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

Abstract

In order to understand the hydrogeological structure and orientation of the study area, an attempt was made to do well correlation and aquifer characterization. A total of nine composite logs comprising of Resistivity and Gamma Ray logs were used for the study. Three correlation panels trending in different directions were prepared. Each panel consists of series of well logs arranged horizontally in accordance with the spatial distribution of the wells on the field. This assisted in carrying out litho-stratification delineation that was used to identify fluid types obtainable in the area. Two parameters (aquifer potential and vulnerability) were used to characterize the delineated aquifer in the area. The aquifer thickness was used to produce the aquifer potential map for the area. The GALDIT-Index (GI) score was computed for each borehole and this was used to evaluate the aquifer vulnerability 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 were delineated and these were: saline water, brackish water and fresh water with bulk resistivity respectively ranging from 0 - 80Ωm, 80 – 120 Ωm and 120 – 620 Ωm. The depth to saline/fresh water interface range from 123 – 167m. The study shows that 5.7% of the study area is of low potential, 45% is of intermediate potential while 49.3% is of high potential. The GI scores obtained for the area vary between 2.67 and 5.5 suggesting that two vulnerability classes (moderate and low vulnerability) were obtainable. The study established that the groundwater system within the region is under pressure and boreholes in the area are vulnerable to saline water contamination if left without necessary monitoring.

Key words: Aquifer Characterization, Aquifer Vulnerability, Aquifer Potential, Formation Fluids and Stratification Sequence