INTEGRATED GEOPHYSICAL AND REMOTE SENSING INVESTIGATION FOR GROUNDWATER POTENTIAL OF OKENE AND ITS ENVIRONS, SOUTHWESTERN NIGERIA

Abstract

This work involved the integration of remote sensing and geophysical techniques in assessing the groundwater potential of Okene and its environ known to be confronted with limited water resources, mainly as a result of the nature of its geology, high relief and difficult terrain. Landsat imagery acquired over the study area was processed using ILWIS 3.2 and Arcgis 9.2 softwares to map lineaments and drainage networks of the study area were. Digital aeromagnetic data over the area was analysed and the derivative maps produced from the enhancement procedures include upward / downward continuation, second order vertical derivative as well as the generation of spectral analysis. Azimuth-frequency plot of the lineaments observed on the imagery show dominant trends in the NE-SW and NNE-SSW directions. The lineament density of the area was calculated using the number of lineaments per unit area of grid. Lineament density maps produced show that lineament density is high in areas like Egbeda, Beliko, Kakun, Ohaketi, Ego, Obele, Ogale, Ugungwe, Obangune, and Aiyatorobabunda when compared to other areas within the study area. The total magnetic intensity map (TMI) of the Okene and its environ is characterized by both low and high magnetic anomalies with their values ranging from -4.160 to 211.784nT. Its derivatives maps show similar signatures except that their anomalies were well pronounced. From the radial average power spectrum, the depth to magnetic sources in the area were determined to range from 100 m to 2500m.Vertical Electrical Sounding (VES) and bore-hole yield data for some regions within the study area were quantitatively interpreted and statistically analyzed. Three to four subsurface layers were identified from the VES data which are topsoil, lateritic layer, weathered layer, partly weathered/fractured basement and the fresh bedrock. The resistivity of the topsoil ranges from 70Ωm to1614Ωm while the thickness varies from 0.1m to 1.4m. The lateritic layer which is not observed in all the areas has resistivity values ranging from703.9Ωm to 1152.7Ωm while the thickness ranges from 3.2m to 13.3m. The weathered layers which are clayey in some places ranges in resistivity from 13Ωm to 200Ωm and 1.8m to 17.3m in thickness. The partly weathered/fractured layer resistivity values range from 262Ωm to 541.4Ωm while their thickness varies from 24.2m to 32.7m. The fresh bedrock has infinitely high resistivity between 975Ωm to 16624Ωm. The weathered layer and partly weathered/fractured bedrock constitute the aquifer units. Bore-hole yield with their depth ranges from 2 to 48 litre/minute and 25m to 48 m respectively. Comparing the results, it could be concluded that groundwater potential is higher in areas with high lineament density, low/magnetic intensity values and thick weathered layer / fractured basement