Question regarding Errors and trustworthiness of measured Waves

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Ellie Best

• June 19, 2020 11:59
• Edited

We have received the following query from an AWAC with AST user:

We are having some mooring problems. The device pitches and rolls due to the tidal current. It then exceeds the +-5° limit for pitch and roll, which is described inside the device manual. During heavy weather the device can reach pitch and roll levels above 20°.

May I send you the measured data and can you have a look at it? I need to know how trustworthy the measured data is. Can you give us rating of our moorings quality?

There is [also] a bigger storm inside the measurements (07.02. – 12.02.). During that storm it happens quite often, that the pressure sensor is detecting higher waves than the AST.

How can this be? Shall I trust the pressure Sensor more than the AST in this case?

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• Official comment

  

  Torstein Pedersen

  • June 26, 2020 06:26

  I had a closer look at this data and the bottom line is that the data looks to be very good - given the buoy motion that you have experienced. 

  The issue with tilt is that something that effects different measurements differently.  For example, the individual beams used to estimate current profiles will be subject to different measurement locations in the vertical, which is less than ideal if we assume the location and ensonified volume to be similar for all beams.  Another ill effect is that the AST beam becomes less orthogonal to the surface with increased tilt and consequently can lead to worse detection of the surface.  The tilt limits that you refer, should be treated as best practice.  I believe the +-5 degree tilt is suggested as a deployment goal, and would have a mild effect on the measurements.  The wave processing has a limit where a warning is issued and this is 10 degrees - but it does not mean the data is poor - it reflects a limit where we have historically seen more challenges for consistently good AST performance.

  Look at the data.  This is the key to determining if it is sensible and good. 

  Again your data is really quite good. Below is a screen shot from the Storm software.  The AST time series shows the "despiked" version in red and blue is the raw version.  This time series comes from the maximum tilt (10 deg Pitch, 11 deg roll - which by the way is well under 20 degrees total tilt, it is not additive).  The number of "bad detects" is less than 1%, and very good.

  The pressure spectrum can sometimes yield overestimated spectra.  This is because the process of defining a point to cut off the spectrum is difficult - particularly when the waves are short and small.  You see in the plot below we seem to "catch" the pressure spectral peak and extrapolate before it increases wildly.  But this is not always the case.  I should also mention that it is difficult to use pressure consistently from a subsurface buoy since the buoy motion can potentially modulate the pressure signal also. 

  -Torstein

   

  

   

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