## 4 Near-Surface Defect Detection and Identification
### Near-Surface Defect Detection Based on SST  
The Complex Morlet wavelet is utilized for CWT and SST. Results for the signal processing of all five types of defect signals are presented in Figures 4-8, respectively. The general defect signals are slightly distanced from the blind zone signal, whereas the blind zone defect signals are overlaps with each other. The maximum curves of CWT and SST of the defect area were compared. Figure 4: CWT and SST results of the ND signal and zoomed view of the blind zone: (**a**) original signal; (**b**) zoomed signal of the blind zone; (**c**) maximum curves of the blind zone after CWT and SST; (**d**) CWT of the signal; (**e**) zoomed CWT of the blind zone; (**f**) zoomed CWT-3D of the blind zone; (**g**) SST of the signal; (**h**) zoomed SST of the blind zone; and (**i**) zoomed SST-3D of the blind zone. Figure 3: Flowchart of near-surface defect detection (**right**) and identification (**left**). Figure 5: CWT and SST results of the GPD signal and zoomed view of the defect area: (**a**) original signal; (**b**) zoomed signal of the defect area; (**c**) maximum curves of the defect area after CWT and SST; (**d**) CWT of the signal; (**e**) zoomed CWT of the defect area; (**f**) zoomed CWT-3D of the defect area; (**g**) SST of the signal; (**h**) zoomed SST of the defect area; and (**i**) zoomed SST-3D of the defect area. Figure 6: CWT and SST results of the GVD signal and zoomed view of the defect area: (**a**) original signal; (**b**) zoomed signal of the defect area; (**c**) maximum curves of the defect area after CWT and SST; (**d**) CWT of the signal; (**e**) zoomed CWT of the defect area; (**f**) zoomed CWT-3D of the defect area; (**g**) SST of the signal; (**h**) zoomed SST of the defect area; and (**i**) zoomed SST-3D of the defect area. The original signal's main frequency component is approximately 1.25 MHz, which is basically consistent with the transducer's center frequency. In Figure 4c, the maximum values of SST and CWT near the blind zone both gradually decrease without observable drastic changes. In Figures 5 and 6, there is a noticeable energy dispersion and convergence phenomenon between the defect signal and blind zone signal, as shown in the zoomed CWT and SST results. In Figures 7 and 8, the CWT result shows that the energy is nearly  
Figure 8: CWT and SST results of the VDBZ signal and zoomed view of the defect area: (**a**) original signal; (**b**) zoomed signal of the defect area; (**c**) maximum curves of the defect area after CWT and SST; (**d**) CWT of the signal; (**e**) zoomed CWT of the defect area; (**f**) zoomed CWT-3D of the defect area; (**g**) SST of the signal; (**h**) zoomed SST of the defect area; and (**i**) zoomed SST-3D of the defect area. Figure 7: CWT and SST results of the PDBZ signal and zoomed view of the defect area: (**a**) original signal; (**b**) zoomed signal of the defect area; (**c**) maximum curves of the defect area after CWT and SST; (**d**) CWT of the signal; (**e**) zoomed CWT of the defect area; (**f**) zoomed CWT-3D of the defect area; (**g**) SST of the signal; (**h**) zoomed SST of the defect area; and (**i**) zoomed SST-3D of the defect area. smooth and gradually decreases, but the SST result shows that there is an area with high brightness when the defect signal appears, indicating that the energy has converged in the defect area. The maximum curves of the defect area show that both CWT and SST can accurately determine the presence of general defects. However, when analyzing the blind zone defect signal, the maximum curve of CWT steadily decreases, while SST has more obvious convex areas. Additionally, the location of the defect can be clearly seen by SST. Therefore, SST is more suitable for detecting defects, especially blind zone defects, due to its excellent energy concentration ability. Another noteworthy point is that CWT transforms the entire signal, resulting in every time-frequency position having a value, where some values may be close to zero.