Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
We use 3 yr of surface absolute gravity measurements at three sites on the Larzac plateau (France) to quantify the changes induced by topography and the building on gravity time-series, with respect to an idealized infinite slab approximation. Indeed, local topography and buildings housing ground-based gravity measurement have an effect on the distribution of water storage changes, therefore affecting the associated gravity signal. We first calculate the effects of surrounding topography and building dimensions on the gravity attraction for a uniform layer of water. We show that a gravimetric interpretation of water storage change using an infinite slab, the so-called Bouguer approximation, is generally not suitable. We propose to split the time varying gravity signal in two parts (1) a surface component including topographic and building effects (2) a deep component associated to underground water transfer. A reservoir modelling scheme is herein presented to remove the local site effects and to invert for the effective hydrological properties of the unsaturated zone. We show that effective time constants associated to water transfer vary greatly from site to site. We propose that our modelling scheme can be used to correct for the local site effects on gravity at any site presenting a departure from a flat topography. Depending on sites, the corrected signal can exceed measured values by 5-15 μGal, corresponding to 120-380 mm of water using the Bouguer slab formula. Our approach only requires the knowledge of daily precipitation corrected for evapotranspiration. Therefore, it can be a useful tool to correct any kind of gravimetric time-series data.
ISSN: 0956-540X

hal-00795590
https://hal.archives-ouvertes.fr/hal-00795590
DOI : 10.1093/gji/ggs007

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Random walk (RW) or Continuous Time Random Walk (CTRW) are recurring Monte Carlo methods used to model convective and diffusive transport in complex heterogeneous media. Many applications can be found, including fluid mechanic, hydrology, and chemical reactors modeling. These methods are easy to implement, very versatile and flexible enough to become appealing for many applications because they generally overlook of deeply simplify the building of explicit complex meshes required by deterministic methods. RW and CTRW provide a good physical understanding of the interactions between the space scales of het-erogeneities and the transport phenomena under consideration. In addition, they can result in efficient up-scaling methods, especially in context of flow and transport in fractured media. In the present study, we review the applications of RW or CTRW for several situations coping with various spatial scales, and different insights into up-scaling applications. RW and CTRW advantages and downsides are also discussed, thus providing a few avenues for further works and applications.
ISSN: 0169-3913

hal-01449131
https://hal.archives-ouvertes.fr/hal-01449131
https://hal.archives-ouvertes.fr/hal-01449131/document
https://hal.archives-ouvertes.fr/hal-01449131/file/0025972-02.pdf
DOI : 10.1007/s11242-016-0693-z

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
During compressive events, deformation in sedimentary basins is mainly accommodated by thrust faulting and related fold growth. Thrust faults are generally rooted in the basement and may act as conduits or barriers for crustal fluid flow. Most of recent studies suggest that fluid flow through such discontinuities is not apparent and depends on the structural levels of the thrust within the fold-and-thrust belt. In order to constrain the paleofluid flow through the Jaca thrust-sheet-top basin (Paleogene southwest-Pyrenean fold-and-thrust belt) this study compares on different thrust faults located at different structural levels. The microstructures in the different fault zones studied are similar and consist of pervasive cleavage, calcite shear veins (SV1), extension veins (EV1) and late dilatation veins (EV3). In order to constrain the nature and the source of fluids involved in fluid-rock interactions within fault zones, a geochemical approach, based on oxygen and carbon stable isotope and trace element compositions of calcite from different vein generations and host rocks was adopted. The results suggest a high complexity in the paleohydrological behaviors of thrust faults providing evidence for a fluid-flow compartmentalization within the basin. Previous studies in the southern part of the Axial Zone (North of the Jaca basin) indicates a circulation of deep metamorphic water, probably derived from the Paleozoic basement, along fault zones related to the major basement Gavarnie thrust. In contrast, in northern part of the Jaca basin, the Monte Perdido thrust fault is affected by a closed hydrological fluid system involving formation water during its activity. The Jaca and Cotiella thrust faults, in turn, both located more to the south in the basin, are characterized by a composite fluid flow system. Indeed, stable isotope and trace element compositions of the first generations of calcite veins suggest a relatively closed paleohydrological system, whereas the late calcite vein generations, which are probably associated with the late tectonic activity of the basin, support a contribution of both meteoric and marine waters. Based on these results, a schematic fluid-flow model is presented. This model allows visualization of three main fluid flow compartments along a N-S transect.
ISSN: 0264-8172

hal-00963606
https://hal.archives-ouvertes.fr/hal-00963606
DOI : 10.1016/j.marpetgeo.2013.09.005

