The first Chinese coastal acoustic tomography experiment

The first Chinese coastal acoustic tomography (CAT) experiment was successfully carried out during July 12 to 13, 2009 with seven acoustic stations for mapping the tidal currents in the Zhitouyang Bay near the Zhoushan Island. The CAT results showed the strong eastward tidal current which entered into the tomography site from the Loutou Channel at the western part of the bay, and separated into two branches, flowing into the Laoshumen Channel, Qingzimeng Channel and Fodu Channel during the ebb tide. On the other hand, the strong westward tidal current, joined with the currents from Laoshumen Channel, Qingzimeng Channel and Fodu Channel, went back to the Loutou Channel during the flood tide. A clockwise tidal vortex of diameter about 5 km was generated in the eastern part of the bay in the transition phase from the ebb to flood. No tidal vortices were generated in the weak tidal currents in the transition phase from the flood to ebb, changing the current direction from westward to eastward. The range-averaged current obtained along the transmission lines after the inverse analysis was in agreement with the acoustic Doppler current profiler (ADCP) results, producing the root-mean-square difference (RMSD) of 1.77 cm/s. The horizontal distributions of tidal current calculated by the inverse analysis were in rough agreement with that of the shipboard ADCP velocity. The RMSD is 11.70 cm/s for the eastward current and 16.50 cm/s for the northward current. It is suggested that the CAT is a powerful instrument for continuously mapping the horizontal tidal current structures in coastal regions in China with heavy shipping traffic and active fishing.