Présentations Colloques

Oral Presentation
Session 8.07: Hydrogeophysics: innovative non-invasive technologies for groundwater resources exploitation and management
Allé Iboukoun Christian
Specific yield estimation using resistivity and chargeability in tropical basement rock aquifers- experimental relations in Benin, West Africa.
Groundwater in hard rocks is the only water source for many human communities in Africa. 80% of Benin surface area is underlined by hard rocks which are deeply weathered as in many tropical climate areas. 40% of the wells drilled in hard rocks of Benin are negatives (yield less than 700 l per hour) although implemented with the use of electrical geophysical method. **In this study, we explore the usefulness of an additional geophysical parameter, i.e. the chargeability, that can be measured jointly with resistivity (using the same device). Particularly, we look for a possible link between geophysical parameters (electrical resistivity and chargeability) and hydrogeological parameter (specific yield). We focus on weathered zone (saprolite) that constitutes the main reservoir of groundwater. Electrical resistivity, chargeability and specific yield are obtained by interpretation of boreholes logging and pumping tests carried out at six experimental sites. The resistivity values are corrected from borehole diameter effect, and the IP time-domain relaxation curves are analyzed using a Cole-Cole dispersion model. The clay content of saprolite is estimated by the use of the “bleu of methylene” test carried out on borehole cuttings,**First, we observe that mean chargeability and mean resistivity values tend to increase with specific yield at the 6 experimental sites. This observation is confirmed on 7 other sites where specific yield is derived from magnetic resonance soundings. Second, we find that higher the clay content, less the chargeability and resistivity values. **We conclude that the measurement of electrical chargeability in addition to resistivity may improve the knowledge of weathered hard rock aquifers, and we recommend confirming our observations with new experiments carried out in new locations.**