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We aim at testing and validating a mobile-immobile mass transfer model from a set of single-well injection withdrawal tracer tests in a heterogeneous porous aquifer. By varying the duration of the injection phase, different volumes of aquifer are investigated by the tracer. Hence, we focus the transport model validation not only on reproducing a single breakthrough curve (BTC) but also on the model's capacity to predict the amount of mixing as a function of the volume visited by the tracer. All the BTCs are strongly asymmetric, as expected when dispersion is controlled by diffusive mass transfers between the mobile water and the immobile water part of the porosity. However, the BTC cannot be modeled by a conventional mobile-immobile mass transfer model with a simple power law memory function. To account for that, we implement a continuous time random walk model in which the transition time distribution y (t), which is related to the excursion time probability of the tracer in the immobile domain, is a dual-slope power law distribution. The model best fits the BTC data set with a transitional regime controlled by y(t) t2 and an asymptotic regime characteristic of the conventional double-porosity model with y(t) t1.5 . This work emphasizes that high-resolution concentration measurement and multiple-scale tracer tests are required for assessing solute dispersion models in heterogeneous reservoirs and for subsequently obtaining reliable predictions.
ISSN: 0043-1397

insu-00373698
https://hal-insu.archives-ouvertes.fr/insu-00373698
https://hal-insu.archives-ouvertes.fr/insu-00373698/document
https://hal-insu.archives-ouvertes.fr/insu-00373698/file/Borgne_et_al-2008-Water_Resources_Research.pdf
DOI : 10.1029/2007WR006279

Éditeur(s) : HAL CCSD BioMed Central
Résumé : International audience
UNLABELLED: ABSTRACT: BACKGROUND: Malaria is a major health issue in French Guiana. Amerindian communities remain the most affected. A previous study in Camopi highlighted the predominant role of environmental factors in the occurrence of malaria. However, all parameters involved in the transmission were not clearly identified. A new survey was conducted in order to clarify the risk factors for the presence of malaria cases in Camopi. METHODS: An open cohort of children under seven years of age was set up on the basis of biologically confirmed malaria cases for the period 2001-2009. Epidemiological and observational environmental data were collected using two structured questionnaires. Data were analysed with a multiple failures multivariate Cox model. The influence of climate and the river level on malaria incidence was evaluated by time-series analysis. Relationships between Anopheles darlingi human biting rates and malaria incidence rates were estimated using Spearman's rank correlation. RESULTS: The global annual incidence over the nine-year period was 238 per 1,000 for Plasmodium falciparum, 514 per 1,000 for Plasmodium visa and 21 per 1,000 for mixed infections. The multivariate survival analysis associated higher malaria incidence with living on the Camopi riverside vs. the Oyapock riverside, far from the centre of the Camopi hamlet, in a home with numerous occupants and going to sleep late. On the contrary, living in a house cleared of all vegetation within 50 m and at high distance of the forest were associated with a lower risk. Meteorological and hydrological characteristics appeared to be correlated with malaria incidence with different lags. Anopheles darlingi human biting rate was also positively correlated to incident malaria in children one month later. CONCLUSIONS: Malaria incidence in children remains high in young children despite the appearance of immunity in children around three years of age. The closeness environment but also the meteorological parameters play an important role in malaria transmission among children under seven years of age in Camopi.
ISSN: 1475-2875

inserm-00633771
http://www.hal.inserm.fr/inserm-00633771
http://www.hal.inserm.fr/inserm-00633771/document
http://www.hal.inserm.fr/inserm-00633771/file/1475-2875-10-246.pdf
DOI : 10.1186/1475-2875-10-246

