ELECTRICAL ANISOTROPY CHARACTERIZATION OF FRACTURED CRYSTALLINE BASEMENT ROCK AT IGARRA, NIGERIA

Abstract

Electrical anisotropy, which refers to the variation of electrical resistivity (or conductivity) with direction, is exhibited by most rocks. In crystalline rocks, electrical anisotropy may result from the preferred orientation of mineral crystal (banding and foliation), cracks, fractures and cleavages which are products of pressure and dynamic processes. Surface geological and geophysical surveys were done at locations within the study area, with the aim of identifying and characterizing electrical anisotropy within the crystalline basement rocks at Igarra. Electrical anisotropy was quantified from the geophysical data by the use of anisotropic parameters of Percentage of Anisotropy, Apparent Anisotropy and Coefficient of Anisotropy. All three parameters showed that electrical anisotropy increases with depth while inhomogeneity is maximum at shallow depth. Analysis and correlation of the surface geological and geophysical data showed that the major cause of electrical anisotropy is the presence of fractures in the rock mass. These fractures which have dominant strike orientation in the N-S direction causes a significant decrease in apparent resistivity in the orientation parallel to its strike and a corresponding increase in apparent resistivity in the direction perpendicular to its strike direction. The increase in electrical anisotropy with depth suggests that the intensity of fracturing (fracture density) increases with depth