An electromagnetic method of geophysical investigation for preliminary stage of groundwater development was carried out using conductivity measurement for the purpose of locating fractured/fissured zones and associated groundwater media at Oru-Ijebu, South-West Nigeria. The area is typical basement complex rocks of South-West, Nigeria with the local geology predominantly dominated by granite gneiss with associated comprising of granite, banded gneiss, pegmatite, and undifferentiated migmatite and these have been intruded by quartz veins and pegmatite veins. Measurements of the ground conductivity were carried out with Geonics EM 34-3 along 10 traverses whose profile lengths varied between 160 and 200 m. Intensive geophysical fieldworks were performed utilizing Frequency Domain Electromagnetic Method (FDEM) using Geonics-EM-34 to determine the vertical and lateral variations of subsurface conductivity. The EM data were acquired at 500 m intervals along 20 profiles (2 on each traverse) with acquired data further processed and interpreted. The processed data were further presented as horizontal and vertical plots on a line graph. In the 10 m coil spacing, the highest and lowest subsurface conductivity is exhibited by EMORU1 and EMORU6 with the values of 164mmho/m and 68mmho/m respectively in HDM. The highest and lowest conductivity is exhibited by EMORU4 and EMORU10 with the values of 139mmho/m and 70mmho/m respectively in VDM. Also, in 20m inter coil spacing, the highest conductivity is jointly observed in EMORU2 with 190mmho/m and 200mmho/m in HDM and VDM respectively. The lowest conductivity is equally observed in EMORU6 with values of 56mmho/m and 80mmho/m for HDM and VDM respectively with the analysis identified key lateral distances for groundwater prospecting, particularly between 72 m to 154 m along the profiles. This study has therefore investigated and characterized the study area for possible groundwater exploration, with varying conductivity values with locations of high conductivity reflection points and cross-over points are prospective sites for groundwater exploration while locations of similar trends in both HDM and VDM possess the same lithology. The qualitative interpretation of EM results identified areas of hydrogeologic importance and forms a predictive and suggestive basis for Vertical Electrical Sounding (VES) investigation; points of positive EM anomalies were considered as priority area for electrical resistivity sounding and prospective groundwater development, since they suggest lithological variations within the unconsolidated overburden and/or water–filled fissures in the bedrock. The identified major geological interfaces were suspected to be of weathered zones.
Keywords: dipole, electromagnetic survey, fissures, geonics, regoliths, true conductivity
