## 4 Case Study
### Result Analysis  
In this region, the geological strata exhibit complexity with layers of varying thickness. Based on columnar data from borehole M1Z3-TGJ-18 near the testing site, the depth of the boulder ranges from approximately 12.9 m to 14.5 m. The stratification from top to bottom includes approximately 1.8 m of clay, followed by 11.5 m of residual sandy clay, and finally, weathered granite at the bottom. The groundwater depth varies from 0 to 1.5 m, with corresponding resistivity values ranging from 0 to 1 \(\Omega\)\(\cdot\)m for Quaternary overburden and strongly weathered granite layers, and from 60 to 120 \(\Omega\)\(\cdot\)m for slightly weathered granite. The resistivity of the boulder is expected to match that of slightly weathered granite due to its location in loose residual soil and strongly weathered layers. This results in higher inversion resistivity values influenced by electrode proximity and varying groundwater conditions. Following inversion, 3D electrical resistivity results between boreholes are obtained, enabling the creation of horizontal resistivity slice maps at different depths. For instance, the depth slice map from 8 m to 15.2 m, depicted in Figure 8, shows inversion  
Figure 7: Drilling location plan for site implementation. resistivity values ranging from 20 \(\Omega\cdot\)m to 190 \(\Omega\cdot\)m across the survey area. A significant high-resistance anomaly is observed between ZK4 and ZK2 boreholes at depths ranging from \(-11\) m to \(-15.7\) m. This anomaly exhibits resistivity values exceeding 80 \(\Omega\cdot\)m, centered approximately 7.5 m in the ZK4-ZK2 direction, interpreted as a solitary rock consistent with observations in the ZK2 borehole. Furthermore, the ZK3 borehole displays a high-resistance anomaly at depths of 14 m to 15.7 nm, albeit with a smaller anomaly range, potentially due to boundary effects at the borehole's bottom. The detection results partially illustrate the spatial distribution of underground boulders. Variations in resistivity between boulders and surrounding rock and soil layers stem from several factors, including residual soil heterogeneity, the composition of completely or strongly weathered rocks, differences in structural planes, variations in groundwater development, and the presence of high-resistivity PVC casing in boreholes. Furthermore,  
Figure 8: Slice diagrams of inversion results at different depths. the complex underground electric field in urban areas significantly influences detection outcomes and the accuracy of inversion results. Utilizing 3D electrical resistivity tomography (ERT) facilitates comprehensive data acquisition and allows sensors to be positioned closer to target bodies. This approach enhances the resolution of electrical detection, enabling clearer delineation and characterization of subsurface anomalies such as boulders. To further validate the results of the 3D cross-hole ERT, a measurement line was arranged in the area around borehole ZK2, which is known to contain isolated boulders. The boulder extends approximately 5 to 8 m to the north side of the measurement line. The line is 31 m long with a 1 m electrode spacing, using a total of 32 electrodes arranged in a single deployment. Due to the cement pavement on site, conventional copper electrodes, which are typically used, would be damaging and thus not suitable for use. Instead, bag electrodes filled with yellow clay were employed on site for coupling with the ground. Figure 9 shows the high-density DC resistivity profiling section. Within the shallow 3-m range, the resistivity values are greater than 3000 \(\Omega\)-m, indicating high-resistivity zones corresponding to the cement pavement and its underlying layers.