Non-contact acoustic–vibration identification and spatial delineation of multiple shallow subsurface delaminations in tunnel linings

Subsurface delamination in tunnel linings can reduce local stiffness and promote cracking and spalling, threatening structural integrity and operational safety. Existing acoustic–vibration studies have mainly examined single delaminations or isolated measurement points, leaving the spatial behavior of multiple coexisting delaminations insufficiently understood. This study develops a non-contact acoustic–vibration detection framework integrating directional acoustic excitation and laser Doppler vibrometry with vibration energy ratio (VER)-based grid scanning, supported by three-dimensional acoustic–structure coupled simulations. Laboratory tests were conducted on two C30 concrete lining specimens comprising 12 experimental cases to investigate spectral characteristics, measurement-location effects, and interactions among adjacent delaminations. Delaminated regions exhibited distinct narrowband resonance peaks at 1361–3224 Hz, primarily associated with local flexural modes of the overlying concrete layer. Responses were strongest above delamination centers and attenuated toward the boundaries, while higher-order and asymmetric modes contributed to multi-peak spectra. Under areal acoustic excitation, multiple delaminations were activated simultaneously, but their dominant responses remained largely localized, indicating limited interference within the tested configurations. VER mapping converted localized responses into spatial anomaly maps, enabling delamination localization and boundary delineation. With VER ≥ 10 dB as the high-anomaly threshold, cases with a delamination depth H ≤ 30 mm and a lateral dimension A ≥ 200 mm generally achieved anomaly coverage ratios above 70%. These results demonstrate the feasibility of extending non-contact acoustic–vibration detection from isolated pointwise identification to spatial characterization of multiple subsurface delamination regions.