# Research on the Application of THz-TDS in Coal-Rock Interface Recognition
## 3 Results and Discussion
### THz Spectral Properties of Coal-Rock and Coal-Rock Mixtures
#### 3.1.2 THz Spectral Properties of Coal-Rock Mixtures  
The THz time-domain spectral system is consistent with previous configurations, and the measurement and calculation results for the coal-rock mixture sample are shown in Figures 3-6. The resulting THz spectra were analyzed using the quartz sandstone-lignite mixture samples, as shown in Figure 3. Figure 3a shows the THz time-domain signal of six samples with the mass ratio of coal powder to coal-rock powder displayed in the upper right corner.  
Figure 3: Quartz sandstone-Lignite. (**a**) Time-domain spectrum. (**b**) Frequency domain spectrum. (**c**) Refractive index. (**d**) Absorption coefficient of coal and rock mixture samples with different ratios of coal.  
Figure 4: Limestone-Lignite. (**a**) Time-domain spectrum. (**b**) Frequency domain spectrum. (**c**) Refractive index. (**d**) Absorption coefficient of coal and rock mixture samples with different ratios of coal.  
Figure 5: Quartz sandstone-Anthracite. (**a**) Time-domain spectrum. (**b**) Frequency domain spectrum. (**c**) Refractive index. (**d**) Absorption coefficient of coal and rock mixture samples with different ratios of coal.  
It can be seen from Figure 3a that the attenuation of THz waves is different when passing through the coal and rock mixed samples. The frequency master diagram of the coal-rock mixed sample is obtained by applying fast Fourier transform, as shown in Figure 3b. From the figure, all samples of the coal-rock mix peak at 0.2 THz. As the mass ratio of coal increases, the peak size gradually decreases, which is consistent with the time-domain variation in Figure 3a. In addition, with the increase of the mass ratio of coal, the bandwidth of the sample also decreases gradually, and the bandwidth of pure lignite sample is the shortest. The refractive index and absorption coefficient of coal and rock mixed samples are obtained by calculation. It can be observed from Figure 3c that the refractive index of the six samples is basically stable, and the difference in refractive index can be attributed to the difference in the main composition of coal and rock materials.  
Figure 3d is the absorption coefficient diagram for the coal-rock mixture samples. From the figure, the absorption coefficients of all samples increase with the increase of frequency. The increase in absorption becomes more pronounced as the coal content in the sample increases. In the THz band, no obvious absorption peaks are observed in the coal and rock mixed samples. This could be due to the following three reasons: (1) there are no EM resonances of either coal or rock molecules in the THz range; (2) there is no component that strongly absorbs THz waves in the sample; (3) there are multiple components in the samples to absorb the THz waves in such a way that peak values cannot be obtained clearly. Based on the optical data obtained from the THz detection of coal-rock mixture samples, the refractive index and absorption coefficient of coal-rock mixture can serve as the reference for identifying the coal-rock interface.