# Integration of ERT, IP and SP Methods in Hard Rock Engineering
## 1 Introduction  
Given natural heterogeneity, the evaluation of hard rock sites for the construction of engineered structures is a challenging task for planners and engineers. The sustainable design of structures depends on the accurate evaluation of the subsurface geological model [1]. The uncertainty in the subsurface interpretation may cause structure failure [2, 3]. A geotechnical site investigation requires time, expenses, and evaluation of subsurface heterogeneity. There is a need to use techniques so that planners can overcome the challenges of a costly and longtime commitment [4]. The construction sites without any proper geotechnical investigation result in building collapse in many developing countries [5]. In the hard rock terrains, the subsurface geological knowledge about the weathered and unweathered rock and the fractures/faults needs to be known for the success of the engineering construction [6]. The weathered zones and fractures/faults are mainly created by tectonic, chemical, erosion and weathering processes. The weathered/fractured zones are mainly associated with groundwater occurrence in the hard rock [7, 8]. Therefore, identification of the weathered/fractured zones saturated with groundwater is essential to avoid locating engineered structures within potential failure zones [3, 6]. Traditionally, boreholes are carried out to acquire subsurface geological knowledge [9; 10]. However, given safety and accessibility issues, it is a challenging task to conduct such tests in steep topographic areas [11; 12]. In addition, the drilling approaches are expensive, time-consuming, need heavy equipment, offer low coverage of point measurements only, and cannot provide lateral information on the subsurface [13; 14]. Thus, the conventional tests cause uncertainties in the geological model which may lead planners to locate engineered structures at locations where failure is more likely [15]. Therefore, an alternative approach of traditional geotechnical techniques is necessary, which can delineate the subsurface between borehole locations prior to the engineering construction and thus bring a sigh of relief to the engineers. In the past years, many researchers have used geophysical methods in their investigations [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28]. Geophysical methods, such as electrical resistivity tomography (ERT), induced polarization (IP) and self-potential (SP) are non-invasive, fast, economical, user friendly, and provide lateral and vertical mapping of the subsurface for large coverage areas [22; 29]. ERT is widely used in environmental and geotechnical investigations [3; 17; 20; 23]. ERT measures electrical resistivity which has a wide range of values and varies with rock type. Unweathered rock has high resistivity, while weathered rock has lower values [10]. Thus, ERT can be used to map the weathered and unweathered zones. However, ERT alone cannot distinguish between water and clay content, since low resistivity suggests water, as well as clay content in the saturated zone [3]. The clay content causes instability in the foundation rock of the engineering structures. Recently, the integration between two or more geophysical methods has been very successful to reduce the uncertainty caused by a single geophysical method [9; 24; 25]. Integration between IP and ERT can remove the uncertainty caused by low values of electrical resistivity as high IP values indicate clay content whereas low IP values reveal water content [30; 31]. In addition, ERT coupled with IP can detect the saturated weathered/fractured zones more efficiently since low values of electrical resistivity and chargeability suggest water content [3]. The negative values of SP suggest groundwater flow; thus, SP is very effective to evaluate the groundwater flow mainly along the weathered/fractured zones [10; 32]. In this work, we performed an integrated geophysical survey of ERT, IP and SP at a highly heterogeneous hard-rock terrain in order to evaluate the subsurface for site suitability prior to engineering construction.