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Drawdown of the ADCP rig

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    Torstein Pedersen

    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:

    https://support.nortekgroup.com/hc/en-us/articles/360013110160-Subsurface-wave-measurements-taken-to-new-depths-2009-

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    Atle Lohrmann

    Are you thinking about a variable draw-down during the data collection period or is the draw-down on a longer time scale so that the depth is constant for the time it takes to collect wave data?

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