Time-Lapse Seismic and Electrical Monitoring of the Vadose Zone during a Controlled Infiltration Experiment at the Ploemeur Hydrological Observatory, France

The vadose zone is the main host of surface and subsurface water exchange and has important implications for ecosystems functioning, climate sciences, geotechnical engineering, and water availability issues. Geophysics provides a means for investigating the subsurface in a non-invasive way and at larger spatial scales than conventional hydrological sensors. Time-lapse hydrogeophysical applications are especially useful for monitoring flow and water content dynamics. Largely dominated by electrical and electromagnetic methods, such applications increasingly rely on seismic methods as a complementary approach to describe the structure and behavior of the vadose zone. To further explore the applicability of active seismics to retrieve quantitative information about dynamic processes in near-surface time-lapse settings, we designed a controlled water infiltration experiment at the Ploemeur Hydrological Observatory (France) during which successive periods of infiltration were followed by surface-based seismic and electrical resistivity acquisitions. Water content was monitored throughout the experiment by means of sensors at different depths to relate the derived seismic and electrical properties to water saturation changes. We observe comparable trends in the electrical and seismic responses during the experiment, highlighting the utility of the seismic method to monitor hydrological processes and unsaturated flow. Moreover, petrophysical relationships seem promising in providing quantitative results

Advancing measurements and representations of subsurface heterogeneity and dynamic processes: towards 4D hydrogeology

Essentially all hydrogeological processes are strongly influenced by the subsurface spatial heterogeneity and the temporal variation of environmental conditions, hydraulic properties, and solute concentrations. This spatial and temporal variability generally leads to effective behaviors and emerging phenomena that cannot be predicted from conventional approaches based on homogeneous assumptions and models. However, it is not always clear when, why, how, and at what scale the 4D (3D + time) nature of the subsurface needs to be considered in hydrogeological monitoring, modeling, and applications. In this paper, we discuss the interest and potential for the monitoring and characterization of spatial and temporal variability, including 4D imaging, in a series of hydrogeological processes: (1) groundwater fluxes, (2) solute transport and reaction, (3) vadose zone dynamics, and (4) surface–subsurface water interactions. We first identify the main challenges related to the coupling of spatial and temporal fluctuations for these processes. We then highlight recent innovations that have led to significant breakthroughs in high-resolution space–time imaging and modeling the characterization, monitoring, and modeling of these spatial and temporal fluctuations. We finally propose a classification of processes and applications at different scales according to their need and potential for high-resolution space–time imaging. We thus advocate a more systematic characterization of the dynamic and 3D nature of the subsurface for a series of critical processes and emerging applications. This calls for the validation of 4D imaging techniques at highly instrumented observatories and the harmonization of open databases to share hydrogeological data sets in their 4D components

Suivi de la redistribution spatio-temporelle de l'eau dans le sous-sol par des méthodes sismiques

