INTRODUCTION The identification and characterization of fractures is important in rocks with low primary
The identification and characterization of fractures is important in rocks with low primary (or matrix) porosity because the bulk porosity and permeability are determined mainly by the intensity, orientation, connectivity, aperture, and infill of fracture systems (Skjernaa and Jorgensen, 1993). Groundwater flow through a fracture network is strongly influenced by hydraulic anisotropy resulting from the geometry of the fractures and the preferential strike of fracture sets makes rock to be both electrically and hydraulically anisotropic (Slater et al., 2006). The presence of aligned vertical features and vertical to sub-vertical thin beds causes anisotropic behavior and inhomogeneity’s in rocks. Anisotropy is a property of a rock which shows different measurements when measured along different axes. A rock is said to be electrically anisotropic if the value of vector measurement of its resistivity varies with direction (Sheriff, 2013).
Subsurface anisotropy contains prints of subsurface fracturing, layering, faults and joint systems, grain boundary cracking among others. Besides, the presence of lateral heterogeneities can produce significant pseudo-anisotropy effect. Anisotropy is therefore jointly influenced by these factors. It is generally assumed that the anisotropy is caused by presence of fluid – filled fractures in a relatively resistive rock or soil (Van-Dycke, 2015)
Azimuthal resistivity method is an improvement of resistivity method which has utility for hydrogeological and geotechnical studies because it provides a method for measuring the in-situ anisotropy and directional nature vertically dipping fracture systems at depths of 70m or less, they are useful for preliminary site evaluations because they indicate presence of intensity of jointing areas lacking surface exposure or subsurface information. Anisotropy, fracture continuity, and directional connectivity have typically been difficult to estimate from conventional joint studies but are critical parameters in many fracture flow. (Taylor and Fleming, 1988). Rocks have varying physical properties giving various anisotropies (electrical, magnetic, gravity etc.) depending on the rock type, structure, mineral assemblages, fluid content, porosity etc. Electrical resistivity of rock varies in space giving the rock its characteristic electrical anisotropy. The anisotropy that can be observed due to a dipping geological layer where bedding or schistosity results in a higher resistivity perpendicular to the layering compared to that in the plane of the layering (Habberjam1972, 1975).
In the study area the crossed square array method was used because, the square array has been shown to be more sensitive to anisotropy than the Schlumberger or Wenner arrays (Habberjam, 1972: Darboux-Afouda and Louis, 1989). The square array geometry is more compact than schlumberger or wenner arrays for Azimuthal survey because it requires less than 65 percent less surface area than the equivalent collinear arrays (Habberjam and Watkins, 1967), and is less likely to be obscured by heterogeneities in bedrock or overburden bedrock relief, cultural noise, electrode placement errors or other source of noises. (Lane et al., 1995). Different authors have shown the usefulness of azimuthal resistivity survey in determining the principal direction of electrical anisotropy (Taylor and Fleming 1988; Ritzi and Andolsek 1992; Skjernaa and Jorgensen 1993; Bayewu and Olasehinde 2011, AlHagrey 1994; Odoh 2010; Ehirim and Essien 2009, Ajibade at al 2012, Ramanujam et al.2006. Th