## 3 Results
### Investigation in Field Measurement  
Filed measurement was conducted in an underground cavern in Huangdao, China to evaluate the surrounding rock stability before grouting reinforcement. Figure 10 illustrates a brief scheme of detecting surrounding rock sections. A GPR instrument is attached on the cavern wall to transmit electromagnetic waves into the surrounding rocks. Once encountering the rock cracks, the electromagnetic waves will be reflected back to the receiving antenna (RA) and the response signals are recorded on the vertical trace. Owing to the wide transmission angle of the transmitting antenna (TA), reflections from non-vertical directions will generate 'pseudo' signals that affect the identification of those from vertical directions. Details of basic detection principles and 'pseudo' signals can be seen in [25]. During the investigation of the entire surrounding rock section, the GPR instrument will continuously move along the measurement direction while acquiring numerous vertical traces. In our research, a MALA 2-dimensional GPR instrument assisted by the data acquisition software 'Groundvision' is adopted. The central frequency of the GPR antenna is 250 MHz, and the horizontal resolution (intervals between neighbouring vertical traces) is 3 cm. A representative subterranean section is firstly investigated to analyze the detectability of shattered rock masses by the developed approach, which is compared with the classic migration approaches (e.g., [33; 34; 35]) utilizing the commercial software 'Reflexw'. The original radargram is presented in Figure 11a. Although strong reflections occur in the near-surface layers (amplitudes over 20,000),  
Figure 10: A brief scheme of detecting surrounding rock sections by GPR. Figure 9: IMFés amplitude profile of the simulated cracked surrounding rock section. signals beneath 2 m are too weak (amplitudes below 100) for post-processing. The visible distribution in Figure 10(a) will differ significantly from the actual distribution due to electromagnetic attenuation, and later arrivals beneath 6 m nearly disappear in this situation. Nonetheless, the surrounding rock conditions are complicated as intensive responses have been received from most areas. Figure 10(b) shows the pre-processed radargram of the surrounding rock section, with the measurement length \(L\) (\(0\leq L\leq 20\) m) and detection depth \(Z\) (\(0\leq Z\leq 6.86\) m). The pre-processing parameters are \(\alpha=0.56\), \(\varphi=0.1\) and \(Z_{0}=0.69\) m. The depth-variant amplitude gain function has reconstructed the signals to normal levels (amplitudes around 10,000), while signals weaker than 200 (2% of normal amplitudes) have been eliminated to avoid noises. Although the right-middle area \(H\) is conveniently recognized as intact rock masses, other areas are almost covered by intensive signals, which increases difficulties in identifying the unstable structures with fragmented rocks. Two representative measurement traces from Figure 10(b), Line 1 with intermittent signals and Line 2 with later heavy signals, are decomposed to compare the local mode difference, as shown in Figures 12 and 13. Besides the added white noises, IMF1s include the local mode in the near-surface layers, but this aberrant strong reflection area is not considered in the identification. Similarly, the compositions after IMF4s are also mostly occupied by the near-surface signals, unable to recognize the joint fissures at deep locations. In contrast, the IMF2 of Line 2 has captured the discernible fluctuation after 3.5 m, and this composition is chosen for further analysis. Utilizing EEMD to decompose all vertical measurement traces, the IMF2s are preserved to reconstruct the amplitude profile of the surrounding rock section, seen in Figure 14. like the previous procedure, the IMF2 signals weaker than the added white noise (amplitudes less than 200) has been removed. For comparison, Figure 15 presents the imaging results finely processed by the commercial GPR software.