Granite microporosity changes due to fracturing and alteration: secondary mineral phases as proxies for porosity and permeability estimation

Several alteration facies of fractured Lipnice granite are studied in detail on borehole samples by means of mercury intrusion porosimetry, polarized and fluorescent light microscopy, and microprobe chemical analyses. The goal is to describe the granite void space geometry in the vicinity of fractures with alteration halos and to link specific geometries with simply detectable parameters to facilitate quick estimation of porosity and permeability based on, for example, drill cuttings. The core of the study is the results of porosity and throat size distribution analyses on 21 specimens representing unique combinations of fracture-related structures within six different alteration facies basically differing in secondary phyllosilicate chemistry and porosity structure. Based on a simple model to calculate permeability from the measured porosities and throat size distributions, the difference in permeability between the fresh granite and the most fractured and altered granite is 5 orders of magnitude. Our observations suggest that the porosity, the size of connections and the proportion of crack porosity increase with fracture density, while precipitation of iron-rich infills as well as of fine-grained secondary phyllosilicates acts in the opposite way. Different styles and intensities of such end-member agents shape the final void space geometry and imply various combinations of storage, transport and retardation capacity for specific structures. This study also shows the possibility to use standard mercury intrusion porosimetry with advanced experimental settings and data treatment to distinguish important differences in void space geometry within a span of a few percent of porosity.</p

Sensitivity of photonic crystal fiber modes to temperature, strain and external refractive index

Several strong narrowband resonances are observed in the transmission spectra of fiber Bragg gratings photo-written in photonic crystal fiber that has a refractive index-neutral germanium/fluorine co-doped core. Experimental results for the strain, temperature and refractive index sensitivities of these mode resonances are reported and compared to those of conventional single mode fiber. In particular, we identify three kinds of resonances whose relative sensitivities to strain, temperature and refractive index are markedly different and present numerical simulations to explain these properties. Potential multiparameter optical sensor applications of these mode resonances are briefly discussed

Why pharmacokinetic differences among oral triptans have little clinical importance: a comment

Triptans, selective 5-HT1B/1D receptor agonists, are specific drugs for the acute treatment of migraine that have the same mechanism of action. Here, it is discussed why the differences among kinetic parameters of oral triptans have proved not to be very important in clinical practice. There are three main reasons: (1) the differences among the kinetic parameters of oral triptans are smaller than what appears from their average values; (2) there is a large inter-subject, gender-dependent, and intra-subject (outside/during the attack) variability of kinetic parameters related to the rate and extent of absorption, i.e., those which are considered as critical for the response; (3) no dose-concentration–response curves have been defined and it is, therefore, impossible both to compare the kinetics of triptans, and to verify the objective importance of kinetic differences; (4) the importance of kinetic differences is outweighed by non-kinetic factors of variability of response to triptans. If no oral formulations are found that can allow more predictable pharmacokinetics, the same problems will probably also arise with new classes of drugs for the acute treatment of migraine

Crise climatique et mutations de l'agriculture : l'émergence d'un grenier céréalier entre Tchad et Chari

The Sahelian climatic crises of the 1970's and 80's have paradoxically had positive consequences on the southern banks of Lake Chad. The withdrawal of the Lake waters has opened vast superficies favorable to subsidence cultures. In collaboration with the Chadian state, Non-Governmentable Organisations and international financial backers have settled on these banks populations from other Sahelian regions hit by the drought. The adoption of new agricultural applications coincides with the replacement of the agriculture of self consomption by a commercial agriculture, vastly oriented towards the N'Djamena's market. The improvement of transport conditions has played an important role in the development of these new relations between the Chadian capital (population 600 000) and the south banks of the Lake. These agricultural mutations lead to important social mutations. Farmers associations allow the emergence of a dynamic peasant middle class, while those who cannot have access to the best subsidence lands are marginalised. The economical success of the farmers associations is frail. Also, their capacity to promote new social relations, independant from traditional structures, is an important point of interrogation. One must bear in mind that these processes are fragile; they could be cancelled by a rising level of the Lake.Les crises climatiques sahéliennes des années 1970-80 ont eu, paradoxalement, des conséquences positives au sud du lac Tchad. Le recul des eaux du lac a ouvert de vastes superficies propices aux cultures de décrue. En collaboration avec l'État tchadien, des O.N.G. et des bailleurs de fonds ont installé sur ces rives des populations originaires d'autres régions sahéliennes frappées par la sécheresse. L'adoption de nouvelles pratiques culturales coïncide avec le remplacement de l'agriculture d'autoconsommation par une agriculture commerciale, largement orientée vers le marché de N'Djaména. Ces mutations agricoles s'accompagnent de mutations sociales importantes. Des groupements villageois permettent l'émergence d'une classe moyenne paysanne dynamique, tandis que ceux qui ne peuvent accéder aux meilleures terres de décrue sont marginalisés. Ces processus sont néanmoins fragiles ; ils pourraient être remis en cause par une remontée du niveau lacustre.Géraud Magrin Y. Crise climatique et mutations de l'agriculture : l'émergence d'un grenier céréalier entre Tchad et Chari . In: Annales de Géographie, t. 105, n°592, 1996. pp. 620-644

Porosity and Compaction State at the Active Pāpaku Thrust Fault in the Frontal Accretionary Wedge of the North Hikurangi Margin

International audienceCharacterization of the porosity evolution across the sedimentary section entering subduction zones and accreted sediments provide valuable information for understanding the deformation history at accretionary margins and the physico-chemical processes in and around fault zones. International Ocean Discovery Program Expeditions 372 and 375 drilled, logged, and cored the reference section on the incoming plate (Site U1520) and across the active Pāpaku thrust (Site U1518), which is a <30° westward-dipping splay fault in the frontal accretionary wedge at the north Hikurangi margin in a region where tsunami earthquakes and recurrent slow slip events are documented. We observe strong variations of physical properties across the thrust fault. In particular, interstitial porosity increases by ∼10% through the fault zone while it exponentially decreases with depth, showing lower values in the hanging-wall (HW; on average, ∼36%) than in the footwall (∼42%). Based on comparisons of porosity with the compaction curve at Site U1518 with that of reference Site U1520, we infer an overcompaction in the HW and a nearly normal compaction in the footwall of the Pāpaku thrust. We suggest that the porosity pattern across the thrust infers differences in the maximum burial depth of sediments, characterized by an overcompacted HW that has been uplifted, thrusted, and concomitantly tectonically eroded above the footwall. Porosity data indicate hydrostatic conditions around the thrust fault contrasting with commonly assumed excess pore pressure at the plate interface