GEOELECTRIC INVESTIGATION OF SALINE WATER INTRUSION IN THE COASTAL AREA OF ONDO STATE, SOUTHWESTERN NIGERIA

Abstract

A geophysical investigation of coastal area of Ondo State was carried out using the electrical resistivity sounding method employing Schlumberger configuration and five composite Geophysical logs in mapping the saline water intruded zones in the study area Composite logs used in the research are Self Potential Log, Electrical Restivity Log and Natural Gamma Ray Log. A total of 60 Vertical Electrical Sounding {VES} were carried out. The work involved both quantitative and qualitative interpretation of the Geophysical logs and VES data obtained from the study locations. Geoeletric sections generated from iterated sounding curves revealed four subsurface geoeletric/geologic layer consisting of topsoil, clayey sand, clay and sand, while the lithology of the borehole, as deduced from the Natural Gamma Log, consists of alternating layer of sand, clay, clayey sand and sandy clay, typical of geological formations within the Dahomey Basin. From both the electrical resistivity method (surface) and down hole method (logging), it was observed that sand formations with low resistivity are characterized by saline water and a good example is Asisa where the entire section mapped from 30m-350m are characterized by saline water, in Obenla it ranges from 45m-235m and Rewoye axis ranges from 74.8m-320m, while higher resistivity sand formations at depth are presumed to be saturated with freshwater.