Publications

Muography uses muons contained in the natural cosmic rays to determine the density of rock volumes. The measurements consist of counting the muons emerging from the target to determine the screening effect produced by the rock. Since the larger the rock thickness, the smaller the number of muons able to cross, the time resolution that can be achieved by muography to monitor density changes is on the order of one or two weeks for kilometer-sized volcanoes. This limitation of the method can be reduced by joining muography with high time-resolution measurements like passive seismic monitoring. In the case of structural imaging, muography benefits from the fact that muon trajectories are linear, making the tomography problem simpler than for other geophysical techniques like electrical resistivity tomography. Experiments performed on La Soufrière of Guadeloupe volcano are described to show how muography can be used to contribute to structural imaging of a highly heterogeneous lava dome and to detect abrupt transient hydrothermal phenomena likely to produce dangerous explosive events.

In this chapter, we describe basic features and give some current applications of the most popular detection technology used in muography: the scintillator-based muon detectors, widely used not only in volcanology, where their properties find natural applications, but also in geosciences, archeology, non-invasive industrial control, civil engineering, homeland security, nuclear non-proliferation and more. As we will emphasize in the following sections, there are many advantages in the use of scintillators, which are known to be robust – and therefore usable in harsh environmental conditions – and offer real-time analogic measurement capabilities with a good space and time resolution. The design of such detectors is flexible and may be used in many different ways depending on the target under study, the field conditions, the modularity of the detectors etc. Throughout this chapter, we will focus on one particular muon detector (also referred to as “muon telescope”) originally designed to study the active volcanic dome of the Soufrière of Guadeloupe to show the generic features of this detection technique.

The Strengbach headwater catchment has been monitored since 1986 by the OHGE (Observatoire Hydrogéochimique de l’Environnement) critical zone observatory. Located in the Vosges mountains (northeastern France) with altitudes ranging from 880 m and 1150 m, the 0.8 km² catchment lies on a granitic bedrock. Meteorological and hydrological data are continuously monitored and six boreholes highlight the underground structure down to depths of 50 to 120 m, providing information on the distribution of geological facies. Particularities of this site, such as its relatively thin porous media, compared to sedimentary contexts and its strong topography have encouraged the application of challenging geophysical experiments. Thus, magnetic resonance soundings, which can provide estimates of water contents, were acquired to explore the potential of the method to detect the water content despite the expected low values. Similarly, gravity measurements were acquired to assess whether the method can detect groundwater variations over time despite the strong topography and the poorly known groundwater fluctuations. In addition, seismic refraction tomographies were acquired to provide information on the geometry of hydrofacies. While those experiments were set up separately, we’ll show how they provide complementary insights and could be integrated within a single workflow to solve the hydrological inverse problem at the catchment scale.

Introduction Modelling the hydrological processes in mountainous areas is particularly challenging due to the strong heterogeneity of the underground medium in terms of hydrological properties, and the lack of groundwater observations. Here, we show how geophysical observations provide key information on the geometry of hydrofacies, the estimate of hydrological properties and the monitoring of the groundwater to study the critical zone in the Strengbach mountainous headwater catchment. Studied Site The OHGE (Observatoire Hydrogéochimique de l’Environnement) is a headwater catchment of 0.8 km² that lies on a granitic bedrock (Pierret et al. 2018). This observatory corresponds to the Strengbach catchment and is part of OZCAR, the French network of critical zone observatories. The OHGE is located in the Vosges mountains (northeastern France) with altitudes varying between 880 m and 1150 m (Fig. 1). The catchment topography shows steep slopes of 15° in average that reach up to 30° locally. Learning from scattered geophysical data Meteorological and hydrological data are monitored since 1986 and six boreholes provide the distribution of geological facies at depths of 50 to 120 m (Chabaux et al. 2023). In addition, electrical resistivity and seismic refraction tomographies were acquired to estimate variations in soil and saprolite thickness. These data show soil thickness varying from 50 cm to 5 m, and saprolite thickness ranging from 1 to 16 m (Lesparre et al. 2024). The electrical resistivity tomographies also underline the spatial distribution of the geological facies, as one slope of the catchment shows significantly higher resistivity values than the other (Lajaunie et al. 2024). Despite the relatively thin saprolite, magnetic resonance soundings detected groundwater above the noise level, and revealed a region with higher water content (Lesparre et al. 2020). Gravity data acquired across the whole catchment shows that the method has the sensitivity to distinguish areas with distinct water storage dynamics (Chaffaut et al. 2022). In particular, a region upstream the Strengbach stream exhibits the highest values of water content and the largest variations in water storage. We are using the geophysical data to develop a catchment-scale hydrogeophysical inversion. Hydrofacies geometries will be derived from the tomographies, and local hydrogeophysical experiments will be interpreted together with direct observations to estimate the range of hydrological properties. That information will serve as prior information of the inverse problem that will assimilate magnetic resonance and gravity data to complete piezometer and flow rate data.

Tree transpiration is a critical process of the water cycle and its observation and quantification are essential to better understand terrestrial ecosystem dynamics. While sap flow measurements provide direct estimates of individual tree transpiration, their reliance on empirical calibration and a rather high energy consumption (due to heat generation) can be strong limiting factors. The self-potential (SP) method, a passive geophysical approach, presents a promising alternative for assessing transpiration rates, although the electrophysiological processes driving SP signals in trees remain underexplored (Fig. 1). This study presents a year-long monitoring of SP and sap velocity in three tree species at three Critical Zone Observatories different in a Mediterranean climate (from OZCAR and ICOS research infrastructures). Using wavelet coherence analysis and variational mode decomposition, our findings reveal strong coherence between SP and sap velocity at diurnal time scales, with coherence diminishing and phase shifts increasing under higher water supply conditions. Electrokinetic coupling coefficients, derived from linear regression between SP and sap velocity variations, align with values typical of porous geological media. During dry seasons, the electrokinetic effect dominates SP signals, suggesting its potential as a tool for evaluating transpiration rates. This research demonstrates the feasibility of integrating geophysical techniques into long-term observations of ecohydrological systems to better understand plant-water interactions in the Critical Zone.

