Experimental Study of Electrical Resistivity to Rock Fracture Intensity and Aperture Size

Abstract

Fracture geometric characteristics (FGC) such as fracture intensity, fracture network connectivity and aperture distribution are crucial features controlling the hydraulic and geotechnical properties of rock formations. Geophysical methods have been used to detect contrast in subsurface material. The Electrical Resistivity (ER) method is one of such methods. The method was applied in an experimental test to investigate ER response with varying fracture intensity If and aperture size Aper width in an experimental set-up at Kwame Nkrumah University of Science and Technology (KNUST, Kumasi, Ghana, geotechnical laboratory. The concepts of electrical resistivity variation with rock mass fracture intensity, and aperture were used to obtained experimental data. ER profiles, each measuring 0.9 m long were recorded using 4 electrodes deployed using the Dipole-dipole, Wenner and Schlumberger configurations from the experimental setup. To quantify the relationship between apparent resistivity and fracture intensity, scatter-plots were drawn with apparent resistivity as the abscissa and fracture intensity as well as aperture width as ordinate. There were strong positive linear and regressive correlations between ρa and fracture intensity. Mathematical relationships are established that relate the ER and fracture intensity, and ER with aperture width. The highest coefficient of determination R2 of 0.924 was represented for best fit equation, for relationship between apparent resistivity and fracture intensity. For aperture width, the best fit model was given a logarithmic relation as ρa =-465.46(Aperture width)2 +4880.5(Aperture width)+1295.5. The study demonstrates the potential usefulness of the ER approach in rock fracture characterisation investigations, which is economic, efficient and less time consuming compared to other methods, of subsurface fracture characterization such core drilling.