## 1 Introduction
For damages that are relatively small in scale within the structure, under the point scatterer approximation, the Decomposition of the Time Reversal Operator (DORT) methods that combines multiple signal classification (MUSIC) [20; 30; 31; 32; 33; 34; 35] performs eigenvalue decomposition of the TROs to obtain the eigenvectors of the incident and scattered signals as well as the intensity (corresponding to the eigenvalues) of each scattering source in the measured structure. This allows for selective focusing of the back-propagated wave field at each scattering source without considering the effects of the structure's inhomogeneity and dispersion characteristics on the wave field. [PERSON] and [PERSON]'s teams discussed the applicability of the DORT method under different scenarios, including the separation of scattering sources and the consideration of the Born approximation scattering model under weak secondary scattering [30] as well as closely spaced damages and multiple scattering effects [35]. The results indicated that while the DORT method could selectively invert and focus the wave field beams at each scattering source, its imaging accuracy did not achieve a significant improvement compared to ultrasound phased array methods. To address this, [PERSON] [36] introduced the MUSIC imaging condition, constructing a pseudo-spectral function as an imaging index based on the orthogonality between the signal subspace and the noise subspace after eigenvalue decomposition to achieve super-resolution imaging in the presence of multiple scatterers within the measured structure. [PERSON] [19] introduced the Forward-Propagation-Free Focusing MUSIC (FPF-F MUSIC) algorithm, which leverages virtual TR to compensate for dispersion during back propagation, ensuring phase coherence across signals without additional repro propagation steps. Existing studies have shown that applying the DORT-MUSIC method can utilize the known Green's function for guided wave propagation in the structure (which includes dispersion relationships) to solve for the spatial locations of scattering sources, with imaging accuracy unaffected by the guided wave dispersion relationship. Furthermore, in the context of imaging multiple damages, it can achieve detection resolutions superior to conventional array imaging methods. However, related studies have not revealed the physical mechanisms behind super-resolution imaging in an acoustic context. Building on this, [PERSON] [28] explored the limits imposed by the diffraction limit on scattering source imaging under the Born approximation and proposed that the process in which evanescent waves with high wave numbers transform into propagating waves during multiple scattering carries super-resolution information about the damage, thus providing a basis for the capability of super-resolution imaging. Subsequent research indicated that, compared to the DAS, the DORT-MUSIC method has the advantage of high-resolution imaging but is also more susceptible to noise [37; 38]. Therefore, extracting characteristic information about damage scattering from complex, overlapping signals in the presence of environmental noise is a key issue in ensuring the imaging quality of the DORT-MUSIC method. It is evident that, compared to the DAS, which only inversely reconstructs the paths of guided wave propagation and reflection, the DORT-MUSIC method directly performs inverse calculations of the guided wave field, retrieves damage scattering information from the received signals, and avoids the impact of guided wave dispersion effects on imaging results. Therefore, it often offers a significant advantage in imaging accuracy. This study provides a systematic comparison of time-domain DAS, frequency-domain DAS, and DORT-MUSIC for ultrasonic guided wave damage imaging, explicitly quantifying resolution limits (down to 0.79A) under experimental noise conditions.