Water resources in mountainous areas are of major importance for local ecosystems as well as for human activities. Therefore, it is crucial to monitor the availability of these resources and to be able to predict their evolution accurately in the context of climate change. Hydrologic modeling is a useful tool to achieve this goal. To do so, the models need to be properly parameterized. Geophysical sounding techniques are very useful tools to provide information for the model calibration process. This work focuses on the Surface Nuclear Magnetic Resonance (SNMR) sounding technique. This geophysical method is based on nuclear magnetic resonance and has the advantages of being non-destructive and directly sensitive to the groundwater content (Legchenko and Valla 2002). A time-lapse SNMR survey was conducted in the Strengbach headwater catchment in the Vosges Mountains (France) during the winter 2021 with the aim of following an infiltration event. Before using this data set for hydrologic model calibration, we used Global Sensitivity Analysis (GSA) tools in order to determine which hydrologic parameters were most influential on the geophysical sounding outputs. This first step is useful for estimating parameters' identifiability.

The French OZCAR critical zone network offers the opportunity to conduct multi-site studies and to explore the critical zone functioning under contrasted climate, geology, vegetation and land use. In this study, an integrated modeling of the water cycle is performed with the ecohydrological model EcH2O-iso in three long-term observatories: (1) the Naizin watershed characterized by an oceanic climate, a metamorphic bedrock and an intensive agriculture (north-west of France, AgrHyS observatory); (2) the Aurade watershed, a watershed with a warmer semi-continental oceanic climate, a sedimentary geological substratum and a crop cover with a wheat-sunflower rotation (south-west of France, Aurade observatory) and; (3) the Strengbach watershed characterized by a mountain climate, a granitic bedrock, and a beech-spruce forest cover (north-east of France, OHGE observatory).Modeling robustness is evaluated by taking advantage of the large database for critical zone sciences including stream flow, water level in piezometers, and evapotranspiration fluxes measured from climatological stations and flux-towers located in the watersheds. Our comparative study brings these general outcomes: (1) the long term CZ evolution controlling the regolith thickness strongly impacts the total water storage in watersheds; (2) the Quaternary geomorphological evolution influences the current hydrological partitioning and the separation of hydrologically active and inactive water storage; (3) Both internal watershed characteristics and external forcings, such as current atmospheric forcing and recent land use need to be considered to infer stream persistence and to understand hydrological diversity; and (4) the observed hydrological diversity cannot be fully understood without considering a continuum of time scales in CZ evolution. Overall, this work illustrates the strength of critical zone networks, allowing a new level of multi-site and comparative studies that are crossing several observatories and encompassing a wide diversity of geology and climate.

Des mesures de résonance magnétique protonique ont été acquises de manière répétée lors d'un suivi de fonte de neige sur le bassin versant du Strengbach (site de l'Observatoire Hydro-Géochimique de l'Environnement, Vosges) pendant l'hiver 2021. Nous utilisons ces mesures géophysiques pour caractériser les propriétés de la zone vadose, en estimant les paramètres d'un modèle hydrologique unidimensionnel (Richards 1D) grâce à une approche de modélisation couplée hydrologique et géophysique. La méthode d'estimation de paramètres choisie (inférence bayésienne) permet à la fois d'estimer les paramètres du modèle et de quantifier l'incertitude sur cette estimation.

Les mesures de potentiel spontané électrique sont influencées par la variation des flux hydriques circulant dans le sol et au sein des arbres. Nous étudions ici comment ces données acquises en milieu forestier sont sensibles aux différents processus de transferts hydriques qui ont lieu dans le continuum subsurface-végétation-atmosphère. Pour cela, nous avons conçu et installé une expérience de mesures de potentiel spontané sur le tronc et au pied de plusieurs arbres d'une même parcelle forestière et complété ces mesures par des observations des variables environnementales.

;Estimating evapotranspiration (ET) is a primary challenge in modern hydrology. Hydrogravimetry is an integrative approach that provides highly precise continuous measurement of gravity acceleration. However, large-scale effects (e.g. tides, polar motion, atmospheric loading) limit the fine time-scale interpretation of this data and processing leads to residual signal noise. To circumvent this limitation, we exploited the difference between two superconducting gravimeters located 512 m apart on the same vertical. The difference calculation makes it possible to remove shared large-scale effects. Daily variation of this gravity difference is significantly correlated with daily evapotranspiration as estimated using the water balance model SimpKcET (p-value = 4.10<sup>-10</sup>). However, this approach is effective only during rain-free periods. In the future, comparison with direct ET measurements (e.g. eddy-covariance, scintillometer) may confirm and strengthen our interpretation. Improved hydrogravimetric data processing will allow to extend this approach to other experimental sites equipped with a single superconducting gravimeter.

Combining hydrogeological models with hydrological and geophysical measurements helps constraining the geometry of aquifers and estimating the hydraulic parameters. We propose to explore here how the parameters of a hydrological model, coupling surface and underground water flows can impact Magnetic Resonance Sounding (MRS) data. MRS is a geophysical method that is directly sensitive to the water content below the recording station in both the saturated and non-saturated media. Thus, MRS data complement piezometric datasets that render the temporal variations of the water table position but do not offer any insight on the water content quantity in the porous media. The hydrological model is applied to the Strengbach headwater catchment, with meteorological forcing measured on the field. The hydrological model provides daily maps of the underground water content as output. From such maps, MRS data can directly be estimated at specific stations distributed over the catchment using petrophysical laws to evaluate the relaxation time. The hydraulic parameters of the model such as the aquifer thickness, porosity or hydraulic conductivity at saturation (Ks) can then be explored to infer how the MRS data are sensitive to those parameters. The aquifer is divided in vertical layers: the soil and the saprolite, which thickness variation over the catchment is defined by generating geostatistical fields from the analysis of seismic refraction data. The thickness of each element of the hydrological model follows roughly a normal distribution. The porosity and Ks are considered as homogeneous over the catchment for each layer. The porosity parameter follows a uniform distribution, while Ks is distributed with a log-uniform law. The underground medium is defined with a porosity and Ks that both increase from depth to surface in order to mimic the scheme of the underground critical zone alteration degree. A global sensitivity analysis (GSA) approach is used in order to distinguish the respective contribution of the different parameters on the MRS signal variability. The objective is also to identify the temporal fluctuation of the MRS signal sensitivity to the different hydraulic parameters. Indeed, some parameters can be more or less influent on the MRS signal depending on the meteorological forcing. In that goal, we use polynomial chaos expansion (PCE) as surrogate models trained from the analysis of MRS signal estimated after running the hydrological model with several hundreds of parameters sets. The contribution of each parameter variability on the MRS signal variance is then quantified directly by Sobol indices from the PCE coefficients. PCE allows also to quantify the variance related to the interactions between parameters. We highlight here the complementarity between MRS and piezometric data by applying the GSA on both data types. Results show that in the context of a small headwater catchment with a small inertia, MRS data are particularly sensitive to the medium porosity and Ks, while piezometric data are mainly controlled by Ks.

