Weak spots

For down-looking Vector probes, the presence of a boundary close to the Vector sampling volume may give rise to problems.  This is especially the case if the boundary is hard (rocks, concrete, glass, etc.) and/or the water echo is weak.  

For each velocity range, there are one or two distances that give rise to problems. The existence of these “weak spots” can be identified in the data record by a decrease in the correlation and an increase in the velocity variance. The problem is mostly seen in flumes with a hard bottom but has also been observed in the field, especially at the higher velocity ranges.

For a standard Vector, the distances from the sampling volume to the boundary that should be avoided are:

Range     "Weak spots"
7 m/s:    2 cm  and 4 cm
4 m/s:    3 cm  and 6 cm
2 m/s:    5 cm  and 9 cm
1 m/s:    8 cm  and 20 cm
0.3 m/s   20 cm
0.1 m/s   46 cm
0.01 m/s  312 cm

The distances are approximate and has a vertical extent of about 1 cm


The problem is less acute at the lower ranges and this - by itself - is a good reason to avoid the higher ranges unless needed.  Another way out of the problem is to use side-looking probes; the issue goes away if there are no boundaries in the path of the transmit pulse.

Can you explain how the heights for the weak spots are calculated and why the numbers you provided seem to differ from those given in the nortek manual (pg 18)?

Hi

The weak spots are related to the spatial separation between the pulse pairs transmitted by your velocimeter.  To be more precise, a weak spot occurs when the first pulse hits the bottom as the second pulse goes through the sampling volume. The position is thus deterministic and can be calculated.

The numbers in the manual refer to the NDV whereas the previous posting refers to the Vector.  That is why they are different.

- Atle Lohrmann

For the Vectrino, the corresponding numbers are:

Range        "Weak spots"
4 m/s:       2 cm  and 5 cm
2.5 m/s:    3 cm  and 10 cm
1 m/s:       5 cm  and 12 cm
0.3 m/s:    10 cm  and 23 cm
0.1 m/s:    23 cm and 45 cm
0.03 m/s:  38 cm and 75 cm

The distances are approximate. The vertical extent depends on the bottom composition and is about 0.5 cm for a flat bottom.

Please note that the Vectrino has some degree of self-adjustment.  Also, the numbers are subject to change since are configured with the Vectrino software.  The above values refer to Vectrino software version 1.05

As point of clarification. For lab work (ie in a flume or tank) are the weak spots restrained to a 1 cm vertical extent.

ie if I was using a Vector in a flume, mounted such that there was 5 cm between the sampling volume and the flume bottom, for a velocity range of 0.1 m/s, would this suffer from boundary echo effects? From my reading of the manual and earlier posting, this would only be a problem if the sampling volume was 45.5 - 46.5 cm from the bottom, ie above where I would like to mount the Vector. Is this correct?

Thanks, R Waters

Hi

The vertical extent of the weak spots is a function of the bottom composition.  If the bottom is well defined (e.g. sand) the extent is no bigger than the transmit pulse or about 1 cm.  If the bottom is rough, the vertical extent can be larger.  It is also a matter of the relative strength between the water scattering and the bottom echo - if the water scattering is high the whole issue goes away.

Quote

From my reading of the manual and earlier posting, this would only be a problem if the sampling volume was 45.5 - 46.5 cm from the bottom, ie above where I would like to mount the Vector. Is this correct?

Yes, this is correct.

- Atle Lohrmann

The first ping hits the bottom and the reflected signal of the first ping reaches the sampling volume at the same time as the second pulse goes through the sampling volume.

- Sven

I've become familiar with the problem of weak spots with the traditional downward looking probe.  We're currently looking into testing a sideways looking probe but it could take some time before we actually get it.  I was wondering, in the meantime, would tilting the downward looking probe so that it's 15, 30 or 45 degrees from vertical do much to change the interference of weak spots?  We're working with a gravel bed (D50= 7.1mm).  Would it make a difference if the sample volume is pointed upstream or downstream of the probe?  Thanks.

Hi there

The problem of boundary influence is felt for each individual receiver element.  If you reduce the effect in one beam (and remember they are all tilted about 30 degree relative to the vertical), then it is likely to show up stronger in one of the others.  And for downlooking probes you need all three receivers to correctly measure the velocity.

The answer, as you suggest, to problem with gravel beds (and high velocities) are side-looking probes.  With these you only have to worry about the distance to the side-wall and this is generally much more manageable than looking down into a gravel bed.

Best regards, Atle Lohrmann