Images du quotidien et patrimoines sociaux : la mise en images d’archives

L’objet du chapitre est de faire émerger des éléments de convergence quant aux étapes de la mise en images d’archives. D’abord, la conservation. Que l’on se situe dans une dialectique du passé à travers des images anciennes ou que l’on ait affaire au désir de transmission d’images récentes, l’impératif de la pérennisation joue et l’injonction des archives est à l’œuvre. Ensuite, la monstration. Que l’image originelle soit intégrée au décor d’un restaurant ou partagée dans une communauté virtuelle, il n’est guère d’image d’archives que vue et offerte au regard. Le statut d’image d’archives est conféré par un regard extérieur, en dehors du cercle de production et de diffusion initial – la famille par exemple. Enfin, la modification. L’image d’archives ne se conçoit que médiatisée, insérée dans un discours et presque toujours modifiée : découpée, agrandie, teintée, détournée de son objet initial, décryptée, c’est véritablement une autre image qui est produite à partir de l’image primaire. L\u27image créée et mise en scène joue sur des codes convenus, dans une allusion explicite ou plus discrète, à ceux des patrimoines savants. Les procédés de médiation co-construisent l\u27image d\u27archives parce qu\u27ils la mettent en exposition et ainsi déclenchent son accréditation comme objet patrimonial. Le statut d\u27image d\u27archives ne serait donc que transitoire, valable uniquement au moment et dans l’instant de son usage, contrairement à celui de document d’archives ou d’image archivée, défini par un lieu de conservation fixe et des modalités d’accès stables

Images archivées, images d’archives : fortunes terminologiques

On ne peut manquer d’être frappé de la quasi-totale absence dans l’institution archivistique française et dans les milieux professionnels qui l’animent, du moins jusqu’à une date récente, non seulement de l’expression « image d’archives » mais encore du mot même « image ». Cette constatation constitue le point de départ d’une enquête terminologique et lexicologique : d’où vient cette expression « images d’archives » ? Quelles sont les étapes et les raisons de sa diffusion 

Ephemerality of discrete methane vents in lake sediments

Methane is a potent greenhouse gas whose emission from sediments in inland waters and shallow oceans may both contribute to global warming and be exacerbated by it. The fraction of methane emitted by sediments that bypasses dissolution in the water column and reaches the atmosphere as bubbles depends on the mode and spatiotemporal characteristics of venting from the sediments. Earlier studies have concluded that hot spots—persistent, high-flux vents—dominate the regional ebullitive flux from submerged sediments. Here the spatial structure, persistence, and variability in the intensity of methane venting are analyzed using a high-resolution multibeam sonar record acquired at the bottom of a lake during multiple deployments over a 9 month period. We confirm that ebullition is strongly episodic, with distinct regimes of high flux and low flux largely controlled by changes in hydrostatic pressure. Our analysis shows that the spatial pattern of ebullition becomes homogeneous at the sonar's resolution over time scales of hours (for high-flux periods) or days (for low-flux periods), demonstrating that vents are ephemeral rather than persistent, and suggesting that long-term, lake-wide ebullition dynamics may be modeled without resolving the fine-scale spatial structure of venting.National Science Foundation (U.S.) (1045193)United States. Department of Energy (DE-FE001399

On switching response surface models, with applications to the structural health monitoring of bridges

Structural Health Monitoring (SHM) is the engineering discipline of diagnosing damage and estimating safe remaining life for structures and systems. Often, SHM is accomplished by detecting changes in measured quantities from the structure of interest; if there are no competing explanations for the changes, one infers that they are the result of damage. If the structure of interest is subject to changes in its environmental or operational conditions, one must understand the effects of these changes in order that one does not falsely claim that damage has occurred when changes in measured quantities are observed. This problem – the problem of confounding influences – is particularly pressing for civil infrastructure where the given structure is usually openly exposed to the weather and may be subject to strongly varying operational conditions. One approach to understanding confounding influences is to construct a data-based response surface model that can represent measurement variations as a function of environmental and operational variables. The models can then be used to remove environmental and operational variations so that change detection algorithms signal the occurrence of damage alone. The current paper is concerned with such response surface models in the case of SHM of bridges. In particular, classes of response surface models that can switch discontinuously between regimes are discussed. Recently, it has been shown that Gaussian Process (GP) models are an effective means of developing response surface or surrogate models. However, the GP approach runs into difficulties if changes in the latent variables cause the structure of interest to abruptly switch between regimes. A good example here, which is well known in the SHM literature, is given by the Z24 Bridge in Switzerland which completely changed its dynamical behaviour when it cooled below zero degrees Celsius as the asphalt of the deck stiffened. The solution proposed here is to adopt the recently-proposed Treed Gaussian Process (TGP) model as an alternative. The approach is illustrated here on the Z24 bridge and also on data from the Tamar Bridge in the UK which shows marked switching behaviour in certain of its dynamical characteristics when its ambient wind conditions change. It is shown that treed GPs provide an effective approach to response surface modelling and that in the Tamar case, a linear model is in fact sufficient to solve the problem

