# Integration of ERT, IP and SP Methods in Hard Rock Engineering
## 5 Conclusions  
In this work, we used an integrated approach of three geophysical methods namely electrical resistivity tomography (ERT), induced polarization (IP) and self-potential (SP). We used this geophysical approach to assess a highly heterogeneous site in South China for the successful construction of the engineered structures. Traditionally, drilling tests are conducted to get the subsurface geological information. However, such tests suffer limitations in terms of cost, coverage and topographic constraints, etc. Furthermore, it may not be possible for planners to drill a large number of wells that provide the point measurements vertically. On the other hand, geophysical methods are non-invasive, cheaper, and provide subsurface geological knowledge both laterally and vertically. Such methods can be integrated with the available geological information obtained from the limited boreholes in order to investigate the large area entirely. Figure 7: (**a**) A four layered-conceptual model of the investigated area obtained using three geophysical methods (ERT, IP and SP) for the delineation of subsurface layers, fractures/faults, clay and groundwater flow, (**b**) Water seepage during the construction as identified by the interpreted fault F4 along profile J, and (**c**) A change in construction design was made by constructing a water channel for water infiltration from the identified faults/fractures on southwest side of the project site. In this work, we calibrated ERT with the local geological information of eight boreholes and constrained the subsurface into four discrete layers with specific values range of resistivities, such as the topsoil layer above water table with a resistivity between 18-180,500 \(\Omega\)m, the weathered layer with resistivity ranging from 18 and 375 \(\Omega\)m, the weathered/unweathered transition zone (partly weathered rock) with resistivity ranging from 375 to 1000 \(\Omega\)m, and the unweathered layer with a resistivity between 18-545,650 \(\Omega\)m. The interpreted 2D ERT models delineated the subsurface layers over the entire area, such as the topsoil layer with an average thickness of 3 m, the weathered layer with an average thickness of 14 m, transition zone with an average thickness of 8 m, and the unweathered layer at an average depth of 25 m. IP was integrated with ERT to delineate clay since ERT alone cannot differentiate between water and clay because low resistivity shows either clay or water. Clay was identified with chargeability greater than 40 ms and resistivity less than 1000 \(\Omega\)m. The deep weathered zones were delineated as the main faults and localized fractures in the project site. Our observation suggests that low values of geophysical parameters (resistivity and chargeability) suggest the faults/fractures saturated with water, such as ERT less than 1000 \(\Omega\)m, IP less than 15 ms and SP less than 0 mV. Groundwater flow along the faults/fractures was mapped by SP. Negative SP mapped the groundwater accumulation mainly along the faults/fractures. Based on the results of ERT, IP and SP, we recommend the most suitable places for the development of engineering structures along each profile (Table 2). Our findings will help the site engineers with the accurate design of engineering structures in the investigated area. Results of this study including the conceptual model (Figure 7a) will also be useful for the interpretation of nearby sites. Our results were confirmed by the actual situation of the investigated site during the construction, such as groundwater seepage during the site construction was clearly observed from the identified fractures/faults (Figure 7b), and the construction design was modified based on our geophysical results (Figure 7c). Our approach of integrated geophysical methods provides better insights into the subsurface for the construction of engineering structures in highly heterogeneous hard rock terrains. Conceptualization, P.S., Y.S. and M.H. ; methodology, P.S., Y.S. and M.H. ; software, P.S., Y.S., M.H., X.Y.