ambigous Current Profiles measured with a 2MHz HR ADCP

Hello ,

I am using a 2MHz Aquadopp HR current profiler downward looking mounted 50cm from the bottom of sandy/silty sediment in the mouth of an estuary. The settings for the measurement are stated below:

Measurement interval  (s) : 120
Cell size             (m) : 0.02
Orientation               : DOWNLOOKING
Distance to botttom   (m) : 0.50
Pulse distance        (m) : 0.60
Profile range         (m) : 0.48
Horiz. vel. range   (m/s) : 1.54
Vert. vel. range    (m/s) : 0.65
Number of cells           : 24
Average interval      (s) : 30
Blanking distance     (m) : 0.10
Measurement load      (%) : 47
Reduced power first ping  :
Samples per burst         : N/A
Sampling rate        (Hz) : N/A
Compass upd. rate     (s) : 30
Coordinate System         : XYZ
Speed of sound      (m/s) : MEASURED
Salinity            (ppt) : 35
Analog input 1            : NONE
Analog input 2            : NONE
Analog input power out    : DISABLED
File wrapping             : OFF
Serial output/TellTale    : OFF

My first question concerns the blanking distance. It is automatically calculated and as shown above stated to be 0.10m. But if I calculate the cell size and distances from bottom: 24cells*0.02m cell size=0.48m, which would only leave me with a blanking distance of 0.02m.

Does that mean that the cells from 0.48m until 0.40m should be excluded from the analysis or have a high probability of being biased?

My second question concerns the measured current velocity profile. As visible below(see attachment) does the profile seem to behave normal until it reaches 0.06m distance from bottom,(where it reaches zero)but then it seems that velocity values are increasing again, which I am not sure how to interpret. The esutary I am working on has a very high sediment load, could that be the reason for an influenced near bottom signal or is it possible that a disolved sediment layer near the bottom influences the signal resulting in these measurements??

Did you ever notice this phenomen?

Thanks in advance and I am looking forward to your reply,

Christian Schwarz

I think there is a bug in the header information. We had  another question recently on cell position and noticed there were two different values reported for the location of range cells.

Dear Mr. Russello,

Thank you for your fast reply.

I now increased my filter threshold for amplitude values which resulted in a much better profile(the data at the bottom below the zero values is now removed). Thank you for your help.

Can I further assume, that as soon as the amplitude signal goes below a certain threshold(in my case around 150), this is the region of my bottom and I can therefore calculate the profile based on this point, or is it more straight forward to calculate it using the blanking distance from above?

Thank you very mucgh for your help so far.

Regards,

Christian

Cell locations can be calculated using the blanking distance and cell size. What I have done in the past is look for an amplitude peak and assumed the bottom location corresponds directly to this peak location. Since I know which way the instrument is pointing, I remove cells past the bottom.

I'd be hesitant to use an amplitude threshold because this will vary at least a little from site to site based on environmental conditions and probably the specific instrument used, and there is always the possibility of cells within the profile being accidentally marked as bad.

Either one will work, the amplitude threshold just seems like it will be a little more problematic.

P.J.

Dear Mr. Russello,

I also used the method with the amplitude peak to find the bottom and it worked quite well.

Thank you very much for your help