The characterization and monitoring of subsurface water systems are fundamental to groundwater resources conservation and management. To this end, hydrogeophysics provides a suite of non-invasive methods to study the shallow subsurface environment and the processes occurring therein over multiple scales. Time-lapse hydrogeophysical applications are notably useful to monitor water dynamics and follow temporal variations in water content. Largely dominated by electrical and electromagnetic methods, these applications are being increasingly explored with seismic methods. The seismic signal is dependent on the mechanical properties of the medium which are in turn affected by changes in water content. Consequently, seismic responses are also influenced by hydrological properties and state variables. Nonetheless, complexities in describing the mechanical behavior of partially saturated shallow materials have limited the quantitative characterization of the subsurface and associated water dynamics by means of seismic methods. Here we investigate the evolution of seismic responses with varying water content in time-lapse field contexts, analyzing both data and inverted parameters, and compare the resulting trends with established petrophysical relationships. We show that seismic time-lapse inversions of P-wave refraction data and corresponding changes in wave propagation velocity enable the recognition of preferential water flow paths in the subsurface, highlighting the potential of seismic methods to monitor hydrological processes and unsaturated flow. Overall, qualitative agreements between seismic velocity trends and theoretical petrophysical relationships still eclipse accurate quantitative estimations of water content from inverted seismic parameters. We anticipate further time-lapse seismic field studies to help bridge the gap between qualitative and quantitative observations. In the wake of the recent advancements in seismic equipment and techniques, appropriate field-scale petrophysical relationships will play an important role in the development of seismic methods for hydrological applications.La caractérisation et la surveillance des systèmes d'eau souterraine sont fondamentales pour la conservation et la gestion des ressources en eau souterraine. Dans cette intention, l'hydrogéophysique fournit une série de méthodes non invasives pour étudier l'environnement souterrain peu profond et les processus qui s'y déroulent sur plusieurs échelles. Les applications hydrogéophysiques à méthode dites time-lapse sont notamment utiles pour surveiller la dynamique de l'eau et suivre les variations temporelles de la teneur en eau. Largement dominées par des méthodes électriques et électromagnétiques, ces applications sont de plus en plus explorées avec des méthodes sismiques. Le signal sismique dépend des propriétés mécaniques du milieu qui sont à leur tour affectées par les changements de teneur en eau. Par conséquent, les réponses sismiques sont également influencées par les propriétés et variables hydrologiques. Néanmoins, les complexités liées à la description du comportement mécanique de matériaux peu profonds et partiellement saturés limitent la caractérisation quantitative de la subsurface et la dynamique de l'eau associée par les méthodes sismiques. Dans ce travail, nous étudions l'évolution temporelle des réponses sismiques en fonction des variations de teneur en eau par la méthode de time-lapse acquise sur le terrain. Nous analysons à la fois les données et les paramètres inversés et nous comparons ensuite les tendances résultantes avec des relations pétrophysiques établies. De cette façon, nous montrons que les inversions sismiques en time-lapse des données de réfraction de l'onde P et les changements correspondant à la vitesse de propagation des ondes permettent la reconnaissance des chemins d'écoulement préférentiels de l'eau dans la subsurface, mettant ainsi en évidence le potentiel des méthodes sismiques pour surveiller les processus hydrologiques et les écoulements non saturés. De manière générale, on observe un fossé d'observations entre les estimations quantitatives de la teneur en eau obtenue par les paramètres sismiques inversés et les corrélations qualitatives, jusqu'à présent dominantes, reliant la vitesse sismique aux relations pétrophysiques théoriques. Cet écart d'observation pourra être comblé par de nouvelles études sismiques en time-lapse sur le terrain. À la suite des progrès récents d'équipement et techniques sismiques, les relations pétrophysiques à l'échelle du terrain joueront un rôle important dans le développement de méthodes sismiques pour des applications hydrologiques

Suivi de la redistribution spatio-temporelle de l'eau dans le sous-sol par des méthodes sismiques

The characterization and monitoring of subsurface water systems are fundamental to groundwater resources conservation and management. To this end, hydrogeophysics provides a suite of non-invasive methods to study the shallow subsurface environment and the processes occurring therein over multiple scales. Time-lapse hydrogeophysical applications are notably useful to monitor water dynamics and follow temporal variations in water content. Largely dominated by electrical and electromagnetic methods, these applications are being increasingly explored with seismic methods. The seismic signal is dependent on the mechanical properties of the medium which are in turn affected by changes in water content. Consequently, seismic responses are also influenced by hydrological properties and state variables. Nonetheless, complexities in describing the mechanical behavior of partially saturated shallow materials have limited the quantitative characterization of the subsurface and associated water dynamics by means of seismic methods. Here we investigate the evolution of seismic responses with varying water content in time-lapse field contexts, analyzing both data and inverted parameters, and compare the resulting trends with established petrophysical relationships. We show that seismic time-lapse inversions of P-wave refraction data and corresponding changes in wave propagation velocity enable the recognition of preferential water flow paths in the subsurface, highlighting the potential of seismic methods to monitor hydrological processes and unsaturated flow. Overall, qualitative agreements between seismic velocity trends and theoretical petrophysical relationships still eclipse accurate quantitative estimations of water content from inverted seismic parameters. We anticipate further time-lapse seismic field studies to help bridge the gap between qualitative and quantitative observations. In the wake of the recent advancements in seismic equipment and techniques, appropriate field-scale petrophysical relationships will play an important role in the development of seismic methods for hydrological applications.La caractérisation et la surveillance des systèmes d'eau souterraine sont fondamentales pour la conservation et la gestion des ressources en eau souterraine. Dans cette intention, l'hydrogéophysique fournit une série de méthodes non invasives pour étudier l'environnement souterrain peu profond et les processus qui s'y déroulent sur plusieurs échelles. Les applications hydrogéophysiques à méthode dites time-lapse sont notamment utiles pour surveiller la dynamique de l'eau et suivre les variations temporelles de la teneur en eau. Largement dominées par des méthodes électriques et électromagnétiques, ces applications sont de plus en plus explorées avec des méthodes sismiques. Le signal sismique dépend des propriétés mécaniques du milieu qui sont à leur tour affectées par les changements de teneur en eau. Par conséquent, les réponses sismiques sont également influencées par les propriétés et variables hydrologiques. Néanmoins, les complexités liées à la description du comportement mécanique de matériaux peu profonds et partiellement saturés limitent la caractérisation quantitative de la subsurface et la dynamique de l'eau associée par les méthodes sismiques. Dans ce travail, nous étudions l'évolution temporelle des réponses sismiques en fonction des variations de teneur en eau par la méthode de time-lapse acquise sur le terrain. Nous analysons à la fois les données et les paramètres inversés et nous comparons ensuite les tendances résultantes avec des relations pétrophysiques établies. De cette façon, nous montrons que les inversions sismiques en time-lapse des données de réfraction de l'onde P et les changements correspondant à la vitesse de propagation des ondes permettent la reconnaissance des chemins d'écoulement préférentiels de l'eau dans la subsurface, mettant ainsi en évidence le potentiel des méthodes sismiques pour surveiller les processus hydrologiques et les écoulements non saturés. De manière générale, on observe un fossé d'observations entre les estimations quantitatives de la teneur en eau obtenue par les paramètres sismiques inversés et les corrélations qualitatives, jusqu'à présent dominantes, reliant la vitesse sismique aux relations pétrophysiques théoriques. Cet écart d'observation pourra être comblé par de nouvelles études sismiques en time-lapse sur le terrain. À la suite des progrès récents d'équipement et techniques sismiques, les relations pétrophysiques à l'échelle du terrain joueront un rôle important dans le développement de méthodes sismiques pour des applications hydrologiques

