## 4 Data Processing and Imaging Results
### Apparent Resistivity Imaging Results
The lowest resistivity of the D1 low-resistivity anomaly is about 158 \(\Omega\)m, and the highest is about 250 \(\Omega\)m. It is an anomaly of the vertical long strip trap, and resistivity is low at the center. The outcrop sandstone stratum experienced strong weathering. The rocks developed large amounts of fractures and joints, which resulted in large quantities of water occurring in the rock-breaking cracks. Based on the above field characteristics, we speculate that the D1 low-resistivity anomaly is caused by fragmented rocks with plenty of water filling them. The D1 low-resistivity anomaly is close to the F2 fault shown in Figure 1. Therefore, the broken rocks may be affected by the fault, proving the accuracy of the resistivity imaging results. The tunnel body passes through this anomaly, and, thus, considerable attention should be given to this anomaly. The lowest value of the D2 low-resistivity anomaly is about 125 \(\Omega\)m, while the highest value is about 200 \(\Omega\)m. In accordance with the aerial photography data, the area is low-lying, with two water-rich ditches above the tunnel body. The rocks at K51+000-K52+080 experienced strong erosion by mountain top drainage for a long time, which resulted in a lot of water in the shallow part of the area. The outcrop sandstone stratum experienced strong weathering, which resulted in the breaking rocks developed plenty of cracks. Due to the relatively low altitude, a great deal of water collects in the D2 anomaly area. Therefore, the D2 anomaly was caused by fragmented rocks and water. The D2 low-resistivity anomaly is close to the F3 fault in Figure 1. Therefore, the broken rocks may be affected by the fault, proving the accuracy of the resistivity imaging results. The tunnel body passes through or is directly above the tunnel body, and considerable attention should be given to this anomaly. In order to meet the needs of disaster prevention and rescue in the tunnel, a parallel pilot tunnel is set up 35 m to the right side of the tunnel. The apparent resistivity results of the parallel pilot tunnel are shown in Figure 10. The overall apparent resistivity characteristics of the parallel pilot tunnel and the tunnel body are basically consistent, and the apparent resistivity profile shows that the background resistivity value of the rock in this section is above 80 \(\Omega\)m. According to the shape and characteristics of apparent resistivity, two low-resistivity anomalies are delineated as D1 and D2, and resistivity is less than 300 \(\Omega\)m.  
Figure 9: Apparent resistivity results: (**a**) apparent resistivity slices of the tunnel; (**b**) apparent resistivity map at the depth of the tunnel. The apparent resistivity between D1 and D2 low-resistivity anomalies is relatively high, with a minimum resistivity of about 400 \(\Omega\)m and a maximum resistivity of about 1600 \(\Omega\)m. It is similar to the resistivity distribution characteristics of the tunnel body, with apparent sandstone resistivity layering. In the field observation, the sandstone stratum shows the characteristic of stratification with different grain size. Furthermore, the study area experienced a strong fold deformation event, which resulted in different-grain sandstones showing different features of apparent resistivity. Therefore, we speculate the stratification characteristics of apparent resistivity caused by the rocks with different degrees of crushing strength. Similarly, in accordance with the resistivity profile of this section and the resistivity distribution of the T2 line profile, rocks in the middle of this section may have developed plenty of joints and cracks. Simultaneously, due to the deep burial depth in this section, local small-scale faults and folds cannot be precluded. Therefore, attention should be given to the area of relatively low apparent resistivity, and protection work should be implemented well.