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Caractérisation multi-échelle des structures hydrogéologiques en contexte volcanique insulaire par électromagnétisme héliporté : application à l’île de La Réunion

Abstract : In 2014, Reunion Island was covered by a vast airborne geophysics campaign. The SkyTEM device mapped magnetic anomalies and bulk resistivity at an extra 300 m depth throughout the island. The resulting dataset revealed the issue of interpreting airborne electromagnetic data at the regional scale. The interpretation of a 3D resistivity model wrapping 2 500 km2 requires the implementation of innovative methodologies to compare it with geological, climatic and hydrogeological data. The aim of the thesis is to enhance this geophysical dataset at the regional scale by integrating hydrogeological analysis at different spatial scales. To answer these issues, this thesis revolves around a multi-scale hydrogeophysical methodology, which allows to analyze how aquifers are distributed and their behaviors on unprecedented scales. A statistical approach has been developed to synthesize 3D geo-electrical contrasts in 2D to interpret the 350 000 soundings. All outcomes are consistent with the geological and hydrogeological knowledge of the island, enabling us to study how regional geological and climatic parameters impact the distribution of aquifers and weathering processes. In the coastal zone, resistivity data are compared with groundwater electrical conductivity logs to validate the interpretation of geophysics. Thus, the resistivity model helped mapping saltwater intrusion and analyzing its control parameters at the island scale. Inland, the combined use of airborne electromagnetic and magnetic data made it possible to characterize the geometry of geological structures and to image weathering profiles and paleo-profiles. Combining results gathered on various scales allows the analysis of weathering impact on aquifers of shield volcanoes. On the coastal zone, un-weathered and highly permeable lithological units (up to 10-1 m/s) are subject to saltwater intrusion, even with recharge rates up to 6 to 8 m per year. With decreasing transmissivity, aquifers become less exposed to saltwater intrusion. According to geophysics outcomes, the contacts between different eruptive phase products at high altitude are characterized by specific weathering paleo-profiles which create permeability contrasts conducive to perched aquifers establishment. Examining the geophysical response in different areas of the island made it possible to analyze the dynamics of weathering processes, initially controlled by the age of the formations and secondly, by the rainfall rates and the temperature. These results showed that weathering and structure of basaltic volcanoes depend on their age and on windward/leeward slopes. Thus, our research helped improve the accuracy of the conceptual hydrogeological model including geological structuring and time evolution of basaltic volcanic aquifers in an insular environment.
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https://tel.archives-ouvertes.fr/tel-01940195
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Submitted on : Friday, November 30, 2018 - 9:54:12 AM
Last modification on : Monday, April 6, 2020 - 2:49:38 PM
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2018LARE0010_MDumont.pdf
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  • HAL Id : tel-01940195, version 1

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Marc Dumont. Caractérisation multi-échelle des structures hydrogéologiques en contexte volcanique insulaire par électromagnétisme héliporté : application à l’île de La Réunion. Hydrologie. Université de la Réunion, 2018. Français. ⟨NNT : 2018LARE0010⟩. ⟨tel-01940195⟩

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