INTERFÉROMÉTRIE RADAR APPLIQUÉE AUX VOLCANS : CAS DE L’ETNA ET DES CHAMPS PHLÉGRÉENS (ITALIE)

During the last few years, the radar images collected by the European satellites ERS1 and ERS2, the Japanese satellite JERS and the Canadian satellite RADARSAT have been used with success to create interferograms. This technique has been applied for geophysical applications like co-seismic deformation mapping, volcano deformation monitoring, landslides monitoring, mining subsidence detection, glaciers monitoring. Here we report the research carried out by our group on Etna volcano (Italy) and in the area of Naples (Italy) where are located several potentially active volcanoes (Vesuvius, Ischia) and where a subsidence of the caldera of Campi Flegrei is still on going in response to the 1982-1984 seismic crisis. Etna is the volcano that has been studied first using ERS SAR interferometry. Using this method, a large scale deflation of the volcano associated with the large 1991-1993 eruption was detected in data covering the second half of the eruption. Further studies showed that the local deformation fields located in Valle del Bove (East of the volcano) where associated with the compaction of the 1986-1987 and 1989 lava fields and also partly with a subsidence of the surrounding terrain in response to the load of the new deposited material. Other local deformation fields have been identified, corresponding to the 1983, 1981 and 1971 lava fields. However, due to its strong topography, interferograms of Etna are affected by tropospheric effects. Those effects must be eliminated in order to correctly interpret the fringes pattern. The problem of the troposphere has been first investigated from its theoretical point of view and using existing local meteorological data as well as radio-soundings data. Recently, thanks to the large amount of available interferograms, another approach has been investigated, consisting in the research of a correlation fringe/elevation in the interferograms themselves. This approach, operated either in automatic mode (automatic fringe unwrapping) or in manual mode proved to be efficient for most of the coherent interferograms. After removal of the tropospheric correction, the evolution of the deformation of the volcano at large scale between 1992 and 1998 has been inferred. The subsidence occurring during the second half of the 1992 eruption as well as the uplift preceding the 1995 unrest of the Southeast crater are visible, but their amplitude is less than previously estimated. The depth of the modelled source of subsidence/uplift related to the large scale deformation is of the order of 6 to 8 km, not well constrained by the data. The study of the correlation fringe/elevation was possible only after a detailed analysis of the spatial and temporal properties of coherence of the Etna area. Indeed, the technique of fringe unwrapping for fringe/elevation correlation analysis is possible only if the poorly coherent pixels are eliminated. A map of the most coherent pixels of the volcano was produced. The recent lava fields as well as the towns and villages surrounding the volcano are the most coherent areas. The quality of the interferograms is also enhanced when high accuracy DEM (Digital Elevation Model) are used. Using kinematic GPS data collected along more than 100 km of road around the volcano, we assessed the accuracy of several DEMs of Etna. The most accurate DEM was produced by digitising 1/25.000e maps of Etna. This DEM does not take into account the topographic changes due to the recent eruptions. Merging other more recent DEMs corresponding to those areas, we produced an updated relatively high accuracy DEM (±3 m) of Etna. In the Naples area, we analysed interferograms in the period 1993-1996 and show that the Campi Flegrei caldera is still subsiding at a rate of about 30 mm/year

Les retards de recours et de soins à Kinshasa en cas de maladie sévère chez la femme en âge de procréer

