Decline analysis of electrical resistivity curves for subsurface evaluation in parts of Choba, Rivers State

This study evaluates subsurface geological conditions and environmental vulnerability in Choba, Rivers State, using decline analysis of electrical resistivity cross plots. The primary objective was to characterize the rate of change in apparent resistivity with increasing depth to identify potential risks to groundwater quality and infrastructural stability. Vertical Electrical Sounding (VES) data was acquired using the Schlumberger array configuration, processed with IPI2WIN software and interpreted qualitatively through VES cross plots. The three VES cross plots reveal a complex multilayered subsurface within the Benin Formation, identifying KH-type, K-type, and Q-type curves. The VES cross plots demonstrate significant stratigraphic heterogeneity, with four-layer and three-layer models exhibiting varying resistivity trends (ρ1 < ρ2 > ρ3 < ρ4 for KH-type; ρ1 < ρ2 > ρ3 for K-type; ρ1 > ρ2 < ρ3 > ρ4 for Q-type). The critical decline segments observed in the curves, where sharp decreases in resistivity indicate high conductivity zones, suggest the absence of protective overburden seals and the presence of conductive fluids, industrial leachates or mineralized groundwater such as iron enrichment. The decline analysis highlights severe subsurface vulnerability and heterogeneity of the subsurface layers in Choba, confirming the lack of protective clay layers that facilitate rapid contaminant infiltration, posing a substantial risk to groundwater potability. This study underscores the necessity for dense geophysical site evaluations before borehole drilling or engineering construction to navigate the region's unpredictable lithological variability.