Characterization of electrical anisotropy in North Yorkshire, England using square arrays and electrical resistivity tomography

Fracture characteristics were quantified at a site underlain by limestone bedrock in Spaunton Moors, North Yorkshire, England, with the use of electrical resistivity tomography and azimuthal square arrays. The electrical resistivity tomography survey was conducted to generate a two-dimensional picture of the subsurface. The azimuthal square array survey was conducted to characterize electrical anisotropy due to similarly oriented fractures within the Coralline limestone. Values of fracture strike obtained from graphical analysis of the square array data corresponded with values obtained from geological mapping of the quarry face. On the square array polar plots, the fracture zone was where resistivity was lowest. Secondary porosity was estimated to range between 20 and 27%. This range of values was interpreted to be a result of foliation. The azimuthal inhomogeneity ratio, computed to estimate the severity of lateral resistivity variations on the site, had a value averaging 0.7. This implies that the effects of lateral resistivity variations are not very severe. Electrical anisotropy observed on the study location is due to the presence of fractures. The lateral variations in resistivity are due to lithological heterogeneities within the bedrock. Square array detected anisotropic behavior within the bedrock in spite of overburden effects. The azimuthal square array technique is therefore useful for characterizing fractures and for quantifying anisotropy within the bedrock to generate results consistent with that obtained from geological mapping of existent outcrops.