Poster: Hydrogeophysical monitoring of a controlled infiltration experiment at the Ploemeur Hydrological Observatory (Brittany, France)

Event: Journées des Doctorants – Ecole Doctorale Géosciences, Ressources Naturelles et Environnement – Paris (France), 19/03/2019
Poster by Lara Blazevic, Ludovic Bodet, Laurant Longuevergne, Sylvain Pasquet, Damien Jougnot


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

Event: AGU Fall Meeting 2018, Washington DC (USA)
poster by Lara Blazevic, Ludovic Bodet, Damien Jougnot, Laurent Longuvergne

Abstract

Seismic 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.


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Presentation: Potential Impact of Groundbased Gravity Gradiometer for Subsurface Reservoir Monotoring

Event: EGU General Assembly, Vienna (Austria) 2019
Presentation by Anne-Karin Cooke, Cédric Champollion, Pierre Vermeulen, Nicolas Le Moigne and Sébastien Merlet

Abstract

Ground-based gravity measurements can provide accurate constrains on the water storage dynamics of subsurface reservoirs. At the scale of the measurements time-lapse gravity experiments allow to monitor the water mass balance taking into account both the saturated and the unsaturated zone.

One major characteristic of the gravity measurement is the integration of all water masses across scales: gravity variations can be the effect of continentalscale soil humidity or aquifer (such as seen by GRACE measurements) and of local effects (such as the umbrellaeffect of a building or reservoir heterogeneities). The vertical gravity gradient is similar to gravity while showing a higher sensibility to local masses. The interest of the gradient for subsurface features such as caves is well known.

The objective of the presentation is the specific potential of gravity vertical gradient monitoring for water reservoir studies. The study is first based on existing measurements of gravity gradients time series (with a relative spring gravimeter): can significant hydro-logical signals be detected? Then based on numerical simulations, the potential of future ground-based for reservoir monitoring is investigated.

The capability of the gravity gradient method to detect heterogeneities (contrary to theclassical gravity) is evident: for example the classical Bouguer plate approximation often used to convert gravityinto equivalent water height (even in heterogeneous area such as karst) has no effect on the gravity gradient, hence new models need to be applied that combine gravity and gravity gradient measurements.


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Publication: A cross-validation framework to extract data features for reducing structural uncertainty in subsurface heterogeneity

in Advances in Water Resources Vol 133 (November 2019)
by Jorge Lopez-Alvis, Thomas Hermans, Frédéric Nguyen
https://doi.org/10.1016/j.advwatres.2019.103427

Abstract

Spatial heterogeneity is a critical issue in the management of water resources. However, most studies do not consider uncertainty at different levels in the conceptualization of the subsurface patterns, for example using one single geological scenario to generate an ensemble of realizations.

In this paper, we represent the spatial uncertainty by the use of hierarchical models in which higher-level parameters control the structure. Reduction of uncertainty in such higher-level structural parameters with observation data may be done by updating the complete hierarchical model, but this is, in general, computationally challenging.

To address this, methods have been proposed that directly update these structural parameters by means of extracting lower dimensional representations of data called data features that are informative and applying a statistical estimation technique using these features.

The difficulty of such methods, however, lies in the choice and design of data features, i.e. their extraction function and their dimensionality, which have been shown to be case-dependent. Therefore, we propose a cross-validation framework to properly assess the robustness of each designed feature and make the choice of the best feature more objective. Such framework aids also in choosing the values for the parameters of the statistical estimation technique, such as the bandwidth for kernel density estimation.

We demonstrate the approach on a synthetic case with cross-hole ground penetrating radar traveltime data and two higher-level structural parameters: discrete geological scenarios and the continuous preferential orientation of channels.

With the best performing features selected according to the cross-validation score, we successfully reduce the uncertainty for these structural parameters in a computationally efficient way. While doing so, we also provide guidelines to design features accounting for the level of knowledge of the studied system.

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Publication: Application of Stable Isotopes of Water to Study Coupled Submarine Groundwater Discharge and Nutrient Delivery

in Water 11:9 (September 2019)
by Carlos Duque, Soren Jessen, Joel Tirado-Conde, Sachin Karan and Peter Engesgaard
https://doi.org/10.3390/w11091842

Abstract

Submarine groundwater discharge (SGD)—including terrestrial freshwater, density-driven flow at the saltwater–freshwater interface, and benthic exchange—can deliver nutrients to coastal areas, generating a negative effect in the quality of marine water bodies. It is recognized that water stable isotopes (18O and 2H) can be helpful tracers to identify different flow paths and origins of water. Here, we show that they can be also applied when assessing sources of nutrients to coastal areas.

A field site near a lagoon (Ringkøbing Fjord, Denmark) has been monitored at a metric scale to test if stable isotopes of water can be used to achieve a better understanding of the hydrochemical processes taking place in coastal aquifers, where there is a transition from freshwater to saltwater.

Results show that 18O and 2H differentiate the coastal aquifer into three zones: Freshwater, shallow, and deep saline zones, which corresponded well with zones having distinct concentrations of inorganic phosphorous. The explanation is associated with three mechanisms: (1) Differences in sediment composition, (2) chemical reactions triggered by mixing of different type of fluxes, and (3) biochemical and diffusive processes in the lagoon bed.

The different behaviors of nutrients in Ringkøbing Fjord need to be considered in water quality management. PO4 underneath the lagoon exceeds the groundwater concentration inland, thus demonstrating an intra-lagoon origin, while NO3, higher inland due to anthropogenic activity, is denitrified in the study area before reaching the lagoon.

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