The effects of mobile scatterers on the quality of ADCP data in differing marine environments

Doppler Current Profilers are now regularly used to measure current profiles in waters from the shallowest coastal region to the most extreme deepwater site. Various different frequencies of instrument are employed to cover vertical bin depths of a few centimetres to many tens of metres. The common factor between them all is that they rely on particles (scatterers) within the water column to reflect back the acoustic signal transmitted by the instrument. In order for the technique to function, these scatterers must be assumed to drift with the ocean flow. Current velocity data have been collected in a number of geographical areas where these scatterers appear to move, independent of the current flow. Several different effects have been identified, the commonest being the diurnal vertical migration of plankton and grazers, which can be characterised by a high vertical velocity at dawn and dusk. At times, this situation is further complicated by animals that appear to swim with a significant horizontal component to their velocity against or at least non-coincident with the main current flow. This can be seen as troughs in the horizontal current speed trace from the ADCP at the same times each day. While the diurnal transports of plankton and nekton are well known, the paper focuses on the current velocity data. Examples are shown of the main types of effect on data sets recorded in some of the major deepwater exploration regions of the world and looks at methods of removing the erroneous data from the real current velocity.