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 communiti es (particularly in sub-Saharan Africa) look to increasingly marginal aquif ers for supply, many of the standard procedures for locating groundwater be come 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 t o assess both the groundwater potential and methods for siting wells and bo reholes in low permeability sediments in Oju, southeast Nigeria. The Oju area suffers from an acute water shortage during a five-month dry s eason. Low permeability Cretaceous shares, 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 s hales; (ii) fracture zones in areas where the shales are lithified; and (ii i) fractures associated with dolerite dykes and sills. The geophysical tech niques used to identify these groundwater targets comprise frequency domain conductivity using the Geonics EM34, vertical electrical resistivity sound ing (VES) and magnetic profiling (using a proton precession magnetometer). Three areas were studied in detail using a combination of geophysical surve ys, exploratory drilling of the characteristic geophysical anomalies identi fied and test pumping. In the interbedded share and sandstone areas, sandstones were distinguished as low conductivity zones ( < 20 mmhos m(-1)) using elecromagnetic and res istivity techniques. In the lithified mudstones, fracture zones were readil y identified using electromagnetic methods as negative anomalies or smaller amplitude 'noisy' profiles. Dolerite intrusions within soft shales were id entified by their lower electrical conductivity and distinct magnetic anoma lies. (C) 2001 NERC. Published by Elsevier Science Ltd. All Rights Reserved .