## 5 Discussions and Conclusions  
Accurate detection of boulder quantities and sizes before shield construction is crucial for effective planning and risk mitigation. Early identification of potential boulders enables construction teams to make informed decisions regarding equipment use and excavation methods, thereby reducing the likelihood of unexpected obstacles and minimizing associated delays. Urban environments present unique challenges for traditional geophysical methods due to dense infrastructure, high-rise buildings, and complex subsurface geology. Interference factors, such as electromagnetic noise from power lines, vibrations from traffic, and the heterogeneous nature of the subsurface, can obscure valuable geophysical signals. Therefore, there is a pressing need for geophysical techniques specifically tailored to urban settings. Geophysical methods, such as ground-penetrating radar (GPR) and seismic refraction, have limitations in urban environments. GPR, while effective for detecting nonconductive materials and providing high-resolution images, suffers from signal attenuation in conductive soils. Seismic refraction provides valuable data on subsurface layering and material properties but often struggles with detecting smaller or deeply buried boulders. In contrast, electrical resistivity tomography (ERT) offers several advantages, including the ability to detect deeper underground structures and effectively identify both conductive and nonconductive boulders. ERT's strength lies in its ability to infer subsurface structures through resistivity measurements, which are relatively insensitive to variations in depth compared with other methods. The 3D cross-hole ERT configuration has shown promising results in urban environments. By drilling holes near the target bodies and placing sensors within, this method significantly reduces external interference, leading to more accurate and reliable measurements. Physical model tests and field detections confirm the effectiveness of 3D cross-hole ERT in detecting boulders and suggest its potential for mapping other subsurface features, such as utility lines, voids, and groundwater distribution. However, despite its advantages, the research on 3D cross-hole ERT in urban environments is still in its infancy, and several research gaps remain. Firstly, the resolution and accuracy of the method can be further improved, especially in complex geological settings with high levels of heterogeneity and interference. Secondly, the cost and time associated with drilling boreholes for 3D cross-hole ERT can be prohibitive for some urban projects, necessitating the development of more cost-effective and efficient survey designs. Furthermore, the integration of ERT with other geophysical methods, such as seismic imaging and ground-penetrating radar, could provide a more comprehensive understanding of the subsurface, but this multimethod approach is still underexplored. Future research should focus on addressing these gaps by developing advanced inversion algorithms capable of handling heterogeneous and noisy data, optimizing borehole drilling strategies to reduce costs and time, and exploring the synergies between ERT and other geophysical techniques. Additionally, more extensive field validation studies are necessary to demonstrate the reliability and applicability of 3D cross-hole ERT in various urban environments. With these advancements, 3D cross-hole ERT has the potential to become a standard tool in urban geophysical surveys, offering critical information for infrastructure planning, construction, and maintenance. Continued development and refinement of this technique will contribute significantly to improving the safety and efficiency of urban excavation and construction projects. **Author Contributions:** Data collection, analysis, and writing--original draft preparation, M.Y. ; writing--review and editing, X.W. ; funding acquisition, H.G. All authors have read and agreed to the published version of the manuscript.