## 2 Geological Prospecting and Field Monitoring
### Arrangement of the Geological Microtremor Exploration and Field Monitoring Points  
The microtremor exploration system is composed of a wireless digital geophone, a data acquisition station, etc. The wireless digital detector has a built-in detector, acquisition circuit, wireless synchronization circuit, digital processing circuit, and communication circuit for high-precision acquisition of micro-motion signals. A microtremor signal is a complex vibration composed of a body wave (P wave and S wave) and surface wave (Rayleigh wave and Love wave), in which surface wave energy accounts for more than 70% of the signal. Microtremor exploration mainly uses the dispersion information of the surface wave (Rayleigh wave) to calculate the velocity structure of the underground medium. The results obtained by the geological drilling exploration method are point-like distributions. Under complex engineering geological conditions, there may be local geological mutations between these points. The method of geological drilling exploration will produce a lot of blind spots. Combined with the geological advanced exploration technology-VIDO microtremor exploration system, it can comprehensively understand the geological conditions, improve the efficiency of shield construction, and ensure safety. This study explored the geological conditions of Rings 62 to 429 from 2 # to 3 #, working well along the tunneling direction. In this direction, the spatial coincidence exploration of four boreholes, ZK082, ZK085, ZK088, and ZK093, was carried out to verify the reliability of geological advanced exploration technology. Microtremor exploration uses a circular array, and the radius distance is selected from 1 to 5 m according to the depth; moreover, the point circles of adjacent arrays are tangent. The alignment direction of microtremor exploration is approximately along the axis direction of the tunnel to detect the geological conditions within the depth range of the tunnel. The microtremor exploration depth can reach 20-30 times the array radius. As the tunnel depth is 20 m in this case, the array radius can be selected as 1-1.5 m. Moreover, a high-resolution frequency wavenumber domain (HRFK) algorithm is used to extract the Rayleigh wave dispersion curve from data. Calculating visual wave velocity or directly using phase velocity and then obtaining a two-dimensional visual wave velocity profile or phase velocity contour map through interpolation and smooth calculation can objectively and intuitively reflect the velocity structure of underground rock and soil layers. The array arrangement is shown in Figure 3. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Geotechnical Category**} & **Natural Unit** & **Cohesion/** & **Cohesion/** & **Cohesion/** & **Cofficient of** & **Horizontal** & **Vertical** & **Permability** & **Friction** & **Compression** & **Rock Compressive** \\  & **Weight/** & **(kN/m\({}^{3}\))** & **Earth Pressure** & **Cofficient** & **Cofficient/** & **Cofficient/** & **angle/(\({}^{\prime}\))** & **Moulus/** & **Strength/** \\  & **kN/m\({}^{3}\)** & & & **(cm/s)** & **(cm/s)** & **(cm/s)** & **MPa** & **MPa** & **MPa** \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Soil parameters. Figure 3: Array arrangement. The electronic level with an accuracy of 0.1 mm/km was used to obtain the surface settlement values in the field. In addition, steel tapes and leveling staff were also used as secondary measuring tools. Nine monitoring points are arranged between \(-\)10 m and 30 m along the vertical direction of the 101 Ring and 130 Ring of the tunnel axis. Each monitoring point is drilled with a drilling rig in advance to fix steel bars longer than 1.5 m in the undisturbed soil of the monitoring point. The external earth pressure of the tunnel segment is obtained by automatic monitoring based on the earth pressure gauge. The positioning ring is welded and fixed; the earth pressure gauge is installed and connected to the automatic acquisition instrument.