A high resolution near bed coherent acoustic Doppler current profiler for measurement of turbulent flow

The design and performance of a prototype high resolution current profiler, using coherent acoustic Doppler techniques for measurement of turbulent flow in the bottom 1.5 metres of water above the sea bed, is discussed. The instrument transmits at 524 kHz and flow is measured in 32 range gated bins of 46 mm length, using pulse to pulse coherent-acoustic Doppler techniques. The instrument has been tested in a laboratory test tank, and in the sea off the leg of a pier. In both situations the transducer was deployed looking downwards, and cable linked to the transmit/receive electronics and thence to the data processing computer. Using the present system of uncoded pulses, the maximum unambiguous detectable velocity is +or-0.37 metres/sec. Typically, results from 128 pulses are averaged, giving a velocity reading in each bin every 0.25 sec. Early results show the minimum measurable averaged velocity to be 5 mm/sec, and the noise level to be +or-5 mm/sec. Simple anti-aliasing and tracking methods should allow measurement of velocities up to 1 metre/second. These methods are readily implemented in the vertical, or near vertical configuration.<>