Éditeur(s) : HAL CCSD Springer
Résumé : International audience
Thiswork examines the relevance of the inclusion of ground-based gravity data in the calibration process of a global rainfall-discharge reservoir model. The analysis is performed for the Durzon karst system (Larzac, France). The first part of the study focuses on the hydrological interpretation of the ground-based gravity measurements. The second part of the study investigates further the information content of the gravity data with respect to water storage dynamics modelling.The gravity-derived information is found unable to either reduce equifinality of the single-objective, dischargebased model calibration process or enhance model performance through assimilation.
ISSN: 1866-6280

hal-00812624
https://hal-brgm.archives-ouvertes.fr/hal-00812624
DOI : 10.1007/s12665-012-1856-z

Éditeur(s) : HAL CCSD
Résumé : 1 p.
For groundwater issues (potential leakages in waste repository, aquifer management ...), the development of modeling techniques is far ahead of the actual knowledge of aquifers. This raises two fundamental issues: 1) which and how much data are necessary to make predictions accurate enough for aquifer management issues; 2) which models remain relevant to describe the heterogeneity and complexity of geological systems. The French observatory H+ was created in 2002 with the twofold motivation of acquiring a large database for validating models of heterogeneous aquifers, and of surveying groundwater quality evolution in the context of environmental changes. H+ is a network of 4 sites (Ploemeur, Brittany, France; HES Poitiers, France; Cadarache, France; Campos, Mallorca, Spain) with different geological, climatic, and economic contexts. All of them are characterized by a highly heterogeneous structure (fractured crystalline basement for Ploemeur, karstified and fractured limestone for Poitiers, Cadarache and Mallorca), which is far to be taken into account by basic models. Ploemeur is exploited as a tap-water plant for a medium-size coastal city (15,000 inhabitants) for 20 years. Each site is developed for long term investigation and monitoring. They involves a dense network of boreholes, detailed geological and geophysical surveys, periodic campaigns and/or permanent measurements of groundwater flow, water chemistry, geophysical signals (including ground motions), climatic parameter, etc. Several large-scale flow experiments are scheduled per year to investigate the aquifer structure with combined geophysical, hydrogeological, and geochemical instruments. All this information is recorded in a database that has been developed to improve the sustainability and quality of data, and to be used as a collaborative tool for both site researchers and modelers. This project lasts now for 5 years. It is a short time to collect the amount of information necessary to apprehend the complexity of aquifers; but it is already enough to obtain a few important scientific results about the very nature of the flow heterogeneity, the origin and residence time of water elements, the kinetic of geochemical processes, etc. We have also developed new methods to investigate aquifers (in-situ flow measurements, flow experiment designs, groundwater dating, versatile in-situ probes, etc.). This experience aiming at building up long term knowledge appears extremely useful to address critical issues related to groundwater aquifers: the structure and occurrence of productive aquifer in crystalline basement, the assessment of aquifer protection area in the context of highly heterogeneous flow, the biochemical reactivity processes, the long term evolution of both water quantity and quality in the context of significant environmental changes, for instance.
AGU, Fall Meeting 2008
San Francisco, United States

insu-00375980
https://hal-insu.archives-ouvertes.fr/insu-00375980

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Random walk (RW) methods are recurring Monte Carlo methods used to modelconvective and diffusive transport in complex heterogeneous media. Many applications canbe found, including fluid mechanic, hydrology and chemical reactors modeling. These methodsare easy to implement, very versatile and flexible enough to become appealing for manyapplications because they generally overlook or deeply simplify the building of explicit complexmeshes required by deterministic methods. RWprovides a good physical understandingof the interactions between the space scales of heterogeneities and the transport phenomenaunder consideration. In addition, they can result in efficient upscaling methods, especiallyin the context of flow and transport in fractured media. In the present study, we review theapplications of RW to several situations that cope with diverse spatial scales and differentinsights into upscaling problems. The advantages and downsides of RW are also discussed,thus providing a few avenues for further works and applications.
ISSN: 0169-3913

insu-01308060
https://hal-insu.archives-ouvertes.fr/insu-01308060
DOI : 10.1007/s11242-016-0693-z

