J. Davies

Geophysical methods for locating groundwater in low permeability sedimentary rocks: examples from southeast Nigeria

Geophysical techniques have long been used to help locate rural groundwater supplies in crystalline basement environments. However, as local communities (particularly in sub-Saharan Africa) look to increasingly marginal aquifers for supply, many of the standard procedures for locating groundwater become inappropriate. Areas underlain by low permeability sediments (such as shales and siltstones) are particularly difficult for locating groundwater resources. In response to these difficulties, this study was commissioned to assess both the groundwater potential and methods for siting wells and boreholes in low permeability sediments in Oju, southeast Nigeria. n nThe Oju area suffers from an acute water shortage during a five-month dry season. Low permeability Cretaceous shales, siltstones and sandstones, with occasional intrusions of basic igneous rocks, underlie the area. Three main targets for groundwater have been identified: (i) sandy units within the shales; (ii) fracture zones in areas where the shales are lithified; and (iii) fractures associated with dolerite dykes and sills. The geophysical techniques used to identify these groundwater targets comprise frequency domain conductivity using the Geonics EM34, vertical electrical resistivity sounding (VES) and magnetic profiling (using a proton precession magnetometer). Three areas were studied in detail using a combination of geophysical surveys, exploratory drilling of the characteristics geophysical anomalies identified and test pumping. n nIn the interbedded shale and sandstone areas, sandstones were distinguished as low conductivity zones (< 20 mmhos m−1) using electromagnetic and resistivity techniques. In the lithified mudstones, fracture zones were readily identified using electromagnetic methods as negative anomalies or smaller amplitude ‘noisy’ profiles. Dolerite intrusions wihin soft shales were identified by their lower electrical conductivity and distinct magnetic anomalies.