A neural surveyor to map touch on the body

Perhaps the most recognizable sensory map in all of neuroscience is the somatosensory homunculus. Although it seems straightforward, this simple representation belies the complex link between an activation in a somatotopic map and the associated touch location on the body. Any isolated activation is spatially ambiguous without a neural decoder that can read its position within the entire map, but how this is computed by neural networks is unknown. We propose that the somatosensory system implements multilateration, a common computation used by surveying and global positioning systems to localize objects. Specifically, to decode touch location on the body, multilateration estimates the relative distance between the afferent input and the boundaries of a body part (e.g., the joints of a limb). We show that a simple feedforward neural network, which captures several fundamental receptive field properties of cortical somatosensory neurons, can implement a Bayes-optimal multilateral computation. Simulations demonstrated that this decoder produced a pattern of localization variability between two boundaries that was unique to multilateration. Finally, we identify this computational signature of multilateration in actual psychophysical experiments, suggesting that it is a candidate computational mechanism underlying tactile localization

CO2 streams containing associated components—A review of the thermodynamic and geochemical properties and assessment of some reactive transport codes

AbstractModelling of the impact on storage of “ CO2-associated components” has rarely been addressed so far. This review, performed within the European research project CO2ReMoVe, exposes a selection of CO2 streams compositions coming from thermal power plants emissions and those injected in pilot sites part of the CO2ReMoVe project. It highlights the lack of data coming from laboratory experiments to describe properly the physical properties of some relevant gas mixtures. The geochemical impact of only 2 components (SO2 and H2S) is evidenced by some geochemical studies. Concerning the numerical modelling, four reactive transport codes (PHREEQC, SCALE2000, TOUGHREACT and COORES) were assessed. Actual limitations lie mainly in the capacity of calculating the physical properties of the whole set of gases (CO2–O2–SO2–N2–Ar–NOx–H2S–COS–CO–H2–HCl–NH3–CH4–C2H6–H2O). The new data acquired within on-going French projects will complete the knowledge of such complex gas mixtures behaviour

Hydrogen solubility in pore water of partially saturated argillites: Application to Callovo-Oxfordian clayrock in the context of a nuclear waste geological disposal

In nuclear waste geological disposals, large amounts of hydrogen (H2) are expected to be produced by different (bio-)geochemical processes. Depending on the pressure generated by such a process, H2 could be produced as a gas phase and displace the neighbouring pore water. As a consequence, a water-unsaturated zone could be created around the waste and possibly affect the physical and physic-chemical properties of the disposal and the excavation disturbed zone around it. The present study is the first part of an ongoing research program aimed at evaluating the possible chemical evolution of the pore water-minerals-gas system in such a context. The goal of this study was to evaluate, in terms of thermodynamic equilibrium conditions, the geochemical disturbance of the pore water due to variations in hydrogen pressure, temperature and relative humidity. No heterogeneous reactions involving mineral phases of the clayrock or reactive surface sites were taken into account in the thermodynamic analysis. In the case sulphate reduction reaction is allowed, geochemical modelling results indicate that the main disturbance is the increase in pH (from around 7 up to more than 10) and an important decrease in the redox potential (Eh) related to hydrogen dissolution. This occurs from relatively low H2 partial pressures (∼1bar and above). Then, temperature and relative humidity (expressed in terms of capillary pressure) further displace the thermodynamic equilibrium conditions, namely the pH and the aqueous speciation as well as saturation indices of mineral phases. Finally, the results suggest that the generation of hydrogen, combined with an increase in temperature (between 30°C and 80°C) and a decrease in relative humidity (from 100% to 30%), should increase the chemical reactivity of the pore water-rock-gas system. © 2011 Elsevier Ltd

Improvement of the Calculation Accuracy of Acid Gas Solubility in Deep Reservoir Brines: Application to the Geological Storage of CO