Background. - Mortality of adult females is very high in Democratic Republic of Congo and often caused by diseases that could have been controlled if treated in time. This is a qualitative study on delays and their causes in the care-seeking process offal women who died prematurely in Kinshasa from non-immediately fatal causes. Method. - This study concerned 60 women identified in two Kinshasa mortuaries in March and April 2004 who had died at the age of IS to 49 years. Deaths considered to be unavoidable were excluded. The history of their disease and death was collected from family members and community leaders, and from the available medical records. The analysis focused on delays occurring at the different stages of the women's care seeking process, from the first signs of danger until death. Results. - The analysis identified different delays: the delay in danger awareness, the delay in taking the care-seeking decision and the delay due to alternative care linked to cultural perceptions of the disease, the delay in reaching a medical facility related to lack of money or vehicles, the delay in patient care related to an absent or incompetent health staff or by inappropriate choice of structure, and finally the delay in administration of the prescribed treatment. In Kinshasa, emergency care may be delayed by slow awareness of danger, but most of all by the poor quality and poor organisation of the health services. On the other hand, the use of non-medical alternatives and a poor perception of the medical services do rarely interfere in the decision to seek medical care. Conclusion. - In Kinshasa, to guarantee the patients rights to quality health care, one must first strengthen and control medical services. One should also teach people to identify services appropriate to medical emergencies. Transportation and pre-financing of emergency care should be organised by local authorities. (C) 2010 Elsevier Masson SAS. All rights reserved

Sr isotopic evidence for ion-exchange buffering in tropical laterites from the Parana, Brazil

International audienceSr isotopes have been measured in laterite sequences and in associated waters from the region of Ribeirao Preto (Parana basin, Brasil), in order to characterize the water-rock interactions corresponding to the successive stages of weathering of these continental flood basalts, We analyzed detailed sections of concentric rims of alteration in ball-shaped spheroids developed around cores of fresh basalts. Waters were also analyzed on the different types of facies in this region: unaltered basalts, thick weathered profiles of lateritic material, and surrounding sandstones, The results show that a major part of the initial Sr is removed from the basalt during the earliest stages of weathering, within the narrow grey rims surrounding the unweathered core. Radiogenic Sr is also leached from hydrothermal minerals located in basalt fractures or disseminated in the basalt groundmass at this early stage. Very high water/rock ratios (of at least 100,000) are calculated for this process, from the Sr-87/Sr-86 in the related waters. A general trend of increasing Sr-87/Sr-86 ratios is observed in the increasingly weathered facies (inner yellow cortexes, red kaolinitic soils and matrices) and in their related waters. The very high Sr-87/Sr-86 ratios of the red kaolinitic horizons probably result from isotopic exchange with radiogenic rainwaters or sandstone waters, and also from the presence of residual or neoformed radiogenic minerals. Budget calculations, as well as experimental leaching of kaolinite samples, suggest that significant amounts of radiogenic Sr can be leached from these facies during weathering, Thus, the kaolinitic facies of tropical laterite profiles are able first to trap strontium from surficial waters, then to provide radiogenic Sr to the waters that weather the fresh basalts at the weathering front. Therefore, the thick weathered horizons must be considered as a natural cation-exchange layer, that play an active role in the weathering process

XPS study of Au/GaN and Pt/GaN contacts

36 hlm. : ilus. ; tab. ; 29 cm

INTERFÉROMÉTRIE RADAR APPLIQUÉE AUX VOLCANS : CAS DE L’ETNA ET DES CHAMPS PHLÉGRÉENS (ITALIE)

