Développement d'une approche « time-lapse » des méthodes sismiques pour l'hydrogéophysique et la compréhension de la dynamique des hydrosystèmes

The characterization and understanding of hydrosystems is part of the "Critical Zone" (CZ) study. They consist in an important issue for the management and protection of surface water and groundwater resources. Geophysics and hydrogeophysics are among the tools for studying this fraction of the CZ and its processes. The near-surface seismic methods are suggested for the imaging of the CZ in order to describe its geometry and the nature of its compartments. Since seismic methods are based on the mechanical properties of the medium, the measured signal is also influenced by the water content: its analysis therefore makes it possible to describe its spatial variation. The time-lapse application of this approach is proposed here in order to follow the temporal variations in the water content of the hydrosystems. A processing workflow based on a statistical study is developed to ensure the significance of the temporal variations in the data with respect to measurement errors. It is shown that when the time-lapse variations are greater than the estimated errors, they reflect the hydrological behaviors of the structures identified and provide new information on their dynamics. Quantifying these results by inverting the seismic data recorded at each time step is however not obvious, even in the knowledge of measurement errors, in particular due to a lack of prior information. In these cases, the posterior uncertainties may be too high to compare the temporal variations of the inverted parameters. However, when the study area is particularly constrained, a thorough inversion at each time step of the data estimated as significant by our approach is possible. In this case, we suggest a method of interpolation of the depth of the saturated zone in the vicinity of known water table levels, from the seismic images. This information can then be used to constrain the hydrodynamic modeling by proposing: (i) a "high resolution" definition of the geometry of the hydrogeological compartments and their facies and (ii) new boundary and initial conditions.La caractérisation et la compréhension des hydrosystèmes font partie intégrante de l’étude de la « Zone Critique » (ZC). Elles représentent un enjeu important pour la gestion et la protection des ressources en eaux de surface et en eaux souterraines. La géophysique et l’hydrogéophysique font partie des outils d’étude de cette fraction de la ZC et des processus qu’elle referme. Les méthodes sismiques de subsurface sont proposées afin de l’imager pour en définir la géométrie et la nature de ses compartiments. Comme la sismique repose sur les paramètres mécaniques du milieu, le signal mesuré est également influencé par la teneur en eau : son analyse permet donc d’en décrire la variation spatiale. L’application « time-lapse » de cette approche est proposée ici afin de suivre les variations temporelles de contenu en eau des hydrosystèmes étudiés. Une chaîne de traitements basée sur une étude statistique est développée afin de s’assurer de la significativité des variations temporelles des données par rapport aux erreurs de mesures. On démontre que lorsque les variations « time-lapse » sont supérieures aux erreurs estimées, elles reflètent les comportements hydrologiques des structures identifiées et apportent de nouvelles informations sur leur dynamique. Quantifier ces résultats par inversion des données sismiques acquises à chaque pas de temps n’est cependant pas évident, même en connaissance des erreurs de mesures, notamment par manque d’information a priori. Dans ces cas, les incertitudes a posteriori peuvent s’avérer trop élevées pour pouvoir comparer les variations temporelles des paramètres inversés. Toutefois, lorsque la zone d’étude est particulièrement bien contrainte, une inversion méticuleuse à chaque pas de temps des données estimées comme significatives par notre approche, est possible. Dans ce cas, on propose une méthode d’interpolation de la profondeur de la zone saturée au voisinage de niveaux piézométriques connus, à partir des images sismiques. Ces informations peuvent ensuite être utilisées afin de contraindre la modélisation hydrodynamique en proposant : (i) une définition « haute résolution » de la géométrie des compartiments hydrogéologiques et de leur faciès et (ii) de nouvelles conditions aux bords et initiales

Development of a time-lapse approach of seismic methods for hydrogeophysics and the understanding of hydrosystem dynamics