Éditeur(s) : HAL CCSD
Résumé : Karst systems are the most important groundwater reservoirs in Mediterranean area. Beyond saturated zone, unsaturated zone constitute an entity where water storage and transfer play a major role in a global behavior of spring water. This structure and functioning are complex and not well known because of the high heterogeneity created and organized by groundwater flow. In this study, we used gravimetric and hydrogeologic method to directly observe and quantify water storage and transfer processes in unsaturated zone of karst system. Surface to depth gravity measurements on several karst system allow quantifying seasonal water storage changes in unsaturated zone. We show that these variations occur in the first tens meters of unsaturated zone. Furthermore, water storage capacity of unsaturated zone seems to be influenced by lithology (limestone or dolomite). Finally, direct observations of water flow in unsaturated zone are done using caves access. We show that some characteristics of unsaturated water flow such as bipolar slow-fast flow can be widespread for whatever depth of measurement and recharge area. Model of this flow, using a simple modeling scheme, show the complexity of water transfer processes in unsaturated zone of karst system.
Les aquifères karstiques constituent l'essentiel des ressources en eau du pourtour Méditerranéen. Au-delà de la zone saturée de ces systèmes, la zone non saturée constitue une entité importante quant au transfert et au stockage temporaire de l'eau. La structure et le fonctionnement de cette entité du karst sont complexes et mal connus à cause du processus de karstification qui hiérarchise la circulation hydrodynamique en son sein. Dans cette étude nous utilisons des méthodes géodésiques et hydrogéologiques afin d'observer de façon directe et de quantifier les processus de transfert et de stockage de la zone non saturée. Les mesures gravimétriques en surface et profondeur sur différents systèmes karstiques ont permis de quantifier les variations du stockage en eau saisonnier dans la zone non saturée. La gravimétrie différentielle montre que ces variations sont réparties dans les quelques premiers mètres de la zone non saturée. Il semble, de plus, que la capacité de stockage de la zone non saturée soit fonction de la lithologie de l'encaissant (calcaire et dolomie). Enfin, des observations directes du flux d'eau en zone non saturée ont été effectuées par le biais de mesures dans différentes cavités. Nous montrons que certaines caractéristiques du flux d'eau en zone non saturée, comme la présence d'un flux bipolaire lent-rapide, peuvent être généralisées quelque soit la profondeur de mesure et la surface d'impluvium. La modélisation de ce flux, à partir d'un schéma simple, a montré la complexité des processus de transfert engendrés au sein de la zone non saturée.
https://tel.archives-ouvertes.fr/tel-00829346

tel-00829346
https://tel.archives-ouvertes.fr/tel-00829346
https://tel.archives-ouvertes.fr/tel-00829346/document
https://tel.archives-ouvertes.fr/tel-00829346/file/manuscrit_deville_finish.pdf

Éditeur(s) : HAL CCSD springer
Résumé : International audience
In this paper, we address the problem of flood prediction in complex situations. We present an original solution in order to achieve the main goals of accuracy, flexibility and readability. We propose the SM2D modular data driven approach that provides predictive models for each sub-process of a global hydrological process. We show that this solution improves the predictive accuracy regarding a global approach. The originality of our proposition is threefold: (1) the predictive model is defined as a set of aggregate variables that act as classifiers, (2) an evolutionary technique is implemented to find best juries of such classifiers and (3) the flood process complexity problem is addressed by searching for sub-models on sub-processes identified partly by spatial criteria. The solution has proved to perform well on flash flood phenomena of tropical areas known to be hardly predictable. It was indeed successfully applied on a real caribbean river dataset after both preprocessing and preliminary analysis steps presented in the paper.
Lecture notes in computer science
Singapour, Singapore

hal-00878780
https://hal.archives-ouvertes.fr/hal-00878780
DOI : 10.1007/978-3-642-40897-7_13