Characterizing and monitoring water flow in the critical zone is of uttermost importanceto understand the water cycle. Water link several process within critical zone from aquiferrecharge and solute transfer to eco-hydrology, many eco-systemic services and biogeochemical reactions. However, the in situ quantification of water flow is technically challenging using traditional hydrological methods and numerous gaps of knowledge remain. The self-potential (SP) method is a passive geophysical method that relies on the measurement of naturally occurring electrical field. One of the contributions to the SP signal is the streaming potential, which is of particular interest in hydrogeophysics as it is directly related to both the water flow and porous medium properties. Unlike tensiometers and other point sensors, which use the measurement of state (e.g., matric pressure) at different locations to infer the intervening processes, the SP method measures signals generated by dynamic processes (e.g. water movement). However, the amplitude of the SP signal depends on multiple soil properties which are dependent to soil type, moisture content, and water chemistry (composition and pH). During the last decades, manymodels have been proposed to relate the SP signal to the water flow. In this contribution, we will present a soil-specific petrophysical model to describe the electrokinetic coupling generated from different water fluxes in the critical zone: rain water infiltration and water uptake from tree-roots. We tested a fully coupled hydrogeophysical approach on a large SP dataset collected in a two-dimensional array at the base of a Douglas-fir tree (Psuedotsuga menziesii ) in the H.J Andrews Experimental Forest in central Oregon, USA. We collected SP measurements over five months to provide insight on the propagation of transpiration signals into the subsurface with depth and under variable soil moisture. The coupled model, which included a root-water up-take term linked to measured sap flux, reproduced both the long-term and diel variations inSP measurements, thus confirming that SP has potential to provide spatially and temporally dense measurements of transpiration-induced changes in water flow. Similar set-ups are being installed on several OZCAR test-sites (Larzac, LSBB, Strengbach). This will allow us to test the approach under different climatic conditions, different soil types and in different ecohydrological systems.

Springwater is the only drinking water resource available in mountains. It is indispensable for local agriculture, industrial water supply and municipal water supply, but mountain water is also an important source of freshwater for the adjacent lowlands. However, the water storage dynamics is difficult to assess in mountainous areas because the topography give rise to major lateral redistribution of water and considerable heterogeneity on all scales, which limits the areal representativeness of any point measurement to assess the water storage dynamics. Gravimetry appears as a very promising method for measuring variations in water content in mountainous soil as it is an integrative method which is directly sensitive to temporal variations in total water content. Physically based distributed hydrological models are key tools to assess hydro-system dynamics but such models are usually under-constrained because of the lack of adapted observational constrains. Here we show that hybrid gravimetry -which combines continuous gravity monitoring at a reference station with relative time-lapse gravity made with a field gravimeter over a network of micro-gravimetric stations - is a valuable tool to calibrate the distributed and physically based NIHM (Normally Integrated Hydrological Model) applied to the study of the Strengbach mountain catchment (Vosges mountains, France). We demonstrate that: (i) gravimetry is sensitive to NIHM parametrization and (ii) gravimetry allows to identify preferential water storage area within the Strengbach catchment. Our study therefore indicates that gravimetry is a promising tool to calibrate distributed hydrological model, especially since it is a cost-effective and non-intrusive method, in contrast with the traditional calibration approach which relies on observations in wells.

Environmental seismology” is a rapidly growing field that aims to provide new observables and proxies to characterize and monitor earth surface processes, especially when subject to anthropogenic or climate forcing. It is mostly based on the recording and analysis of ambient ground vibrations (sometimes referred as ”seismic noise”) induced by such human and environmental sources, which, in turn, can also be used to image the elastic properties of the medium. Since the end of 2020, we have initiated a project named SiPaZoc (passive seismology for critical zone studies) to evaluate the ability of different passive seismic approaches to characterize the critical zone, the evolution of its water resource, and the dynamic of surface transport of the Strengbach watershed (Haut-Rhin, France), a small headwater catchment hosting the OHGE French national observatory. The project mostly consists of a series of deployments of miniaturized seismometers (”nodes”) borrowed from the DENSAR park managed by Ecole et Observatoire des Sciences de la Terre -A network of 99 nodes, distributed across the entire watershed along a homogeneous grid and three denser profiles. This network, deployed for ̃3 weeks in Nov-Dec. 2020, aimed to record ambient seismic noise, characterize the main local sources generating this noise, and implement cross-correlation approaches between pairs of sensors to build a 3D velocity (Vs) model of the first ̃50 meters at depth of the whole basin. - A network of 6 nodes located in the central part of the watershed and operating continu- ously for several months (measurements started in Feb. 2021). This network aims to take advantage of the pervasive aspect of the ambient seismic noise to determine, by interferometric approaches on ambient noise cross-correlations, small temporal variations of seismic velocities induced by variations of the water content in the subsurface

We apply a global sensitivity analysis of magnetic resonance sounding (MRS) data to the hydraulic parameters of a hydrological model at a catchment scale. The methodology used provides a quantitative estimate of the proportion to which the explored parameters influence the simulated MRS response. The sensitivity analysis is performed through a variance-based approach that provides indicators to infer how the MRS sensitivity to the different parameters might vary in space and time. We also apply the sensitivity analysis to water table depth estimates. We show that MRS measurements complement fruitfully classical water table depth acquisitions. Indeed, MRS is sensitive to the model water content at saturation while this parameter does not influence significantly the water table depth estimates.

Deterministic geophysical inversion suffers from a lack of realism because of the regularization, while stochastic inversion allowing for uncertainty quantification is computationally expensive. In this contribution, we propose to use Bayesian Evidential Learning as an alternative to hydrogeophysical coupled inversion. We demonstrate the ability of the approach to successfully predict a hydrogeological target from time-lapse ERT data in the context of a heat injection and storage experiment.

