The tenfold increase in borehole siting in Northern Nigeria during the last few years has resulted in an increase in the use of electromagnetic (EM) methods and a corresponding decrease in the more traditional but less cost effective resistivity methods. The role of geophysics in exploration is optimally in combination with direct geological observation. Two types of aquifer which are amenable to geophysical investigation in the Nigerian environment are weathered, jointed crystalline rocks and alluvium.
Location of joints and associated narrow zones of deep weathering in crystalline terrain is best achieved by EM techniques with resistivity backup where needed. Resistivity alone does not always reveal such features. Location of shallow aquifers in alluvium is amenable to a combination of EM and resistivity techniques. However, the complexities of alluvial and underlying Quaternary sedimentation necessitate extensive computer modelling of both sets of data in order to optimize the field techniques and interpretation methods. In particular the use of selected combinations of intercoil spacings to estimate maximum thicknesses of aquifers shows some promise.