## 2 Materials and Methods  
### Geological Setting in Study Area  
The Guanghua Basin is located in the northern part of the Pearl River Delta, with a dense river network, abundant precipitation, tectonic control by the Guangcong fault, and the tectonic pattern is dominated by the NE direction. The central and southern part of the basin is mainly composed of huge thick carbonate deposits accumulated in the Late Devonian period, and has experienced many times of transgression and crustal uplift in the geological history [37; 38]. The above conditions are extremely favorable for the formation of cover karst.  
The detection application research area is a subway station of Line 18 of Guanghua Intercity Railway, which is located in the center of Huadu District, Guangzhou. Guangzhou North Railway Station is 3 km to the west, and Guangzhou Baiyun International Airport is 7 km to the southeast. It is an important hub connecting the Tianhe and Huadu administrative regions (Figure 1). The shield tunneling machine started from this station, and the subway line passes through Tianmeixin village. According to the previous drilling results, the strata distributed in the study area are mainly Quaternary Holocene (Q\({}_{4}\)) and Upper Pleistocene (Q\({}_{3}\)), and the Middle Pleistocene (Q\({}_{2}\)) and Lower Pleistocene (Q\({}_{1}\)) are missing. The Holocene is composed of artificial fill, a silt layer, and a silty sand layer, while the Upper Pleistocene is mainly an alluvial-flood soil layer and residual soil layer. The lower bedrock of the exploration study area is mainly the limestone of the Lower Carboniferous Dating Terrace (C\({}_{1}\)ds) Member. The thickness of the Quaternary cover layer is 8-30 m, the cover karst is relatively developed, and the hole-finding rate of the drilling is 49.43%. The karst cave exposed by drilling is generally in a semi-filled state, and the filling material is mostly gravel and limestone debris. In addition, according to a large number of dropdown and water leakage events in the drilling construction records, it is judged that the beaded karst caves are connected to each other, which brings safety risks such as gusher sand to the construction.  
The construction site is a typical urban village (Figure 2), the construction is dense, the roadway is narrow, the flow of people is large, a considerable part of the area is difficult to carry out drilling construction for underground detection, and there is a natural pond and a river in the south of it, the groundwater is shallow, and it is very likely that there is a water diversion channel formed after the dissolution of limestone, so that a water inrush accident may occur during shield construction, and it is urgent to select appropriate geophysical  
Figure 1: Schematic diagram of internal traffic and carbonate distribution in Guanghua Basin (modified from Meng, 2020 [38]).  
detection methods to evaluate the development of underground karst, and make targeted treatments, to find potential water-conducting structures and karst fissures, and to draw and locate karst caves, so as to provide a reference for the next step of cross-well seismic computed tomography borehole layout and other karst treatment methods.  
According to the detailed geotechnical investigation report of this section and the geotechnical engineering investigation code for urban rail transit [39], the physical property parameters of the main geological bodies in the study area are shown in Table 1. There are obvious differences in longitudinal wave velocity and resistivity between different geological bodies, especially the karst caves and dissolution areas which have obvious characteristics of low resistivity and low wave velocity compared with intact bedrock, which have the prerequisites for corresponding geophysical exploration and research.