Cette étude présente les méthodes utilisées pour modéliser des expériences d'infiltration d'eau dans un sol, suivies par radar de sol et le schéma d'inversion construit en couplant les modèles hydrogéologiques et géophysiques. Les résultats préliminaires des tests d'inversion, réalisés pour ajuster des données synthétiques simulées, montrent qu'on peut estimer la conductivité hydraulique à saturation.

The use of compressional wave velocity (Vp) images is widely recognized to supply a strong insight on the depth variability of interfaces structuring the critical zone. Indeed, Vp is highly sensitive to the porosity and to the degree of weathering of the sub-surface. Here, we deduce from ten Vp profiles covering a watershed, the respective geometry of interfaces separating the regolith, the saprolite and the weathered bedrock. We reconstruct then the catchment geometry using a geostatistical analysis of the Vp images, considering the geomorphological characteristics of the studied site. An adapted geostatistical model (truncated power variogram) is used to consider the scale variability of the Vp measurement support. A sensitivity analysis is performed to study the impact on hydrogeophysical data of the Vp limits used to identify the interfaces’ depth, the randomness of the geostatistical interpolation as well as the porosity and hydraulic conductivity parameters. The simulated hydrogeophysical data correspond to flow rate at the catchment outlet and magnetic resonance soundings (MRS) distributed on the catchment. Then, the likelihood of the hydrologic model parameters is estimated by comparing the simulated hydrogeophysical data to field measurements. The fitting of both flow rate and MRS data show an equifinality issue as the thickness of the aquifer layers is strongly correlated with the aquifer’s porosity and hydraulic conductivity when simulating such hydrogeophysical signals. This equifinality is greatly reduced by the analysis of the Vp characteristics that helps determining the aquifer layers thicknesses distribution. So the aquifer porosity and hydraulic conductivity are determined more reliably when fitting the outlet flow rate and the MRS data after constraining the aquifer geometry from Vp images. The methodology proposed here is assessed on the small 1 km² Strengbach headwater catchment, located in a mid-mountain environment and laying on hard-rock

Transpiration is a crucial process in the water cycle and its quantification is essential for understanding terrestrial ecosystem dynamics. Solely relying on sap flow measurements may not fully assess tree transpiration due to its complexity. Self-potential (SP), a passive geophysical method, may provide constraints on transpiration rates even if many questions remain about tree electrophysiological effects. In this study, we continuously measured tree SP and sap velocity on three tree species for one year in a Mediterranean climate. Using wavelet coherence analysis and variational mode decomposition, we explored the empirical relationship between tree SP and transpiration. Our analysis revealed strong coherence between SP and sap velocity at diurnal time scales, with coherence weakening and phase shifts increasing on days with higher water supply.We estimated electrokinetic coupling coefficients using a linear regression model between SP and sap velocity variations at the diurnal scale, resulting in values typically found in porous geological media. During a dry growing season, the electrokinetic effect emerges as the primary contribution to tree SP, indicating its potential utility in assessing transpiration rates. Our results emphasize the need for improved electrode configurations and physiochemical modeling to elucidate tree SP in relation to transpiration.

Understanding the critical zone processes related to groundwater flows relies on subsurface structure knowledge and its associated parameters. We propose a methodology to draw the patterns of the subsurface critical zone at the catchment scale from seismic refraction data and show its interest for hydrological modelling. The designed patterns define the structure of a physically based distributed hydrological model applied to a mountainous catchment. With that goal, we acquired 10 seismic profiles covering the different geomorphology zones of the studied catchment. We develop a methodology to analyse the geostatistical characteristics of the seismic data and interpolate them over the whole catchment. The applied geostatistical model considers the scale variability of the subsurface structures observed from the seismic data analysis. We use compressional seismic wave velocity thresholds to identify the depth of the soil and saprolite bottom boundaries. Assuming that such porous compartments host the main part of the active aquifer, their patterns are embedded in a distributed hydrological model. We examine the sensitivity of classical hydrological data (piezometric heads) and geophysical data (magnetic resonance soundings) to the applied velocity thresholds used to define the soil and saprolite boundaries. Different sets of hydrogeological parameters are used in order to distinguish general trends or specificities related to the choice of parameter values. The application of the methodology to an actual catchment illustrates the interest of seismic refraction in constraining the structure of the critical zone subsurface compartments. The sensitivity tests highlight the complementarity of the analysed hydrogeophysical data sets.

An integrated ecohydrological modeling approach was deployed in three long‐term critical zone (CZ) observatories of the French CZ network (CZ Observatories—Application and Research) to better understand how the CZ heterogeneity modulates the water cycle within territories. Ecohydrological simulations with the physically based model EcH 2 O‐iso constrained by a wide range of observations crossing several disciplines (meteorology, hydrology, geomorphology, geophysics, soil sciences, and satellite imagery) are able to capture stream water discharges, evapotranspiration fluxes, and piezometric levels in the Naizin, Auradé, and Strengbach watersheds. In Naizin, an agricultural watershed in northwestern France with a schist bedrock underlying deep weathered materials (5–15 m) along gentle slopes, modeling results reveal a deep aquifer with a large total water storage (1,080–1,150 mm), an important fraction of inactive water storage (94%), and relatively long stream water transit times (0.5–2.5 years). In the Auradé watershed, representative of agricultural landscapes of the southwestern France developed on molasse, a relatively shallow regolith (1–8 m) is observed along hilly slopes. Simulations indicate a shallow aquifer with moderate total water storage (590–630 mm), an important fraction of inactive water storage (91%), and shorter stream water transit times (0.1–1.3 years). In the Strengbach watershed, typical of mid‐mountain forested landscapes developed on granite, CZ evolution implies a shallow regolith (1–5 m) along steep slopes. Modeling results infer a shallow aquifer with the smallest total water storage (475–575 mm), the shortest stream water transit times (0.1–0.7 years), but also the highest fraction of active water storage (18%).

