## 3 Results and Discussion
### Experiment Results  
Based on the experimental data from the 11 constructed PZT unit drives and 11 sensing points using SLDV sensing, frequency-domain DAS and DORT-MUSIC methods were employed to image dual damages located at different distances with a radius of 2.5 mm. Since the out-of-plane velocity signals from SLDV sensing are primarily composed of \(A_{0}\) mode guided wave components, the imaging focused mainly on the \(A_{0}\) mode guided wave components. The results are shown in Figures 13-16, with a comparison of the results presented in Table 3.  
Figure 13: Imaging results and corresponding indicator profile from experimental data with two damages separated by 20 mm. (**a**,**c**) Frequency-domain DAS. (**b**,**d**) DORT-MUSIC. The color scale represents normalized intensity, and the red circles and lines indicate damage edges. Spatial coordinates are normalized relative to the wavelength (\(\lambda\)). Figure 14: Imaging results and corresponding indicator profile from experimental data with two damages separated by 12 mm. (**a**,**c**) Frequency-domain DAS. (**b**,**d**) DORT-MUSIC. The color scale represents normalized intensity, and the red circles and lines indicate damage edges. Spatial coordinates are normalized relative to the wavelength (\(\lambda\)). Figure 15: Imaging results and corresponding indicator profile from experimental data with two damages separated by 10 mm. (**a**,**c**) Frequency-domain DAS. (**b**,**d**) DORT-MUSIC. The color scale represents normalized intensity, and the red circles and lines indicate damage edges. Spatial coordinates are normalized relative to the wavelength (\(\lambda\)). Our results demonstrate distinct performance characteristics between the imaging methods under experimental noise conditions. While numerical simulations have shown that under the condition of dual damage, the time-domain DAS cannot clearly distinguish the presence of dual damage, both frequency-domain DAS and DORT-MUSIC achieve superior results by incorporating dispersion relations for solving. The frequency-domain DAS still maintains a high positioning accuracy, while the maximum deviation in the relative array radial (y-direction) positioning is 4 mm. When the damage distance is relatively far, the peak-to-center intensity difference index is less affected by experimental measurement data noise. However, due to the relatively weak scattering from artificial damage, it was difficult to distinguish the presence of dual damage in the experimental measurements for the case of d = 6 mm, resulting in a decrease in resolution compared to numerical calculations. Figure 16: Imaging results and corresponding indicator profile from experimental data with two damages separated by 6 mm. (**a**,**c**) Frequency-domain DAS. (**b**,**d**) DORT-MUSIC. The color scale represents normalized intensity, and the red circles and lines indicate damage edges. Spatial coordinates are normalized relative to the wavelength (\(\lambda\)). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Damage Center** & \multicolumn{2}{c}{**Damage Localization**} & \multicolumn{2}{c}{**Peak-to-Center Intensity**} & \multicolumn{2}{c}{**Array Performance**} \\
\begin{tabular}{c} **DORT-** \\ **MUSIC** \\ \end{tabular} \\ \hline
20 &
\begin{tabular}{c} (0, 0) \\ (0, 1) \\ (0, 0) \\ (\(-\)0.6, 1.2) \\ (0, 0) \\ (\(-\)0.4, 0) \\ (\(-\)0.2, 0.8) \\ (\(0, -\)1) \\ (\(0, -\)1) \\ (\(0, -\)2, 0.6) \\ NA \\ (\(-\)0.2, 0.8) \\ \end{tabular} & \(-26.2\) & \(-8.9\) & \(1.30\) & \(1.17\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of experimental imaging results for dual damage at different distances. In contrast, DORT-MUSIC shows remarkable super-resolution capability, resolving 6 mm spacings in simulations (\(\tau=-9.2\) dB). The super-resolution capability stems from the interaction of evanescent waves' high wave number components localized near the damage with propagating waves during multiple scattering events, as theorized by [PERSON] [28].