# Reconstruction and Measurement of Irregular Karst Caves Using BLST along the Shield Metro Line
## 1 Introduction
The rapid development and wide application of contactless drilling laser measurement technology provides a solution for the exploration of karst caves. The biggest advantage of this method is that the microlaser probe can adapt to various kinds of narrow channels and boreholes and can be inserted deeply into a cave to obtain the point cloud coordinate data. This method has been widely employed in many professional instruments for the exploration of poor geological structures, such as the Cavity Monitoring System (CMS) [14] and Cavity Autoscanning Laser System (C-ALS) [15]. [PERSON] [16] took the lead in applying borehole scanning technology in goaf detection, stability calculation, monitoring, and early warning. [PERSON] [17] investigated the formation rules of point cloud grids and maximum angle triangles, the dominant vertex segmentation strategy, and the irregular triangle optimization method, and realized the three-dimensional modeling and visualization of scattered point clouds in complex goaf sections. Many scholars have also carried out a lot of research on point cloud data acquisition, reconstruction, and visualization [18, 19]. At present, the research and relevant applications are mainly focused on the detection of goaf sections in relatively favorable environments, while the occurrence environment of karst caves along subway lines is extremely complex. Many scholars have carried out studies on laser transmission characteristics in water [20, 21, 22], most of which focused on the attenuation characteristics and detection effects of lasers in ocean water and showed that a \"blue-green window\" exists in ocean water; that is, blue and green lasers experience the least attenuation in ocean water [23]. However, the water environment inside a water-filled cave is much more complex than that in the ocean. Most of the media in a water-filled cave, such as CaCO\({}_{3}\), clay, and silt, are suspended. As a result, the optical reaction between the above-mentioned suspended media and the laser during its transmission is more complicated, and the attenuation characteristic of the laser is also more obvious, which result in the detection distance being extremely limited, and thus not meeting the requirements for karst cave detection. Moreover, the influence of complex environmental factors, such as cave humidity, dust concentration, water-filled media, and turbidity, on the laser point cloud detection and reconstruction is not clear. As a result, problems related to multinoise, multidistortion, and multinail factors in the karst cave model have not been effectively solved [24]. In summary, the cavity laser scanning method has the following advantages: (1) the ability to realize the quantitative and precise detection of cave position, size, filling state, and connectivity; (2) the ability to establish an actual and accurate cave model with a better visualization effect; (3) the ability to provide an accurate data basis to rationalize a treatment plan and guide the field workers to form reasonable treatment plans. For example, grout always runs out in the process of grouting the karst caves, which is mainly caused by groundwater flow or cracks in the connected pipes [25]. If the boundary shape, depth, size, volume, and internal connectivity of the karst cave can be accurately obtained, the stability evaluation of the surrounding rock in the shield tunnel will be largely improved, which is of great significance to the safe treatment of the karst areas. Therefore, we carried out an experimental study on the attenuation of a multiwavelength laser in complex karst cave water environment, the results of which will be used to guide the selection and optimization of laser detection in water-filled caves, in order to realize the maximum laser detection distance and the best detection effect of water-filled caves. The research achievements will be also applied to the Jinan subway project.