Analysis of ADCP data above a bottom observatory

A 300-kHz ADCP was set on GEOSTAR, a six-m3 deep-sea observatory. It was operated with cells of 80 cm during a three-week test experiment at 42-m water depth in the northern Adriatic sub-basin. Although it provided valuable data about the horizontal current field over most of the water column, it also allowed specifying the wake disturbances induced by the observatory. These disturbances are characterised by vertical velocities that are significant up to ~20 m above seafloor (echo intensity data suggest that the wake can even reach the surface), and by inclinations of the bottom nepheloïd layer (as deduced from differences in echo intensities from beam to beam). Our analysis is validated by consistent relationships between the horizontal current direction and speed on one side and the characteristics of both dynamical (vertical velocity) and non-dynamical (echo intensity) parameters on the other side. It is in good agreement with the simulations from a numerical model, and hence specifies the sensitivity (especially with respect to echo intensity) and accuracy of an instrument usually operated within fields of current and scatterers not disturbed by the device supporting it. In addition, the error velocity parameter displays specific characteristics that easily allow specifying the thickness of the layer disturbed by the observatory, thus providing a technique to validate the quality of data acquired in similar conditions