## 3 Results
### Identifying Unstable Rock Structures  
Two surrounding rock sections were investigated for further applying the developed approach in identifying unstable rock structures.  
Figure 16 shows the original and pre-processed radargrams of a surrounding rock section, with the measurement length \(L\) (\(0\leq L\leq 16\) m) and detection depth \(Z\) (\(0\leq Z\leq 6.86\) m), and the reconstructed profile by IMF2s is illustrated in Figure 17. The original radargram is insufficient for post-processing as the signals beneath 2 m are over 200 times weaker than those in the near-surface layer (amplitudes over 20,000). Later arrivals beneath 6 m are nearly invisible owing to the electromagnetic attenuation. This issue has been handled by the amplitude gain function, with pre-processed results shown in Figure 16b. However, rock identification is still difficult since intensive interference signals have nearly occupied the entire section. Contrarily, the cracked and intact rock masses are distinguishable in the reconstructed profile (Figure 17). The intact portions I, J and K that present evidence in Figure 16b have been entirely revealed, while the intact portions M and N covered by intensive signals in Figure 16b also become visible. Other areas are identified as unstable rock structures with different cracked situations, thus requiring further reinforcement measures. The deepest 10% area marked in the red rectangle is difficult to identify since the end effects of EEMD would affect this area.  
Figure 14: IMF2s amplitude profile of the cavern detection. Signals with amplitudes less than 200 are removed.  
Figure 15: Imaging results of finely processed GPR data by GPR commercial software.  
Figure 18 shows the original and pre-processed radargrams of another surrounding rock section, with the measurement length \(L\) (\(0\leq L\leq 18\) m) and detection depth \(Z\) (\(0\leq Z\leq 6.26\) m), and the reconstructed profile by IMF2s is illustrated in Figure 19. Similar to the previous results, the signals beneath 2 m in the original radargram are over 200 times weaker than those from the near-surface layer (amplitudes over 20,000), which results from the electromagnetic attenuation. This issue has been handled by the amplitude gain function, with pre-processed results shown in Figure 18b. However, intensive signals have nearly occupied the entire section, making our identification difficult. Stable rock structures have been revealed after reconstructing the radargram. Different from the previous rock section in Figure 18, the deep portion R around 4-6 m of this section is almost cracked, which means the rock structure is unstable. The intact portions P and Q that show evidence in Figure 18b have been entirely revealed, while the intact portion O covered by intensive signals in Figure 18b also become visible. Therefore, our developed approach shows good practice in unstable rock structure identification. The deepest signals in the red rectangle are affected by the end effects of EEMD, and therefore the GPR time window should be 10% larger than the targeted depth in further applications.  
Figure 16: (a) Original GPR radargram of a surrounding rock section. (b) Corresponding pre-processed radargram (signals with amplitudes less than 500 are removed).  
Figure 17: IMF2s amplitude profile of the surrounding rock section in Figure 16. Signals with amplitudes less than 500 are removed.