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.
