One or More? Physics-Guided Super-Resolution Decomposition of Underwater Multispectral LiDAR Echoes

Underwater multispectral LiDAR jointly captures range and spectral information, providing multidimensional observations for detailed underwater sensing. However, echoes from closely spaced targets can overlap within a single waveform, while water-induced temporal broadening further reduces peak separability. These effects make accurate target counting and physical spacing estimation challenging. To study this problem under realistic conditions, we develop an underwater multispectral LiDAR system and use it to acquire two real-world full-waveform datasets: a single-target reference library spanning target ranges, reflectances, and laser intensities, and a multiband multi-target library covering different degrees of echo overlap with physical spacing annotations. We propose Aquatic Super-Resolution (AquaSR), a physics-guided framework for underwater echo super-resolution and decomposition. AquaSR incorporates a backbone-agnostic prior-constrained joint multiband decomposition (PC-JMD) module. Its central principle is to share target count and physical spacing across bands while retaining band-specific echo shapes, amplitudes, first-target delays, and backgrounds. A frozen decoder pretrained on the single-target library provides a continuous single-echo shape prior that constrains component reconstruction. The module determines target count by comparing reliability-weighted reconstruction errors under single- and dual-target hypotheses and estimates shared spacing by fusing cross-band geometric evidence. Physical-attribute experts then refine the spacing using echo morphology and energy features, followed by component reconstruction. Experiments on measured data show that AquaSR outperforms representative classical waveform decomposition methods and deep learning baselines and recovers target range structure at sub-pulse-width scales. Experiments with multiple backbone configurations further demonstrate the cross-architecture compatibility of PC-JMD. These results support precise ranging and structural analysis of closely spaced underwater targets.