La caractérisation et la compréhension des hydrosystèmes font partie intégrante de l’étude de la « Zone Critique » (ZC). Elles représentent un enjeu important pour la gestion et la protection des ressources en eaux de surface et en eaux souterraines. La géophysique et l’hydrogéophysique font partie des outils d’étude de cette fraction de la ZC et des processus qu’elle referme. Les méthodes sismiques de subsurface sont proposées afin de l’imager pour en définir la géométrie et la nature de ses compartiments. Comme la sismique repose sur les paramètres mécaniques du milieu, le signal mesuré est également influencé par la teneur en eau : son analyse permet donc d’en décrire la variation spatiale. L’application « time-lapse » de cette approche est proposée ici afin de suivre les variations temporelles de contenu en eau des hydrosystèmes étudiés. Une chaîne de traitements basée sur une étude statistique est développée afin de s’assurer de la significativité des variations temporelles des données par rapport aux erreurs de mesures. On démontre que lorsque les variations « time-lapse » sont supérieures aux erreurs estimées, elles reflètent les comportements hydrologiques des structures identifiées et apportent de nouvelles informations sur leur dynamique. Quantifier ces résultats par inversion des données sismiques acquises à chaque pas de temps n’est cependant pas évident, même en connaissance des erreurs de mesures, notamment par manque d’information a priori. Dans ces cas, les incertitudes a posteriori peuvent s’avérer trop élevées pour pouvoir comparer les variations temporelles des paramètres inversés. Toutefois, lorsque la zone d’étude est particulièrement bien contrainte, une inversion méticuleuse à chaque pas de temps des données estimées comme significatives par notre approche, est possible. Dans ce cas, on propose une méthode d’interpolation de la profondeur de la zone saturée au voisinage de niveaux piézométriques connus, à partir des images sismiques. Ces informations peuvent ensuite être utilisées afin de contraindre la modélisation hydrodynamique en proposant : (i) une définition « haute résolution » de la géométrie des compartiments hydrogéologiques et de leur faciès et (ii) de nouvelles conditions aux bords et initiales.The characterization and understanding of hydrosystems is part of the "Critical Zone" (CZ) study. They consist in an important issue for the management and protection of surface water and groundwater resources. Geophysics and hydrogeophysics are among the tools for studying this fraction of the CZ and its processes. The near-surface seismic methods are suggested for the imaging of the CZ in order to describe its geometry and the nature of its compartments. Since seismic methods are based on the mechanical properties of the medium, the measured signal is also influenced by the water content: its analysis therefore makes it possible to describe its spatial variation. The time-lapse application of this approach is proposed here in order to follow the temporal variations in the water content of the hydrosystems. A processing workflow based on a statistical study is developed to ensure the significance of the temporal variations in the data with respect to measurement errors. It is shown that when the time-lapse variations are greater than the estimated errors, they reflect the hydrological behaviors of the structures identified and provide new information on their dynamics. Quantifying these results by inverting the seismic data recorded at each time step is however not obvious, even in the knowledge of measurement errors, in particular due to a lack of prior information. In these cases, the posterior uncertainties may be too high to compare the temporal variations of the inverted parameters. However, when the study area is particularly constrained, a thorough inversion at each time step of the data estimated as significant by our approach is possible. In this case, we suggest a method of interpolation of the depth of the saturated zone in the vicinity of known water table levels, from the seismic images. This information can then be used to constrain the hydrodynamic modeling by proposing: (i) a "high resolution" definition of the geometry of the hydrogeological compartments and their facies and (ii) new boundary and initial conditions

Développement d'une approche « time-lapse » des méthodes sismiques pour l'hydrogéophysique et la compréhension de la dynamique des hydrosystèmes

