# Integration of ERT, IP and SP Methods in Hard Rock Engineering
## 3 Materials and Methods  
In the past decades, ERT has been widely used to assess hard-rock sites for the evaluation of weathered and unweathered layers, discontinuities, joints and fractures/faults [3; 7; 10; 18; 19; 21; 22; 34; 35]. It has been highly efficient in the areas dominated by the hydrogeological significance of weathering processes [9; 10]. In ERT surveys, electrical resistivity is measured (in \(\Omega\)m), which depends on the subsurface variations mainly associated with the type of rock, fractures, joints, lithology, water content, porosity, and permeability [3; 36]. ERT data is collected using different electrode configurations which provide different resolutions [36]. ERT is affected by lithology and electrolytic conduction related to the size and configuration of the pore space, which is dependent on the size and shape of subsurface grains [9]. In general, electrical resistivity decreases with increasing water and clay content [37]. The resistivity of metamorphic and igneous rocks is higher than that of sedimentary rocks; similarly, the resistivity of unweathered rocks is higher than that of weathered/fractured rocks [10]. Thus, a huge contrast between resistivity values of weathered and unweathered rock is useful for the site suitability of the engineered structures [3; 9]. ERT surveys are performed by injecting electric current into the subsurface via a pair of current electrodes, and the potential difference is measured through a pair of potential electrodes. The spacing between the electrodes controls the subsurface volume/area of influence. As the electrode spacing increases, the volume/area increases. The electrical resistivity measured in the field is known as the apparent resistivity (\(\rho\)a) and is equal to:  
\[\rho\text{a}=\frac{KV}{I} \tag{1}\]  
where \(V\) is the potential difference obtained between two potential electrodes, \(I\) is the direct electric current injected through two current electrodes, and \(K\) is the geometric factor that depends on the electrode spacings used to collect resistivity measurements. Figure 1: Location of the investigated area including the geophysical profiles (yellow lines) of ERT, IP and SP measurements, and boreholes (white circles). The electrical resistivity measured in the field does not show the true resistivity of the subsurface layers [10; 38]. The apparent resistivity assumes a homogeneous subsurface. To solve for the true resistivity, apparent resistivities must be inverted [3; 37]. IP is a method in which the subsurface materials get polarized as a result of the injection of an electric current into the subsurface [39; 40]. Chargeability (in ms) is measured in the time-domain IP surveys. The electric field causes ions to displace within the EDL (electrical double layer). For time-domain IP, the measurement is the signal decay where displaced ions return back to the original state [41]. IP measurements are affected by the electrolyte properties in pore spaces, size of the grain, type of minerals, surface-water interaction, and the ratio of internal surface area to volume [42]. IP has higher polarization values for clay, sulfides and graphite; whereas its low values suggest the water content associated with the weathered/fractured zones in the hard rock terrains [30; 31]. Thus, IP measurements have been very successful to differentiate between clay (with high chargeability and low resistivity) and weathered/fractured rock saturated with water (for low chargeability and low resistivity) [3]. In this investigation, IP was used as a supporting method of ERT to differentiate between water and clay. SP is a passive geoelectrical technique, which measures natural potential (in mV) between two points on the ground [43]. SP anomalies are sensitive to the streaming potentials (e.g., groundwater flow). The high potential anomalies are mainly caused by conductive minerals, magnetite, sulfide ore bodies, graphite and groundwater flow [44; 45; 46].