# Reconstruction and Measurement of Irregular Karst Caves Using BLST along the Shield Metro Line
## 3 Test Results
### Optimization Scheme of Longest Laser Detection Distance  
First of all, the selection of the laser wavelength is mainly based on the suspended medium and turbidity of the cave water environments. Table 2 gives the maximum detection ranges of a multiwavelength laser in different suspended media and with different turbidity levels. It can be seen from Table 2 that no matter what kind of suspended medium is used, the 450 nm wavelength laser is suitable in water environments with low turbidity, while for clay and silt media, when the turbidity is about 25 NTU, the detection distance of the 650 nm laser is longer. Combined with the field geological data and actual borehole data, by distinguishing the type of suspended medium and turbidity grade of water-filled karst caves, the laser with the appropriate corresponding wavelength is selected to increase the maximum detection distance.  
Figure 7: Attenuation coefficient—turbidity curve with lasers of different power levels: (**a**) CaCO\({}_{3}\); (**b**) silt; (**c**) clay; (**d**) SiO\({}_{2}\).  
Secondly, in the selection of laser power, Table 3 gives the maximum detection distances of lasers of various power levels in different suspended media with different turbidity grades. When only considering the maximum detection distance, a laser with higher power should be selected to meet the long detection distance requirements for water-filled caves. However, to ensure the safety of personnel, the selected laser power is generally lower than 5 milliwatt, as it will otherwise cause harm to the operator's eyes. Many devices put an emphasis on eye protection, such as the C-ALS produced by MDL (Measurement Devices Ltd) Company (UK) adopting FDA (Food and Drug Administration) IEC (International Electrotechnical Commission) first-class laser eye protection and the CMS system produced by Optech Company of Canada adopting FDA 21 CFR1040 first-class laser eye protection. On the premise of ensuring the safety of personnel, a laser with higher power should be selected.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Medium**} & \multirow{2}{*}{**Length (nm)**} & \multirow{2}{*}{**Turbidity (NTU)**} & \multirow{2}{*}{**10**} & \multirow{2}{*}{**20**} & \multirow{2}{*}{**30**} & \multirow{2}{*}{**40**} & \multirow{2}{*}{**50**} \\  & & & & & & & \\ \hline \multirow{4}{*}{CaCO\({}_{3}\)} & 405 & 1.63 & 0.85 & 0.58 & 0.44 & 0.35 \\  & 450 & 1.95 & 1.03 & 0.70 & 0.53 & 0.43 \\  & 532 & 0.83 & 0.54 & 0.40 & 0.32 & 0.27 \\  & 650 & 1.61 & 0.79 & 0.52 & 0.39 & 0.31 \\  & 808 & 0.85 & 0.57 & 0.43 & 0.34 & 0.29 \\ \hline \multicolumn{2}{c}{Laser wavelength suggested (nm)} & **450** & **450** & **450** & **450** & **450** \\ \hline \multirow{4}{*}{Silt} & 405 & 1.02 & 0.61 & 0.44 & 0.34 & 0.28 \\  & 450 & 3.11 & 1.03 & 0.62 & 0.44 & 0.34 \\  & 532 & 1.12 & 0.70 & 0.50 & 0.40 & 0.32 \\  & 650 & 1.65 & 0.96 & 0.68 & 0.52 & 0.43 \\  & 808 & 1.05 & 0.55 & 0.37 & 0.28 & 0.23 \\ \hlineThirdly, the evaluation of the detection environment is mainly based on the type of suspended medium in the water-filled cave and the corresponding turbidity. The specific types of suspended media can be preliminarily judged in combination with the sampled water quality. If the field conditions permit, the water quality turbidity can be sampled and detected for further quantitative grade classification [26].