During the last few years, the radar images collected by the European satellites ERS1 and ERS2, the Japanese satellite JERS and the Canadian satellite RADARSAT have been used with success to create interferograms. This technique has been applied for geophysical applications like co-seismic deformation mapping, volcano deformation monitoring, landslides monitoring, mining subsidence detection, glaciers monitoring. Here we report the research carried out by our group on Etna volcano (Italy) and in the area of Naples (Italy) where are located several potentially active volcanoes (Vesuvius, Ischia) and where a subsidence of the caldera of Campi Flegrei is still on going in response to the 1982-1984 seismic crisis. Etna is the volcano that has been studied first using ERS SAR interferometry. Using this method, a large scale deflation of the volcano associated with the large 1991-1993 eruption was detected in data covering the second half of the eruption. Further studies showed that the local deformation fields located in Valle del Bove (East of the volcano) where associated with the compaction of the 1986-1987 and 1989 lava fields and also partly with a subsidence of the surrounding terrain in response to the load of the new deposited material. Other local deformation fields have been identified, corresponding to the 1983, 1981 and 1971 lava fields. However, due to its strong topography, interferograms of Etna are affected by tropospheric effects. Those effects must be eliminated in order to correctly interpret the fringes pattern. The problem of the troposphere has been first investigated from its theoretical point of view and using existing local meteorological data as well as radio-soundings data. Recently, thanks to the large amount of available interferograms, another approach has been investigated, consisting in the research of a correlation fringe/elevation in the interferograms themselves. This approach, operated either in automatic mode (automatic fringe unwrapping) or in manual mode proved to be efficient for most of the coherent interferograms. After removal of the tropospheric correction, the evolution of the deformation of the volcano at large scale between 1992 and 1998 has been inferred. The subsidence occurring during the second half of the 1992 eruption as well as the uplift preceding the 1995 unrest of the Southeast crater are visible, but their amplitude is less than previously estimated. The depth of the modelled source of subsidence/uplift related to the large scale deformation is of the order of 6 to 8 km, not well constrained by the data. The study of the correlation fringe/elevation was possible only after a detailed analysis of the spatial and temporal properties of coherence of the Etna area. Indeed, the technique of fringe unwrapping for fringe/elevation correlation analysis is possible only if the poorly coherent pixels are eliminated. A map of the most coherent pixels of the volcano was produced. The recent lava fields as well as the towns and villages surrounding the volcano are the most coherent areas. The quality of the interferograms is also enhanced when high accuracy DEM (Digital Elevation Model) are used. Using kinematic GPS data collected along more than 100 km of road around the volcano, we assessed the accuracy of several DEMs of Etna. The most accurate DEM was produced by digitising 1/25.000e maps of Etna. This DEM does not take into account the topographic changes due to the recent eruptions. Merging other more recent DEMs corresponding to those areas, we produced an updated relatively high accuracy DEM (±3 m) of Etna. In the Naples area, we analysed interferograms in the period 1993-1996 and show that the Campi Flegrei caldera is still subsiding at a rate of about 30 mm/year.Comité National Français de Géodésie et GéophysiquePublished3.6. Fisica del vulcanismoope

XPS study of Au/GaN and Pt/GaN contacts

Au/GaN and Pt/GaN contacts have been studied with XPS. According to XPS depth profiling, the N signal is weak in the region below the metal contact and the Pt or Au signal decreases much more slowly than expected for a sharp interface. Next, we have performed in situ studies of the formation of Au contacts on GaN. In contrast to the results from depth profiling, we observe 2D growth and little or no chemical interaction between Au and GaN. This suggests that conventional calculations of sputtering yields and ion-beam-induced mixing cannot be applied to the analysis of noble metal/GaN depth profiles. Heating during or after Au deposition results in strong clustering, observed by both XPS and AFM. The Schottky barrier height measured by XPS is 1.15 eV.</jats:p

Volcano-glacier interactions on composite cones and lahar generation: Nevado del Ruiz, Colombia, case study

International audienceThe catastrophic lahars triggered by the 13 November 1985 eruption of the ice-clad Nevado del Ruiz volcano, Colombia, demonstrate that the interaction of hot pyroclasts with snow and ice can release 30-50 million m3 of meltwater in 30-90 minutes. The 1985 eruption caused a 16% loss in area and a 9% loss in volume of snow, firn and ice. Turbulent pyroclastic density currents mechanically mixed with snow and produced meltwater at a rate of 0.5-1.6 mm s−1. Laboratory experiments suggest that turbulent, fluidized pyroclastic density currents exert mechanical and thermal scour, thereby efficiently transferring heat from hot pyroclasts to snow. Ice cap loss at Nevado del Ruiz continued between 1985 and 2000, representing a ∼52% decline in area and a ∼30% fall in volume. Ice 60-190 m thick caps the east and southeast summit plateau, whereas an ice field < 30 m thick and devoid of snow is retreating on the north, northeast and west edges. This asymmetrical distribution of ice reflects combined long-term effects of the 1985 eruption and of the post-1985 ice cap retreat. Should volcanic activity resume, steep-sided glaciers can fail and pyroclastic flows and surges can sweep the snowpack and generate mixed avalanches and lahars. Although the potential source of meltwater has decreased since 1985, extensive debris at the ice cap margins can be incorporated to future lahars