A coastal acoustic tomography experiment in the Sanmen Bay, China

Abstract

The coastal acoustic tomography (CAT) experiment was successfully carried out during September 6 to 9, 2009 in the Sanmen Bay near the Taizhou city, China. Seven sets of CAT systems were set up around the bay for mapping the tidal currents at 3-min interval. The 5 kHz sound, phase-modulated by the 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> order M sequence was transmitted from the broadband transducer. The M sequences with different codes were assigned for each of acoustic stations to resolve multi arrivals. During the CAT experiment, a downward looking acoustic Doppler current profiler (ADCP) was deployed from an anchored boat inside the tomography domain and collected the data for comparing with the CAT data. An inverse model is adopted to reconstruct the tidal current distributions using travel-time differences obtained from the reciprocal sound transmission between the paired CAT stations. The inverse results showed the strong northwestward tidal current entered into the tomography site through the two channels at the southeastern part of the bay during the ebb tide. On the other hand, the strong southeastward tidal current flowed back during the flood tide. The horizontal distributions of tidal current calculated by the inverse analysis were in good agreement with that of the ADCP velocity. The RMSD is 13.05 cm/s for the eastward current and 0.60 cm/s for the northward current, respectively. It is suggested that the CAT is a powerful instrument for continuously mapping the horizontal tidal current structures in coastal regions in China.