Finding bed with downlooking aquadopp

Hi there. 

I'm trying to figure out how high a downlooking aquadopp was deployed above the bed.  

The high backscatter from the bed was loud & clear.  I took the bin of highest backscatter as the bed location (rangebins were 1.5cm, using HR instrument), but when I tested this approach against direct measurements (from tape measures), I found that the backscatter-estimated elevation was a few cm too high.  I'm trying to figure out why.  

A few dumb questions about things I might have messed-up:

Is range measured from the flat triangle of plastic between the three beams?

Does the z distance to the center of 1st rangebin equal     (blanking distance)+(rangebin size)/2?

Is the point of highest backscatter usually slightly below the bed?

Thanks very much!

Steve Henderson

Hi Steve,

I think the offset you're experiencing is as expected. It would be more correct to use the start of the amplitude increase. 
Since it is easier to find the maximum value the easiest way to implement this is "calibrate" your measurements. Find the offset between max peak and seabed location and compensate for this in your results.

There are more sophisticated techniques around. Some of them are kind of business secrets, but they are also much more complicated to implement.

Best regards

Jonas Røstad

Thanks Jonas!

Doing an on-site calibration to determine offset is out of the question, but doing a calibration somewhere else would be easy.  There might be a difference in sediment type between calibration site & field site.  Is this likely to have much effect?  Thanks.

Steve H.

Previously Jonas Røstad wrote:

Hi Steve,

I think the offset you're experiencing is as expected. It would be more correct to use the start of the amplitude increase. 
Since it is easier to find the maximum value the easiest way to implement this is "calibrate" your measurements. Find the offset between max peak and seabed location and compensate for this in your results.

There are more sophisticated techniques around. Some of them are kind of business secrets, but they are also much more complicated to implement.

Best regards

Jonas Røstad

 

Dear Steve,

How much this offset will differ between different sediment types is uncertain, but it shouldn't be too much for small cells. If you check 2-3 different sites with different sediment types I guess you would gain the needed experience in this.

A different approach is to check your data and compare them to the known distance to the bottom. You might find a point on the rising edge that you can detect. 

Best regards and good luck with your trials.

Jonas Røstad