# Recognition of Tunnel Fracture Zones in Seismic Waves and Ground-Penetrating Radar Data
## 3 Recognition of Tunnel Fracture Zones
### Recognition of Fracture Zones in GPR Data  
Electromagnetic waves exhibit a significant increase in reflection strength upon encountering interfaces with distinct electrical properties. This noticeable rise in amplitude is a critical feature in the identification algorithm for fracture zones. Figure 8a shows the waveform of a radar single-channel reflection signal.  
When ground-penetrating radar detects a tunnel, electromagnetic waves initially penetrate the surrounding rock from the air. Due to the difference in the dielectric constant between the air and the surrounding rock, the initial phase of the A-scan image is negative, as described by Equation (6). This negative phase occurs when electromagnetic waves transition from a medium with a smaller dielectric constant to that with a larger one. As the electromagnetic wave enters the fracture zone from the intact surrounding rock, the reflection coefficient changes due to the presence of voids in the fracture zone. This change is reflected in the A-scan image as a positive phase, opposite to the initial negative phase. The alteration in the phase of the electromagnetic wave entering the fracture zones from the intact surrounding rock is a characteristic for identifying the fracture zones. This characteristic condition is utilized to achieve the identification of fracture zones.  
Figure 8: (**a**) Recognition results of surrounding rock fracture zones in A-scan. (**b**) B-scan of tunnel envelope. The red line shows the location of the A-scan. (**c**) The blue area is the result of identifying the fracture zones in the B-scan.  
Figure 7: Three-dimensional image of tunnel.  
Utilizing the high-center-frequency characteristics of electromagnetic waves in fracture zones, identifying fracture zones within a single-channel waveform involves comparing the center frequency of a preliminarily selected area with that of an unselected area. Figure 9 illustrates the normalized center spectrograms in selected fracture zones versus unselected areas, demonstrating higher center frequencies in the selected regions compared to the unselected ones. This center-frequency comparison identifies the region of broken bands localized by phase and amplitude.  
The statistical analysis of each radar signal in the data quantifies the average amplitude of signal points in both the broken and unselected areas. A comparison curve is established for radar waveform amplitude data. Figure 10 shows that the two curves exhibit a distinct energy difference, where the selected fracture zones display higher energy than the unselected area. This observation indicates a notable increase in amplitude within the fracture zones.  
Figure 10: Amplitude comparison curves for fracture zones and intact surrounding rock.  
Figure 9: (a) Spectrogram of selected areas. (b) Spectrogram of unselected areas.  
The stacking of multiple A-scans depicts the locations of characteristic points of fracture zones in the B-scan, as shown in Figure 8c. The B-scan has frequent recognition feature points, indicating their alignment with the characteristic features of fracture zones. The discontinuity in phase axes points to a lack of continuity in the surrounding rock within the measurement area, suggesting the frequent development of rock fracture zones.