## 4 Discussion  
Figure 12 shows that as water is continuously injected into the coal rock, the water pressure increases, decreases, and then rises again. The rise in water pressure is due to the continuous injection of water into the coal rock while it remains unbroken. The drop in water pressure is caused by the rock breaking and water seeping out. There are two typical water flow signals, Water flow signal 1 and Water flow signal 2. Within 30 min of the water pressure dropping, there were 15 water flow signals. According to the test records, water droplets flowed out of the surrounding rock during the time range of the water pressure drop. This suggests that when water flow signal occurs, it often corresponds to the penetration of hydraulic cracks and the seepage of water. Figure 13a-c display time-domain pictures of microseismic waveforms. On the other hand, Figure 13f-h represent time-frequency pictures that correspond to Figure 13a, 13b, and 13c, respectively. Figure 13d shows the time-domain image of blasting, while Figure 13i is the time-frequency figure that corresponds to Figure 13d. Figure 13e displays the time-domain picture of water flow signal, and Figure 13j is the time-frequency figure that corresponds to Figure 13e. We selected a display frequency band of 0-250 Hz for the time-frequency figure. The microseismic waveforms induced by borehole HF in coal seam have varying shapes. Figure 13a-c show that the SNR gradually decreases. The duration  
Figure 12: Water flow signal during pressure drop stage. of the waveforms is also less than 1 s, with waveform duration decreasing continuously with decreasing SNR. The three waveforms have durations of 0.696 s, 0.267 s, and 0.1452 s, respectively, with a main frequency of about 100 Hz. Figure 13d-i indicate that the duration of the blasting waveform is shorter than that of the microseismic waveforms. This duration is similar to the weak microseismic waveform shown in Figure 13c. The frequency of the blasting waveform is higher, around 400 Hz. This is because the microseismic waveform occurs due to the slow initiation, development, and penetration of microcracks in the coal rock. On the other hand, the blasting waveform has a shorter duration and faster mutation characteristics. Figure 13: _Cont._  
Figure 13: Time-domain and frequency-domain images of microseismic waveforms. **(a)** Time-domain figure of Microseismic waveform 1; **(b)** Time-domain figure of Microseismic waveform 2; **(c)** Time-domain figure of Microseismic waveform 3; **(d)** Time-domain figure of the blasting waveform; **(e)** Time-domain figure of the water flow signal; **(f)** Wavelet transformation of Microseismic waveform 1; **(g)** Wavelet transformation of Microseismic waveform 2; **(h)** Wavelet transformation of Microseismic waveform 3; **(i)** Wavelet transformation of blast waveforms; **(j)** Wavelet transformation of water flow signal. Water flow signal is a type of microseismic waveform that is generated through the friction between water flow with high pressure and rock after a penetrated hydraulic crack is formed. It lasts much longer than microseismic and blasting waveforms, as seen in Figure 13e,j, and can last up to approximately 11.534 s. The main frequency of water flow signal is approximately 75 Hz, which is slightly lower than the main frequency of microseismic waveforms. By using time-frequency transform, the frequency characteristics of the waveforms can be described with the change in time. This allows the waveform dividision into distributed frequency bands, and their frequency characteristics can be accurately quantified through waveform reconstruction. Figure 14 depicts the energy distributions of three types of waveforms. The blasting waveform has a wide range of frequencies, mainly in higher-frequency bands. On the other hand, microseismic waveforms and water flow signal are primarily distributed in the low-frequency range. Specifically, the blasting waveform energy in the frequency band above 700 Hz is negligible, accounting for only 0.021% of the total energy.