The characterization and understanding of hydrosystems is part of the "Critical Zone" (CZ) study. They consist in an important issue for the management and protection of surface water and groundwater resources. Geophysics and hydrogeophysics are among the tools for studying this fraction of the CZ and its processes. The near-surface seismic methods are suggested for the imaging of the CZ in order to describe its geometry and the nature of its compartments. Since seismic methods are based on the mechanical properties of the medium, the measured signal is also influenced by the water content: its analysis therefore makes it possible to describe its spatial variation. The time-lapse application of this approach is proposed here in order to follow the temporal variations in the water content of the hydrosystems. A processing workflow based on a statistical study is developed to ensure the significance of the temporal variations in the data with respect to measurement errors. It is shown that when the time-lapse variations are greater than the estimated errors, they reflect the hydrological behaviors of the structures identified and provide new information on their dynamics. Quantifying these results by inverting the seismic data recorded at each time step is however not obvious, even in the knowledge of measurement errors, in particular due to a lack of prior information. In these cases, the posterior uncertainties may be too high to compare the temporal variations of the inverted parameters. However, when the study area is particularly constrained, a thorough inversion at each time step of the data estimated as significant by our approach is possible. In this case, we suggest a method of interpolation of the depth of the saturated zone in the vicinity of known water table levels, from the seismic images. This information can then be used to constrain the hydrodynamic modeling by proposing: (i) a "high resolution" definition of the geometry of the hydrogeological compartments and their facies and (ii) new boundary and initial conditions.La caractérisation et la compréhension des hydrosystèmes font partie intégrante de l’étude de la « Zone Critique » (ZC). Elles représentent un enjeu important pour la gestion et la protection des ressources en eaux de surface et en eaux souterraines. La géophysique et l’hydrogéophysique font partie des outils d’étude de cette fraction de la ZC et des processus qu’elle referme. Les méthodes sismiques de subsurface sont proposées afin de l’imager pour en définir la géométrie et la nature de ses compartiments. Comme la sismique repose sur les paramètres mécaniques du milieu, le signal mesuré est également influencé par la teneur en eau : son analyse permet donc d’en décrire la variation spatiale. L’application « time-lapse » de cette approche est proposée ici afin de suivre les variations temporelles de contenu en eau des hydrosystèmes étudiés. Une chaîne de traitements basée sur une étude statistique est développée afin de s’assurer de la significativité des variations temporelles des données par rapport aux erreurs de mesures. On démontre que lorsque les variations « time-lapse » sont supérieures aux erreurs estimées, elles reflètent les comportements hydrologiques des structures identifiées et apportent de nouvelles informations sur leur dynamique. Quantifier ces résultats par inversion des données sismiques acquises à chaque pas de temps n’est cependant pas évident, même en connaissance des erreurs de mesures, notamment par manque d’information a priori. Dans ces cas, les incertitudes a posteriori peuvent s’avérer trop élevées pour pouvoir comparer les variations temporelles des paramètres inversés. Toutefois, lorsque la zone d’étude est particulièrement bien contrainte, une inversion méticuleuse à chaque pas de temps des données estimées comme significatives par notre approche, est possible. Dans ce cas, on propose une méthode d’interpolation de la profondeur de la zone saturée au voisinage de niveaux piézométriques connus, à partir des images sismiques. Ces informations peuvent ensuite être utilisées afin de contraindre la modélisation hydrodynamique en proposant : (i) une définition « haute résolution » de la géométrie des compartiments hydrogéologiques et de leur faciès et (ii) de nouvelles conditions aux bords et initiales

About the use of near-surface seismic data to better constrain hydrogeological models

International audience<p>Over the past decade, we have done our best to develop alternative methods to image the heterogeneities of the critical zone, describe the dynamics of its hydrosystems, and add seismic techniques to the hydrogeophysics toolbox. With the growth of long-term observation infrastructures in this field, the geophysical tools recently developed by the community tend to be viewed as state-of-the-art geophysical characterization methods mainly deployed to augment observatory and network databases. A major problem is that geophysical results are mostly just sets of parameters, in other words "models", deduced from sparse data sets and poorly posed problems. They certainly cannot be considered as data by observatories. In order to better transport information from the data into models that could be safely exploited by non-geophysicists, we need to: increase the extent and throughput of our surveys; optimize our acquisition configurations with respect to the target of interest; greatly increase our spatial and temporal sampling capabilities; automate our tedious processing workflows; and improve, if not completely revise, our inversion tools. We illustrate this last point with examples from the field. They show how a thorough interpretation of geophysical models can provide valuable prior information on the distribution of hydrofacies and calibrate the hydrogeological modeling domain. In addition, we raise the question of the propagation of uncertainty from the geophysical data to the hydrogeological model and suggest the use of alternative petrophysics to better interpret the data collected in the partially saturated zone.</p&gt

Time-lapse seismic experiments to constrain hydrodynamic parameters at the stream-aquifer interface

International audienceVP/VS or Poisson’s ratio estimated from active seismic methods recently proved to be efficient in the imaging ofthe critical zone and associated hydrosystems. We suggest here a time-lapse application of this approach to provideboth spatial and temporal constraints on the hydrodynamic model of the Avenelles experimental basin (Seine etMarne, France). The preliminary studies of this hydrosystem relied on typical combined interpretation of sparsegeological and hydrological data. Geophysical surveys, performed throughout the watershed, helped delineatingthe different compartments and identifying their connectivity with the stream network. Once a basin-scale globalhydrogeological model established, hotspots were targeted with local high frequency monitoring stations to inves-tigate its stream-aquifer exchanges. At these stations, recorded data (bank piezometers, stream water temperatureand level, temperature profiles in the hyporheic zone) clearly showed contrasts in the dynamic of the hydrosystemalong the stream network. However, the nature of the compartments and their associated properties, observed at thebasin-scale, would not explain the data observed at the local scale. It highlighted the need for detailed descriptionof the hydrosystem, at the stream-aquifer interface. One specific hotspot was thus selected to perform soundingsand geophysical measurements of higher resolutions. Thanks to electrical resistivity tomography, P-wave refrac-tion and surface-wave seismic imaging, we provided a description of the local heterogeneities both in terms oflithology and water content. The seismic experiments were then repeated with a two-month time step. At eachtime step, pseudo-2D sections of Poisson’s ratio clearly showed strong spatial and temporal variations in satura-tion of the vadose zone. These results then helped providing updated constraints and boundary conditions to thehydrodynamic mode

How Groundwater Models Can Benefit from Near-Surface Seismic Data ?

