Lateral uniformity of India Plate strength over central and eastern Nepal

The current understanding of the Himalayan lithosphere stems mostly from cross-sections through the range at the longitude of the Kathmandu Basin. In this paper we laterally extend the analyses of structures and rheology along the Nepal Himalayas between the Pokhara valley and the Arun river. We take advantage of available information and a new data set including gravity measurements and a receiver function profile. It appears that the geometry of the Moho inferred from seismological profiles and long-wavelength gravity anomalies does not exhibit major East-West variations within the 350-km-wide study area. Using thermomechanical modelling, we show that the northward deepening of the Moho observed along profiles perpendicular to the main thrust faults can be interpreted simply as the bending of a strong India Plate. This result suggests a gradual mechanical decoupling between the crust and the mantle, leading to a northward decrease of the effective elastic thickness of the Indian lithosphere from∼75 km to∼25 km beneath the Ganga Basin and the Tibetan Plateau, respectively. Our results also confirm (partially) eclogitized lower Indian crust beneath southern Tibet. At shorter wavelengths, the observed gravity profiles exhibit some small lateral variations that can be interpreted in terms of east-west variations of the thickness of subsurface geological structures such as the Ganga Basin and the Tethyan Sedimentary Sequenc

Monitoring of groundwater redistribution in a karst aquifer using a superconducting gravimeter

Geodetic tools monitor the earth’s deformation and gravity field. They are presently sensitive enough to record subtle changes triggered by hydrological processes, thus providing complementary data to standard hydrological measurements. Among these tools, superconducting gravimeter (SG) have proven useful to unravel groundwater redistribution, which significantly alter the gravity field. In the frame of the EquipEx MIGA (Matter wave-laser based Interferometer Gravitation Antenna) project, one SG (iOSG-24) was set up in July 2015 in the Low-noise Underground Laboratory (LSBB) at Rustrel, France, in a gallery located 500 m beneath the surface. In this work, we analyse the underground iOSG-24 gravity time series together with hydro-meteorological data and basic gravity modelling. We find that the gravimeter recorded the redistribution of water in the ground and that most of this redistribution occurs in the unsaturated zone located above the gravimeter. Nevertheless, residuals between our model and the gravity data suggest the occurrence of large lateral fluxes and rapid runoff not considered in our model. We discuss how the setting of a second SG, planned in July 2018, at the surface of the LSBB could help unravelling such hydrological processes

Quantification de la vapeur d'eau troposphérique par GPS (modèles 2D et tomographies GPS) - Application aux précipitations intenses.

Crée le 16/01/2006modifié le 30/01/2006 (pb typo)The water vapour plays a unique role in the climate and the meteorology of the earth. It is a key variable to understand the convection and the intense precipitations. Now the quantitative precipitation forecast does not improve since about twenty years partly because the measure of the water vapour variability is a lack of the current observing systems. The accuracy of integrated water vapour GPS measurements has been shown since about ten years. The GPS tomography allows nowadays to retrieve the 3D field of water vapour. This dissertation presents the theory (least square method and Kalman filtering), the validation and the applications of the GPS tropospheric tomography software LOFTT_K developed during this thesis. The tomography has been validated with simulations, meteorological modelling and independent measurements. The tomography has been then used to understand the small scale interactions between the local (the breezes) and mesoscale (the mistral) dynamics. The meteorological GPS has also allowed to characterize the humidity variations during rain events in the Cévennes (South France). Finally the GPS tomography at mesoscale has shown a mechanism of humidity « loading » of the atmosphere before a convection initiation case.La vapeur d'eau joue un rôle majeur dans le fonctionnement climatique et météorologique de la Terre. C'est notamment une variable clé pour comprendre les phénomènes de convection à l'origine des précipitations intenses. Or les prévisions quantitatives des pluies ne s'améliorent pas depuis 20 ans parce que - entre autres - la mesure de la variabilité de la vapeur d'eau est une lacune des systèmes d'observations. La précision de la mesure du contenu intégrée en vapeur d'eau par le système GPS est démontrée depuis une dizaine d'années. La tomographie GPS permet aujourd'hui de restituer le champ 3D de vapeur d'eau. Ce mémoire présente la théorie (méthodes des moindres carrés et filtre de Kalman), la validation et les applications du logiciel LOFTT_K de tomographie troposphérique GPS développé pendant cette thèse. La tomographie a été validée grâce à des simulations, des modèles météorologiques et des mesures indépendantes. Elle a ensuite été utilisée pour comprendre les interactions petites échelles entre les dynamiques locales (les brises) et de mésoéchelle (le mistral). Le GPS météorologique a permis aussi de caractériser les variations d'humidité pendant les pluies cévenoles. Enfin la tomographie GPS régionale a mis en évidence un mécanisme de chargement en humidité de l'atmosphère avant l'initiation de la convection.A l'avenir, les données GPS seront assimilées dans les modèles opérationnels de prévision météorologique. Elles doivent aussi servir à approfondir la connaissance des relations entre les précipitations, la convection et la dynamique de la vapeur d'eau

