Geophysical Investigation of Deep Sedimentary Structures in the Chad Basin, Yobe State

The Chad Basin in Yobe State, North East Nigeria, contains a thick succession of continental to marginal marine sediments whose deep structural architecture has important implications for groundwater resource potential, petroleum prospectivity, and understanding of the Cretaceous-Tertiary tectonic evolution of the West African platform. This study applies seismic refraction tomography and integrated borehole log analysis to investigate deep sedimentary structures within the Nigerian portion of the Chad Basin in Yobe State. Seismic refraction profiles were acquired along a grid of lines totalling two hundred and forty kilometres, with first arrival traveltimes inverted using tomographic methods to produce two-dimensional velocity models. Velocity anomalies are interpreted in terms of lithological variations within the Chad and Fika formations. Available borehole lithological logs and formation water chemistry data from the Yobe State water well database were used to calibrate seismic velocity models and constrain subsurface layer correlations. The study presents a series of geological cross-sections illustrating basin structure at depths of up to three kilometres. Identified structural features including half-grabens and reactivated basement highs are evaluated for their hydrocarbon and groundwater prospectivity. These findings are submitted to the Nigerian Geological Survey Agency as a contribution to the national deep basin mapping programme. Keywords: seismic refraction, Chad Basin, deep structures, Yobe State, basin analysis.