Intégration de données géophysiques dans la réalisation de microzonages

International audienceLors d’études récentes de microzonage sismique, le BRGM a utilisé deux méthodes géophysiques dans l’objectif de mieux contraindre les données géologiques et de suppléer au manque de sondages géotechniques. La technique H/V bruit de fond et la méthode SASW sont théoriquement complémentaires : La méthode H/V doit donner la fréquence de résonance du sol, reliée à l’épaisseur des couches et à la vitesse des ondes de cisaillement dans ces couches, paramètres qui peuvent être estimés par la méthode SASW. Cependant, leur mise en application, même si elle a apporté une aide très significative pour la réalisation des microzonages, a également soulevé beaucoup de questions. A partir de quelques exemples, nous proposons de montrer les apports réciproques de ces deux méthodes au travers de la réalisation de ces microzonages

Initial growth phases of two bloom-forming cyanobacteria (

Proliferations of cyanobacteria have detrimental effects on ecosystem functioning, and on the global freshwater food chain. Many studies have focused on the “in situ” dynamics of bloom-forming cyanobacteria, including Cylindrospermopsis raciborskii and Planktothrix agardhii. Few have used experimental assays to explore the fast-growing ability of naturally co-occurring species. Here we investigated the growth of these species when exposed separately (i.e., in monocultures) to a range of light and nutrient conditions, plus their interactive performances in mixed cultures in a short-time experiment (6 days). The use of microplates made it possible to carry out multiple measurements of in-vivo fluorescence (IVF), and to monitor species-dependent biovolumes. No allelopathic effect was significantly observed for any target species, while significantly lower growth rates were obtained in mixed cultures, which may reflect other interference interactions between the species. We showed that Planktothrix grew faster with low light intensity and high nutrient concentrations, and was drastically inhibited by nitrogen deprivation, in contrast to Cylindrospermopsis. However, Cylindrospermopsis outgrew Planktothrix at high NH4+ concentrations, suggesting that this species may be a serious competitor for the native species in many water systems.- Initial fast-growing abilities of bloom-forming species assessed in mono vs mixed cultures. - A low growth rate recorded in mixed cultures, suggesting possible competitive interactions under specific conditions. - P. agardhii outcompete C. raciborski at low light intensity and high NO3- source; while C. raciborski outgrow at high NH4+ levels

Seismic Microzonation: A Comparison between Geotechnical and Seismological Approaches in Pointe-à-Pitre (French West Indies)

International audiencePointe-à-Pitre, the main city of Guadeloupe in the French West Indies, has on several occasions been partially destroyed by major historical earthquakes. Moreover, a post-seismic assessment of the damage from the 1985 Montserrat earthquake indicates that the town is prone to site effects. Consequently, from 1996 to 1998, BRGM conducted a seismic microzonation study based on geotechnical and geological data. At the same time, three seismological studies were being conducted – two based on earthquake recordings using a time-series analysis and the classical spectral ratio (CSR) method (CETE/LCPC and BRGM), and the third based on noise measurement at 400 points using the horizontal-to-vertical noise ratio (HVNR) method (CETE/LCPC). The objective of this paper is not to carry out a new microzonation study by taking into account all the results, but rather to show in what respects the results of these different methods are in agreement or not. A comparison of the results of the seismological studies with the geotechnical microzonation shows that they are in fairly good agreement, albeit with some discrepancies. The results indicate that the seismological methods and the geotechnical data are highly complementary and should be used together in compiling seismic transfer-function microzonation maps