Groundwater (GW) systems exist in dynamic balance with the climate and human pressure, connecting interfacing zones of recharge and discharge with multiple feedbacks. Quantifying the water flows at these interfaces is a key issue for hydrogeologists to consider for safe yield and good water quality. These interfaces are composed of various morpho-sedimentary units with highly contrasting geometries and lithologies. GW recharge and SW-GW exchanges cannot be directly measured. Consequently, it is necessary to model these fluxes, as they are dependent on the boundary conditions and spatial description of the hydrofacies, which are largely unknown and typically estimated via model calibration using conventional data (hydraulic heads and river discharge). Over the past decade, we have done our best to develop alternative methods to image the heterogeneities of the critical zone, describe the dynamics of its hydrosystems, and add seismic techniques to the hydrogeophysics toolbox.We illustrate this last point with examples from the field. They show how a thorough interpretation of geophysical models can provide valuable prior information on the distribution of hydrofacies and calibrate the hydrogeological modeling domain. In addition, we raise the question of the propagation of uncertainty from the geophysical data to the hydrogeological model and suggest the use of alternative petrophysics to better interpret the data collected in the unsaturated zone.especially in view of the ongoing changes in climate and land useOver the past decade, we have done our best to develop alternative methods to image the heterogeneities of the critical zone, describe the dynamics of its hydrosystems, and add seismic techniques to the hydrogeophysics toolbox. With the growth of long-term observation infrastructures in this field, the geophysical tools recently developed by the community tend to be viewed as state-of-the-art geophysical characterization methods mainly deployed to augment observatory and network databases. A major problem is that geophysical results are mostly just sets of parameters, in other words "models", deduced from sparse data sets and poorly posed problems. They certainly cannot be considered as data by observatories. In order to better transport information from the data into models that could be safely exploited by non-geophysicists, we need to: increase the extent and throughput of our surveys; optimize our acquisition configurations with respect to the target of interest; greatly increase our spatial and temporal sampling capabilities; automate our tedious processing workflows; and improve, if not completely revise, our inversion tools. We illustrate this last point with examples from the field. They show how a thorough interpretation of geophysical models can provide valuable prior information on the distribution of hydrofacies and calibrate the hydrogeological modeling domain. In addition, we raise the question of the propagation of uncertainty from the geophysical data to the hydrogeological model and suggest the use of alternative petrophysics to better interpret the data collected in the partially saturated zone

Assessing Surface Water and Groundwater Interactions Using Long-Term Hydrological and Time-Lapse Seismic Data in the Orgeval Critical Zone Observatory

International audienceQuantifying the water and heat fluxes at the interface between surface water (SW) and groundwater (GW) is a key issue for hydrogeologists to consider for safe yield and good water quality. However, such quantification with field measurements is not straightforward because the SW-GW changes depend on the boundary conditions and the spatial description of the hydrofacies, which aren't well known and are usually guessed by calibrating models using standard data like hydraulic heads and river discharge. We provide a methodology to build stronger constraints to the numerical simulation and the hydrodynamic and thermal parameter calibration, both in space and time, by using a multi-method approach. Our method, applied to the Orgeval Critical Zone Observatory (France), estimates both water flow and heat fluxes through the SW-GW interface using long-term hydrological data, time-lapse seismic data, and modeling tools. We show how a thorough interpretation of high-resolution geophysical images, combined with geotechnical data, provides a detailed distribution of hydrofacies, valuable prior information about the associated hydrodynamic property distribution. The temporal dynamic of the WT table can be captured with high-resolution time-lapse seismic acquisitions. Each seismic snapshot is then thoroughly inverted to image spatial WT variations. The long-term hydrogeological data (such as hydraulic head and temperature) and this prior geophysical information are then used to set the parameters for the hydrogeological modeling domain. The use of the WT geometry and temperature data improves the estimation of transient stream-aquifer exchanges

How Groundwater Models Can Benefit from Near-Surface Seismic Data ?

