Application of integrated geophysical techniques for monitoring wells site selection in the White Volta river basin
AbstractThe CSIR-Water Research Institute has established thirteen (13) borehole monitoring stations in the White Volta River Basin of the three northern regions since 2005. Eight (8) of the monitoring boreholes are located in the Upper East and Upper West Regions, whilst the remaining five
(5) are sited in the Northern Region of the basin, including Wa, Tumu, Bawku and Bugya-Pala near Walewale. The objective, among others, is to generate groundwater levels and temperature fluctuations data as well as obtain information on water quality trends for the effective management
of the groundwater resources of the basin. Crystalline igneous and metamorphic rocks as well as consolidated sedimentary formations underlie the basin. The drilling sites were selected using integrated geophysical survey techniques. Inductive electro-magnetic (EM) profiling and electrical resistivity methods including 2-D profiling/sounding and vertical electrical sounding (VES) were used in areas underlain by sandstone, igneous and metamorphic rocks. However, in areas underlain by sedimentary shale and mudstone rocks, where conductivity contrast between their fractured portions and the fresh rock is known to be lacking, only resistivity siting methods were employed. The results of the integrated geophysical methods showed strong correlation in
delineating potential anomalies (fractures) in sandstone, igneous and metamorphic rocks, confirming the aquifer depths of existing boreholes in these rocks, which ranged from 15 m to 40 m deep. Unlike crystalline basement rocks (igneous and metamorphic) and sandstone, where low
bedrock resistivity response values (<20 Ohm-m) at depth indicated fractures, such low resistivity values in sedimentary shale and mudstone revealed rocks with high clay content that were not necessarily due to fractures.