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Characterization of karstic aquifer and localization of conduits using geophysics.

Girard Jean-François

Karstic conduits play a crucial role for water supply along the mediterranean coast. Karst aquifers are generally tapped at their springs. However, it can be convenient to collect water directly inside the conduit. In order to drill a borehole, one must know the exact position of the karst conduit within the saturated zone of the aquifer. Water-filled karst conduits can be located either by underground topographical survey if there is access for cave divers, or by using surface geophysical survey methods.

Magnetic Resonance Sounding (MRS) is a surface geophysical method designed for imaging of water bearing structures. It was recently applied on 2 test sites near Montpellier and Rocamadour (France). Both numerical and experimental results confirm the efficiency of the developed approach. Nowadays, methodological improvement allowed a 2D imaging of the conduit. Jointly used with electrical methods it also provides a characterization of the surrounding limestone matrix and epikarst.

However, the imaging of such targets is generally a difficult task. For MRS, the accuracy of the results depends on the EM noise level (anthropic activities) and the depth and volume of the karstic conduit. Technical performances are continuously improving and will provide a valuable tool for the water ressource prospection in karstic environment in the coming years.

Boucher M., Girard J-F., Legchenko A., Baltassat J-M., Dörfliger N., Chalikakis K. (2006) Application of magnetic resonance soundings to localization of a water filled karst network. Hydrology journal 330, 413-421.
Girard J-F., Boucher M., Legchenko A., Baltassat J-M. (2007) - 2D Magnetic Resonance Tomography applied to karstic conduit imaging. Journal of Applied Geophysics (accepted).