## 3 Results and Discussion  
### Measured Analysis of Advance Microtremor Exploration  
Geological exploration is one of the most critical preparations in tunnel construction. The scope of shield tunneling excavation face includes mudstone, sandstone, gravelly soil, pebble, clay, and other complex geology, which may have a local geological mutation. The combination of advanced microtremor and borehole exploration can clarify the geological situation. Timely adjusted construction parameters according to geological conditions, such as mud water pressure, jack thrust, grouting pressure, etc., will make shield tunneling more efficient and reliable.  
It is divided into four parts along the tunnel axis to analyze and compare the results of microtremor exploration. They are Rings 62-118, 121 to 217, 290 to 317, and 390-429, as shown in Figure 5. The result of the surface wave phase velocity from microtremor exploration is coincident with that from borehole exploration. The difference between the two is that the geological conditions between boreholes are estimated. At the same time, the results obtained by microtremor exploration are continuous and have an enormous scope, showing more details of the actual stratum. Among them, the radius of the microtremor exploration array from Ring 62 to Ring 118 is 2 m. When the shield machine reaches and drives out of the geological range, the wave velocity reflected by the geological conditions at the bottom of the tunnel is low, i.e., 300-400 m/s. This indicates that the soil in this part of the stratum is relatively loose, which will likely change the shield machine's tunneling attitude. Therefore, construction parameters such as slurry balance and grouting pressure need to be adjusted in advance. The geology of Rings 64 to 116 reflects normal rock wave velocity without obvious abnormality. The radius of the array from Rings 121 to 217 is 3 m. The results show that there are inhomogeneous bodies with low wave velocity in Rings 142, 145, 181, 184, and 187, and that the wave velocity is 300-400 m/s. Construction parameters  
Figure 4: The layout of monitoring points for surface settlement and pressure gauges outside tunnel segments.  
shall be adjusted timely when shield tunneling approaches this section. The radius of the array from Rings 290 to 317 is 3 m, and the geological wave velocity of the Ring 302 section is low, i.e., 400-500 m/s. The radius of the micro motion array of Rings 390-429 is 3 m. At Ring 390 along the tunnel direction, the interface of the soil rock composite stratum began to appear, the wave velocity of the stratum decreased to 300-400 m/s, and the subsequent stratum results were roughly the same as the distribution trend of the geological profile obtained by drilling.  
Figure 5: Cont.  
In order to verify the reliability of microtremor geological exploration, an array with a radius of 2 m was set up at the position of holes ZK082, ZK085, ZK086, and ZK093 along the tunnel. The comparison between the geological results measured by borehole and the results obtained by microtremor exploration is shown in Figure 6. The figure also shows the change in geological wave velocity obtained by microtremor exploration. It can be seen from the comparison results in the figure that the stratum structure information obtained by microtremor exploration in this study is highly consistent with the analysis data of the drill core, which can further verify the reliability of this method.  
Figure 5: Comparison between microtremor exploration results and geological profile: (**a**) Rings 62 to 118; (**b**) Rings 121 to 217; (**c**) Rings 290 to 317; (**d**) Rings 390 to 429.  
Figure 6: _Cont._