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We examine absolute gravity (AG) and vertical Global Positioning System (GPS) time series between 1995 and 2010 at eight collocated sites in mid-continent North America. The comparison of AG and GPS rates aligned to ITRF2005 yields a gravity/uplift ratio of -0.17 +/- 0.01 mu Gal mm(-1) (1 mu Gal = 10 nm s(-2)) and an intercept of -0.1 +/- 0.5 mm yr(-1). In contrast, aligning the GPS velocities to ITRF2000 results in a gravity/uplift intercept of -1.3 +/- 0.5 mm yr(-1). The near-zero gravity/uplift offset for the ITRF2005 (or ITRF2008) results shows a good alignment of the GPS vertical velocities to Earth's center of mass, and confirms that GPS velocities in this reference frame can be compared to predictions of geodynamic processes such as glacial isostatic adjustment (GIA) or sea-level rise. The observed gravity/uplift ratio is consistent with GIA model predictions. The ratio remains constant in regions of fast and slow uplift, indicating that GIA is the primary driving process and that additional processes such as local hydrology have a limited impact on a decadal time-scale. Combining AG and GPS measurements can provide significant constraints for geodetic, geodynamic, and hydrological studies. Citation: Mazzotti, S., A. Lambert, J. Henton, T. S. James, and N. Courtier (2011), Absolute gravity calibration of GPS velocities and glacial isostatic adjustment in mid-continent North America, Geophys. Res. Lett., 38, L24311, doi:10.1029/2011GL049846.
ISSN: 0094-8276

hal-00689010
https://hal.archives-ouvertes.fr/hal-00689010
DOI : 10.1029/2011GL049846

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Mobile-immobile mass transfer is widely used to model non-Fickian dispersion in porous media. Nevertheless, the memory function, implemented in the sink/source term of the transport equation to characterize diffusion in the matrix (i.e., the immobile domain), is rarely measured directly. Therefore, the question can be posed as to whether the memory function is just a practical way of increasing the degrees of freedom for fitting tracer test breakthrough curves or whether it actually models the physics of tracer transport. In this paper we first present a technique to measure the memory function of aquifer samples and then compare the results with the memory function fitted from a set of field-scale tracer tests performed in the same aquifer. The memory function is computed by solving the matrix diffusion equation using a random walk approach. The properties that control diffusion (i.e., mobile-immobile interface and immobile domain cluster shapes, porosity, and tortuosity) are investigated by X-ray microtomography. Once the geometry of the matrix clusters is measured, the shape of the memory function is controlled by the value of the porosity at the percolation threshold and of the tortuosity of the diffusion path. These parameters can be evaluated from microtomographic images. The computed memory function compares well with the memory function deduced from the field-scale tracer tests. We conclude that for the reservoir rock studied here, the atypical non-Fickian dispersion measured from the tracer test is well explained by microscale diffusion processes in the immobile domain. A diffusion-controlled mobileimmobile mass transfer model therefore appears to be valid for this specific case.
ISSN: 0043-1397

insu-00373705
https://hal-insu.archives-ouvertes.fr/insu-00373705
https://hal-insu.archives-ouvertes.fr/insu-00373705/document
https://hal-insu.archives-ouvertes.fr/insu-00373705/file/Gouze_et_al-2008-Water_Resources_Research.pdf
DOI : 10.1029/2007WR006690

Éditeur(s) : HAL CCSD Elsevier
Résumé : TAG LITHO
In this study we attempt to understand the water storage variations in a karst aquifer on the Larzac Plateau (South of France) using absolute gravimetry. On this karst system, water input is exclusively rainfall and draining occurs at the Durzon perennial spring in a karstic valley. Our basis assumption is that water storage in the vadose zone (epikarst and infiltration zone) has a significant effect on surface gravity through Newtonian attraction. The karst aquifer is hence being monitored since January 2006 with monthly absolute gravity measurements at three sites. The gravity measurements are corrected for regional scale gravity using water storage models in order to obtain gravity variations related exclusively to the local water storage variations. The gravity variations exhibit an important seasonal component (10-15 mu gal corresponding to a variation of equivalent water stab thickness of similar to 24-36 cm). Water storage variation seen by gravimetry is not spatially uniform on the studied karst system, corroborating current geomorphologic observations and interpretations. Finally, we use a global mass balance consideration linking rainfall, evapotranspiration and spring discharge to provide a conceptual framework for the understanding of observed gravity variations.
ISSN: 0022-1694

hal-00411884
https://hal.archives-ouvertes.fr/hal-00411884
DOI : 10.1016/j.jhydrol.2008.06.020

