Fracture Detection and Secondary Porosity Estimation in Basement Terrain Using Crossed Square Array Resistivity Method

Summary Electrical resistivity method of crossed square array was used to delineate subsurface fractures and estimate the secondary porosity in gneissic rocks (migmatite, granite and banded gneiss) of southwestern Basement Complex of Nigeria. Resistivity measurements were carried out at ten (10) locations using Ohmega resistivity meter, survey was done about a common center with an electrode array expanded in four different azimuths namely 0°, 45°, 90°and 135°, at electrode spacing increment of A(2)1/2. The coefficient of anisotropy, azimuthal inhomogeneity ratio (AIR), azimuthal resistivity, fracture strike, bedrock anisotropy, and secondary porosity were then calculated from the measured apparent resistivity. The study revealed that the measured apparent resistivity varies with the orientation of the array indicating fracture anisotropy in the subsurface. The results also showed the presence of fracture zone which are mainly oriented NE –SW direction, with few in the NW – SE direction. The coefficient of anisotropy (λ) ranges between 0.51 and 2.38, bedrock anisotropy (N) varies from 1.006 to 1.58 while calculated secondary porosity ranges between 0.00006 to 0.112. Crossed square array data analyses evidently indicate the presence of multiple subsurface fracture zones and showed that the study area has good potentials for groundwater development.