## 4 Results Interpretation and Drilling Verification
### Results Interpretation
The resistivity of the first depression has no obvious zonal distribution characteristics and the resistivity step changes slowly, which is inferred to be caused by the stratum. In the second depression, a low resistivity zone, extending in an east-southeast direction, has an obvious resistivity step zone change. It is inferred that the structure in the deep bedrock is of low resistivity due to the water leakage developed in the shallow surface. The deep strata between \(-650\)-\(-950\) altitude are characterized by a high resistivity, which can be inferred to be Cambrian limestone, dolomitic limestone, and other strata. However, it is worth noting that there are several small closed anomalies in the shape of a \"bull's eye\" in the inversion profile with the controlled-source audio magnetotelluric method, and the distribution is irregular, such as \(2520\)-\(2620\) measure point altitude \(-200\) and \(2820\)-\(2920\) measure point altitude \(-150\) m. It is confirmed that the controlled-source audio-frequency magnetotelluric method is seriously disturbed by the magnetic field in an urban area, which causes the jump of the electromagnetic field signal, a fact that may bring inconvenience to interpretation. In order to further strengthen the geological understanding of the test section, the electric field data collected by GDP32 and processed by Professor [PERSON] from  
Figure 2: Geological interpretation of G4 test profile in the study area (**a**) High-density electrical processing results; (**b**) Controllable-source audio-magnetotelluric processing results; (**c**) Wide-area electromagnetic processing results; (**d**) Geological profile of G4. Central South University were used to obtain the data processing results of the wide-area electromagnetic method, as shown in Figure 2c. It is obvious that the graph data is smooth and stable, and the stratification is also more obvious. Combined with the previous two methods, the section is interpreted comprehensively, and the results are shown in Figure 2d. It is Quaternary above \(-\)100 m and there is only a high resistance at the large measure point between 3220 and 3720. According to the field investigation, it may be interfered by the industrial stray current in the shallow surface because the data were collected at the roadside of an industrial development zone where human activities are concentrated. The Cretaceous strata underlying Quaternary show relatively moderate resistivity at around 600 \(\Omega\)-m. The middle part is relatively missing, and it is inferred that there are Cretaceous strata before the 2120 measure point and after the 2720 measure point. The middle measure points between 2120 and 2520 are arched by the high resistivity body of the section, which is inferred to be Cambrian strata, and the top of which is in unconformable contact with Quaternary in deep Cambrian strata. In combination with the results of the regional geological survey, it is inferred that there is a fault fracture zone with a width of about 80 m and a southeast dip in the depth of the 2520 measure point. The dolomitic limestone of the Tanxi formation and the micrite argillaceous limestone of the Wunitang formation were transformed by tectonic movement. In the deep and on the southeast of the fault, Cambrian Niutitang formation carboniferous strata exist with a dip angle of about 30\({}^{\circ}\)and a lean to the northwest. It can be seen based on the above results that the resistivity detection method has a good resolution for identifying large geological structures and stratum distribution, but it is difficult to figure out more detailed stratification within the Quaternary, which is the more prominent in the Dongting Basin Changde area where the typical water system of the Huxiang Basin developed. In order to provide multi-parameter information for bedrock detection, natural surface wave microtremor detection was completed in the vicinity of a sk09 verification borehole (measure point 2120) and sk10 (measure point 3700), respectively.