International audienceGroundwater (GW) systems exist in dynamic balance with the climate and human pressure, connecting interfacing zones of recharge and discharge with multiple feedbacks. Quantifying the water flows at these interfaces is a key issue for hydrogeologists to consider for safe yield and good water quality. These interfaces are composed of various morpho-sedimentary units with highly contrasting geometries and lithologies. GW recharge and SW-GW exchanges cannot be directly measured. Consequently, it is necessary to model these fluxes, as they are dependent on the boundary conditions and spatial description of the hydrofacies, which are largely unknown and typically estimated via model calibration using conventional data (hydraulic heads and river discharge). Over the past decade, we have done our best to develop alternative methods to image the heterogeneities of the critical zone, describe the dynamics of its hydrosystems, and add seismic techniques to the hydrogeophysics toolbox.We illustrate this last point with examples from the field. They show how a thorough interpretation of geophysical models can provide valuable prior information on the distribution of hydrofacies and calibrate the hydrogeological modeling domain. In addition, we raise the question of the propagation of uncertainty from the geophysical data to the hydrogeological model and suggest the use of alternative petrophysics to better interpret the data collected in the unsaturated zone.especially in view of the ongoing changes in climate and land useOver the past decade, we have done our best to develop alternative methods to image the heterogeneities of the critical zone, describe the dynamics of its hydrosystems, and add seismic techniques to the hydrogeophysics toolbox. With the growth of long-term observation infrastructures in this field, the geophysical tools recently developed by the community tend to be viewed as state-of-the-art geophysical characterization methods mainly deployed to augment observatory and network databases. A major problem is that geophysical results are mostly just sets of parameters, in other words "models", deduced from sparse data sets and poorly posed problems. They certainly cannot be considered as data by observatories. In order to better transport information from the data into models that could be safely exploited by non-geophysicists, we need to: increase the extent and throughput of our surveys; optimize our acquisition configurations with respect to the target of interest; greatly increase our spatial and temporal sampling capabilities; automate our tedious processing workflows; and improve, if not completely revise, our inversion tools. We illustrate this last point with examples from the field. They show how a thorough interpretation of geophysical models can provide valuable prior information on the distribution of hydrofacies and calibrate the hydrogeological modeling domain. In addition, we raise the question of the propagation of uncertainty from the geophysical data to the hydrogeological model and suggest the use of alternative petrophysics to better interpret the data collected in the partially saturated zone

Etude de la zone critique : Variations temporelles des données sismiques

National audienceNear-surface seismic methods are mainly used to determine the geometricalcharacteristics of hydrosystems. However, they have been recently suggested toinvestigate the mechanical properties of the Critical Zone (CZ) influenced bywater content, thanks to the combined estimation of P- and S-wave velocities. Wepropose here a time-lapse application of this approach. Two seismic acquisitionswere carried out under distinct hydrogeological conditions along the same lineat the Ploemeur hydrogeological observatory (France). Vertical componentseismic data were recorded to extract: (i) P-wave first arrival times and (ii)Rayleigh-wave phase velocities. The significant variations with time and space,of both datasets, indicate marked changes in mechanical properties of the CZ tobe compared with hydrological data.Les méthodes sismiques de subsurface sont souvent utilisées pour lacaractérisation géométrique des hydrosystèmes. De récentes recherches mettenten évidence leur potentiel pour l’étude des propriétés mécaniques de la zonecritique (ZC) influencées par la teneur en eau, grâce à l’estimation conjointe desvitesses des ondes P et S. Une approche « time-lapse » est ici proposée. Deuxacquisitions sismiques selon le même profil ont été réalisées sous conditionshydrogéologiques distinctes à l’observatoire hydrologique de Ploemeur(Morbihan). L’utilisation de géophones à composante verticale permetl’extraction (i) des temps des premières arrivées des ondes P et (ii) des vitessesde phase des ondes de Rayleigh. La variation significative des données dans letemps et l’espace, indique des changements notoires des propriétés mécaniquesde la ZC, à comparer avec les données hydrogéologiques

Geophysical characterization of the vadose zone above an abandoned underground quarry of Chalk

An abandoned underground quarry of Chalk located near Beauvais (France) is of particular interest to study infiltration and dissolution processes in the vadose zone. The presence of permanent underground lakes created by the outcroping water table allows to describe the spatial and temporal variability of groundwater geochemistry within the quarry. Geophysical measurements were carried out at the surface above the quarry to characterize the geometry and the physical properties of the vadose zone and study their influence on the groundwater quality variations. Electromagnetic induction (EMI) mapping performed at the surface provided a spatial description of the thicknesses of soil and clay-with-flints superficial formation covering the Chalk. Electrical resistivity tomography (ERT) emphasized deeper geological structures along a transect located directly above an underground lake. Finally, the combined use of pressure-wave traveltime tomography and surface-wave profiling along the same transect highlighted strong lateral variations of the Poisson's ratio corresponding to significant water content variations within the clay-with-flints and the Chalk formations