## 4 Discussion  
In the process of subway construction, it is very important to grasp the karst regional scope and karst distribution law along the line in advance for subsequent karst detection planning and reducing project risks. Dense infrastructure, high-rise buildings, and complex road conditions in urban environments pose challenges for drilling and many geophysical detection methods [40]. Electromagnetic noise from residential electricity, vibration from traffic, and interference such as steel and building materials can cover useful information. It greatly limits the detection effect of the geo-radar method, microseismic method, and transient electromagnetic method. Therefore, in order to realize the accurate detection of the karst distribution range and law, the appropriate combination of geophysical prospecting means should be selected according to the conditions of the construction site. As a significant geophysical exploration technique, the electrical resistivity tomography method is widely used in the field of mineral resources and water-bearing structure detection, which is based on the resistivity difference in geological bodies to detect underground structures. Taking advantage of its strong anti-interference ability and large  
Figure 7: Cross-well seismic computed tomography detection data inversion results and geological interpretation map. (**A**) TMCT 1–TMCT 2 section; (**B**) TMCT 3–TMCT 4 section; (**C**) TMCT 5–TMCT 6 section. detection range, this paper sets up multiple survey lines in Tianmeixin village and near the southern pond and compares and analyzes the change trend of resistivity of each section. It is concluded that the karst development degree of the village is gradually increasing from north to south, and it is inferred that the connection between the southern pond and the underground water in Tianmeixin village is the main reason controlling the karst development. This indicates that the karst in this area may still be in the process of development, and the area is gradually increasing, which causes security risks to the long-term stability and subsequent operation of the subway tunnel. However, the measurement line of the electrical resistivity tomography method must be straight and has a minimum detection length requirement, which is often limited by the detection environment. Moreover, as the detection depth increases, the lateral detection range in the deep section decreases, and the interpretation results show an inverted trapezoidal shape with large detection blind spots on both sides. In addition, the resolution of the electrical resistivity tomography method is low, so it can only roughly locate the karst distribution area. It is difficult to describe the karst boundary precisely, and it is difficult to meet the detection requirements of engineering only using this method [41]. The cross-well seismic computed tomography method is selected as the final karst detection method because of its strong anti-interference ability and high detection accuracy. However, the cross-well seismic computed tomography method needs to carry out detection in the borehole, and the cost is high. In this detection, the narrow streets and alleys in the urban villages greatly limit the area where drilling construction can be carried out, so it is necessary to narrow the detection area as much as possible to reduce the amount of drilling construction. Therefore, on the basis of the electrical resistivity tomography detection results, this paper uses several groups of representative detection method profiles that are easy to carry out to verify the inference about the range of karst development, and the results of the three geophysical exploration methods are in good agreement. It also proves the reliability of the inversion results of the two geophysical prospecting methods. Finally, the karst development zone in the south of Tianmeixin village is drawn as the key detection area, and reasonably reduces the workload of cross-well seismic computed tomography detection.