# Fine Exploration and Control of Subway Crossing Karst Area
## 2 Quantitative Exploration Method of Karst Cave Based on Three-Dimensional Laser Measurement
### Location Exploration Method of Karst Cave  
Table 1 mainly introduces six common geophysical methods under different detection principles. Each of them has different characteristics. In accordance with the characteristics of geophysical prospecting and the engineering geology of the R1 line, the most suitable geophysical method was selected. First, the subway was built in the urban area, where the surface soil is thicker. The seismic wave reflection method and the ground-penetrating radar method lead to more disturbances. Therefore, these two methods are not applicable. Second, because of the abundance of groundwater in the interval, the microgravity method is not applicable. Third, the upper part of the subway line is a highway, and the high-density electrical method leads to certain destructiveness on the road surface. Finally, the transient electromagnetic method is more suitable for the comparison between the land sonar method and the transient electromagnetic method. The principle of the transient electromagnetic method is that a pulsed magnetic field is transmitted to the front of the working face using an ungrounded line. When the current in the transmission line is abruptly disconnected, the two eddy fields will be excited to maintain the magnetic field (that is, one field) before the disconnected current, the size, and attenuation of the two eddy current field and the electricity of the surrounding medium. This is related to the electrical distribution of the surrounding medium. The variation of the secondary field with time is observed during the first field intermittence. After processing, the electric property, the scale, and the form of the underground medium can be understood to achieve the aim of detecting the target. Figure 11: Exposed water-bearing karst cave. Figure 10: Geologic profile of the distance between the Wangfu village station and the Dayang village station. The main reason for choosing high-density resistivity method and transient electromagnetic method for comprehensive detection is that the resistivity of underground rock is mainly influenced by mineral composition, water content, and temperature of rock. These complex factors result in the instability of rock resistivity and change in a large range. When there are fissures or fault zones in underground rocks and water-rich rocks with water-conducting channels, the resistivity of natural rocks located in these structural areas will be significantly reduced. There are obvious electrical differences with surrounding strata. Previous geological survey and site exposure show that it has the geological and physical basis of electrical detection. Each geophysical prospecting method has its own advantages and disadvantages. It is often difficult to satisfy the requirement of detection accuracy by using a single method. The combination of various geophysical methods can complement each other and verify each other. We have made a total of 500 meters of detection, from L30+900-L31+450. Figure 12b transient electromagnetic (TEM) locations are from L30+900-L31+40 and L31+110-L31+290, totaling 500 m. Five sets of high-density resistivity have been made, each of which is 100 m. Figure 12a shows the third group, which is from L31+150-L31+250. By means of resistivity anomaly, areas with strong karst development are detected. In these areas, we determined the locations of five boreholes, h1(110), h2 (230), h3 (280), h4 (315), and h5 (350) in Figure 12b, respectively. From the results of core drilling, it can be seen that all five boreholes encounter karst caves.