The first Chinese coastal acoustic tomography (CAT) experiment for mapping the tidal currents in Zhitouyang Bay near Zhoushan Island was successfully performed with seven acoustic stations from July 12 to 13, 2009. Using CAT, the horizontal distributions of the tidal currents in the tomography domain were calculated by the inverse analysis, in which the travel time differences for sound traveling reciprocally between the station pairs are used as data. The specified tidal current structures, such as the strong east-west oscillation of the tidal current, the branched current, and the tidal vortices, were reconstructed as snapshots at the successive tidal phases. The relative vorticity calculated from the inverted current fields served to specify the current structures, such as tidal vortices. The inversion-estimated uncertainty of (0.02-0.08) m s^-1 narrowed the root-mean-square difference (RMSD) of (0.00-0.11) m s^-1 between the 3-min interval original data and the hourly mean data for all the sampled data, which may be regarded as a measure of error. Throughout the tidal phases, the divergence from the inverted current showed a positive (negative) distribution in the shallow (deep) region as an overall view. However, the divergence for the entire tomography domain was nearly equal to zero, corresponding to no net transport. This result implies that the observational errors are quite small for the present experiment. This experiment reaffirms that coastal acoustic tomography is an accurate and efficient observational method for continuously mapping tidal current structures in coastal regions that are characterized by heavy shipping traffic and active fishing.

The subsurface structures of internal solitary waves were observed over two days from 27 to 29 February 2019, by two bottom‐moored coastal acoustic tomography systems over a path length of 18.286 km in the Lombok Strait, Indonesia. One‐way travel times along the transmission path between the acoustic stations were determined for the first three arrival peaks. The resulting three travel times were used to execute a four‐layer (0–50, 150–300, 300–450, and 400–603 m) temperature inversion, constructing an underdetermined problem. The inverted four‐layer temperatures show three positive peaks and two negative peaks during the observation period, implying the sequential passage of internal solitary waves with depressed and elevated interfaces. The generation of these temperature peaks is synchronized with the diurnal tides, and the peak heights decrease with depth. The inverted four‐layer temperatures are in excellent agreement with conductivity‐temperature‐depth data obtained near the acoustic stations.

This study investigates techniques for the acoustic mapping of ocean currents in a shallow-water environment using moored and ship-towed stations. The currents are estimated using the differential travel times (DTTs) observed in the reciprocal acoustic transmissions between those stations. Due to the relative motion induced by the ship-towed station, the Doppler shift estimated from the measured channel impulse response is used to compensate for the arrival patterns and to correct the influence of the relative speed on the DTTs. Furthermore, to estimate the DTTs from the arrival patterns received in the reciprocal directions, a method is devised which involves the time-evolving cross-correlation function of the reciprocal arrival patterns. A feasibility test was carried out at Sizhiwan Marine Test Field, Kaohsiung, Taiwan in September of 2015. The currents were estimated using the data collected from one ship-towed and four moored stations. The estimated currents are consistent with the direct measurements from a shipboard acoustic Doppler current profiler and reveal the spatial distribution of the currents.

Ocean current profiling using ocean acoustic tomography (OAT) was conducted in the Kuroshio Current southeast of Taiwan from August 20 to September 15, 2009. Sound pulses were transmitted reciprocally between two acoustic stations placed near the underwater sound channel axis and separated by 48 km. Based on the result of ray simulation, the received signals are divided into multiple ray groups because it is difficult to resolve the ray arrivals for individual rays. The average differential travel times from these ray groups are used to reconstruct the vertical profiles of currents. The currents are estimated with respect to the deepest water layer via two methods: An explicit solution and an inversion with regularization. The strong currents were confined to the upper 200 m and rapidly weakened toward 500 m in depth. Both methods give similar results and are consistent with shipboard acoustic Doppler current profiler results in the upper 150 m. The observed temporal variation demonstrates a similar trend to the prediction from the Hybrid Coordinate Ocean Model.