Three-Dimensional Current And Scattering Strength Distribution Mapping System

It is very important to observe the current structure when making the oceanographic investigations. We have developed a current and scattering strength distribution mapping system. This system consists of an Acoustic Doppler Current Meter, a ship position Measuring Unit and a Mapping and Recording Unit. This system functions to map the distribution of the current and scattering strength, its profile and its time variations on real-time basis on the ship. Our basic experiment we have made in advance has shown that the fluctuation of the doppler information available from the backscattering object is larger than that of the reflected signal from the sea bottom and we have confirmed that a long averaging time is required to estimate the current velocity, compared with that for the bottom track speed estimation. This system can estimate the mean value of the current velocity in 60 seconds in average with a precision of 5cm/s by controlling the transmission repetition period of the acoustic wave within the range from 0.3 sec. to 1.5 sec. and increasing the doppler signal data. The experiment was made in November and December 1982 in Hario Seto in Kyushu, Japan. The strait showed a complicated current state because the tidal current flowed from the narrow strait to the bay like a jet-stream; the maximum speed of 9.6 knots was observed. In the measurement of the vertical section at the outlet of the narrow strait a 36m wide jet stream was observed up to 30m depth of the strait. In the measurement of the horizontal distribution at the bay, the main stream and eddy were clearly observed. Further, our experiment made clear that there was a relation between the current velocity distribution and the scattering strength distribution. As described above, the system that we have developed can easily observe the structure of the current distribution in a coastal water area. This paper will describe the system design and experimental results.