## 4 Data Processing and Imaging Results
### Apparent Resistivity Imaging Results  
For each measuring line, we calculated the apparent resistivity and apparent depth of HTEM using the measured electromagnetic induction data (some lines are shown in Figure 5) and obtained the imaging results within the range of 0-300 m [25]. Simultaneously, we calculated the apparent resistivity and apparent depth of GAFEM using the measured electromagnetic field data (some lines are shown in Figure 6) and obtained resistivity results of 200-2200 m [23; 31]. For the steep terrain of Mount Hora, we used known terrain parameters, flight height, coil attitude angle, etc., when modeling HTEM and GAFDEM. Therefore, the apparent resistivity imaging results calculated using electromagnetic response data only include the influence of geological structure anomalies. Finally, we used the Kriging interpolation method to fuse the apparent resistivity results obtained by HTEM and GAFEM, and obtained the apparent resistivity imaging results in the entire survey area. The apparent resistivity imaging results of survey lines L2, L4, L6, and L8 are presented in Figure 7. The 3D slices T1-T4 of the horizontal apparent resistivity are shown in Figure 8, where the L serial number represents the survey line and the T serial number represents the horizontal resistivity slice. Figures 7 and 8 illustrate that the apparent resistivity of K46+040-K51+000 is high, with the lowest resistivity of about 400 \(\Omega\)m and the highest resistivity of about 1600 \(\Omega\)m. Resistivity exhibits evident stratification. Yellow indicates relatively high resistance from top to bottom, while green indicates relatively low resistance. At K50+640-K51+200, the tunnel body's crossing resistivity is high. However, an evident low-resistance anomaly of equivalent coil closure is notable directly above the tunnel's main body in this area. The two low-resistance values are significantly characterized by downward extension, and their center positions have low resistance, with a resistivity of about 200 \(\Omega\)m. At K52+080-K53+490, the apparent resistivity distribution map shows that the shallow low resistivity is characterized by a downward extension. Combined with the aerial images, the low-lying areas are  
Figure 6: Field measured data of the GAFEM system. (**a**) L3 line. (**b**) L4 line. characterized by a downward extension of low-resistivity anomalies. At K53+120-K53+300 and K53+400-K53+490, the tunnel body passes through the low-resistivity abnormal area. In accordance with the overall apparent resistivity imaging results presented in Figures 7 and 8, we obtained the resistivity distribution of the tunnel body's survey line, as shown in Figure 9. The resistivity distribution map of the tunnel trunk's survey line consists of three parts: the entrance turning, the tunnel trunk, and the exit turning. The turning point at the entrance (K44+970-K45+120, total of 150 m) was obtained by processing the  
Figure 8: A 3D slice of the apparent resistivity. Figure 7: Apparent resistivity result of the survey line. (**a**) L2 line. (**b**) L4 line. (**c**) L6 line. (**d**) L8 line. data of the survey lines adjacent to L6. The main body of the tunnel was obtained from processing the data of the L6 survey line. The turning point at the exit (K52+970-K53+470, total of 500 m) was obtained by processing the survey lines adjacent to L6. Figure 9b shows a tile map of apparent resistivity at the depth wherein the tunnel body is located. The colors ranging from purple to red represent the resistivity value from low to high. Analyzing the image can obtain information, such as apparent resistivity morphology and characteristics. As shown in the image, the background value of resistivity of the rock mass in this section is above 80 \(\Omega\)m. The shallow part of the mountain surface generally exhibits low resistivity. Another two low-resistivity anomalies are D1 and D2, respectively, with resistivity values less than 300 \(\Omega\)m, which can be interpreted by highly jointed and saturated sandstone deposits.