Drawdown of the ADCP rig
A customer recently asked the following about measuring waves from a Signature500:
Is it very important that the distance between the instrument head and the seabed (mounting height) is very accurate? We do not have very accurate bottom measurements, besides that there has been a little pull-down in the rig, so the instrument depth has varied somewhat? I’m not entirely sure if I understood quite how to know, in general, if there is too much drawdown to trust the data.
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Official comment
When it comes to measuring waves from a subsurface buoy, the draw down is of course an issue. The drag from the mean background current will pull down the buoy and therefore we will assume first of all that this is more or less in static equilibrium. The primary concern is that the alitimeter is not out of range of the ocean surface.
The more troubling type of draw down or displacement is when the buoy oscillates. This can occur if wave-induced currents apply a force on the buoy. The buoys motion can create perceived current profile that would appear to be wave signal. You can pretty much see if it is present by evaluating the Direction Spectrogram. If there is an oscillation present, then you will see a constant band in the directional spectrogram that has a different direction from the neighboring frequecies - a constant and inconsistent direction. Bouy oscillations and the consequences are discussed in the following conference proceeding:
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