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The marine Messinian deposits of Tunisia cover a narrow littoral strip some 300 km long between the northern Bizerte and Cap Bon areas and the central-eastern Sahel region. Litho- and biofacies analysis of six stratigraphic sections reveals the distinctive features of these deposits.;The lower Messinian deposits are characterized by ubiquitous siliciclastics and abundant oolitic/bioclastic limestones organized in an eastward facing ramp. Westward (landward), this ramp changes into coastal lagoons, sometimes containing evaporites. Eastward, the ramp passes to the reefal Pelagian Platform extending as far as Lampedusa.;Two main sedimentary cycles are distinguished: 1) an early Messinian siliciclastic retrogradational then oolitic/bioclastic progradational cycle (Beni Khiar Formation and lower Oued bel Khedim Formation): 2) a late Messinian brackish to continental cycle that probably accumulated in rapidly subsiding lagoons (Cued el Bir Formation and upper Oued bel Khedim Formation). The Tunisian early Messinian cycle is partly eustatically controlled, but the late Messinian cycle cannot be confidently correlated to other well-known Messinian series because of tectonic movements.;The lower Messinian deposits of Tunisia are also characterized by abundant suspension-feeding organisms (molluscs and bryozoans) and rare corals, calcareous algae, echinoids, and larger benthic foraminifers. The proposed palaeoenvironmental model shows that the lower Messinian ramp of Tunisia was located on a current-protected margin and subjected to continent-derived sediment and nutrient supply. Eastward, nutrient influx diminished and a shallow-water isolated carbonate platform with coralgal facies developed between the western and the eastern Mediterranean basins. The main hydrological connection between these two basins occurred through a narrow seaway situated to the northeast of the Pelagian Platform, south of Sicily and Malta.
ISSN: 0031-0182

hal-00475583
https://hal.archives-ouvertes.fr/hal-00475583
DOI : 10.1016/j.palaeo.2009.10.028

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Tilt fluctuations can potentially reflect the response of hydrosystems to important rainfall. In this context, long baseline tiltmeters have been installed in an underground tunnel penetrating the Fontaine de Vaucluse karst to study the medium deformation related to solicitations exerted by water infiltrating the hydrosystem. The instruments monitor the tilt as well as its spatial variation. Northward tilts reaching a 1 μrad amplitude are observed consecutively to rainfalls. The tilt amplitude is highly correlated with the Fontaine de Vaucluse outlet flow fluctuations. The measured tilt signal is also relatively homogeneous over a 150 m length. Different types of structure likely to produce such observations are tested in order to identify their location with respect to the tiltmeters, their dimension as well as the amount of water level variation in the structure. Following rainfalls, the infiltration of water modifies the pore pressure, inducing a medium deformation. The hypothesis of an homogeneous surface loading on the Vaucluse plateau is first refuted since the related tilt is much lower than the one measured. The water supplied by rainfalls has to accumulate in discontinuities in order to generate a higher tilt. So, the deformation related to a pressure exerted on a fracture filled by water is assessed. A first study reveals the interest of the tilt homogeneity information that constrains strongly the fracture properties. Thus, the fracture must be located at a distance more than a few hundreds metres from the tiltmeters in order to produce a tilt homogeneous in space. If the fracture is initially dry, it must also be filled on a height higher than 150 m consecutive to a rainfall in order to generate a tilt amplitude in the same magnitude as the one measured. Then, we explore the influence of water level variations on the tilt produced by a fracture located at the interface between the saturated and unsaturated zones, which are thereby permanently flooded. Since several parameters of that model satisfactorily explain the field observations, we discuss how simultaneous geodetical observations could provide complementary information that would further constrain the geometry of the structure at the origin of the medium deformation.
ISSN: 0956-540X

insu-01408482
https://hal-insu.archives-ouvertes.fr/insu-01408482
https://hal-insu.archives-ouvertes.fr/insu-01408482/document
https://hal-insu.archives-ouvertes.fr/insu-01408482/file/Lesparre-2017.pdf
DOI : 10.1093/gji/ggw446

