Présentations Colloques

Oral Presentation
8.07
Session 8.07: Hydrogeophysics: innovative non-invasive technologies for groundwater resources exploitation and management
Lawson Fabrice Messan Amen
Efficiency of Magnetic Resonance Sounding to characterize hydrogeological properties of weathered hard rock aquifers
As compared to other non invasive geophysical methods, Magnetic Resonance Sounding (MRS) is selective to groundwater and has the potential for quantifying aquifer properties. We developed a MRS measurement methodology adapted to African hard rock context, i.e. low specific yield, daily fluctuation of the earth magnetic field [1] and electromagnetic noise. **Weathering process of hard rocks results in a typical two-layers groundwater reservoir where an unconsolidated saprolite layer is located just above a fissured layer [2]. We carried out numerous MRS in hard rock aquifers in Benin (West Africa), and we compared MRS and pumping test results. We found that parameters integrated over the saturated thickness, i.e. transmissivity and aquifer storage, can be estimated from MRS with an acceptable accuracy when weathered layers are thick enough. When the saprolite layer is stripped, transmissivity and storage are mainly controlled by the fissured layer and are poorly estimated from MRS. **We conducted numerical modeling to assess the conditions when the properties of each layer can be defined separately. We found that specific yield and hydraulic conductivity of each layer cannot be accurately estimated yet.**Finally, we conclude that MRS is nowadays efficient to quantify transmissivity and storage in deeply weathered bed rock aquifers. However, development in MRS methods is still needed to improve the estimate of the distribution of specific yield and hydraulic conductivity with depth.****REFERENCE-** [1] Legchenko, A., Vouillamoz, J.M., Lawson, F.M.A., Allé, C., Descloitres, M., Boucher, M., in press. Magnetic resonance measurements in varying Earth’s magnetic field. GEOPHISICS.**[2] Lachassagne, P., Wyns, R., Dewandel, B., 2011. The fracture permeability of Hard Rock Aquifers is due neither to tectonics, nor to unloading, but to weathering processes. Terra Nova 23, 145–161.**
Benin