Ship-induced effects on bottom-mounted acoustic current meters in shallow seas

The echo-amplitude of a 23-m-deep bottom-mounted acoustic Doppler current profiler (ADCP) shows regular spikes up to 30 dB above background level when a ship passes nearby, due to deep penetration of bubble clouds. This is evidenced from regularly occurring spikes in echo-data that are simultaneous with ferry crossings in a narrow sea-strait. The bubbles can nearly reach the bottom and are comparable in magnitude to near-bottom scattering off suspended material in vigorous tidal currents exceeding 1 m s−1 in magnitude. The bubble clouds mask the sea surface from the echo-amplitude, which hampers the use of an ADCP for estimating atmospheric parameters and near-surface currents, under such conditions. The echo-spikes associated with the ferry are confirmed with coinciding dips in bottom pressure up to 1200 N m−2 and with deviations up to 10° in the ADCP's heading due to pressure waves and magnetic field disturbances from under the ferry and from its rear, respectively.