In this study, we investigate the use of ground-penetrating radar (GPR) time-lapse monitoring of artificial soil infiltration experiments. The aim is to evaluate this protocol in the context of estimating the hydrodynamic unsaturated soil parameter values and their associated uncertainties. The originality of this work is to suggest a statistical parameter estimation approach using Markov chain Monte Carlo (MCMC) methods to have direct estimates of the parameter uncertainties. Using the GPR time data from the moving wetting front only does not provide reliable results. Thus, we propose to use additional information from other types of reflectors to optimize the quality of the parameter estimation. Water movement and electromagnetic wave propagation in the unsaturated zone are modeled using a one-dimensional hydrogeophysical model. The GPR travel time data are analyzed for different reflectors: a moving reflector (the infiltration wetting front) and three fixed reflectors located at different depths in the soil. Global sensitivity analysis (GSA) is employed to assess the influence of the saturated hydraulic conductivity Ks, the saturated and residual water contents θs and θr, and the Mualem–van Genuchten shape parameters α and n of the soil on the GPR travel time data of the reflectors. Statistical calibration of the soil parameters is then performed using the MCMC method. The impact of the type of reflector (moving or fixed) is then evaluated by analyzing the calibrated model parameters and their confidence intervals for different scenarios. GSA results show that the sensitivities of the GPR data to the hydrodynamic soil parameters are different between moving and fixed reflectors, whereas fixed reflectors at various depths have similar sensitivities. Ks has a similar and strong influence on the data of both types of reflectors. Concerning the other parameters, for the wetting front, only θs and α have an influence, and only at long time steps since the total variance is zero at the very beginning of the experiment. On the other hand, for the fixed reflectors, the total variance is not zero at the very start and the parameters θs, θr, α and n can have an influence from the very beginning of the infiltration. Results of parameter estimation show that the use of calibration data from the moving or fixed reflectors alone does not enable a good identification of all soil parameters. With the moving reflector, the error between the estimated mean value and the exact target value for θr and α is 9 % and 45 %, respectively, and less than 3 % for the other parameters. The best reduction of the size of the parameter distribution is obtained for n, with a posterior distribution 9 times smaller than the prior one. For the others, this reduction ratio varies between 1 and 5. For the fixed reflectors, the estimated mean values are far from the target values for α, θr and n, representing for a reflector located at 120 cm 15 %, 27 %, and 121 %, respectively. On the other hand, when both data are combined, all soil parameters can be well estimated with narrow confidence intervals. For instance, when using both data from the moving wetting front and a fixed reflector located at 120 cm for calibration, the estimated mean values of the errors of all parameters are less than 5 %. Moreover, all parameter distributions are well reduced, with a maximum reduction for Ks, leading to a posterior distribution being 46 times smaller than the prior one, and the worst but still satisfactory being for θr for which the posterior distribution is 8 times smaller than the prior one. The methodology was applied to fine, medium, and coarse sands with very good results, particularly for the finest soil. The thickness of the unsaturated zone was also tested (0.5, 1, and 2 m) and a better estimation of the hydrodynamic parameters is obtained when the water table is deeper. In addition, the height of water applied in the infiltrometry test influences the speed of the test without affecting the performance of the proposed method.

Temperature logs are an important tool in the geothermal industry. Temperature measurements from boreholes are used for exploration, system design, and monitoring. The number of observations, however, is not always sufficient to fully determine the temperature field or explore the entire parameter space of interest. Drilling in the best locations is still difficult and expensive. It is therefore critical to optimize the number and location of boreholes. Due to its higher spatial resolution and lower cost, four-dimensional (4D) temperature field monitoring via time-lapse Electrical Resistivity Tomography (ERT) has been investigated as a potential alternative. We use Bayesian Evidential Learning (BEL), a Monte Carlo-based training approach, to optimize the design of a 4D temperature field monitoring experiment. We demonstrate how BEL can take into account various data source combinations (temperature logs combined with geophysical data) in the Bayesian optimal experimental design (BOED). To determine the optimal data source combination, we use the Root Mean Squared Error (RMSE) of the predicted target in the low dimensional latent space where BEL is solving the prediction problem. The parameter estimates are accurate enough to use in BOED. Furthermore, the method is not limited to monitoring temperature fields and can be applied to other similar experimental design problems. The method is computationally efficient and requires little training data. For the considered optimal design problem, a training set of only 200 samples and a test set of 50 samples is sufficient.

In this study, the interest of ground penetrating radar (GPR) time-lapse measurements for the estimation of hydrodynamic unsaturated soil parameters is investigated using synthetic infiltration experiments. Water movement and electromagnetic wave propagation in the unsaturated zone are modeled using a one-dimensional hydrogeophysical model. The GPR travel time data are analyzed for different reflectors: a moving reflector (the infiltration wetting front) and three fixed reflectors located at different depths in the soil. Global sensitivity analysis (GSA) is employed to assess the influence of the saturated hydraulic conductivity, the saturated and residual water contents, and the Mualem–van Genuchten shape parameters α and n of the soil on the GPR travel time data of the reflectors. Statistical calibration of the soil parameters is then performed using the Markov chain Monte Carlo (MCMC) method. The impact of the type of reflector (moving or fixed) is then evaluated by analyzing the calibrated model parameters and their confidence intervals for different scenarios. GSA results show that the sensitivities of the moving and fixed reflectors data to the hydrodynamic soil parameters are different whereas the fixed reflectors have similar sensitivities. Results of parameter estimation show that the use of only data from the moving or fixed reflectors does not allow a good identification of all soil parameters. When both data are combined, all soil parameters can be well estimated with narrow confidence intervals.

In mountain areas, both the ecosystem and the local population highly depend on water availability. However, water storage dynamics in mountains is challenging to assess because it is highly variable both in time and space. This calls for innovative observation methods that can tackle such measurement challenge. Among them, gravimetry is particularly well-suited as it is directly sensitive–in the sense it does not require any petrophysical relationship–to temporal changes in water content occurring at surface or underground at an intermediate spatial scale (i.e., in a radius of 100 m). To provide constrains on water storage changes in a small headwater catchment (Strengbach catchment, France), we implemented a hybrid gravity approach combining in-situ precise continuous gravity monitoring using a superconducting gravimeter, with relative time-lapse gravity made with a portable Scintrex CG5 gravimeter over a network of 16 stations. This paper presents the resulting spatio-temporal changes in gravity and discusses them in terms of spatial heterogeneities of water storage. We interpret the spatio-temporal changes in gravity by means of: (i) a topography model which assumes spatially homogeneous water storage changes within the catchment, (ii) the topographic wetness index, and (iii) for the first time to our knowledge in a mountain context, by means of a physically based distributed hydrological model. This study therefore demonstrates the ability of hybrid gravimetry to assess the water storage dynamics in a mountain hydrosystem and shows that it provides observations not presumed by the applied physically based distributed hydrological model.

