Geophysical and Borehole Log Evaluation of Groundwater Prospect in Parts of Lagos State, Southwestern Nigeria

Twelve (12) vertical electrical sounding were obtained in parts of Lagos State. Nineteen (19) deep boreholes within the Dahomey basin using resistivity and natural gamma ray wirelogs data were obtained. The vertical electrical sounding obtained were interpreted by WIN resist software. Four layers were obtained from the correlated geo-electric section which are topsoil, sand, clay and sandy clay. The obtained borehole data were interpreted using the surfer 2012 software program. Lithological units and saturation fluids were delineated. The lithological logs showed the fluid contents have various aquifers. The results of the subsurface geophysical investigation carried out within the study area show geo-electric layers characterized by low to moderately high resistivity materials (7-1314) Ωm indicative of sand top soil with thickness ranging from (1.0-2.6) m. The second layer is characterized by low resistivity materials (1.0-34) Ωm, indicative of clay materials with thickness ranging from (5.1-191) m. The last layer is characterized by low to moderately high resistivity (0.4-900) Ωm, indicative of sand materials. The borehole data interpretation result shows that the study area is underlain by clays, sandy clays, clayey sand and sand. The clay occurs as lenses within the clay formation. Based on this, five (5) aquifers were delineated in the study area along traverse AB, five (5) aquifers delineated along CD, Six (6) aquifers along EF. One parametric VES and borehole were correlated and it shows the increment of the lithologic units almost at the same depth which allows for the correct interpretation of the subsurface. The sand layer constitutes the major units in the study area while sandy clay and clayey sand constitute the minor aquifer units. This research work revealed that fresh water can be tapped from the entire well in the study area at various depths depending on the area where the well is located.