Evaluation of Groundwater Level Using Combined Electrical Resistivity Log with Gamma (Elgg) around Ikeja, Lagos State, Southwestern Nigeria

This study has been able to demarcate the subsurface geologic units based on the lithologic characters. The subsurface geology of Lagos Main land is made up of complex lithologies of an alternating sequence of clay and sand deposits. The borehole lithologic correlation shows that the subsurface geologic sequence is composed of alternations of sand, clay, lenses of clayey sand and sandy clay. All the boreholes correlated were located within the Ikeja, Lagos State. The borehole depths vary from 119 m to 127 m and the sand and clay though variable show lateral continuity across the section. Four major fresh water sand aquifer layers were delineated within the upper 127 m of the subsurface sequence. The first aquifer layers were located at depth ranging from 61 m to 71 m below the ground level and thickness of 10 m with the resistivity varying from 190 Ωm to 320 Ωm. The second aquifer layers were located at depth ranging from 79 m to 87 m below the ground level and thickness of 8 m with the resistivity varying from 500 Ωm to 700 Ωm. The third aquifer layers were located at depth ranging from 85 m to 92 m below the ground level and thickness of 7 m with the resistivity varying from 300 Ωm to 590 Ωm while the fourth aquifer layers were located at depth ranging from 82 m to 89 m below the ground level. It has a thickness of 7 m with the resistivity varying from 300 Ωm to 610 Ωm. The aquifers were very heterogeneous as indicated by wide range of aquifer characteristics. Due to the occurrence of the sand formations in the study area, the aquifer level is between the depths of 61 m to 92 m. Hence, if there is a need for industrial purpose the aquifer should be tapped at the depth between 61 m to 89 m, and also screened for water supply. Relatively higher resistivity indicates significant formation fluid.