## 4 Results
### Data Acquisition Layer
#### 4.1.2 Site Information Collection  
Site information of an underground utility tunnel can be collected through a variety of methods as shown in Table 5 below. Given the diverse nature of an underground utility tunnel, sensors can be attached based on their location and functionality characteristics, rather than their performance. The sensors can be classified into two categories--those inside and outside the underground utility tunnel--and further categorized based on whether they are fixedly installed, installed on moving robots, carried by people, or need to be connected to external systems.  
For fixed sensors, multisensors and cameras are repeatedly arranged at certain intervals to collect information in real time. There are three major types of sensors used inside an underground utility tunnel: environmental (CO2, O2, temperature and humidity, smoke, and flame) and flooding sensors (ultrasonic sensors), location sensors (BLE sensors), access  
\begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt} p{142.3 pt}} \hline \hline
**Monitoring Location** & **Monitoring Means and Method** & **Collected Data** \\ \hline \multirow{8}{*}{\
\begin{tabular}{} \end{tabular} } & Fixed sensor: multisensors and cameras are repeatedly arranged at certain intervals in tens of km along the utility tunnel, and information is collected in real time \\ \cline{2-3}  & Fixed sensor: sensor is arranged in facilities with a high risk & Vibration sensor of a water supply line, partial discharge sensor of a power line, water level sensor of a collection well, acoustic sensor \\ \cline{2-3}  & Movable robot: robot moves on the ceiling rate over tens of km along the utility tunnel to collect information in real time & Temperature, humidity, oxygen, carbon dioxide, carbon monoxide, hydrogen sulfide, nitrogen dioxide, low-luminance image, thermal image, LiDAR sensor \\ \cline{2-3}  & Manned patrol: patrol moves on foot and visually observes tens of km along the utility tunnel & Abnormalities in structures, firefighting facilities, power facilities, and communications facilities \\ \hline \multirow{8}{*}{Outside
\begin{tabular}{} \end{tabular} } & Fixed sensor: sensors and cameras are arranged at the entrance/exit of the management office and underground utility tunnel to collect information & Access security sensor, detection sensor of safety device \\ \cline{2-3}  & & Weather information such as temperature, humidity, precipitation, wind direction, wind speed, earthquake of a certain region, construction information, and terrorism risk information of the nearby areas \\ \hline \hline \end{tabular}
\end{table}
Table 5: Overview of site information collection for underground utility tunnels.  
sensors (RFID sensors), vibration sensors (gyro sensors), image sensors (low-luminance CCTV, thermal imaging cameras, and LiDAR sensors), and acoustic sensors.  
Movable robots move along the ceiling rail to collect information in real time while the same tools as fixed sensors are used. Furthermore, various kinds of information are collected via manned patrol, which is a traditional method, and via connection with external (weather data) systems.