Quantification de la vapeur d'eau troposphérique par GPS (modèles 2D et tomographies 3D) (application aux précipitations intenses)

La vapeur d eau joue un rôle majeur dans le fonctionnement climatique et météorologique de la Terre. C est notamment une variable clé pour comprendre les phénomènes de convection à l origine des précipitations intenses. Or les prévisions quantitatives des pluies ne s améliorent pas depuis 20 ans parce que -entre autres - la mesure de la variabilité de la vapeur d eau est une lacune des systèmes d observations. La précision de la mesure du contenu intégrée en vapeur d eau par le système GPS est démontrée depuis une dizaine d années. La tomographie GPS permet aujourd'hui de restituer le champ 3D de vapeur d eau. Ce mémoire présente la théorie (méthodes des moindres carrés et filtre de Kalman), la validation et les applications du logiciel LOFTT_K de tomographie troposphérique GPS développé pendant cette thèse. La tomographie a été validée grâce à des simulations, des modèles météorologiques et des mesures indépendantes. Elle a ensuite été utilisée pour comprendre les interactions petites échelles entre les dynamiques locales (les brises) et de mésoéchelle (le mistral). Le GPS météorologique a permis aussi de caractériser les variations d humidité pendant les pluies cévenoles. Enfin la tomographie GPS régionale a mis en évidence un mécanisme de chargement en humidité de l atmosphère avant l initiation de la convection. A l avenir, les données GPS seront assimilées dans les modèles opérationnels de prévision météorologique. Elles doivent aussi servir à approfondir la connaissance des relations entre les précipitations, la convection et la dynamique de la vapeur d eauThe water vapour plays a unique role in the climate and the meteorology of the earth. It is a key variable to understand the convection and the intense precipitations. Now the quantitative precipitation forecast does not improve since about twenty years partly because the measure of the water vapour variability is a lack of the current observing systems. The accuracy of integrated water vapour GPS measurements has been shown since about ten years. The GPS tomography allows nowadays to retrieve the 3D field of water vapour. This dissertation presents the theory (least square method and Kalman filtering), the validation and the applications of the GPS tropospheric tomography software LOFTT_K developed during this thesis. The tomography has been validated with simulations, meteorological modelling and independent measurements. The tomography has been then used to understand the small scale interactions between the local (the breezes) and mesoscale (the mistral) dynamics. The meteorological GPS has also allowed to characterize the humidity variations during rain events in the Cévennes (South France). Finally the GPS tomography at mesoscale has shown a mechanism of humidity loading of the atmosphere before a convection initiation case. In a near future, the GPS data will be assimilated in the operational weather forecasting models. They should also be useful to improve the knowledge between the rain, the convection and the water vapour dynamicMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

GPS monitoring of the tropospheric water vapor distribution and variation during the 9 September 2002 torrential precipitation episode in the Ce´vennes (southern France)

On 8–9 September 2002, torrential rainfall and flooding hit the Gard region in southern France causing extensive damages and casualties. This is an exceptional example of a so-called Cévenol episode with 24 hour cumulative rainfall up to about 600 mm at some places and more than 200 mm over a large area (5500 km2). In this work we have used GPS data to determine integrated water vapor (IWV) as well as horizontal wet gradients and residuals. Using the IWV, we have monitored the evolution of the convective system associated with the rainfall from the water vapor accumulation stage through the stagnation of the convective cell and finally to the breakup of the system. Our interpretation of the GPS meteorological parameters is supported by synoptic maps, numerical weather analyses, and rain images from meteorological radars. We have evidenced from GPS data that this heavy precipitation is associated with ongoing accumulation of water vapor, even through the raining period, but that rain stopped as soon as the weather circulation pattern changed. The evolution of this event is typical in the context of the Cévenol meteorology. Furthermore, we have shown that the horizontal wet gradients help describe the heterogeneity of the water vapor field and holds information concerning the passage of the convective system. Finally, we have noticed that the residuals, which in theory should be proportional to water vapor heterogeneity, were also highly perturbed by the precipitation itself. In our conclusions we discuss the interest of a regional GPS network for monitoring and for future studies on water vapor tomography

First evaluation of an absolute quantum gravimeter (AQG#B01) for future field experiments

