Borehole Geophysics for Hydrostratigraphic Characterisation in Lagos State

This study applies borehole geophysical logging to characterise the hydrostratigraphic sequence in existing water supply boreholes in Lagos State, South West Nigeria. Increasing water demand in the Lagos metropolis and surrounding areas has driven rapid borehole development, but many boreholes are completed without geophysical logging that would enable optimal screen placement and capacity assessment. Retroactive geophysical logging of existing boreholes provides subsurface stratigraphic information that improves understanding of aquifer geometry and groundwater resource management. This study conducts geophysical logging in 12 selected boreholes across Lagos Island, Epe, and Ikorodu local government areas using a multi-parameter downhole tool recording natural gamma, single-point resistance, spontaneous potential, and fluid temperature and conductivity logs. Log data are acquired at 0.1-metre depth increments using a Century Geophysical logging system. Formation tops, lithological boundaries, and permeable zones are picked from the combined log signatures. Comparison between logged and drilled borehole descriptions evaluates interpretation accuracy. Findings reveal that natural gamma logs effectively discriminate between sand (below 30 API) and clay (above 70 API) throughout the Coastal Plain Sands sequence to depths up to 65 metres. Fluid conductivity logs in two boreholes show saline water intrusion in the bottom 10 to 15 metres of previously productive freshwater sections, indicating ongoing saltwater encroachment. Screen intervals in six boreholes are found to be poorly matched to productive sand horizons identified by the logs, explaining their suboptimal yields. The study recommends routine borehole logging for all new Lagos water supply boreholes.

Keywords: borehole geophysics, hydrostratigraphy, Lagos State, gamma log, saline intrusion.