# 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
#### 3.1.3 Application of THz-TDS in Coal-Rock Interface Identification  
In the actual working environment of a coal mine, continuous mining machines need to make timely judgements about the interface while cutting the coal seam. These machines  
Figure 6: Limestone-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. work underground in coal seams, where the material being cut is general coal. However, when a continuous mining machine encounters a coal-rock working face, it starts cutting a mixture of coal and rock. At this point, it is crucial for the shearer to stop mining the current working face and proceed to judge and mine the next working face instead of completely cutting the rock before starting [29]. In this section, THz time-domain spectroscopy technology is used to assist the continuous mining machine in making real-time assessments of coal-rock working faces; a flow chart is shown in Figure 7. When the continuous mining machine cuts the working face, it generates a mixture of coal/rock/coal-rock mixed powder. This powder is collected and sieved. With particle size below 74 \(\upmu\)m, the dry treatment is carried out to ensure the reliability for THz detection. Subsequently, the samples undergo THz testing to obtain the time-domain spectrum signal. After testing, the samples are separated from the remaining powder collected earlier, humidified, and removed. The purpose of this process is to avoid retesting the collected and tested powder, which could potentially impact the subsequent mining machine's real-time assessment of the coal-rock working face and lead to incorrect judgment. Then, the time-domain spectral signals of the samples are subjected to fast Fourier transform to obtain frequency domain spectra. The optical parameters such as refractive index and absorption coefficient are calculated. All the THz data from the samples are sent to the database for analysis. The analysis database is built by integrating all the time-domain signals, frequency domain signals and multiple optical parameters of coal, rock and coal-rock mixture in Sections 3.1.1 and 3.1.2. In practice, the situation of coal-rock working surface is relatively complex, which requires us to judge a variety of coal-rock combinations in addition to coal and rock. The core function of the database is to assist the continuous mining machine in distinguishing complex coal-rock interface situations, providing a wealth of information. The THz information of the samples is compared and analyzed against the existing information in the database to determine whether the tested sample is coal, rock, or a coal-rock mixture and draw a conclusion. The resulting determination can be transmitted as instructions to the continuous mining machine, which takes the next steps accordingly. Subsequently, the conclusions can be recorded in the database to provide more abundant data analysis for future tests. Through this process, the THz time-domain spectroscopy technology can be effectively applied to the coal-rock interface identification. Figure 7: The flow chart of real-time assessments of coal–rock working faces. In order to verify the experimental results, we used the simulation software Computer Simulation Technology (CST 2021) to simulate the experimental process of measuring coal seams with different thicknesses by THz time-domain spectroscopy technology, and compared the simulated time-domain spectra with the actual samples. Fourier transform of the time-domain signal of the sample was used to obtain the effective spectrum range of the coal slice, which is 0.2-1.5 THz. The simulation model of rectangular waveguide was established using CST. The waveguide can guide the directional transmission of electromagnetic waves.