GEOPHYSICAL INVESTIGATION FOR DELINEATING SOURCE OF SALINE WATER IN AGBABU, ONDO STATE, NIGERIA.

Abstract

The aim of this study is to determine the source of saline water in the inland aquifers of Agbabu, Southwestern Nigeria. A combination of aeromagnetic data covering the eastern Dahomey basin, 23 Vertical Electrical Soundings data and hydrochemical analysis of 11 subsurface water samples were utilized in this study. Analysis of the aeromagnetic maps which included the magnetic intensity maps, the derivative maps and the depth solution maps revealed the inland extension of the Chain Fracture Zone (CFZ). This NE-SW trending fracture zone cut across the entire Eastern Dahomey Basin including Agbabu and its environs. Four geoelectric layers were delineated across Agbabu, namely, the topsoil, clayey sand, clayey and bituminous sand layers. The resistivity of these layers varied respectively from 4 - 2806 Ωm, 3 - 1280 Ωm, 3 - 161 Ωm and 1 - 10390 Ωm while the thickness of the upper three layers varied respectively from 0.3 - 3.9 m, 3 - 8.1 m and 0.9 - 178.1 m. Within the third layer, zones of anomalously low resistivity (1 - 55 Ωm) considered to be saline water zones were observed across the traverses. The Salinity Extent Chart generated from the VES results revealed a possible boundary between saline and non-saline water zones in Agbabu and indicates that the saline source is from the eastern end of the town. Similarly, water analysis results confirmed the presence of saline water zones. Based on the combination of the deductions made from interpretations of all the methods used and the identification of the inland extension of the Chain Fracture Zone (CFZ), two possible sources or origin of the saline water zones in Agbabu are suggested. First, flow of saline bottom water from the ocean through the CFZ and second, connate water trapped in the interstices of the sedimentary rocks