## 5 Discussion  
This paper comprehensively analyzes the characteristics of underground cavities in the study area by integrating ERT inversion results, geological survey data, and borehole data. The ERT inversion results reveal the presence of six low-resistivity zones in the study area. Since the field measurements were conducted during the rainy season and the site experienced continuous rainfall in the week prior to the survey, it is inferred that the low-resistivity zones were formed by rainwater infiltrating underground cavities. Therefore, these low-resistivity zones are presumed to indicate the locations of underground cavities. Based on Figure 3, the two cavities in Zone-1 are located within the limestone formation, and borehole ZK1 data confirm that V1 is a water-filled karst cave. Thus, the two low-resistivity zones near K32+690 and K32+810 are inferred to be water-filled karst caves. The analysis of Figure 3 indicates that the low-resistivity zones in Zone-2 are situated within the mudstone formation (the stratum containing refractory ore). Among these, the low-resistivity zone near K33+80 is not fully connected and consists of two smaller low-resistivity zones, which appear as two independent zones (V1 and V2) in line L1. Borehole ZK2 confirms that this low-resistivity zone is indeed a water-filled mining tunnel, while the intact core from ZK3 suggests that the F3 low-resistivity zone is not a fully developed goaf. Therefore, it is inferred that the low-resistivity zone near K33+80 comprises two separate water-filled mining tunnels. By combining Figure 3 and borehole ZK4 data, the low-resistivity zone near K33+190 is confirmed to be a water-filled mining tunnel. The large low-resistivity zone between K33+260 and K33+350 exhibits some variations in the inversion results across different survey lines. In line L1, V6 shows three smaller low-resistivity zones, with the two closer to the starting point of the line appearing to be connected. In line L3, F5 also displays three smaller low-resistivity zones, with the two closer to the end of the line appearing to be connected. In line L5, K5 presents as three independent low-resistivity zones. Borehole ZK5 confirms that this zone is indeed a water-filled mining tunnel. Borehole ZK6, located at the junction of the two smaller low-resistivity zones in F5, indicates that these zones are not fully connected. Therefore, it is inferred that the low-resistivity zone between K33+260 and K33+350 consists of three independent water-filled mining tunnels. Analysis of the ERT inversion results and borehole data reveals that, when multiple low-resistivity zones are close to each other, they may appear as a single large low-resistivity zone in the inversion results. Tables 1 and 2 indicate that the mudstone formation containing the tunnels and the underlying sandy shale formation are relatively fractured. Rainfall infiltration not only saturates the tunnels but also increases the overall water content of the formation. When the tunnels are saturated, water within them diffuses into surrounding fractures, causing the surrounding bedrock to also exhibit low resistivity in the inversion results. In summary, multiple closely spaced low-resistivity zones in the study area may appear as a single large low-resistivity zone in the inversion results. To better observe the relationship between the underground cavities and the resistivity profiles, this study arranged the ERT profiles of the five measurement lines according to their relative positions to construct a three-dimensional schematic. Based on the previous interpretation of the ERT profiles, the karst caves and abandoned mining tunnels were also delineated (Figure 8). This figure clearly demonstrates that the five profiles show consistency at the same distances, which further validates that the ERT profiles can accurately reflect the subsurface physical property variations.