Application of azimuthal Schlumberger-resistivity survey in Southeastern Nigeria. – C.N. Ehirim and J.O. Ebeniro
ABSTRACT
Southeastern Nigeria was divided into four geologic basins namely: the Niger delta, Proto-delta, Anambra and the Abakaliki basins. These basins were subsequently divided into grids cells of 100km2 areas and in each cell, azimuthal Schlumberger electrical survey was carried out to determine the degree of electrical anisotropy related to the structural texture and degree of tectonism. A maximum electrode expansion of AB/2 = 100m and MN/20m were employed and the maximum depth penetrated was 33m, which is appropriate for the investigation of geological and structural hazards related to electrical anisotropy in the basins. Results indicate that the basins are electrically anisotropic, probably due to bedding laminations, aligned mineral grains, grain boundary cracks and fractures in preferred directions. The Niger delta basin has the least mean coefficient of anisotropy of 1.05, percent anisotropy of 10.50%, and highest mean resistively of 4160.38Ωm; while the Abakaliki basin has the highest coefficient of anisotropy of 1.14, percent anisotropy of 24.66%, and least mean resistively of 55.50Ωm. The decrease in the coefficient of anisotropy and percent anisotropy with increased electrode expansion within each basin and the relatively low values of the coefficient of anisotropy in the basins imply that, these structures are less penetrative and as such, the basins are likely to be less intensely fractured and fairly homogeneous. The Niger delta basin is the most homogeneous, while the Abakaliki basin is the least homogeneous. The principal anisotropic axes lies in the NW-SE in the Niger delta and NE-SW in the other basins. These revelations will be of immense benefit to civil engineers, environmental and engineering geophysicists.