Re: Poor quality 2MHz Aquadopp profiler data

Hi,

I have just carried out a brief test deployment of a brand new 2MHz Aquadopp profiler in the surf and shoaling wave zone of sandy beach and am a little worried by the apparent poor quality of the data that has been recorded.

The instrument was deployed upward-looking on a bed-frame and programmed with: Profile interval = 30 s; Average interval = 15 s; Cell size = 0.1 m; Blanking distance = 0.1 m; and 96 cells.

The attached figure plots the Amplitude and Velocity data from each of the 3 beams (in XYZ mode) for a single burst when the instrument was in 4.5 m of water (outer surf/shoaling wave zone) and is typical of the entire dataset. Looking at the Amplitude, there are good acoustic returns from the lower 1.5 m of the profile, before the signal decays towards the noise floor of the instrument, and then a return from the water surface at 4-5 m elevation. Being in shallow and relatively energetic flow (1 m waves) I would have expected plenty of scatterers in the water column and no decay to the noise floor before the surface return?

Moving on to the Velocity data, there appears to be no structure to the signal from any of the three beams, which is particularly worrying in the region 0.5-1.5 m in the profile when the Amplitude is high.

Two questions that spring to mind are whether the instrument is sampling rapidly enough (the .hdr file reports 16 pings per burst) and whether the profile/average intervals are sufficiently long to resolve the signals. Assuming a wave period of ~7 s, we should be sampling over 3 wave periods so should observe some vertical structure in the water column, particularly since independent measurements with an electromagnetic current meter located several metres away observe consistent mean currents in the X and Y directions of around 10 cm/s.

I'd be grateful to hear any thoughts on this issue.

With kind regards

Martin Austin 

I have also uploaded the .hdr file containing the full instrument deployment settings

Hi Jonas,

Thanks for the rapid reply. I have uploaded the .prf file to the ftp server.

The thought of sand on the transducers might be a good one. I know that the instrument rig was moving around during the deployment and actually flipped over just before the end of the deployment. Initially the transducer face was 18 cm above the bed, but it might have settled into the bed somewhat. We are currently re-desiging the instrument rigs to prevent this.

Best wishes

Martin

Hi Martin,

Most definitely sand. The shape of the amplitude signal looks very much like other sets I've seen where sand is covering the transducers. Your description of the site and flow supports this.

Just a tiny layer of sand may kill the acoustic signal.

Best regards

Jonas

Hi Jonas,

Many thanks for taking a look at this issue for me. Hopefully the re-designed rigs can alleviate the problem.

Best wishes

Martin

Hi Martin,

Not a problem at all - this is what we are here for.

Good luck with future deployments.

Best regards

Jonas