marcellin Boule, qui a fondé l'Institut de Paléontologie humaine à Paris. On a essayé de replacer l'évolution culturelle dans le cadre de la stratigraphie du Quaternaire. Mais aujourd'hui, on peut parler des sciences préhistoriques car la préhistoire est au carrefour de très nombreuses disciplines, sciences de la Terre, sciences physiques, sciences de la Vie, sciences humaines
Contribution of a three-dimensional regional scale basin model to the study of the past fluid flow evolution and the present hydrology of the Paris basin, France
No full textInternational audienceA basin model was built to simulate in three dimensions the 248 Myr geological history of the Paris basin, France, i.e. sedimentation, erosion, compaction heat and fluid flow. This multidisciplinary study was based on a detailed stratigraphic database of more than 1100 well logs together with a hydrodynamic database of 1000 data (heads and permeabilities). The region covers a maximum surface area of 700 000 km(2). The NEWBAS code of the Ecole des Mines de Paris was used in order to simulate compaction and heat and fluid flow. Three examples of the use of this model are given to illustrate different features of the geological functioning of the basin. (i) By modelling processes such as sedimentation, compaction, fluid and heat flow, the model provides estimates of the hydraulic conductivity fields within one order of magnitude from observations at the regional scale. This permeability field can reproduce the present-day observed pressures and fluxes in the basin. (ii) Observed excess pressures in the main aquitards are considered as possible consequences of the geological history of the basin. The calculated excess pressures are small and stay within the range of the measured values, between 0 and 2.75 MPa, close to the pressures in the aquifers. However, the weak excess pressures measured in the Callovo-Oxfordian sequence in the eastern part of the basin are not reproduced by the model. Mechanisms other than compaction disequilibrium must be invoked. (iii) This model also calculates regional-scale palaeofluid flow whose value is currently arbitrarily assumed by geochemists when studying diagenetic processes. Hence, it provides a hydrologic background for diagenetic models. The cementation in the western Keuper reservoirs was investigated. Topographically driven flow during tectonic inversion periods, e.g. the Lower Cretaceous and Early Tertiary, is shown to be a plausible cause of brine migrations. This brine displacement would then explain the high salinities recorded in the fluid inclusions trapped in the Keuper cements. The conditions for the migration would have been most favourable at the time of the maximum burial, i.e. the Early Tertiary and not the Early Cretaceous as previously suggested
From Galileo to Convexity: some key ideas in structural mechanics
No full textThe main topic treated in the First Day and the Second Day of Galileo’s Dialogues is the resistance that solids offer to fracture with special consideration to prisms and cylinders submitted to axial tensile loading or to “transverse”, i. e. bending, forces. Although no consideration is given to deformation of the solid before fracture one may say that Galileo implicitly introduces the concept of a Continuum within which coherence forces do act in order to maintain the filaments, fibres or any other constituent particles together. Thus, he opens the way to the concept of stress, which was settled explicitly some 200 years later. Having recognised that coherence forces and gravity forces in a solid are not related in the same way to its geometric scale, he performs what can be considered as the first striking example of dimensional analysis with application to similarity. In its celebrated analysis of the resistance of a cantilever beam submitted to bending, Galileo gives a first attempt to deriving the resistance of a whole solid submitted to some kind of loading from the resistance of its constituent material determined from another test. Together with Coulomb’s celebrated Essay these are two milestones of the Theory of yield design, the fundamental root of Ultimate Limit State Design, which is presently introduced in international codes for civil engineerin
