# Reconstruction and Measurement of Irregular Karst Caves Using BLST along the Shield Metro Line
## 3 Test Results  
### Attenuation Law of Lasers with the Same Wavelength in Different Suspended Media  
Based on the above-mentioned test system, we recorded the laser emission power and attenuation power under the influence of different laser wavelengths, different suspended media, and different water turbidity grades, and calculated the corresponding laser attenuation coefficients using the above method.  
(1) From the fitting curve in Figure 2a, it can be seen that when the laser wavelength is 405 nm, the suspended media are CaCO\({}_{3}\), silt, clay, and fine sand. For the laser suspended in the above media, the attenuation coefficient increases linearly with the increase of turbidity. The R\({}^{2}\) fitting degree values are 0.9956, 0.9802, 0.9945, and 0.9965, respectively. From the slope of the fitting curve, with the increase of turbidity, the increasing rate of the laser attenuation coefficient increases from CaCO\({}_{3}\) to clay, silt, and fine sand in turn. The particle size of CaCO\({}_{3}\) is about 5-10 \(\upmu\)m, and the particle sizes of clay, silt, and fine sand are greater. When the laser wavelength is 405 nm, the rate of the laser attenuation coefficient increases with the increase of the medium particle size; that is, the slope of the fitting curve is \(\gamma\) (CaCO\({}_{3}\)) \(<\gamma\) (clay) \(<\gamma\) (salt).  
(2) When the laser wavelength is 450 nm, the fitting curve in Figure 2b shows that the attenuation coefficient increases linearly with the increase of turbidity in the solution of CaCO\({}_{3}\), silt, clay, and fine sand. The R\({}^{2}\) fitting degree values are 0.954, 0.9559, 0.9562, and 0.9964, respectively. With the increase of water turbidity, the increasing rate of corresponding attenuation coefficient is different in the CaCO\({}_{3}\), silt, fine sand, and clay media: the slope value of the fitting curve is \(\gamma\) (CaCO\({}_{3}\)) \(<\gamma\) (clay) \(<\gamma\) (sand) \(<\gamma\) (salt).  
(3) When the laser wavelength is 532 nm, the attenuation coefficient also increases linearly with the increase of turbidity, and the R\({}^{2}\) fitting degree values are 0.9973, 0.9958, 0.9668, and 0.9938, respectively, as shown in Figure 2c. From the slope of the fitting curve, the attenuation coefficient rate increases from silt to CaCO\({}_{3}\), clay, and fine sand; that is, \(\gamma\) (salt) \(<\gamma\) (CaCO\({}_{3}\)) \(<\gamma\) (clay) \(<\gamma\) (sand).  
(4) The attenuation coefficient of the 650 nm wavelength laser increases linearly with the increase of turbidity in the CaCO\({}_{3}\), silt, clay, and fine sand media, as shown in Figure 2d. The attenuation coefficient rate increases from silt to clay, CaCO\({}_{3}\), and fine sand; that is, \(\gamma\)(salt) \(<\gamma\)(clay) \(<\gamma\)(CaCO\({}_{3}\)) \(<\gamma\)(sand).  
(5) According to Figure 2e, when the laser wavelength is 808 nm, the attenuation coefficient increases linearly with the increase of turbidity, and the R\({}^{2}\) fitting degrees rates are all higher than 0.98. Judging from the slope of the fitting curve, with the increase of water turbidity, the order of the increasing rate of attenuation coefficient from low to high is CaCO\({}_{3}\), fine sand, clay, and then silt. That is, the laser attenuation coefficient rate increases with the increase of the medium particle size.