# Research on the Application of THz-TDS in Coal-Rock Interface Recognition
## 2 Experimental Detection
### Sample Preparation  
In this study, two kinds of coal (anthracite and lignite) and two kinds of rock (quartz sandstone and limestone) were collected from Datong Coal Mine in Shanxi Province. The sample preparation was undertaken using two methods: the first was the preparation of coal and rock powder mixed laminate, while the second part was the preparation of coal flakes (anthracite) with different thickness. The test sample is shown in the Supplementary Materials. In the first part, coal and rock were individually pressed into certain size pellets. Then, in order to simulate different coal-rock mixtures, four combinations were made (quartz sandstone-anthracite, limestone-anthracite, quartz sandstone-lignite, limestone-lignite), and each combination was mixed in four different proportions.  
Before preparation, we used an agate mortar to grind the coal and rock into powder with a particle size of less than 74 \(\upmu\)m, that is, the ground powder must pass through a 200-mesh steel screen. Since THz waves are sensitive to water vapor, in order to make the test results accurate and reliable, the ground and screened powder was dried in a dryer to remove excess water in the sample. During the experiments, coal powder was mixed with rock powder in proportion after grinding. Different proportions refers to the ratio of coal to coal-rock mixed powders. Since the diameter of the tablet press is 5 mm, each sample has the same diameter but different thickness. The thickness of different samples was measured using a digital caliper. The specific information for the samples is shown in Tables 1 and 2. The total mass of each sample was 0.2 g. The signal of THz waves through the air was used as the reference. In this study, we used THz time-domain spectroscopy to detect and identify coal and coal mixture powders. In a typical shearer coal-cutting operation scenario, there can be a large amount of dust and powder nearby the cut solid material and the drum. Therefore, studying the powder of coal, rock and coal-rock mixture is more in line with the situation of mining operation site than without such consideration.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Anthracite** & **Lignite** & **Quartz Sandstone** & **Limestone** \\ \hline Mass (g) & 0.20 & 0.20 & 0.20 & 0.20 \\ \hline Thickness (mm) & 1.02 & 1.02 & 1.00 & 0.95 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Coal and rock sample information. (coal and rock alone).  
The second part is the preparation of flake anthracite samples with different thicknesses to simulate coal seam thickness detection using THz waves, which is more suitable to coal-rock identification scenarios in actual coal mines. First, the original anthracite was cut into various thickness samples using a cutting machine. Then, the surfaces of these plate-like samples were polished using sandpaper to make them relatively smooth and reduce scattering of THz waves on the surface of the sample, minimizing their impact on the test results. The thickness of different samples was measured using a digital caliper. The thickness of each sample was measured three times and the average value was taken. The information for 10 flaky samples is shown in Table 3. THz waves passing through an empty sample stage, representing air, was used as a reference in the time-domain spectroscopy.