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Time-lapse microgravity surveying combined with absolute gravity measurements is used to investigate water storage changes in a karst aquifer of similar to 100 km(2) area. The survey consists of 40 gravity stations measured with a relative gravimeter; absolute gravity is measured at three stations for each survey. In total, four gravity surveys are performed over a 2 year time period during consecutive wet and dry periods. Survey precisions range between 2.4 and 5 mu Gal, enabling statistically significant detection of 10 mu Gal change, i.e., similar to 0.25 m equivalent water level change. Observed gravity changes are coherent between consecutive survey periods, i.e., net water withdrawal and net water recharge is observed, reaching changes as high as 22 mu Gal. Observed gravity changes allow refining evapotranspiration estimates, which may serve to improve the water budget of the aquifer. High-and low-gravity amplitude zones characterize the karst system, demonstrating spatially variable storage behavior. Geomorphologic considerations are invoked to explain the location of preferential zones of water storage, and a conceptual model of water storage is discussed for the studied karst.
ISSN: 0148-0227

hal-00496400
https://hal.archives-ouvertes.fr/hal-00496400
DOI : 10.1029/2009JB006616

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Crystal growth driven by a flowing solution is modeled for a flow with low Reynolds number using a computational dynamic software. Considering equivalent crystallographic faces, the chemical flux is calculated along upstream and downstream faces. Upstream flux is higher compared to downstream and leads to a symmetry breakdown of the crystal shape and develops mirror symmetry parallel to the flow velocity. Moreover the ratio of these two fluxes (upstream / downstream) gives a quantitative relation between the relative crystal growth rate and the flow velocity. Thus, using an inverse method, the flow direction and velocity can be deduced by the study of the variation of the growth band thicknesses of equivalent crystallographic faces. This new method was applied to the formation of metasomatic tourmalinite associated with a leucogranite sill. The approach is complemented by a study of the chemistry of the tourmaline. In the studied case, the application of the new method gives the high fluid velocities in pores (10-3 - 10-4 m/s) during metasomatism. Equivalent Darcy velocities are estimated and discussed accounting for the major role played by the regional deformation. Finally, a two-stage tectono-hydrodynamic model is proposed for the metsomatism. The first stage is genetically linked to the sill injection, and the second is characterized by a wider event with hydrothermal flow passing along the leucogranite sills.
ISSN: 0012-821X

Droits : info:eu-repo/semantics/OpenAccess
insu-00352307
https://hal-insu.archives-ouvertes.fr/insu-00352307
https://hal-insu.archives-ouvertes.fr/insu-00352307/document
https://hal-insu.archives-ouvertes.fr/insu-00352307/file/Sizaret_et_al_final.pdf
DOI : 10.1016/j.epsl.2009.01.013

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We investigate the contribution of atmospheric and its induced non-tidal oceanic loading effects on surface time-varying gravity and tilt measurements for several stations in Western Europe. The ocean response to pressure forcing can be modelled accordingly to the inverted barometer, i.e. assuming that air pressure variations are fully compensated by static sea height changes, or using ocean general circulation models.Wevalidate two runs of theHUGO-mbarotropic ocean model by comparing predicted sea surface height variations with hundred tide-gauge measurements along the European coasts. We then show that global surface pressure field, as well as a barotropic high-resolution ocean model forced by air pressure and winds allow in most cases a significant reduction of the variance of gravity residuals and, to a smaller extends tilt residuals. Wefinally show that precise gravity measurements with superconducting gravimeters allow the observation of large storm surges, occurring in the North Sea, even for inland stations. However, we also confirm that the continental hydrology contribution cannot be neglected. Thanks to their specific sensitivity feature, only tiltmeters closest to the coast can clearly detect the loading due to these storm surges.
ISSN: 0264-3707

hal-00708046
https://hal.archives-ouvertes.fr/hal-00708046
https://hal.archives-ouvertes.fr/hal-00708046/document
https://hal.archives-ouvertes.fr/hal-00708046/file/ETS2008_BOY.pdf
BIBCODE : 2009JGeo...48..182B

