Continental with excessive velocity readings

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Continental with excessive velocity readings

Posted by Tom Van Engeland at May 10. 2017

Hi,

I'm currently working on data from a cabled observatory that is equipped with a Nortek Continental (190kHz). The device is installed in a frame on the bottom (depth ~ 260m) and looks up to the surface. It registers data in ENU coordinates.

The problem: At distances beyond 100m from the transducer, it gives unrealistically high velocities (on the order of 4-6m/s) that are all westward and upward. These velocities are tied to the lowest backscatter values (which are still above the theoretical required minimum that I found on the forum somewhere) in a non-linear way.

After transformation from ENU to beam coords, it became clear that beam 1 was responsible for this, although beam 3 showed a similar pattern but much weaker. For the beam with the strongest effect, observations below 120 counts (backscatter) increased quickly from 0.1 to 2 m/s as the intensity decreases to, say, 90 counts (cf. attached graph). Closer than 100 m from the transducer these relationships between backscatter and velocity do not exist.

 

An aquadopp 600kHz on the same frame shows very strong downward velocities at distances beyond 20 m from the transducer (40 meter total range). Here, the low intensities cause high noise in the data, which is not that odd, but again one of the beams is responsible for the downward velocities.

I understand why low backscatter could be noisy data. I do not understand where this bias comes from.

My hypothesis about the cause:

* partial blocking of the relevant beams by the frame (difficult to see in fotographs from the frame;  no info on deployment, other than the docs on the website)

* transitions between water masses cause the quite sharp threshold around 100m from the transducer.

        spring mixing: freshwater from runoff is mixed downward into atlantic water near the bottom. The non-linear behaviour also appears deeper (closer to the transducer).

        summer stratification:   effect only beyond 100 m from the transducer and stable.

 

Has anyone ever seen such a phenomenon? Any similar experience with a Continental or Aquadopp? Any ideas about potential causes?

 

Thanks,

Tom

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