Suivi de la redistribution spatio-temporelle de l'eau dans le sous-sol par des méthodes sismiques

La caractérisation et la surveillance des systèmes d'eau souterraine sont fondamentales pour la conservation et la gestion des ressources en eau souterraine. Dans cette intention, l'hydrogéophysique fournit une série de méthodes non invasives pour étudier l'environnement souterrain peu profond et les processus qui s'y déroulent sur plusieurs échelles. Les applications hydrogéophysiques à méthode dites time-lapse sont notamment utiles pour surveiller la dynamique de l'eau et suivre les variations temporelles de la teneur en eau. Largement dominées par des méthodes électriques et électromagnétiques, ces applications sont de plus en plus explorées avec des méthodes sismiques. Le signal sismique dépend des propriétés mécaniques du milieu qui sont à leur tour affectées par les changements de teneur en eau. Par conséquent, les réponses sismiques sont également influencées par les propriétés et variables hydrologiques. Néanmoins, les complexités liées à la description du comportement mécanique de matériaux peu profonds et partiellement saturés limitent la caractérisation quantitative de la subsurface et la dynamique de l'eau associée par les méthodes sismiques. Dans ce travail, nous étudions l'évolution temporelle des réponses sismiques en fonction des variations de teneur en eau par la méthode de time-lapse acquise sur le terrain. Nous analysons à la fois les données et les paramètres inversés et nous comparons ensuite les tendances résultantes avec des relations pétrophysiques établies. De cette façon, nous montrons que les inversions sismiques en time-lapse des données de réfraction de l'onde P et les changements correspondant à la vitesse de propagation des ondes permettent la reconnaissance des chemins d'écoulement préférentiels de l'eau dans la subsurface, mettant ainsi en évidence le potentiel des méthodes sismiques pour surveiller les processus hydrologiques et les écoulements non saturés. De manière générale, on observe un fossé d'observations entre les estimations quantitatives de la teneur en eau obtenue par les paramètres sismiques inversés et les corrélations qualitatives, jusqu'à présent dominantes, reliant la vitesse sismique aux relations pétrophysiques théoriques. Cet écart d'observation pourra être comblé par de nouvelles études sismiques en time-lapse sur le terrain. À la suite des progrès récents d'équipement et techniques sismiques, les relations pétrophysiques à l'échelle du terrain joueront un rôle important dans le développement de méthodes sismiques pour des applications hydrologiques.The characterization and monitoring of subsurface water systems are fundamental to groundwater resources conservation and management. To this end, hydrogeophysics provides a suite of non-invasive methods to study the shallow subsurface environment and the processes occurring therein over multiple scales. Time-lapse hydrogeophysical applications are notably useful to monitor water dynamics and follow temporal variations in water content. Largely dominated by electrical and electromagnetic methods, these applications are being increasingly explored with seismic methods. The seismic signal is dependent on the mechanical properties of the medium which are in turn affected by changes in water content. Consequently, seismic responses are also influenced by hydrological properties and state variables. Nonetheless, complexities in describing the mechanical behavior of partially saturated shallow materials have limited the quantitative characterization of the subsurface and associated water dynamics by means of seismic methods. Here we investigate the evolution of seismic responses with varying water content in time-lapse field contexts, analyzing both data and inverted parameters, and compare the resulting trends with established petrophysical relationships. We show that seismic time-lapse inversions of P-wave refraction data and corresponding changes in wave propagation velocity enable the recognition of preferential water flow paths in the subsurface, highlighting the potential of seismic methods to monitor hydrological processes and unsaturated flow. Overall, qualitative agreements between seismic velocity trends and theoretical petrophysical relationships still eclipse accurate quantitative estimations of water content from inverted seismic parameters. We anticipate further time-lapse seismic field studies to help bridge the gap between qualitative and quantitative observations. In the wake of the recent advancements in seismic equipment and techniques, appropriate field-scale petrophysical relationships will play an important role in the development of seismic methods for hydrological applications

