Application of Geophysical Borehole Logs For Aquifer Characterization in Coastal Environment of Lagos, Southwestern Nigeria

Abstract

Well correlation and aquifer characterization were successfully carried out across a part of Lekki, Lagos state, Nigeria. The formation stratification sequence comprises predominantly of sand and clay with intercalations of both occurring as clayey sand and sandy clay. Three distinct fluid types (saline water, brackish water and fresh water) were delineated. The bulk resistivity (formation and fluid resistivity) ranged from 0 - 80Ωm for saline water sand and clay, 80 – 120 Ωm for the brackish water sand and clay and 120 – 620 Ωm for fresh water. Salinity occurrence was observed throughout the entire study area with depth of occurrence ranging between 14 – 152m. The depth to saline/fresh water interface range from 123 – 167m. Two aquifer types (unconfined and leaky confined aquifers) were observed in the study area. A maximum of five aquifers were delineated across the borehole sections. The depths to the delineated aquifers ranged from 163 – 249m with the aquifer thickness ranging from 10 to 56m. The maximum saline water depth extent was at about 152m (BH7 and BH5). Brackish water (the saline water interface) ranges from about 123.5 – 167m while the minimum depth to freshwater was about 163.5m (BH4) and maximum depth was about 175m (BH8) for all observed logs. The hydrogeological importance of the Coastal Plain Sand (a good aquifer unit) in the study area is under severe threat of continued seawater (saline water) intrusion on its coastal flank as observed from the vulnerability analysis of the study area. The study area is under intense stress from probably heavy abstraction as more than 80% of the observed boreholes were characterized as highly vulnerable and all of the observed boreholes already exceeded the standard range of values for some of the vulnerability parameters particularly.