The assessment of the short and long term consequences of CO2 injection in aquifers requires both laboratory experiments and numerical modelling in order to better understand the various physical-chemical processes taking place. Modelling injection in a reservoir, where relatively high temperature (above 50°C), high pressure (several hundreds of bars), and high salinity (greater than that of seawater) conditions are likely to be encountered, thus requires numerical tools able to take into account the specific effects of the various electrolytes dissolved in brines, and the non-ideal behaviour of the CO2 gaseous phase. This study evaluates the consistency of the various corrections (activity, fugacity, influence of pressure on thermodynamic constants) to be taken into account in geochemical models to meet these calculation accuracy requirements. These corrections were implemented in the thermo-kinetic modelling software SCALE2000 (Azaroual et al., 2004a) which was used to check their validity by comparing the calculation results with available experimental observations and other results from CO2 solubility calculation models. An estimation of the relative weight of each of the corrections for a 237 g.l-1 brine (60°C, pCO2 = 200 bar) showed a systematic overestimation (higher than 100%) of CO2 solubility when either salinity (NaCl equivalent) is neglected or gas is considered ideal. The error induced by the NaCl-equivalent approximation compared to real brine is lower (less than 5%). The second part of this study presents an application example of a hypothetical scenario of massive CO2 injection in a carbonated reservoir; data used for the brine composition are actual data (Moldovanyi and Walter, 1992) from the Smackover site (Arkansas, United States). The simulations performed considering a representative elementary volume of saturated bulk rock (porous mineral assemblage saturated with the Smackover brine) with a prescribed constant CO2 pressure of 150 bar, show two distinctively different behaviours whether the system is assumed to be a closed (batch reactor) or an open reactor fed by a constant brine flow rate. In the first case, the calculations performed with SCALE2000 lead to negligible variations in the mineralogy. In the second case, more representative of the dynamical nature of an injection system, the results show major modifications in the mineralogy finally leading to a strong increase in porosity (from 20% initially to 85% after 50 y of simulated time). Further calculations were carried out with SCALE2000, now considering a 1D system constituted of a set of four homogeneous identical reactors connected in series (fluid velocity of 1 m.day-1). With initial and boundary conditions similar to those considered earlier, and prescribing a constant pCO2 in the first reactor only, the results showed that significant dolomite precipitation occurred in the most-downstream reactor hence inducing some CO2 precipitation. Mass balance calculations performed on the four reactors system finally demonstrated a global loss in total mineral carbon with respect to the amounts initially available. However, the evolution trends observed in the most-downstream two reactors indicated that possible trapping might be expected beyond the relatively limited geometrical boundaries considered in the modelled system

Realization and characterization of a beam of titanium atoms

A dense thermal beam of titanium atoms has been realized using a tungsten crucible inside a high temperature oven ($\leq 2\,300$ K). Its flux ($\approx 10^{14}$ atom/cm$^2$/s) and its long term stability have been measured by means of a quartz balance, its angular divergence has been evaluated from the size of a metallic spot deposited on a glass plate. A spectroscopic investigation performed via the laser-induced fluorescence technique has led to the measurement of the isotope shifts between $^{46}$Ti, $^{48}$Ti and $^{50}$Ti, for six visible 3d$^2$ 4s$^2$ a $^3$F$_{\rm J''}\to 3$d$^2$ 4s4p z $^5$D$_{\rm J'}$ transitions. This beam offers the characteristics required to apply high resolution laser techniques to reaction dynamics crossed-beam experiments.Un jet intense de titane atomique a été réalisé à partir d'un creuset de tungstène porté à haute température ($\leq 2\,300$ K). Le flux ($\approx 10^{14}$ atomes/cm$^2$/s) et la stabilité à long terme du jet ont été mesurés avec une balance à quartz ; la divergence angulaire a été évaluée à partir de la dimension de dépôts métalliques sur des lames-témoin. Le jet a également été caractérisé par spectroscopie : la technique de fluorescence induite par laser a permis de mesurer le déplacement spectral dû aux isotopes 46, 48 et 50 de Ti, pour six transitions visibles du type 3d$^2$ 4s$^2$ a $^3$F$_{\rm J''}\to 3$d$^2$ 4s4p z $^5$D$_{\rm J'}$. Le jet réalisé offre les caractéristiques requises pour l'application des techniques laser à haute résolution aux expériences de dynamique réactionnelle en faisceaux croisés

Modélisation thermodynamique de la dégradation de déchet : prévision du devenir à long terme

Le stockage ou la banalisation de déchets contenant (cendres stabilisées, produits vitrifiés) pose le problème de la stabilité dans le temps des structures minérales assurant la stabilisation ou le piégeage des éléments toxiques. Ainsi, à ce jour, en France comme dans tous les pays européens, une réflexion ayant pour thème le devenir à long terme des résidus de l'incinération est en cours. Elle devrait aboutir à l'élaboration d'une batterie de tests (lixiviation, neutralisation acidobasique, sohxlet ...) susceptibles de fournir les paramètres nécessaires à la prédiction de l'évolution chimique de produits soumis à des conditions physicochimiques données. Malheureusement ce type de démarche, ne s'appuyant que sur la mesure de phénomènes immédiats, ne pourra apporter qu'une interprétation imparfaite quant au devenir à long terme. A travers la modélisation thermodynamique de la dégradation des déchets il est possible de proposer une démarche complémentaire à l'approche purement analytique. Les travaux présentés ici font état d'une démarche menée autour d'un vitrifiat de Refiom