Time-reversal acoustic tomography for coastal current monitoring: Theory and initial field validation

This study presents a novel time-reversal coastal acoustic tomography (TR-CAT) system enabling real-time current estimation without instrument recovery. The system employs paired transceivers: a regular station transmitting BPSK-modulated m-sequences (carrier frequency 17.857 kHz, bandwidth 4.5 kHz) and a TR station that time-reverses and retransmits received signals. Theoretical analysis establishes equivalence between time-reversed arrival patterns and conventional cross-correlation functions under weakly reciprocal channel conditions. Field experiments at Bachimen Harbor, Taiwan validated performance under stationary–stationary and stationary–mobile configurations. During 11.5-h static deployments, TR-derived differential travel times (DTTs) demonstrated consistency with conventional cross-correlation results throughout tidal cycles. For mobile deployments with vessel drift up to 0.5 m/s, Doppler compensation (approximately 6 Hz shifts) was achieved through multi-Doppler matched filtering of probe signals, maintaining accuracy despite bidirectional propagation effects. Results demonstrated effective tracking of tidal-induced DTT variations (–0.5 to 0.3 ms) over continuous deployments of 9.5 h. By eliminating the operational complexity of instrument recovery while preserving measurement accuracy, the TR-CAT system offers advantages for autonomous coastal monitoring applications.