Secondary reflection... or not?

Hello,

a colleague and I are of differing opinions and I'm hoping someone can settle our disagreement!

we have received ADP data from a deploment in ~11m depth, from a device configured for ~23m - is there any way the 'dry' depth cells can have influenced the data in the 'wet' cells? The speed information seems reasonable but direction is fairly random.

best wishes, Jeremy

Hello Jeremy,

What you call the "dry" depth cells should not cause any problems with your data from the region within the water column. The NDP knows the speed of sound in the water from the measured temperature and the salinity set by the user, and it uses this to calculate from where in the water column the return signals are coming. Hence, when the sensor is set up to measure with 23 meters range it will not "see" what happens beyond 11 meters until it has finished receiving from the first 11 meters. In other words, using an instrument set up for 23 meters range in 11 meters of water does not damage the data within the water column. If your directional information appears to be random it must be for other reasons. If you need help to find out why you can send us the data file, and we will take a look at it.

Regarding valid regions of the water column you probably know that due to side-lobe effects the last 10% or so of the water column must be used with care. In your case this means that the last meter before the surface may be contaminated. This is a generic problem common to all acoustic current profilers.

Best regards,
Ketil Horn

Hi

Please note that the current direction tends to be a little unsteady in low flow regimes because of the Doppler noise.  When you calculate direction:

Direction = atan (Vy+Vnoise/Ux+Vnoise),

the results can be a little erratic if Ux, Vy, and the noise are all about the same size.  As the current increases, the noise term becomes irrelevant (Vx,Vy>>Vnoise) and the direction estimate becomes more stable.