Key Points: •For the first time, two vertically spaced gravimeters allow to interpret small gravity hydrologically induced signal (<5 nm/s²). •Superconducting gravimetric signal are correlated with evapotranspiration at daily time step. •Gravimetry enables an integrative estimate of evapotranspiration particularly relevant for hydrology. Abstract: Estimating evapotranspiration (ET) is a primary challenge in modern hydrology. Hydrogravimetry is an integrative approach providing highly precise continuous measurement of gravity acceleration. However, large-scale effects (e.g., tides, polar motion, atmospheric loading) limit the fine time-scale interpretation of the gravity data and processing leads to residual signal noise. To circumvent this limitation, we exploited the difference between two superconducting gravimeters (SGs) vertically spaced by 512 m. The gravity difference allows to remove common large-scale effects. Daily variation of the gravity difference is significantly correlated with daily evapotranspiration as estimated using the water balance model SimpKcET (p-value = 4.10(-10)). However, this approach is effective only during rain-free periods. In the future, comparison with direct ET measurements (e.g., eddy-covariance, scintillometer) may confirm and strengthen our interpretation. Improved hydrogravimetric data processing could extend the proposed approach to other experimental sites equipped with a single SG. Plain Language Summary: Land evaporation and vegetation transpiration are crucial parameters in ecohydrology because evapotranspiration constitutes more than two-thirds of precipitated water at the continental scale. However, this invisible flux is difficult to characterize, especially at kilometric scale, and its quantification is challenging for the hydrologist community. Continuous gravity monitoring using a superconducting gravimeter is a direct estimation of the mass change of lands with high precision. At a mountain site in southern France, we highlight a significant association between evapotranspiration calculated by a numerical model and the mass loss of the mountain. This approach provides a novel way to monitor evapotranspiration that will reinforce traditionally used methods.

Assessing the spatial and temporal heterogeneity in subsurface water storage has strong societal and environmental implications, as it is key to assess the water availability for the ecosystem and society. This challenge is especially significant in mountainous areas, where the local population totally depends on springwater as a freshwater resource, while water storage dynamics is complex to evaluate because it exhibits spatiotemporal heterogeneities on all scales as a result of the topography. In this study, we compare the water balance of a headwater granitic catchment (CWB) with water storage changes assessed from in situ continuous gravity monitoring using an iGrav superconducting gravimeter (SG WSC) located at the summit of the catchment. We show that SG WSC and CWB exhibit a similar annual cycle, although they deviate in the months following winter peak flow events. We investigate the reasons for these discrepancies using a tank model adjusted to the SG signal. This shows that during these events, the effective discharge in the SG footprint area is much lower than the catchment streamflow. We attribute this difference in the drainage term to a lower contribution of the upper part of the catchment to the generation of peak flow, compared to the lower part.

Magnetic Resonance Sounding (MRS) measurements are acquired at 16 stations in the Strengbach headwater catchment (Vosges Mountains – France). These data, rendering the vertical distribution of water contents in the subsurface, are used to show their potential in conditioning a hydrological model of the catchment, as described in the article “Magnetic resonance sounding measurements as posterior information to condition hydrological model parameters: Application to a hard-rock headwater catchment” – Journal of Hydrology (2020). Acquisition protocols follow a free induction decay scheme. Data are filtered by applying a band-pass filter at the Larmor frequency. A filter removing the 50 Hz noise is also applied with the exception of data at a Larmor frequency close to the 50 Hz harmonic. The signal envelopes are then fitted by a decaying exponential function over time to estimate the median characteristic relaxation time of each MRS sounding.

In headwater catchment, the calibration of hydrological models is complex due to the scarcity of data in mountainous areas. Here, an innovative methodology is developed to condition hydrological model parameters by using magnetic resonance sounding (MRS) measurements in combination with stream flow rate data. MRS has the specificity in the various geophysical imaging techniques of being mainly sensitive to the vertical distribution of water content among the subsurface. In a way very similar to hydraulic head observations, these local distributions of water content may serve as information in a hydrological model to pattern subsurface flow by seeking model parameters. Simulations are run with different sets of parameters of a hydrological model. Each simulation provides as an output a 4-D map (3-D spatial plus time) of the vertical water content distributions over the whole catchment and their fluctuations over time. This output is then used to simulate the MRS signal that would be produced by the estimated water content. The simulated MRS signal is compared to measured MRS data to determine which hydrological simulations (which model parameters) are close to observations. The approach is applied on a hard-rock headwater catchment housing a very shallow and thin aquifer where an MRS survey covers the whole studied site. Hydraulic parameters of an integrated hydrological model of the catchment are spatially distributed by zones with uniform values, the prior delineation of the zones being guided by pedological studies. As MRS measurements supply local but spatially distributed information, the method conditions the various zones on their parameter values in a much better way than the classical (in headwater catchments) measure of the stream flow rate at the outlet of the system. Finally, hydrological simulation and time-dependent MRS forward calculations can help identifying possible locations for MRS stations to monitor the transient behavior of the hydrological state of the catchment.

Background and aims Monitoring root water uptake dynamics under water deficit (WD) conditions in fields are crucial to assess plant drought tolerance. In this study, we investigate the ability of Electrical Resistivity Tomography (ERT) to capture specific soil water depletion induced by root water uptake. Methods A combination of surface and depth electrodes with a high spatial resolution (10 cm) was used to map 2-D changes of bulk soil electrical conductivity (EC) in an agronomic trial with different herbaceous species. A synthetic experiment was performed with a mechanistic model to assess the ability of the electrode configuration to discriminate abstraction patterns due to roots. The impact of root segments was incorporated in the forward electrical model using the power-law mixing model. Results The time-lapse analysis of the synthetic ERT experiment shows that different root water uptake patterns can be delineated for measurements collected under WD conditions but not under wet conditions. Three indices were found (depletion amount, maximum depth, and spread), which allow capturing plant-specific water signatures based moisture profile changes derived from EC profiles. When root electrical properties were incorporated in the synthetic experiments, it led to the wrong estimation of the amount of water depletion, but a correct ranking of plants depletion depth. When applied to the filed data, our indices showed that Cocksfoot and Ryegrass had shallower soil water depletion zones than white clover and white clover combined with Ryegrass. However, in terms of water depletion amount, Cocksfoot consumed the largest amount of water, followed by White Clover, Ryegrass+White Clover mixture, and Ryegrass. Conclusion ERT is a well-suited method for phenotyping root water uptake ability in field trials under WD conditions.

