Re: Oscillating low correlation regions

Posted by Julia Mullarney at Saturday 00:10

Hi,

I was just looking at some of our HR Aquadopp data and I noticed that there is a 'wave' of low correlations near the bottom.  You can see it in the attached figure (panel 3).  I also plotted pressure (minus an offset) in black on top so we can see that this band correlates pretty well with the surface waves (period of about 1s).  It appears in all three beams.  I was just curious as to what causes this - some sort of reflection?

Cheers,

Julia

Hi Julia,

Posted by Sven Nylund at Tuesday 15:03

The bands with reduced correlation and corresponding noisy velocities are caused by interference from the surface. Your HR Aquadopp has probably been looking up and the reflection of the first ping off the surface has interfered with the return signal of the second ping, in the bands you see in your data. To avoid this you must increase the margin between the pulse distance and the distance to the surface. You can do this for example in the standard menu by using the minimum distance to the surface as the "Distance to surface". This would then be the mean distance to the surface minus Hmax/2. To be on the safe side you must then take into account both the tidal effects as well has an assumption on how large the waves can get during the deployment.

Another approach is to filter out these bands by interpolating points with low correlation.

Cheers,

Sven