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Welcome to ENIGMA ITN Website!

ENIGMA ITN is an Innovative Training Network funded by the European Commission.

ENIGMA ITN ( European training Network for in situ imaGing of dynaMic processes in heterogeneous subsurfAce environments) aims at training a new generation of young researchers in the development of innovative methods for imaging process dynamics in subsurface hydrosystems, in order to enhance understanding and predictive modelling capacities and to transfer these innovations to the economic sector.

The 15 young PhD students  trained within the network contribute to develop the spatial representation of subsurface heterogeneity, fluxes, chemical reactions and microbial activity, through the integration of data and approaches from geophysics, hydrology, soil physics, and biochemistry.
The network ENIGMA gathers 21 partners (15 academic and 6 industrial) from 8 European countries.
Each of the 15 PhD students conducts the research work in 2 or 3 institutions, in collaboration with the industrial partners.

Publication: Inferring geostatistical properties of hydraulic conductivity fields from saline tracer tests and equivalent electrical conductivity time-series

in Advances in Water Resources (2020)
by Alejandro Fernandez Visentini, Niklas Linde, Tanguy Le Borgne, Marco Dentz
https://doi.org/10.1016/j.advwatres.2020.103758

Abstract

We use Approximate Bayesian Computation and the Kullback–Leibler divergence measure to quantify to what extent horizontal and vertical equivalent electrical conductivity time-series observed during tracer tests constrain the 2-D geostatistical parameters of multivariate Gaussian log-hydraulic conductivity fields. Considering a perfect and known relationship between salinity and electrical conductivity at the point scale, we find that the horizontal equivalent electrical conductivity time-series best constrain the geostatistical properties. The variance, controlling the spreading rate of the solute, is the best constrained geostatistical parameter, followed by the integral scales in the vertical direction. We find that horizontally layered models with moderate to high variance have the best resolved parameters. Since the salinity field at the averaging scale (e.g., the model resolution in tomograms) is typically non-ergodic, our results serve as a starting point for quantifying uncertainty due to small-scale heterogeneity in laboratory-experiments, tomographic results and hydrogeophysical inversions involving DC data.

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More on ESR9 research project

Congratulations Doctor Lara Blazevic!

ENIGMA fellow Lara Blazevic successfully defended her thesis “Monitoring spatio-temporal wate redistribution in the subsurface with seismic methods” on Friday 18th September 2020 virtually from Sorbonne University. Congrats!

Abstract
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.


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Publication: Heterogeneity-Induced Mixing and Reaction Hot Spots Facilitate Karst Propagation in Coastal Aquifers

in Geophysical Research Letters (2020)
by Kevin de Vriendt, Maria Pool, Marco Dentz
https://doi.org/10.1029/2020GL087529

Abstract

The freshwater‐seawater mixing zone is a critical region for chemical activity. Yet little is known about the influence of ever present spatial heterogeneity on the dynamics of mixing and calcite dissolution, which play a key role in the understanding of karst development. We analyze the impact of different heterogeneity structures and strengths on the local and global response of mixing and dissolution rates across the saltwater freshwater mixing zone. We find that the initial heterogeneity structure significantly impacts observed dissolution and mixing patterns, which sheds some new light on karst propagation in coastal aquifers.

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More on ESR1 research project

Presentation : “Detection of subsurface water storage dynamics with combined gravity – vertical gravity gradient monitoring and hydrological simulation”

Event: EGU General Assembly 2020 (online)
Presentation by Anne-Karin Cooke, Cédric Champollion, Pierre Vermeulen, Camille Janvier, Bruno Desruelle, Nicolas Le Moigne, Sébastien Merlet
https://doi.org/10.5194/egusphere-egu2020-9020

Abstract

Time-lapse ground-based gravimetry is increasingly applied in subsurface hydrology, providing mass balance constraints on water storage dynamics. For a given water content change as e.g. after a precipitation event, the simplest assumption is that of a homogeneous, infinite slab (Bouguer plate) of water column causing the measurable increase in gravitational attraction. For heterogeneous subsurface environments such as karst aquifers at field scale this assumption may not always hold. The gravity signal is depth-integrated and non-unique, hence indistinguishable from a heterogeneous distribution without further information.

Exploiting the different spatial sensitivities of gravity and vertical gravity gradient (VGG) data can shed light on the following questions:

  • Is the subsurface water content within the gravimeter’s footprint likely to be homogeneous or showing small-scale heterogeneity?
  • If not, at which distance are these mass heterogeneities and how large are they?
  • Which monitoring set-ups (tripod heights, number of and distance between VGG measurement locations) are likely to detect mass heterogeneity of which spatial characteristics?

One year of monthly vertical gravity gradient surveys has been completed in the geodetic observatory in karstic environment on the Larzac plateau in southern France. We interpret the VGG observations obtained in this field study in the context of further available hydraulic and geophysical data and hydro-gravimetrical simulation. Finally, practical applications in view of detecting near-surface voids and reservoirs of different porosities as well as their storage capacity and seasonal dynamics are evaluated.

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More on the ESR8 project

Publication: Continuous dissolved gas tracing for fracture-matrix exchanges

in Geophysical Research Letters (August 2020)
by Richard Hoffmann, Pascal Goderniaux, Pierre Jamin, Eliot Chatton, Jérôme de la Bernardie, Thierry Labasque, Tanguy Le Borgne, Alain Dassargues
https://doi.org/10.1029/2020GL088944

Abstract

Transport in fractured media plays an important role in a range of processes, from rock weathering and microbial processes to contaminant transport, and energy extraction and storage. Diffusive transfer between the fracture fluid and the rock matrix is often a key element in these applications. But the multiscale heterogeneity of fractures renders the field assessment of these processes extremely challenging. This study explores the use of dissolved gases as tracers of fracture‐matrix interactions, which can be measured continuously and highly accurately using mobile mass spectrometers. Since their diffusion coefficients vary significantly, multiple gases are used to probe different scales of fracture‐matrix exchanges. Tracer tests with helium, xenon and argon were performed in a fractured chalk aquifer and resulting tracer breakthrough curves are modelled. Results show that continuous dissolved gas tracing with multiple tracers provide key constrains on fracture matrix interactions and reveal unexpected scale effects in fracture‐matrix exchange rates.

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More on ESR11 research project