Recent developments in uncertainty quantification show that a full inversion of model parameters is not always necessary to forecast the range of uncertainty of a specific prediction in Earth Sciences. Instead, Bayesian evidential learning (BEL) uses a set of prior models to derive a direct relationship between data and prediction. This recent technique has been mostly demonstrated for synthetic cases. This paper demonstrates the ability of BEL to predict the posterior distribution of temperature in an alluvial aquifer during a cyclic heat tracer push-pull test. The data set corresponds to another push-pull experiment with different characteristics (amplitude, duration, number of cycles). This experiment constitutes the first demonstration of BEL on real data in a hydrogeological context. It should open the range of future applications of the framework for both scientists and practitioners.

The temporal variability of transit-time distributions (TTDs) and residence-time distributions (RTDs) has received particular attention recently, but such variability has barely been studied using distributed hydrological modeling. In this study, a low-dimensional integrated hydrological model is run in combination with particle-tracking algorithms to investigate the temporal variability of TTDs, RTDs, and StorAge Selection (SAS) functions in the small, mountainous Strengbach watershed belonging to the French network of critical-zone observatories. The particle-tracking algorithms employed rely upon both forward and backward formulations that are specifically developed to handle time-variable velocity fields and evaluate TTDs and RTDs under transient hydrological conditions. The model is calibrated using both traditional streamflow measurements and magnetic resonance sounding (MRS)-which is sensitive to the subsurface water content-and then verified over a ten-year period. The results show that the mean transit time is rather short, at 150-200 days, and that the TTDs and RTDs are not greatly influenced by water storage within the catchment. This specific behavior is mainly explained by the small size of the catchment and its small storage capacity, a rapid flow mainly controlled by gravity along steep slopes, and climatic features that keep the contributive zone around the stream wet all year long.

Electrical resistivity tomography (ERT) has become an important tool for studying root-zone soil water fluxes under field conditions. The results of ERT translate to water content via empirical pedophysical relations, usually ignoring the impact of roots; however, studies in the literature have shown that roots in soils may actually play a non-negligible role in the bulk electrical conductivity (s) of the soil-root continuum, but we do not completely understand the impact of root segments on ERT measurements. In this numerical study, we coupled an electrical model with a plant-soil water flow model to investigate the impact of roots on virtual ERT measurements. The coupled model can produce three-dimensional simulations of root growth and development, water flow in soil and root systems, and electrical transfer in the soil-root continuum. Our electrical simulation illustrates that in rooted soils, for every 1% increase in the root/sand volume ratio, there can be a 4 to 18% increase in the uncertainty of s computed via the model, caused by the presence of root segments; the uncertainty in a loam medium is 0.2 to 1.5%. The influence of root segments on ERT measurements depends on the root surface area (r = ?0.6) and the s contrast between roots and the soil (r = ?0.9), as revealed by correlation analysis. This study is important in the context of accurate water content predictions for automated irrigation systems in sandy soil. Abbreviations: 3D, three-dimensional; ERT, electrical resistivity tomography.

Applications of time-lapse inversion of electrical resistivity tomography allow monitoring variations in the subsurface that play a key role in a variety of contexts. The inversion of timelapse data provides successive images of the subsurface properties showing the medium evolution. Image quality is highly dependent on the data weighting determined from the data error estimates. However, the quantification of errors in the inversion of time-lapse data has not yet been addressed. We have developed a methodology for the quantification of time-lapse data error based on the analysis of the discrepancy between normal and reciprocal readings acquired at different times. We applied the method to field monitoring data sets collected during the injection of heated water in a shallow aquifer. We tested different error models to indicate that the use of an appropriate time-lapse data error estimate yielded significant improvements in terms of imaging. An adapted inversion weighting for time-lapse data implies that the procedure does not allow an over-fitting of the data, so the presence of artifacts in the resulting images is greatly reduced. Our results determined that a proper estimate of time-lapse data error is mandatory for weighting optimally the inversion to obtain images that best reflect the evolution of medium properties over time.

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.

Les hydrosystèmes de montagne contribuent significativement à l'approvisionnement en eau des populations et constituent des écosystèmes avec une riche biodiversité. Ils sont identifiés comme des sentinelles du changement climatique, au sens où ils sont particulièrement vulnérables à de petites perturbations des variables climatiques. Il est donc nécessaire de déterminer leur fonctionnement hydrologique actuel, dans l'optique d'évaluer leur vulnérabilité au changement climatique et de proposer des mesures d'adaptation. Parmi les différentes méthodes d'observation in-situ disponibles, la gravimétrie est une méthode adéquate.

Evapotranspiration (ET) is a key process of the water cycle in general and in ecohydrology in particular. Measuring ET in forest eco-hydrosystems allows us to gain a better understanding of the response of forests to drought events, and to better anticipate the effects of climate change. Punctual (e.g. lysimeters or sapflow measurements) or integrative measurement methods (e.g. eddy covariance tower) can be used to estimate ET at the forest stand scale but these methods are not without limitations (e.g., resolution issues, representativeness, not adapted to mountainous areas).Superconducting gravimeters can be used to study ET. These gravimeters can be deployed in both flat and mountainous environments. In this work, we studied the hydrological residuals (i.e., hydrologically induced gravity variations) of 5 superconducting gravimeters located in different contexts. We interpreted the daily decreases in the stacked hydrological residual as the loss of water mass due to evapotranspiration. These results were compared with those of the SimpKcET water balance model.The results underline that the detectability of the ET signal depends strongly on the configuration of the gravimetric station, the topography and the type of ecosystem. We show that gravimeters located on summit area and in a forested context can detect the seasonality of ET. Conversely, gravimeters located in flat or underground areas and with a significant masking effect are unable to detect ET.Gravimetry therefore has a strong complementarity with conventional methods used to study ET and could contribute to a better understanding of water fluxes in forested ecosystems.

