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

A high resolution current profiler, using pulse to pulse coherent acoustic Doppler techniques, has been developed primarily for the measurement of turbulent flow in the bottom 1.5 m of water above the sea bed, with suspended sediment concentrations up to 10 grams/litre. The instrument transmits pulses of sound modulated at 524 kHz and flow is measured in 32 range gated bins of 46 mm length. The instrument has been deployed in two contrasting field locations. In both situations the transducer was mounted 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 measured velocity is ±0.37 m/s. Typically the echoes from 128 pulses are processed, giving velocity measurements in each bin every 0.25 s. Results show the noise level to be of the order of ±5 mm/s with this sampling rate. Simple de-aliasing and tracking methods allow radial velocities along the acoustic beam of up to 1 m/s to be measured when deployed so that the sea bed occurs in one of the range bins