# Reconstruction and Measurement of Irregular Karst Caves Using BLST along the Shield Metro Line
## 1 Introduction  
As cities develop and urban populations increase rapidly, traffic congestion has become one of the greatest problems facing many cities in China. Metro systems, or subways, which makes full use of underground space and reduce congestion on the ground, have become an important part of urban infrastructure and a popular traffic choice for Chinese people in the 21 st century [1; 2; 3; 4]. Due to complex urban geologic conditions, a large number of metro tunnels have to pass through underground karst areas, such as the Jinan metro tunnel passing through water-rich hard rock karst cave areas, the Wuhan metro tunnel passing through honeycomb-shaped caves, and the Changsha metro tunnel passing through complicated underwater karst caves. The covertness of typical karst geological bodies such as karst caves increases the permeability of the rock structure and lowers relevant rock mechanicalparameters, which may result in engineering disasters and hazards, including tunnel water bursting and inrush, water leakage, karst surface collapse, and shield cutterhead drooping, particularly when the caves bear both confined water pressure and shield tunneling disturbance [5, 6]. It is no exaggeration that water inrush has become a serious threat to shield tunneling projects [7, 8, 9]. With the rapid development of computer technology, photogrammetry technology, and intelligent control technology, the detection of unfavorable geological bodies in tunnels and underground engineering projects utilizes more digital and quantitative methods and equipment with higher accuracy [10]. Currently, the common methods to detect cavity disaster sources, such as caves developed in the shallow karst areas, are drilling and geophysical exploration. The drilling method, as the most conventional and direct geological survey method, is especially suitable for detection in high-risk areas, such as underground karst caves. With a single borehole, one can obtain the approximate position, development height, and roof thickness of a karst cave at the borehole measuring point, while multiple boreholes can determine the horizontal boundary range of the karst cave. However, the drilling method lacks clear direction and is essentially a type of single-point detection method, during which the connection between boreholes can only be determined by empiricism and relevant calculation. This leads to the inevitability of blind areas, high costs, and consumption of time and labor [11]. The commonly used geophysical exploration methods, classified according to the exploration principles, include electrical methods (such as the high density electrical method), electromagnetic wave methods (such as the transient electromagnetic method and ground penetrating radar method), and seismic wave methods (such as the land sonar method) [12]. In the process of geophysical exploration, the detection capacity of geophysical exploration methods is limited by many factors, including the physical characteristics and spatial morphology of the detection medium or object, the geological structure and structural characteristics of the detection site, the hydrogeological conditions of the detection site, the topographic relief of the working face, the distribution of electromagnetic interference sources and interference bodies, the performance parameters of the devices, and the experience levels of the personnel operating the devices outdoors and processing and analyzing data indoors. Therefore, it is almost impossible to avoid fuzziness, multiplicity, and uncertainty in comprehensive geophysical exploration [13]. Consequently, drilling and comprehensive geophysical exploration methods cannot accurately detect karst cave boundaries in complex geological environments, rather can only deliver two-dimensional or even one-dimensional qualitative data with poor visualization effects and cannot provide quantitative parameters of karst caves, such as the accurate position and size information.