Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Advances in groundwater storage monitoring are crucial for water resource management and hydrological processes understanding. The evaluation of water storage changes (WSC) often involve point measurements (observation wells, moisture probes, etc.), which may be inappropriate in heterogeneous media. Over the past few years, there has been an increasing interest in the use of gravimetry for hydrological studies. In the framework of the GHYRAF (Gravity and Hydrology inAfrica) project, 3 yr of repeated absolute gravity measurements using a FG5-type gravimeter have been undertaken at Nalohou, a Sudanian site in northern Benin. Hydrological data are collected within the long-term observing system AMMA-Catch. Once corrected for solid earth tides, ocean loading, air pressure effects, polar motion contribution and non-local hydrology, seasonal gravity variations reach up to 11 μGal, equivalent to a WSC of 260-mm thick infinite layer of water. Absolute temporal gravity data are compared to WSC deduced from neutron probe and water-table variations through a direct modelling approach. First, we use neutronic measurements available for the whole vertical profile where WSC occur (the vadose zone and a shallow unconfined aquifer). The RMSD between observed and modelled gravity variations is 1.61 μGal, which falls within the error bars of the absolute gravity data. Second, to acknowledge for the spatial variability of aquifer properties, we use a 2-D model for specific yield (Sy) derived from resistivity mapping and Magnetic Resonance Soundings (MRS). The latter provides a water content (θMRS) known to be higher than the specific yield. Hence, we scaled the 2-D model of θMRS with a single factor (α). WSC are calculated from water-table monitoring in the aquifer layer and neutronic measurements in the vadose layer. The value of α is obtained with aMonte-Carlo sampling approach, minimizing the RMSD between modelled and observed gravity variations. This leads to α = Sy/θMRS = 0.63 ± 0.15, close to what is found in the literature on the basis of pumping tests experiments, with a RMSD value of 0.94 μGal. This hydrogeophysical experiment is a first step towards the use of time-lapse gravity data as an integrative tool to monitor interannual WSC even in complicated subsurface distribution.
ISSN: 0956-540X

hal-00821181
https://hal.archives-ouvertes.fr/hal-00821181
DOI : 10.1093/gji/ggt146

Éditeur(s) : HAL CCSD Elsevier
Résumé : High-precision Ni isotopic variations are reported for the metal phase of equilibrated and unequilibrated ordinary chondrites, carbonaceous chondrites, iron meteorites, mesosiderites, and pallasites. We also report new Zn and Cu isotopic data for some of these samples and combine them with literature Fe, Cu, and Zn isotope data to constrain the fractionation history of metals during nebular (vapor/solid) and planetary (metal/sulfide/silicate) phase changes. The observed variations of the Ni-62/Ni-58, Ni-61/Ni-58, and Ni-60/Ni-58 ratios vary linearly with mass difference and define isotope fractionation lines in common with terrestrial samples. This implies that Ni was derived from a single homogeneous reservoir. While no Ni-60 anomaly is detected within the analytical uncertainties, Ni isotopic fractionation up to 0.45 parts per thousand per mass-difference unit is observed. The isotope compositions of Ni and Zn in chondrites are positively correlated. We suggest that, in ordinary chondrites, exchange between solid phases, in particular metal and silicates, and vapor followed by mineral sorting during accretion are the main processes controlling these isotopic variations. The positive correlation between Ni and Zn isotope compositions contrasts with a negative correlation between Ni (and Zn) and Cu isotope compositions, which, when taken together, do not favor a simple kinetic interpretation. The observed transition element similarities between different groups of chondrites and iron meteorites are consistent with the genetic relationships inferred from oxygen isotopes (IIIA/pallasites and IVA/L chondrites). Copper is an exception, which we suggest may be related to separate processing of sulfides either in the vapor or during core formation. (C) 2007 Elsevier Ltd. All rights reserved.
ISSN: 0016-7037

hal-00408304
https://hal.archives-ouvertes.fr/hal-00408304
DOI : 10.1016/j.gca.2007.06.049

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
A network of 54 survey GPS sites, 28 continuous GPS stations and three absolute gravity (AG) observation sites have been set up in the Alborz mountain range to quantify the present-day kinematics of the range. Our results allow us to accurately estimate the motion of the South Caspian block (SCB) for the first time, and indicate rotation of the SCB relative to Eurasia, accounting for the left lateral motion in the Alborz range. In light of these new results, it clearly appears that deformation rates vary along the range, the eastern part accommodating mainly left lateral strike slip (2 mm yr(-1) south of the range and 5 mm yr(-1) north of the range) with a very low range normal shortening rate on the Khazar thrust fault (similar to 2 mm yr(-1)), and the western part accommodating range normal shortening (similar to 6 mm yr-1) on the Khazar thrust fault with a left lateral component of similar to 2 mm yr(-1) north of the range and 1 mm yr(-1) south of the range. These present-day kinematics agree with geomorphologic estimated slip rates, but not the long-term deformation, corroborating the idea that the kinematics of the range have changed recently due to the change of SCB motion.;Modelling of the interseismic deformation suggests a deep locking depth on the central-western segment of the Khazar fault (similar to 30 km) in agreement with the Baladeh earthquake rupture and aftershock ranging between 10 and 30 km. Given this unusual deep locking depth and the 34 degrees dip of the thrust, a large part of the Alborz range is located above the seismically coupled part of the fault. Based on our AG measurements this part of the range seems to uplift at a rate of 1-5 mm yr(-1), in agreement with terrace uplift.
ISSN: 0956-540X

hal-00617146
https://hal.archives-ouvertes.fr/hal-00617146
https://hal.archives-ouvertes.fr/hal-00617146v2/document
https://hal.archives-ouvertes.fr/hal-00617146/file/djamour2010.pdf
DOI : 10.1111/j.1365-246X.2010.04811.x