## 3 Testing Results Analysis
### Analysis of Experimental Results of Upper Step Specim
#### 3.1.3 Analysis of Characteristics of Two-Dimensional Spectrum  
The original waveform file is transformed by a fast Fourier transform to obtain a two-dimensional spectrum diagram, in which the maximum amplitude of the two-dimensional spectrum diagram is the main frequency. Figure 10 is the main frequency diagram of the whole process of the upper step specimen. Table 1 is the proportion of the main frequency distribution of the upper step specimen. Based on the comprehensive analysis of Figure 10 and Table 1, the main frequency distribution of the experiment could be divided into four stages:  
Figure 9: Diagram of acoustic emission absolute energy and accumulated energy about upper step specimen.  
(1) I compaction stage: This stage was the geostress loading stage. The main frequency of this stage was mainly concentrated between 100 and 200 kHz, and 250 and 350 kHz, and the sum of the two was 89.63%. At this stage, there was also a small amount of signal with a main frequency of about 100 kHz, which was mainly caused by the disturbance of the load change to the original crack, resulting in a small amount of large-scale fracture inside the rock.  
(2) II stress concentration stage: This stage was the vertical loading stage and the main stage of rockburst energy accumulation. The bandwidth of the main dominant frequency band remained stable, and there was no large-scale and large-scale fracture in the sample. With the increase in load, the elastic strain energy accumulated rapidly.  
(3) III shock disturbance stage: In this stage, the low-frequency signal and high-frequency signal tended to be stable during the disturbance process, the fluctuation was obviously smaller, and the bandwidth of the main frequency band appeared from wide to narrow. The high-frequency band (450-600 kHz) had experienced a process from existence to nonexistence, which showed that after the optimization design of the step method, the elastic strain energy accumulated in the surrounding rock was released, and the energy accumulated in the rock mass did not exceed its energy storage limit. Under the premise of insufficient energy storage, the transformation from the original small-scale fracture to the larger-scale and large-scale fracture was inhibited. After experiencing multi-level burial depth, the low-frequency signal (100-200 kHz) and the intermediate frequency signal (250-350 kHz) increased slightly at the burial depth of 1250 m. The increase of medium- and low-frequency values meant the occurrence of large-scale and larger-scale fractures, respectively. Combined with the images, it could be seen that the generation of large-scale fracture in the whole process is not a significant effect.  
\begin{table}
\begin{tabular}{c c c c c} \hline  & \multicolumn{4}{c}{**Signal Proportion (\%)**} \\ \cline{2-5}
**Stages** & \begin{tabular}{c} **Low** \\ **Frequency** \\ **(100–200 kHz)** \\ \end{tabular} & \begin{tabular}{c} **Intermediate** \\ **Frequency** \\ **(250–350 kHz)** \\ \end{tabular} & \
\begin{tabular}{c} **High** \\ **Frequency** \\ **(450–600 kHz)** \\ \end{tabular} &
\begin{tabular}{c} **Ultra High** \\ **Frequency** \\ **(600–700 kHz)** \\ \end{tabular} \\ \hline I & 51.70 & 37.93 & 0.97 & 0.00 \\ II & 31.01 & 46.51 & 0.78 & 0.00 \\ III & 32.79 & 56.72 & 0.66 & 0.00 \\ IV & 18.54 & 69.59 & 2.65 & 0.00 \\ \hline \end{tabular}
\end{table}
Table 1: Main frequency distribution ratio of upper step.  
Figure 10: Main frequency diagram in the whole process of upper step specimen.  
and larger-scale fractures released the high stress inside the surrounding rock, promoted the generation of cracks and formed a loose zone, thereby regulating the high elastic energy storage in the high-stress rock mass for pre-splitting rock mass and reducing the risk of rockburst.  
(4) IV rockburst failure stage: At this stage, the acoustic emission frequency band showed a beam-like feature. There was no frequency band above 390-430 kHz and 500 kHz in this experiment, which reflected the non-intensity of the rockburst.