International audienceQuantum gravimeters are a promising new development allowing for continuous absolute gravity monitoring while remaining user-friendly and transportable. In this study, we present experiments carried out to assess the capacity of the AQG#B01 in view of future deployment as a field gravimeter for hydrogeophysical applications. The AQG#B01 is the field version follow-up of the AQG#A01 portable absolute quantum gravimeter developed by the French quantum sensor company Muquans. We assess the instrument's performance in terms of stability (absence of instrumental drift) and sensitivity in relation to other gravimeters. No significant instrumental drift was observed over several weeks of measurement. We discuss the observations concerning the accuracy of the AQG#B01 in comparison with a state-of-the-art absolute gravimeter (Micro-g-LaCoste, FG5#228). We report the repeatability to be better than 50 nm s−2. This study furthermore investigates whether changes in instrument tilt and external temperature and a combination of both, which are likely to occur during field campaigns, influence the measurement of gravitational attraction. We repeatedly tested external temperatures between 20 and 30 ∘C and did not find any significant effect. As an example of a geophysical signal, a 100 nm s−2 gravity change is detected with the AQG#B01 after a rainfall event at the Larzac geodetic observatory (southern France). The data agreed with the gravity changes measured with a superconducting relative gravimeter (GWR, iGrav#002) and the expected gravity change simulated as an infinite Bouguer slab approximation. We report 2 weeks of stable operation under semi-terrain conditions in a garage without temperature-control. We close with operational recommendations for potential users and discuss specific possible future field applications. While not claiming completeness, we nevertheless present the first characterization of a quantum gravimeter carried out by future users. Selected criteria for the assessment of its suitability in field applications have been investigated and are complemented with a discussion of further necessary experiments

Diurnal Cycle of Water Vapor as Documented by a Dense GPS Network in a Coastal Area during ESCOMPTE IOP2

Global positioning system (GPS) data from a dense network have been used for the analysis of the diurnal cycle of water vapor over Marseille, France, during the second intensive observation period (IOP2; 21–26 June 2001) of the Expérience sur Sites pour Contraindre les Modèles de Pollution Atmosphérique et de Transport d'Emission (ESCOMPTE) field experiment. Both tomographic analyses and integrated water vapor (IWV) contents from GPS have been used, in addition to wind profiler data and surface observations. Tomographic analysis of data from the dense GPS network and radiosondes provided the continuous temporal evolution of the vertical distribution of water vapor over the city of Marseille. The city is located on the shore of the Mediterranean Sea in southeastern France and is often under the influence of sea-breeze effects. Two different layers of breeze circulation are identified: a shallow sea breeze, blowing perpendicular to the local coastline, and a deep sea breeze, induced by the regional temperature gradient between sea and land. The origin of water vapor is shown to be mainly due to the advection of marine moist air by these sea-breeze circulations. However, the diurnal cycle of water vapor over Marseille is strongly influenced by the synoptic situation, which changes during the IOP2 (between a northerly mistral in the early stage of the IOP and an easterly wind at the end). It is shown that vertical profiles from tomographic analyses (combined with wind profiler data) allow for a proper interpretation of the diurnal cycle observed in IWV. Two-dimensional maps of IWV are also shown to complement the description of the horizontal advection of moisture by the different circulation regimes

Contribution of artificial tracing and geophysical methods to understanding flows in the non-saturated area of karst (St Ferron site, France)

peer reviewedGeophysical methods allow imaging the structure of the epikarst. However, studying groundwater transfer in a non-saturated zone is still a challenging task. Tracing is then used to remedy this problem and to relate water flow with geological structures. These studies are all the more important than the epikarst is a vulnerable area with important water reserve. In order to validate the hypotheses issued on the basis of the geophysical methods, a multi tracer experiment was realized on the St Ferron site (Larzac, France). The study showed the importance of using tracing in addition to geophysical methods to validate the hypotheses on the flows and thus better know the epikarst and its functioning.Les méthodes géophysiques permettent d’imager la structure de l’épikarst, mais l’étude des transferts dans la zone non saturée reste une tâche difficile. Les traçages sont dès lors utilisés pour remédier à ce problème et relier les écoulements aux structures géologiques. Ces études sont d’autant plus importantes que l’épikarst est une zone vulnérable, présentant d’importants stocks d’eau. Afin de valider les hypothèses émises sur la base des méthodes géophysiques, un multi-traçage a été réalisé sur le site de St Ferron (Larzac, France). L’étude montre l’importance d’utiliser des traçages en complément des méthodes géophysiques pour valider les hypothèses sur les écoulements et ainsi mieux connaître l’épikarst et son fonctionnement

On the use of GPS tomography to investigate water vapor variability during a Mistral//sea breeze event in southeastern France

Global Positioning System (GPS) tomography analyses of water vapor, complemented by high-resolution numerical simulations are used to investigate a Mistral/sea breeze event in the region of Marseille, France, during the ESCOMPTE experiment. This is the first time GPS tomography has been used to validate the three-dimensional water vapor concentration from numerical simulation, and to analyze a small-scale meteorological event. The high spatial and temporal resolution of GPS analyses provides a unique insight into the evolution of the vertical and horizontal distribution of water vapor during the Mistral/sea-breeze transition