Éditeur(s) : HAL CCSD European Geosciences Union
Résumé : International audience
Natural disturbances are the dominant form of forest regeneration and dynamics in unmanaged tropical forests. Monitoring the size distribution of treefall gaps is important to better understand and predict the carbon budget in response to land use and other global changes. In this study, we model the size frequency distribution of natural canopy gaps with a discrete power law distribution. We use a Bayesian framework to introduce and test, using Monte Carlo Markov chain and Kuo–Mallick algorithms, the effect of local physical environment on gap size distribution. We apply our methodological framework to an original light detecting and ranging dataset in which natural forest gaps were delineated over 30 000 ha of unmanaged forest. We highlight strong links between gap size distribution and environment, primarily hydrological conditions and topography, with large gaps being more frequent on floodplains and in wind-exposed areas. In the future, we plan to apply our methodological framework on a larger scale using satellite data. Additionally , although gap size distribution variation is clearly under environmental control, variation in gap size distribution in time should be tested against climate variability.
ISSN: 1726-4170

hal-01590729
https://hal-univ-guyane.archives-ouvertes.fr/hal-01590729
https://hal-univ-guyane.archives-ouvertes.fr/hal-01590729/document
https://hal-univ-guyane.archives-ouvertes.fr/hal-01590729/file/bg-14-353-2017.pdf
DOI : 10.5194/bg-14-353-2017

Éditeur(s) : HAL CCSD
Résumé : Karst systems are the most important groundwater reservoirs in Mediterranean areas. Their structure and functioning are complex and not well known because of the high heterogeneity created and organised by groundwater flow. Water storage entities and water storage changes within remain difficult to assess by direct measurement and modelling. Geodetic monitoring is here used to constrain and bring original insights on karst system functioning. The Durzon karst system (Larzac plateau, Southern France) is monitored by several geodetic techniques: - Monthly absolute gravity measurements at three sites - Time-lapse surface to depth gravity measurements - Time-lapse microgravity surveying - Continuous long base hydrostatic tiltmeter monitoring at two sites Absolute gravity monitoring complemented with relative gravity surveys allows quantification of water storage changes on the studied karst system. Interannual water storage changes as detected by gravity are ~400 mm in amplitude. These storage changes are shown to occur in the epikarst and infiltration zone in certain areas. Water storage variations spatial heterogeneity is also demonstrated on the karst system. Tilt measurements monitor fast and slow infiltration processes within the karst fractured media. Karst system geodetic monitoring brings both quantitative and integrated hydrologic knowledge. Such methods could be applied to other karst aquifer studies.
Les aquifères karstiques constituent l'essentiel des ressources en eau souterraine sur le pourtour de la Méditerranée. Leur structure et leur fonctionnement sont complexes et mal connus à cause du processus de karstification qui hiérarchise la circulation hydrodynamique en leur sein. La localisation du stockage de l'eau et de ses variations au sein de tels aquifères est difficile à mesurer et à modéliser. Nous utilisons les outils de la géodésie pour apporter de nouvelles contraintes sur le fonctionnement hydrologique et les variations de stock d'eau en milieu karstique. Ainsi, le système karstique du Durzon, (Causse du Larzac, France) fait l'objet d'un suivi par plusieurs méthodes géodésiques incluant: - Des mesures de gravimétrie absolue en trois sites répétées de manière mensuelle ; - Des mesures répétées de gravimétrie relative entre la surface et la profondeur du karst ; - Des mesures répétées de microgravimétrie ; - Des mesures inclinométriques longue base en deux sites. Nous démontrons que le couplage entre la gravimétrie absolue et relative permet d'appréhender les variations de stock d'eau sur le bassin d'alimentation étudié. Les variations inter-anuelles observées sont de l'ordre de 400 mm, et nous montrons que celles-ci s'opèrent dans certaines zones au sein des horizons supérieurs du karst : épikarst et zone d'infiltration. Une forte hétérogénéité spatiale des variations de stockage est aussi mise en évidence à l'échelle du bassin d'alimentation. Les mesures inclinométriques permettent de suivre les phénomènes d'infiltration rapide et lente dans la masse karstique fracturée. Le suivi géodésique de ce système karstique permet d'apprécier quantitativement et de façon intégrée son fonctionnement hydrologique et pourra être transposé pour l'étude d'autres aquifère karstiques.
https://tel.archives-ouvertes.fr/tel-00404851

tel-00404851
https://tel.archives-ouvertes.fr/tel-00404851
https://tel.archives-ouvertes.fr/tel-00404851/document
https://tel.archives-ouvertes.fr/tel-00404851/file/THESE_jacob.pdf