Design and implementation of a 3-axis coherent Doppler velocity profiler

Coherent Doppler velocity profiling is playing an increasing role in ocean scientific research. One area of application is near-bed sediment transport studies, as part of coastal erosion and shelf edge monitoring. Here, a coherent Doppler velocity profiling system designed at POL for sediment transport studies in the bottom 1 metre above the seabed, is discussed from a practical point of view. The system uses a central vertically downwards pointing transducer to transmit acoustic pulses into the water column. This transducer, plus two orthogonal receiving transducers, are used to record the acoustic return signal scattered from particulates in suspension. The system records coherent “in-phase” (I) and “quadrature-phase ” (Q) signal components of the detected return signal from which Doppler phase shift and hence velocity can be calculated. Additionally, acoustic backscatter signal strength can be calculated allowing measurement of the suspended particulate concentration. Profiles are formed by recording I and Q components from each transducer at co-located range bins in the water column above the sea-bed. Results from deployments and issues, for example “phase wrap around”, are discussed. To combat phase wrap around, a dual PRF technique is employed and is also discussed along with future developments planned.