Prospections électromagnétiques et forages en zone aride — Kori Teloua (Agadez, Niger)

In the Agadez region of Niger, the alluvial water reservoir of the Teloua is only sporadically recharged during periods of flooding. The town's drinking-water reservoir is contained in the ancient paleochannel meanders underlying the present drainage system. A geophysical exploration project was carried out to attempt to reconstruct these paleochannels using an electromagnetic bipole-bipole method (with a 20 m coil seperation and 2048 Hz frequency). The resulting apparent resistivity map enabled these channels to be distinguished as zones of high apparent resistivity. Nine boreholes were subsequently drilled to confirm these results and to be used as piezometers. The importance of correlating the two methods was highlighted through the comparison of the drillhole flowrates and the geophysical resulting drillhole flowrates of 10 m³/h correlated with areas of apparent resistivity values between 90–200 ω·m. One of the nine drillholes has been developed into a production well (80 m³/h, positioned on a resistivity high of 120 ω·m). The bipole-bipole geophysical method enabled the observation drillholes and production well to be positioned quickly and efficiently.