The challenge of reconstructing complex multi-scale heterogeneities from multi-method geophysical and geological investigations of a karstic fractured limestone

Laurent Gautier and Celine Mallet and Mohamad Abbas and Cedric Bailly and Louis Lefrancois and Arnaud Isch and Clara Jodry and Jean-Michel Baltassat and Jacques Deparis and Simon Andrieu and Florence Quesnel and Eglantine Husson and Mohamed Azaroual. ( 2021 )
in: 2021 RING Meeting, ASGA

Abstract

The OZNS project implements a unique observatory within the Beauce Limestone Formation at Villamblain (Région Centre Val de Loire, France). This observatory aims to investigate the key role of the vadose zone (VZ) for fluid and heat transfers within the Critical Zone, as an interface between atmosphere and aquifers. The studied zone covers an area with a radius of 25 m and spans from the surface down to 25 m depth, reaching the aquifer and the barrier layer of the Molasses du Gâtinais. The instrumented surface, central well, and surrounding drill holes will produce decade-long continuous records of the thermal, hydrological, and biogeochemical state of the vadose zone. However, despite all the instrumentation efforts, the processes at stake within the vadose zone will only be finely understood at the light of an accurate characterisation of the geological architecture hosting this observatory. First investigations, combining direct observations, well-logging, and multimethod-geophysics (electric resistivity and induced polarization tomography, seismic, ground penetrating radar, gravimetry, and nuclear magnetic resonance) with cores sampled from drilled boreholes highlighted the unique complexity of these geological formations. They consist of a series of terrestrial limestones, with palustrine and lacustrine facies, alternating monogenic and polygenic breccias, micritic limestones, erosive surfaces, and microbial formations. The already initially complex limestones are also affected by a long history of fractures and alterations, from early diagenesis to localized silicification and meteoric alterations forming isalterite and alloterite horizons. These alterations developed a complex crypto-karst that follows bedding and possibly fractures, forming a complex network of centimetric to decametric horizontal hollows and vertical conduits. These structures are clearly observed throughout the 9 available drill holes and seem to affect geophysical measures. However, the dimension of the karstic heterogeneities is too small for being accurately imaged by inverting surface geophysical measurements alone. A solution could be provided by considering additional knowledge about the reconstructed objects. Typically, karst develops along preferential structures and will connect entry points in the less conductive surface layers down to pathways leading to the aquifer. We propose to investigate an iterative inversion scheme that progressively incorporates and upscales these knowledges in the joint inversion and fusion of multi-method geophysical data. This methodology first requires to determine the controlling factors (a REV: Representative Elementary Volume) including the spatial heterogeneities within OZNS' fractured and karstic limestones at the various observation scales, from centimetric laboratory experiments to decametric field observations.

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BibTeX Reference

@INPROCEEDINGS{LAURENT_RM2021,
    author = { Gautier, Laurent and Mallet, Celine and Abbas, Mohamad and Bailly, Cedric and Lefrancois, Louis and Isch, Arnaud and Jodry, Clara and Baltassat, Jean-Michel and Deparis, Jacques and Andrieu, Simon and Quesnel, Florence and Husson, Eglantine and Azaroual, Mohamed },
     title = { The challenge of reconstructing complex multi-scale heterogeneities from multi-method geophysical and geological investigations of a karstic fractured limestone },
 booktitle = { 2021 RING Meeting },
      year = { 2021 },
 publisher = { ASGA },
  abstract = { The OZNS project implements a unique observatory within the Beauce Limestone Formation at Villamblain (Région Centre Val de Loire, France). This observatory aims to investigate the key role of the vadose zone (VZ) for fluid and heat transfers within the Critical Zone, as an interface between atmosphere and aquifers. The studied zone covers an area with a radius of 25 m and spans from the surface down to 25 m depth, reaching the aquifer and the barrier layer of the Molasses du Gâtinais. The instrumented surface, central well, and surrounding drill holes will produce decade-long continuous records of the thermal, hydrological, and biogeochemical state of the vadose zone. However, despite all the instrumentation efforts, the processes at stake within the vadose zone will only be finely understood at the light of an accurate characterisation of the geological architecture hosting this observatory. First investigations, combining direct observations, well-logging, and multimethod-geophysics (electric resistivity and induced polarization tomography, seismic, ground penetrating radar, gravimetry, and nuclear magnetic resonance) with cores sampled from drilled boreholes highlighted the unique complexity of these geological formations. They consist of a series of terrestrial limestones, with palustrine and lacustrine facies, alternating monogenic and polygenic breccias, micritic limestones, erosive surfaces, and microbial formations. The already initially complex limestones are also affected by a long history of fractures and alterations, from early diagenesis to localized silicification and meteoric alterations forming isalterite and alloterite horizons. These alterations developed a complex crypto-karst that follows bedding and possibly fractures, forming a complex network of centimetric to decametric horizontal hollows and vertical conduits. These structures are clearly observed throughout the 9 available drill holes and seem to affect geophysical measures. However, the dimension of the karstic heterogeneities is too small for being accurately imaged by inverting surface geophysical measurements alone. A solution could be provided by considering additional knowledge about the reconstructed objects. Typically, karst develops along preferential structures and will connect entry points in the less conductive surface layers down to pathways leading to the aquifer. We propose to investigate an iterative inversion scheme that progressively incorporates and upscales these knowledges in the joint inversion and fusion of multi-method geophysical data. This methodology first requires to determine the controlling factors (a REV: Representative Elementary Volume) including the spatial heterogeneities within OZNS' fractured and karstic limestones at the various observation scales, from centimetric laboratory experiments to decametric field observations. }
}