Event: AGU Fall Meeting 2019, San Francisco (USA) Presentation by Richard Hoffmann, Pascal Goderniaux, Alain Dassargues
Informative reference data for a realistic assessment of aquifer heterogeneity is a prerequisite for robust transport simulations. Structure-based imaging using salt or a dye as tracer with a known concentration and volume to observe transfer times, is a powerful hydrogeological tool in moderate heterogenous media. Solving then the advection-dispersion equation will explain most of the point to point transport behavior. But, once the aquifer heterogeneity is more complex, e.g. in a double porosity medium like chalk, matrix porosity linked to diffusion processes must be taken into consideration to avoid a biased interpretation of the tracer information. Thus, performing additional local process-based imaging using smart tracers as dissolved gas and hot or cold water, assists to explain the late-time tailing behaviors realistically.
Smart tracers were injected in a sub-horizontal fracture connecting
two adjacent wells to provide data about the complementary behaviors of
each tracer and to focus on matrix diffusion processes. One reference
data set is a 70 hours injection of hot water (∆T = + 40 °C)
complemented by two 10 minutes uranine pulse injections within an
inflatable double packer system isolating the sub-horizontal chalk
fracture of interest. The temperature signal arrives at a 7.55 m
distance with a delay of 12 hours compared to the first uranine
injection and shows a rebound after the injection stopped. Useful
reference data for further numerical modelling consists now in (a) local
fracture geometry information deduced from interpretation by analytical
solutions and, (b) matrix diffusion information.
Numerical modelling of those smart tracer experiments may question deterministic models for predictions and motivates for data-driven prediction tools like Monte-Carlo simulation procedures within a direct predictive framework. Distance based global sensitivity analysis (e.g. simultaneous variation of multiple input variables like diffusion coefficient, aperture and matrix storage) will be considered accounting for temperature related changes of viscosity and density. Key information about the most influencing parameters are main model outcomes, as local process understanding is very useful for possible future upscaling in regional models made of structure-based imaging.
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Event: AGU Fall Meeting 2019, San Francisco (USA)
Presentation by Joel Tirado-Conde, Majken Looms, Peter Engesgaard
Wetlands are extremely dynamical systems and their behavior depends on
the characteristics of the surroundings (topography, geology and
vegetation, among others) as well as on meteorological and hydrological
processes. Wetlands are wet partly because they receive groundwater (or
drain water) through diffuse upwelling and through springs. Studying
upwelling is of great importance to e.g. evaluate the overall ecology or
capacity to remove nitrate of the wetland system. One problem is that
diffuse upwelling is difficult locate and measure.
We analyze the temporal dynamics of a groundwater fed wetland in
central Jutland (Denmark) by the use of various thermal methods across a
lowland stream valley. A monitoring system consisting of Distributed
Temperature Sensing (DTS), wells with temperature depth profiles and
thermal infrared (TIR) imaging on a UAV, in conjunction with
hydrological and atmospheric data, provide a quasi 3D time-lapse
characterization of the thermal behavior of the system, both on the
ground and in the subsurface, over a period of around two years.
By analyzing the temporal evolution of the temperature in both the wetland surface and the groundwater, we can infer potential locations of groundwater upwelling to the land surface and subsequent overland flow. This is relevant as previous studies have shown that it is a generally overlooked flow component that may have a big impact relative to base flow. Moreover, it serves as a test for the feasibility of using heat as a tracer to study groundwater – surface water exchanges in wetlands.
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in Geophysical Journal International Volume 220, Issue 2, February 2020, Pages 1187-1196
by Satoshi Izumoto, Johan Alexander Huisman, Yuxin Wu, Harry Vereecken
Induced calcite precipitation is used in geotechnics to modify the mechanical and hydrological properties of the underground. Laboratory experiments have shown that spectral induced polarization (SIP) measurements can detect calcite precipitation. However, the results of previous studies investigating the SIP response of calcite precipitation were not fully consistent.
This study aims to investigate how the SIP response of calcite depends on solute concentration to explain the differences in SIP response observed in previous studies. A four-phase experiment with SIP measurements on a column filled with sand was performed. In phase I, calcite precipitation was generated for a period of 12 d by co-injecting Na2CO3 and CaCl2 solutions through two different ports. This resulted in a well-defined calcite precipitation front, which was associated with an increase in the imaginary part of the conductivity (σ′′σ′′). In phase II, diluted solutions were injected into the column. This resulted in a clear decrease in σ′′σ′′. In phase III, the injection of the two solutions was stopped while calcite precipitation continued and solute concentrations in the mixing zone decreased. Again, this decreased σ′′σ′′. Finally, the injection rate of the Na2CO3 solution was reduced relative to that of the CaCl2 solution in phase IV. This resulted in a shift of the mixing zone away from the calcite precipitation front established in phase I and an associated decrease of σ′′σ′′.
These results imply that the SIP response of calcite is highly sensitive to the solute concentration near the precipitates, which may explain previously reported conflicting results.
Full article here
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Event: Journées des Doctorants – Ecole Doctorale Géosciences, Ressources Naturelles et Environnement – Paris (France), 19/03/2019
Poster by Lara Blazevic, Ludovic Bodet, Laurent Longuevergne, Sylvain Pasquet, Damien Jougnot
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Event: AGU Fall Meeting 2018, Washington DC (USA)
poster by Lara Blazevic, Ludovic Bodet, Damien Jougnot, Laurent Longuvergne
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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