# 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
The rectangular waveguide is composed of a waveguide port for the emission source of electromagnetic waves at the front of the waveguide (THz pulse), the two kinds of dielectric materials in the middle of waveguide (air and anthracite), and a waveguide port of short circuit for realizing the reflection of THz wave. In order to better simulate the experimental process, the materials are divided into two parts, the first part is air to simulate the propagation of THz pulse in free space, and the second part is anthracite. For air material, relative permittivity is 1, conductivity is 0; for the anthracite material, relative permittivity is 3.6, conductivity is 0.00001 [30]. The cross-section length of rectangular waveguide is 0.1956 mm; the width is 0.0978 mm. Mesh properties set as hexahedra, largest cell set as 0.0132 mm, and the background properties set as normal. The direction of the incident electric field is set as the positive direction of \(x\)-axis. At the same time, the boundary conditions of rectangular waveguide set port-dependent incident energy on the y-z and x-z planes. Using a time-domain solver, the frequency range is set to 0.2-1.5 THz, consistent with the effective spectrum range of the coal flake sample. Because in the CST time-domain solver, the incident pulse waveform is controlled by setting the solving frequency range (width of the frequency range), so the incident waveform in this study is fixed. By adjusting the distance of the wind side, the first side of the coal seam can be changed, and the second side can be fixed to achieve the change of the thickness of the coal seam. The experimental process of measuring coal seams with different thicknesses by terahertz time-domain spectroscopy is simulated, and the simulated time-domain spectra are compared with the actual samples. In order to simulate the THz spectra of different thickness samples in the experiment, the length of the waveguide coal seam part (Sr1-Sr6) to be simulated are set as 4.05, 3, 5, 3.50, 3.45 and 3.90 mm, respectively. The average time for simulating coal seams of different thicknesses is 2 min. The simulation results (Sr) are shown in Figure 8b. The peak value of the first surface of the time-domain signal of different thickness coal seams is mainly in the range of 10-20 ps, and the peak value of the second surface is concentrated around 35 ps. Compared with Figure 8a, it can be seen that the time ranges of the two surface peaks in the samples and simulation results are consistent, and the time-domain spectra are the same. Because their position depends on the thickness of the sample, the second peak is identical to the time interval of the first peak. Observation of anthracite flakes Af2, Af3, Af6, Af7 and simulation of their obtained simulation results Sr5, Sr6, Sr1, Sr4 were compared. The thickness comparison between the experiment and simulation are shown in Table 4.  
As can be seen from Figure 8, the simulation waveform is different from the experimental waveform, because the waveform after simulation is different due to the different Settings of materials and coal seam thickness (the Settings are different to simulate the experimental process), which is not fixed and different from the experimental THZ waveform. However, the simulation results (the interval between the two peaks) are consistent \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Experiment**} & \multicolumn{2}{c}{**Simulation**} \\
**Anthracite Flakes (Af)** & **Thickness (mm)** & **Anthracite Flakes (Sr)** & **Thickness (mm)** \\ \hline Af2 & 3.47 & Sr5 & 3.45 \\ Af3 & 3.87 & Sr6 & 3.90 \\ Af6 & 4.03 & Sr1 & 4.05 \\ Af7 & 3.48 & Sr4 & 3.50 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The thickness comparison between the experiment and simulation. with the experimental results, which can achieve the goal of this study. The two peaks correspond to the largest amplitude of the two pulse oscillations.