Mapping Tidal Current Structures in Zhitouyang Bay, China, Using Coastal Acoustic Tomography

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.