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International audience; Source to sink is one of the main concepts in Earth Sciences for a better knowledge of hydrosystems dynamics. Regarding this issue, one challenge consists in the characterization by in-situ measurements of the sources of suspended sediment, or sediment source fingerprinting. Few methods can fully cover such requirements and among them, the methodology using the grain shape deserves to be developed.Indeed, morphometry of particles is widely used in sedimentology to identify different sedimentary stocks, source-to-sink transport and sedimentation mechanisms. Currently, most grain shape analyses are carried out with a scanning electron microscope coupled to image analysis to measure various size and shape descriptors on particles like elongation, circularity, bluntness or fractal dimension. However, complexity and time of analysis are the main limitations of this technique for a long-term monitoring of sediment transfers.Here we present an experimental morphometric approach using a morphogranulometer (a CCD camera coupled to a peristaltic pump). The camera takes pictures while the sample is circulating through a flow cell, leading to the analysis of numerous particles in a short time. The image analysis provides size and shape information discriminating various particles stocks according to their nature by statistical analyses.After lab calibrations, measurements were carried out on various field samples, from suspended sediments to core samples. Results shows that shape characterisation allows to identify the proportions of some sediment fractions, for examples charcoals levels in a sediment core or fine rootlets in suspended sediments. Some punctual multi-tracers tests were also conducted by injecting particles well defined by the morphogranulometer (colored silica sand) and chemical tracers at different hydrological conditions to compare with the previous chemical tracers tests made on site (Duran 2015) and statistical investigations of transport processes (Fournier 2008).These results show the promising ability of the morphogranulometer to trace sediment transfers on various hydrosystems.

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International audience; Source to sink is one of the main concepts in Earth Sciences for a better knowledge of hydrosystems dynamics. Regarding this issue, one challenge consists in the characterization by in-situ measurements of the sources of Suspended Particles Matters (SPM) or sediment source fingerprinting (Collins 2016). Few methods can fully cover such requirements and among them, the methodology using the grain shape deserves to be developed. Indeed, morphometry of particles is widely used in sedimentology to identify different sedimentary stocks, source-to-sink transport and sedimentation mechanisms. Currently, most grain shape analyses are carried out with a scanning electron microscope coupled to image analysis to measure various size and shape descriptors on particles like elongation, circularity, bluntness or fractal dimension. However, complexity and time of analysis are the main limitations of this technique for a long-term monitoring of SPM transfers. Here we present an experimental morphometric approach using a morphogranulometer (a CCD camera coupled to a peristaltic pump). The camera takes pictures while the sample is circulating through a flow cell, leading to the analysis of numerous particles in a short time. The image analysis provides size and shape information discriminating various particles stocks according to their nature by statistical analyses. Lab calibration measurements were carried out on international standard samples of particles commonly found in natural waters. The size and morphological distributions of the different mineral (clay, sand, oxides etc), biologic (microalgae, pollen, etc) and organic (peat, coal, soil organic matter, etc) samples are found statistically independent and can be discriminated on a 3D graph. Local pure samples were able to validate the calibration with 30% recombination. Mixing tests show that the morphogranulometer is able to predict proportions of sources in mixtures of the standard samples. These results show the promising ability of morphogranulometers to identify the sources of particles. Such a development offers great perspectives to use the method for the characterisation of hydrosystems. The perspectives of the study are lab tests on natural samples, and then in-situ field measurements in a local spring for real time monitoring of the SPM sources.

International audience; Source to sink is one of the main concepts in Earth Sciences for a better knowledge of hydrosystems dynamics. Regarding this issue, the present day challenge consists in the characterization by in-situ measurements of the nature and the origin of suspended particles matters (SPM). Few methods can fully cover such requirements and among them, the methodology using the form of particles deserves to be developed. Indeed, morphometry of particles is widely used in sedimentology to identify different sedimentary stocks, source-to-sink transport and sedimentation mechanisms. Currently, morphometry analyses are carried out by scanning electron microscope coupled to image analysis to measure various size and shape descriptors on particles like flatness, elongation, circularity, sphericity, bluntness, fractal dimension. However, complexity and time of analysis are the main limitations of this technique for a long-term monitoring of SPM transfers. Here we present an experimental morphometric approach using a morphogranulometer (a CCD camera coupled to a peristaltic pump). The camera takes pictures while the sample is circulating through a flow cell, leading to the analysis of numerous particles in a short time. The image analysis provides size and shape information discriminating various particles stocks according to their nature and origin by statistical analyses. Measurements were carried out on standard samples of particles commonly found in natural waters. The size and morphological distributions of the different mineral fractions (clay, sand, oxides etc), biologic (microalgae, pollen, etc) and organic (peat, coal, soil organic matter, etc) samples are statistically independent and can be discriminated on a 4D graph. Next step will be on field in situ measurements in a sink-spring network to understand the transfers of the particles stocks inside this simple karstic network. Such a development would be promising for the characterisation of natural hydrosystems.

International audience; Harmonic pumping tests consist in stimulating an aquifer by the means of hydraulic stimulations at some discrete frequencies. The inverse problem consisting in retrieving the hydraulic properties is inherently ill posed and is usually underdetermined when considering the number of well head data available in field conditions. To better constrain this inverse problem, we add self‐potential data recorded at the ground surface to the head data. The self‐potential method is a passive geophysical method. Its signals are generated by the groundwater flow through an electrokinetic coupling. We showed using a 3‐D saturated unconfined synthetic aquifer that the self‐potential method significantly improves the results of the harmonic hydraulic tomography. The hydroelectric forward problem is obtained by solving first the Richards equation, describing the groundwater flow, and then using the result in an electrical Poisson equation describing the self‐potential problem. The joint inversion problem is solved using a reduction model based on the principal component geostatistical approach. In this method, the large prior covariance matrix is truncated and replaced by its low‐rank approximation, allowing thus for notable computational time and storage savings. Three test cases are studied, to assess the validity of our approach. In the first test, we show that when the number of harmonic stimulations is low, combining the harmonic hydraulic and self‐potential data does not improve the inversion results. In the second test where enough harmonic stimulations are performed, a significant improvement of the hydraulic parameters is observed. In the last synthetic test, we show that the electrical conductivity field required to invert the self‐potential data can be determined with enough accuracy using an electrical resistivity tomography survey using the same electrodes configuration as used for the self‐potential investigation.

