Applicantion of the Acoustic Tomography to Velocity Measurement in the Coastal Seas
H. Zheng, N. Gohda, H.Yamaoka, A. Kaneko (Faculty of Engineering, Hiroshima University, Japan)
E-mail : hzheng@ocean.hiroshima-u.ac.jp, ngohda@ipc.hiroshima-u.ac.jp, akaneko@ipc.hiroshima-u.ac.jp, and
T. Takano (Asia Air Survey Co.,Ltd,Japan)
E-mail : ttamu@ari.ajiko.co.jp
ABSTRACT
The multi-station acoustic tomography system composed of five acoustic tranceivers has been constructed for velocity measurement in the coastal seas. This is a moored-type tomography system, which can be applied to measure vortex-included current fields. In this system, all clock signals are synchronized by the GPS clock pulse signals through PLL (phase locked loop). When the GPS signals are unavailable, it is replaced by the high-accuracy time base module (Seascan Inc.) of the long-term stability of which is 5×10-8. The maximum station number of sixteen can be processed for this system. The CDMA (code-division multiple-access) method is applied to identify signals from individual stations with the aid of a set of 10th-order Gold sequence. Every station is desrgned to be equipped with at least one hydrophone and one DSP (digit signal processor). The major role of DSP is to calculate instantly the correlation function of received signals. The number of hydrophone and DSP can be increased up to four, depending on the number of the station.
A full tomography experiment to measure tidal vortices generated in the Seto Inland Sea, Japan will be carried out during Feb. 16-18, 1999.
INTRODUCTION
Since 1994, our group has researched and developed a coastal acoustic tomography system (CATS). The purposes of this research are to develop the CATS for velocity measurement and realize a long-term velocity field measurement in the coastal ocean.
During 1994 to 1997, a pair of velocity tomography system was constructed and its performance was certified through five field experiments with ranges from 2 to 20 km [1][2]. This system was of the direct reading type in which all parts except a transmitter and hydrophones were put onboard the ship. In 1997, the design and construction of a multi-station system of moored type started. Modern technologies related to electronics, acoustics and satellite are applied to this new system. A demonstration experiment will be performed in the Neko Seto Channel of the Seto Inland Sea during Feb. 16-18, 1999, using five sets of the system.
