GROUNDWATER POTENTIAL ZONE IDENTIFICATION USING GEOELECTRICAL SURVEY: A CASE STUDY OF GBONGAN, OSUN STATE, SOUTHWESTERN NIGERIA
Keywords:
Basement complex, hydrogeophysical, lithology, vertical electrical sounding, weathered and fractured.Abstract
Hydrogeophysical study involving the use of Vertical Electrical Sounding (VES) technique was carried out in Ode-
Omu palace area in Gbongan, Osun State with the aim of assessing the subsurface geology and groundwater potential
zones. A total of twenty-seven (27) VES points were occupied across the study area using Schlumberger electrode
configuration, with half electrode separation (AB/2) varying from 1 to 100 m. The depth to subsurface layers, their
thicknesses, geologic structures and aquifer characteristics were determined with the electrical resistivity method.
Three to five lithologic units were identified in the study from the quantitative interpretation of the VES data using
curve matching with Orellana-Mooney master curves and modeling with WinResist 1.0TM software. These include:
topsoil (30-1992m− ), clayey sand (23-60m− )/laterite(213-401m− ), weathered Basement(15-164m−
), fractured Basement(7-121m− ) and fresh bedrock (415-13054m− ) which are predominantly of the
‘H’ curve type (48.1%), followed by ‘HA’ type (14.8%), other curve types include: ‘HKH’ (7.4%), ‘QH’ (7.4%),
‘KH’ (7.4%), ‘AKH’ (3.7%), ‘AK’ (3.7%), ‘AA’ (3.7%) and HAA (3.7%). The weathered, partly weathered and the
fractured Basement constitute the main aquifer units. The aquifers are of generally low resistivity values vis-à-vis
below 100 Ω-m. The depths to fresh bedrock at the chosen VES locations vary from 2.8 to 23.4 m with a mean value
of 8.81 m in the study area. The geoelectrical interpretation results have permitted the delineation of the study area
into shallow north and moderately thick west, southwest, south and southeastern regions in terms of overburden
thickness while the resistivity over the study area is generally low and might favour groundwater prospectivity
(provided the low resistivity values are not caused by clay content). This study is expected to assist in future planning
of groundwater resources.