La science est claire: selon le 6e rapport du GIEC, au rythme des émissions de CO2 actuelles les budgets climats pour rester sous les seuils de 1.5°C et 2°C de réchauffement, seront dépassés dans moins de 10 et 25 ans respectivement. Selon le GIEC et l’Agence Internationale de l’Énergie “rester sur une trajectoire 1,5°C implique de stopper le développement de nouveaux projets pétroliers et gaziers dès 2022.” Pourtant la plupart des industries fossiles (IF), ici définies comme actives dans l’exploration ou l’exploitation d’énergies fossiles carbonées (charbon, gaz, pétrole) continuent à investir dans le développement de nouveaux sites d’extraction et rejettent ces résultats scientifiques sur la base d’arguments douteux voir fallacieux. En parallèle, les IF maintiennent des liens étroits avec l’Enseignement Supérieur et la Recherche (ESR), à travers des projets de recherche, des enseignements ou d’autres partenariats, leur permettant d’accéder à des travailleurs formés, d’orienter les sujets de recherches, d’obtenir des financements publics et peut-être avant tout, de maintenir, voir verdir, leur image en s’associant à l’image positive de l’ESR. Implication pour la recherche: Les universités et organismes de recherche publics sont fondés sur des valeurs incluant l’honnêteté intellectuelle, la droiture morale, l’intégrité éthique et la production de connaissance au service de problématiques sociales. Vu que les industries fossiles assument, de manière de plus en plus évidente (Bonneuil et al., 2021), leur malhonnêteté intellectuelle et leur volonté assumée de préserver leurs intérêts privés (quel que soit le coût du changement climatique pour la société), on pourrait penser que les collaborations entre l’ESR publique et les IF soient dénoncées et interrompues. Pourtant, même si quelques grandes universités dont Cambridge, Oxford, Harvard, ou l’Université Libre d’Amsterdam ont cessé certaines, voire toutes, formes de collaborations, ce type de décision reste rare et n’est pas encore advenu en France. Nous proposons d’enquêter sur les raisons de cette inaction et de cette passivité de l’ESR français vis-à-vis des IF et sur leurs implications en terme de lutte contre le dérèglement climatique. Plusieurs mécanismes peuvent être en jeu, dont au moins i) l’opacité sur les liens entre ESR et IF (qui empêche de les dénoncer), ii) la croyance que les IF et/ou leur projet sont une partie de la solution aux problèmes climatiques (qui pousse à les soutenir) et iii) les conditions de financement difficiles de l’ESR (qui fait craindre l’interruption des collaborations). Pour clarifier l’importance de ces mécanismes et leurs effets nous nous appuyons sur deux approches complémentaires. D’une part une cartographie des liens entre IF et ESR français, inspiré de l’initiative Hollandaise (Mapping Fossil Ties Coalition, 2024). Cette cartographie s’appuiera sur les bases de données des projets financés par l’Europe (plus de 500 projets des Programmes FP7 et Horizon incluaient au moins un établissement Français de l’ESR et une IF) ou par l’ANR ayant des IF pour partenaires, permettant de visualiser les sujets de recherches en jeu, les niveaux de financements allant aux IF et les principales universités partenaires. De plus, l’analyse de sites webs et de rapports d’activités permettront de décrire plus en détails les liens de certaines institutions fortement liées aux IF et leur justification (par ex via les UMR industrielles du CNRS). D’autre part nous analyserons et présenterons les résultats d’un questionnaire anonyme diffusé parmi les membres de l’ESR français visant à clarifier la position des agents vis -à -vis des liens entre IF et ESR, de leur justification et de leurs craintes associées à la rupture éventuelle de ces liens, clarifiant ainsi le rôle des trois mécanismes hypothétiques ci -dessus. Le questionnaire permettra aussi de compléter par une approche bottom-up la cartographie des liens entre IF et ESR.

Le réseau GEOFCAN (approche GEOphysique et structurale de l’organisation spatiale et du Fonctionnement des Couvertures pédologiques Anthropisés et Naturelles) est coordonné par cinq institutions suivantes : le Bureau de Recherches Géologiques et Minières (BRGM), l’Institut National de Recherche pour l’Agriculture, l’alimentation et l’Environnement (INRAE), l’Institut de Recherche pour le Développement (IRD), Sorbonne Université, et l’Université Paris‐Saclay. Les membres du réseau rassemblent leurs compétences scientifiques et leurs moyens techniques pour : (i) une meilleure connaissance des relations entre la constitution des matériaux (nature et mode d’assemblage des constituants, espace poral) et leurs propriétés physiques élémentaires, notamment celles qui entrent dans la constitution du signal donné, par diverses méthodes géophysiques ; (ii) l’adaptation et au développement d’outils géophysiques qui soient notamment adaptés à l’étude de la zone critique, des structures de couvertures pédologiques et des transferts dont elles sont le siège, dans une large gamme de contextes pédoclimatiques ; (iii) un renouvellement des méthodes d’étude du fonctionnement des couvertures pédologiques autour d’un objectif général de spatialisation. Les objectifs du réseau sont de : (i) organiser tous les 2 ans un colloque rassemblant la communauté internationale francophone de géophysique de proche surface et/ou environnementale, (ii) motiver/aider les jeunes géophysiciennes et géophysiciens (notamment les étudiantes et étudiants en master, les doctorantes et doctorants et les post-doctorantes et post-doctorants), (iii) fédérer la communauté internationale francophone de la géophysique de proche surface, (iv) soutenir des projets innovants. Ce 13ème colloque fait notamment suite au 1er qui avait eu lieu à Bondy en 1997, et au 12ème initialement prévu à Grenoble et qui a eu finalement lieu en ligne en 2021. Après le succès en nombre de personnes suivant du dernier colloque en ligne, et en raison de la bonne situation sanitaire actuelle, ce 13ème colloque est organisé en hybride pour combiner la possibilité d’accès à distance pour les personnes éloignées, et l’occasion de se retrouver pour les personnes pouvant venir à Strasbourg. Le choix de Strasbourg, capitale européenne, est lié au pôle ancien et très actif en recherche et en formation en géophysique. Roger GUÉRIN Coordinateur de GEOFCAN