Modeling burial induced changes in physical sandstone properties - A case-study of North Sea and Norwegian Sea sandstone formations

The changes in physical properties of sandstones with burial depth are a result of mechanical and chemical compaction processes. These processes are affected by rock microstructure, pressure regimes and temperature history. Data from 30 wells have been used to investigate and compare the changes in porosity, bulk density, elastic moduli and wave propagation velocities between the mid-Jurassic sandstones of the Etive Fm. in the North Sea and the Garn Fm. in the Norwegian Sea. At shallow burial depths (less than 2 km) the changes of the physical properties are governed by effective stress. A mechanical compaction model is used to describe the porosity loss and the bulk density increase with depth, whereas the friable-sand theory is used to explain the changes in elastic moduli and wave propagation velocities. For both formations, the under predictions by the models in the porosity, bulk moduli and P-wave velocity values from the data suggest high depositional porosities (0.40) and small amounts of quartz cement at depths of 1.6-2.0 km. At greater burial depths and temperatures (greater than 2 km, and greater than 75°C) quartz cementation is the main controlling factor in the changes of the physical properties. The porosity loss and the bulk density increase with depth are explained by means of a quartz cement precipitation model, and the contact-cement theory is used to describe the changes in elastic moduli and wave propagation velocities. High porosities (greater than 0.15) at great burial depths (greater than 4 km) suggest the presence of higher amounts of clay coatings in both formations, and they may also be a result of high overpressures. The great variations in porosity and bulk modulus values for Garn sandstones encountered at same depths, indicate that the Garn Fm. is less well sorted and more affected by different types of quartz deposition than the Etive Fm. The contact-cement model main over prediction trend for the bulk modulus of highly overpressured sandstones enlightens the effects of different pressure regimes in the chemical compaction domain

Finding appropriate rocks physics models to interpret seismic data in hydrogeophysics applications

International audienceSeismic methods have been recently applied to the monitoring of spatial and temporal variations of near surface characteristics for hydrogeological purposes. The seismic signal is certainly related to mechanical properties that partly depend on porosity and saturation. The behavior of pressure (P) and shear (S) waves in the presence of water is partially decoupled, and the ratio of their propagation velocities VP/VS has been used to study water saturation changes. However, the interpretation of the mechanical properties remains complex in unconsolidated near surface materials, limiting the quantitative description of linked hydrodynamic properties. In this study, we investigate the theories behind wave propagation velocities in poorly consolidated media and how they are affected by water content, focusing our discussion on the partially saturated response. We present a field case where we used a Hertz-Mindlin based rock physics model to estimate water saturation from VP and VS from seismic data. The model is able to distinguish between dry and fully saturated areas at two distinct hydrological periods, but fails in identifying partially saturated areas in both cases. This work underlines the need for more elaborated models to infer hydrodynamic properties from seismic data

In situ datasets that couple tracer experiments and geophysical monitoring available

In this deliverable D4.2, we report on the collection of data sets that couple conventional and non-conservative tracer information with geophysical imaging techniques. These in-situ datasets are of particular importance since our conceptual models rely essentially on lab experiments and borehole measurements of chemical species concentrations. The main challenge is to obtain truly quantitative probabilistic estimates of concentration and reaction rate distribution, while coping with the limited spatial resolutions of imaging techniques and the multiple sources of signals associated with transport and reaction processes