GEOPHYSICAL EVALUATION OF COAL DEPOSITS IN PART OF NORTHERN ANAMBRA BASIN NIGERIA, USING ELECTRICAL RESISTIVITY AND BOREHOLE DATA

Abstract

This study integrates geophysical and borehole methods to evaluate coal deposits in part of the Northern Anambra Basin, Nigeria using vertical electrical sounding (VES) and 2D electrical resistivity imaging (ERI) in conjunction with borehole data. The geoelectric survey reveals five to nine distinct lithological layers, comprising of sandy soil, clay, shale, sandstone, sandy shale, shaly sandstone, interbedded sandstone and shale bed and coal. The coal seam is embedded within the alternating sandstone and shale beds. Coal seam thickness ranges from 0.5 to 6.1 meters, with thicker deposits concentrated in the southwestern and northwestern regions. Geoelectric sections were correlated with borehole lithologic logs for subsurface validation and structural interpretation. Utilizing the relationship between resistivity and coal properties, the coal core samples obtained from borehole intersections were subjected to proximate and ultimate analyses, enabling the identification of high-quality coal zones. Overburden thickness varies from 5 meters in the east to 140 meters in the west, influencing mining method selection. Open-pit mining is feasible in the eastern section, while underground mining is preferable in deeper western deposits. Structural controls, including monoclines and potential faults, influence coal thickness and geoelectric layer continuity, while groundwater intrusion affects the coal seam resistivity and moisture content. The study also presents 2D resistivity models, which visually depict coal seam geometry, overburden variations, and groundwater-influenced zones. A total coal resource of approximately 23.86 million metric tonnes is estimated, with an overall strip ratio of 22.63. The study concludes that the identified high-quality coal zones with moderate overburden suggest viable extraction opportunities, while areas with deep overburden require strategic mining plans.