Natural Nuclear Reactor Oklo and Variation of Fundamental Constants Part 1: Computation of Neutronics of Fresh Core

Using modern methods of reactor physics we have performed full-scale calculations of the natural reactor Oklo. For reliability we have used recent version of two Monte Carlo codes: Russian code MCU REA and world wide known code MCNP (USA). Both codes produce similar results. We have constructed a computer model of the reactor Oklo zone RZ2 which takes into account all details of design and composition. The calculations were performed for three fresh cores with different uranium contents. Multiplication factors, reactivities and neutron fluxes were calculated. We have estimated also the temperature and void effects for the fresh core. As would be expected, we have found for the fresh core a significant difference between reactor and Maxwell spectra, which was used before for averaging cross sections in the Oklo reactor. The averaged cross section of Sm-149 and its dependence on the shift of resonance position (due to variation of fundamental constants) are significantly different from previous results. Contrary to results of some previous papers we find no evidence for the change of the fine structure constant in the past and obtain new, most accurate limits on its variation with time: -4 10^{-17}year^{-1} < d alpha/dt/alpha < 3 10^{-17} year^{-1} A further improvement in the accuracy of the limits can be achieved by taking account of the core burnup. These calculations are in progress.Comment: 25 pages, 14 figures, 12 tables, minor corrections, typos correcte

Effect of ship locking on sediment oxygen uptake in impounded rivers

In the majority of large river systems, flow is regulated and/or otherwise affected by operational and management activities, such as ship locking. The effect of lock operation on sediment-water oxygen fluxes was studied within a 12.9 km long impoundment at the Saar River (Germany) using eddy-correlation flux measurements. The continuous observations cover a time period of nearly 5 days and 39 individual locking events. Ship locking is associated with the generation of surges propagating back and forth through the impoundment which causes strong variations of near-bed current velocity and turbulence. These wave-induced flow variations cause variations in sediment-water oxygen fluxes. While the mean flux during time periods without lock operation was 0.5 6 0.1 g m�2 d�1, it increased by about a factor of 2 to 1.0 6 0.5 g m�2 d�1 within time periods with ship locking. Following the daily schedule of lock operations, fluxes are predominantly enhanced during daytime and follow a pronounced diurnal rhythm. The driving force for the increased flux is the enhancement of diffusive transport across the sediment-water interface by bottom-boundary layer turbulence and perhaps resuspension. Additional means by which the oxygen budget of the impoundment is affected by lock-induced flow variations are discussed

Using noble gases to compare parameterizations of air‐water gas exchange and to constrain oxygen losses by ebullition in a shallow aquatic environment

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Biogeosciences 123 (2018): 2711-2726, doi:10.1029/2018JG004441.Accurate determination of air‐water gas exchange fluxes is critically important for calculating ecosystem metabolism rates from dissolved oxygen in shallow aquatic environments. We present a unique data set of the noble gases neon, argon, krypton, and xenon in a salt marsh pond to demonstrate how the dissolved noble gases can be used to quantify gas transfer processes and evaluate gas exchange parameterizations in shallow, near‐shore environments. These noble gases are sensitive to a variety of physical processes, including bubbling. We thus additionally use this data set to demonstrate how dissolved noble gases can be used to assess the contribution of bubbling from the sediments (ebullition) to gas fluxes. We find that while literature gas exchange parameterizations do well in modeling more soluble gases, ebullition must be accounted for in order to correctly calculate fluxes of the lighter noble gases. In particular, for neon and argon, the ebullition flux is larger than the differences in the diffusive gas exchange flux estimated by four different wind speed‐based parameterizations for gas exchange. We present an application of noble gas derived ebullition rates to improve estimates of oxygen metabolic fluxes in this shallow pond environment. Up to 21% of daily net oxygen production by photosynthesis may be lost from the pond via ebullition during some periods of biologically and physically produced supersaturation. Ebullition could be an important flux of oxygen and other gases that is measurable with noble gases in other shallow aquatic environments.NSF. Grant Numbers 1233678, 1238212, DEB 1354494; Woods Hole Oceanographic Institution (WHOI); National Defense Science and Engineering Graduate Fellowship; Northeast Climate Science Center Grant Number: DOI G12AC000012019-03-0

Porewater methane transport within the gas vesicles of diurnally migrating Chaoborus spp.: An energetic advantage

We show that diurnally migrating Chaoborus sp. (phantom midge larvae), which can be highly abundant in eutrophic lakes with anoxic bottom, utilises sediment methane to inflate their tracheal sacs, which provides positive buoyancy to aid vertical migration. This process also effectively transports sediment methane bypassing oxidation to the upper water column, adding to the total methane outflux to the atmosphere