A Note on the Depth of Sidelobe Contamination in Acoustic Doppler Current Profiles

Acoustic Doppler current profilers (ADCP) do not provide reliable water velocity measurements near the sea surface or bottom because acoustic sidelobe reflections from the boundary contaminate the Doppler velocity measurements. The apparent depth of the center of the sidelobe reflection is z(sl) = h(a)[1-2 cos(theta)], where h(a) is the distance from the ADCP acoustic head to the sea surface and theta is the ADCP beam angle. However, sidelobe contamination extends one and a half ADCP bins below zsl as the range gating of the acoustic return causes overlap between adjacent ADCP bins. Consequently, the contaminated region z < z(sl) +3 Delta z/2 is deeper than traditionally suggested, with a dependence on bin size Delta z. Direct observations confirming both the center depth of the sidelobe reflection and the depth of contamination are presented for six bottom-mounted, upward-looking ADCPs. The sidelobe reflection is isolated by considering periods of weak wind stresses when the sea surface is smooth and there is nearly perfect reflection of the main beams away from the ADCP and hence little acoustic return from the main beams to the ADCP.