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Colloque avec actes et comité de lecture. Internationale.; International audience; Laboratory experiments are performed on the physical clogging of a porous medium by injecting polydispersive suspended particles ranged from 1.7 to 40 μm in a column filled with sand (d50=715 µm). Three suspensions have been injected (2g/l, 3g/l and 5g/l) at constant flow velocity in a saturated porous medium. Hydrodynamic effects on particles deposition and porous medium damage were investigated. Particles retentions are significant at the inlet and decreases with depth which induces the drop of permeability and porosity. The coupling between electric and hydraulic flows is electrokinetic in nature, the second objective of this work is to test the sensitivity of the method of self-potential (SP) and to show the usefulness of this method to understand the controlling parameters of the electrical field associated with the filtration of suspended particles through a porous medium. Electrodes have been used to record the SP signal which detect and quantify the clay particles in the porous medium during injection tests. The variations of the SP signals are linearly correlated to the piezometric level changes. SP signals are directly sensitive to deposition particles distribution, and it is high at the inlet of the column where the deposit is large and decreases with depth. For the test injection with 2g/l the SP signal varies between +16 mv and +13 mv for the test with 3g/l it varies between +12 mv and +8mv and that with 5g/l it varies between - 22mv to-19mv. When the concentration of clay injected in the porous medium increases, the SP signal decreases and changes from a positive sign to a negative sign. SP method confirmed the behavior of the porous medium clogging obtained by the hydraulic measurements. The obtained results show that SP signal is sensitive to clay particles, their immediate passage to the inlet of the column and the amount deposited particles in each section of the porous medium.

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In this paper, the temporal dynamics of precipitation in northwestern France in 8 relation to the dominant climatic pattern in Europe were investigated. The general trends 9 and the nonstationary behavior of the North Atlantic Oscillation (NAO) were described 10 using fractal analysis and Fourier spectral and continuous wavelet analysis of a NAO 11 index time series over the 1865–2004 period. The 44-year and 8-year multidecadal 12 components showed a clear increasing power during the second half of the last century. 13 The possible link between rainfall variability and the NAO was then investigated. The 14 links between the NAO and rainfall were not as obvious, as NAO-like components were 15 not easily retrieved in the precipitation records: Relationships between the NAO and 16 rainfall were very likely at certain timescales but were not systematically very obvious. 17 For instance, the characteristic quasi-biennal oscillation (QBO) of the NAO was barely 18 detected in precipitation, but a 6-year fluctuation beginning in the 90s was found to be 19 statistically significant at a 95% confidence limit After investigating a possible link 20 with the Southern Oscillation Index, the occurrence of this fluctuation in the beginning of 21 the 90s could be related to the observed shift of the QBO toward slightly higher 22 frequencies in the NAO time series. On the other hand, a modulation of the QBO by a 23 6-year interannual fluctuation would suggest the existence of a 6-year climate pattern 24 that could affect precipitation and, to a lesser extent, the NAO. Cross-wavelet analysis 25 between the NAO and precipitation revealed a loss in correlation across the 1970–2005 26 period, which seemed to be the fact of a QBO-like fluctuation. This loss of correlation 27 was related to the above-mentioned shift of the QBO and 6-year rainfall interannual 28 band since the 90s.

International audience; A 3D tomography algorithm of self‐potential (SP) signals is applied for the first time to the localization of subsurface cavities. A specific application is made to a marl‐pit in Normandy (North‐West of France). A SP map with a total of 221 (5 m‐spaced) measurements shows a negative anomaly with an amplitude of −8 mV associated with the position of the marl pit. To explain these data, we solved the boundary‐value problem for the coupled hydro‐electric problem associated with the presence of the cavity using a finite‐element code. The numerical simulations point out the role of open conduits in electrical charge accumulation near the roof of the cavity and the resistivity contrast between the cavity and the surrounding formation. We applied successfully a SP tomography algorithm showing that the roof of the cavity was associated with a monopole charge accumulation due to the entrance of the ground water flow in a network of open cracks.

International audience; In karst systems, rain events often result in a decrease in conductivity (a tracer of dissolved phase transport) and an increase in turbidity (a tracer of suspended solids transport) at wells and springs. We investigated the response to rain events at five karst outlets (three springs and two wells) discharging from the cretaceous chalk aquifer of the Haute-Normandie region, France. As the input signal (rainfall) is relatively homogeneous spatially, the differences in the responses at the sites provide information on the internal organization of the karst systems and their hydrodynamic functioning. We used autocorrelation functions and turbidity-conductivity (T-[Delta]C) hysteresis curves to analyze the responses. At short time scales, autocorrelation functions allow estimation of the inertia of the conduit flow system. The T-[Delta]C curves reflect the processes of deposition, resuspension, and direct transport of suspended solids within the karst network. The results show that: (i) a long memory effect for conductivity indicates storage of water in the karst network and deposition of suspended solids, and the transport of the dissolved phase and suspended solids is not the same; (ii) a short memory effect for conductivity indicates that transport of the dissolved phase and suspended solids is synchronous and direct. On the basis of the results at the five sites, we propose three conceptual models of hydrodynamic functioning.

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