# 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
\begin{table}
\begin{tabular}{c c} \hline \hline
**Prediction Method** & **Characteristic** \\ \hline The seismic wave reflection method [20] & The detection accuracy is higher in the range of 20–200 m and the detection effect is better for karst caves. However, the adaptability to thick overburden soil is poor, and energy attenuation is faster. \\ \hline The land sonar method [21] & The prediction of medium and small caverns and fractured zones is effective, but the velocity of each layer cannot be accurately obtained. \\ \hline The high-density electrical method [22] & No filling cavern or water-rich cavern with the obvious electrical difference with surrounding rock stratum is good. It can only be qualitative but not quantitatively explored. \\ \hline The transient electromagnetic method [23, 24] & It is sensitive to large water-filled low-resistivity zones and has the ability to penetrate low-resistance coverage. The depth of detection is large. However, the resolution of empty caves or dry-filling karst caves is not enough, so it is difficult to distinguish. \\ \hline The ground-penetrating radar method [25, 26] & In the high-resistivity area, the detection depth is 20–30 m and the detection effect is good for karst caves and faults. However, it is easy to be affected by the surface soil, and the detection depth is greatly reduced. \\ \hline The microgravity method [27] & Under flat terrain, it is widely used in karst investigation. However, factors such as topography fluctuation, sediment thickness, and fluctuation of diving surface are relatively large. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The characteristics of different geophysical methods. The core of each drill is shown in Figure 13.  
Figure 12: Comprehensive interpretation diagram of the high-density resistivity method and transient electromagnetic. (**a**) The result of high-density resistivity method; (**b**) The result of transient electromagnetic. Figure 13: Drilling core drawing. (**a**) Coring of hole h1; (**b**) Coring of hole h2; (**c**) Coring of hole h3; (**d**) Coring of hole h4; (**e**) Coring of hole h5. Using five target drilling points, through the improvement of exploration equipment and exploration method, using panoramic probe sampling, combined with automatic ray compensation and automatic positioning technology, the acquisition of geological images in a borehole dark environment was achieved. Long distance accurate detection and real imaging of disaster sources such as karst cave were realized by 3D laser scanning (MDL, Edinburgh, Scotland). The characteristics of 3D laser scanning (Figure 14) are as follows: (1) Rapidity: The spatial information of the karst cave can be obtained quickly, and the coordinate information of the karst cave surface can be measured in time. (2) Non-contact: We can scan the targets in a non-contact way. (3) Penetration: Under the water cut condition of the cave, it can cross the water body and reach the target surface. (4) Real-time, dynamic, and active: The target information is obtained by the reflection of light emitted by itself. It cannot be constrained by time and space, and the beam is quasi-parallel light, which avoids the inherent optical distortion error of conventional optical measurement. (5) High density and high precision: Laser scanning obtains target characteristics in a high-density and high-precision way. The laser point cloud is composed of point position coordinate data. Using a large lattice and a certain density grid to describe the entity information, the target information can be described more accurately. Three-dimensional laser scanning devices are placed in five boreholes, as shown in Figure 15.