# Integration of ERT, IP and SP Methods in Hard Rock Engineering
## 4 Results and Discussion
### Delineation of Subsurface Layers  
The 2D ERT models along with all profiles generated by the inversion program of RES2 DINV are shown in Figure 3. The electrical resistivity varies between 18-545,650 \(\Omega\)m along all ERT profiles over the entire investigated site. Based on the correlation between ten ERT models and the local hydrogeological information obtained by eight boreholes, the interpretation was constrained for a four-layered model of topsoil cover, weathered layer (highly weathered rock), weathered/unweathered transition zone (partly weathered  
Figure 2: (**a**) 2D ERT model along profile B, (**b**) 2D ERT model along the same profile B interpreted for subsurface layers and fractures/faults based on the correlation between ERT models and borehole lithological logs, (**c**) 2D IP model along the same profile B interpreted for clay and fractures/faults, and (**d**) 2D SP model along profile B interpreted for groundwater flow path and fractures/faults.  
rock) and unweathered layer. The topsoil cover of dry strata above the water table was delineated with a resistivity less than 180,500 \(\Omega\)m, the weathered layer below the water table underlying the top layer was evaluated with a resistivity between 18-375 \(\Omega\)m, the unweathered rock at the bottom was revealed with a resistivity greater than 1000 \(\Omega\)m and as high as 545,650 \(\Omega\)m, and the transition zone between weathered and unweathered layer (below the water table) was identified with resistivity ranging from 375 to 1000 \(\Omega\)m. The weathered layer, unweathered layer and weathered/unweathered transition zone (partly weathered layer) are clearly identified in 2D ERT models shown in Figure 3.  
All 2D ERT profiles were integrated and interpreted for the delineation of weathered layer, weathered/unweathered transition zone and unweathered rock as shown in Figure 4. Thus, a 3D view of the integrated 2D ERT profiles provides a clearer view of the subsurface layers over the entire area. Each ERT model in Figures 3 and 4 was interpreted for the identification of the most suitable places for the construction of engineering structures. Average depth/thickness of the delineated topsoil cover, unweathered layer, weathered/unweathered transition zone and weathered layer, including most appropriate locations for engineering construction along each profile are given in Table 2. Results of ten ERT models in Table 2 reveal that the topsoil cover is evaluated with an average depth of 3 m, the weathered layer is evaluated with an average thickness of 14 m, the weathered/unweathered transition zone (partly weathered rock) is interpreted with an average thickness of 8 m, and the unweathered layer is delineated at an average depth of 25 m. The results suggest that ERT profiles A, G and H are more suitable for engineering construction compared with other profiles since they reveal only one deep weathered/transition zone and lower thickness of the weathered rock; while profiles profile B, F, I and J provide the most unsuitable places for construction as they delineate several deep weathered/transition zones and thick weathered layer. Looking at the ERT results alone, the southeastern parts of the project area provide more suitable places for engineering structures due to a shallower depth to the unweathered zone.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Profile**} & \multicolumn{4}{c}{**Average Depth/Thickness of Subsurface Layers**} & \multirow{2}{*}{**Most Suitable Places for Engineering Structures (m)**} \\ \cline{2-2} \cline{4-6}  & **Topsoil** & **Weathered Layer** & **Transition Zone** & **Unweathered** & **Engineering Structures (m)** \\  & **Thickness (m)** & **Thickness (m)** & **Thickness (m)** & **Layer Depth (m)** & \\ \hline A & 3 & 10 & 7 & 20 & 60–180, 500–800, 1000–1300 \\ \hline B & 3 & 26 & 6 & 35 & 50–75, 220–275, 430–500 \\ \hline C & 3 & 10 & 7 & 20 & 0–175, 275–375