Variability of the water stock dynamics in karst: insights from surface-to-tunnel geophysics

Fores, Benjamin; Champollion, Cédric; Lesparre, Nolwenn; Pasquet, Sylvain; Martin, Aurélie; Nguyen, Frédéric

Variability of the water stock dynamics in karst: insights from surface-to-tunnel geophysics | Hétérogénéité et dynamique de la ressource en eau karstique: apport de la géophysique surface-tunnel Variabilidad en la dinámica de las reservas de agua en el karst: conocimientos a partir de la geofísica desde la superficie hasta un túnel 喀斯特贮存水动态的变异特征:基于地表到隧道的地球物理学的认识 Variabilitas dinamika stok air di karst: pratinjau dari geofisika permukaan-ke-terowongan Variabilidade da dinâmica do estoque de água no carste: percepções a partir da geofísica da superfície até a cavidade

2021

A hydrogeophysical field experiment was conducted on a karst hydrosystem in the south of France to investigate groundwater transfer and storage variability at a scale of a few hundred meters. A 200-m-long N/S tunnel going through limestone provided the unique opportunity to set up measurements with original configurations inside the unsaturated zone. Three geophysical methods were used: gravimetry, electrical, and seismic. Two-dimensional electrical resistivity and seismic velocity images were retrieved by surrounding the medium with electrodes and geophones, both at the surface and inside the tunnel to improve the sensitivity in depth. This gave information about the weathering state but also about the limestone content and associated porosity characteristics, as the methods are sensitive to distinct properties with different resolution patterns. A time-lapse gravity surface-to-tunnel profile supplied information on the seasonal water mass changes and its variations along the tunnel. Besides, tracers were injected on each side of the profile from the surface and the restitution was sampled in the tunnel drip flows. A contrasting hydrological behavior was evidenced on each side of the tunnel from temporal gravity measurements and tracing tests. The analysis of the whole dataset allowed for better interpretation of the imaged structures, with different hydrological functioning. This study demonstrates the variability of the karst behavior at the scale of a few hundred meters and the benefits of a multi-method approach coupling hydrological and geophysical measurements. This kind of experiment provides fundamental understanding of systems that cannot be directly observed.

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