What are weak spots and how can I avoid them?Follow
The presence of a boundary close to the velocimeter sampling volume may give rise to problems. This is especially the case if the boundary is hard (rocks, metal, concrete, glass etc.) and/or the water echo is weak.
The weak spots (or pulse-to-pulse interference, when discussing profilers) 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 ping hits the bottom and reflects, and this reflected signal reaches the sampling volume at the same time as the second pulse. The position is thus deterministic and can be calculated.
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 higher velocity ranges.
The vertical extent of 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 content and the bottom echo – if the water scattering is high the whole issue goes away.
Keep in mind that weak spot positions are only a general location, and they will vary a bit based on the speed of sound and can extend over a range of about 1 cm. For a standard Vector, the distances from the sampling volume to the boundary that should be avoided are:
|Velocity range [m/s]||Weak spot [cm]|
|7||2 and 4|
|4||3 and 6|
|2||5 and 9|
|1||8 and 20|
The distances are approximate and have a vertical extent of about 1 cm
The problem is less acute at lower velocity ranges and this is in itself a good reason to avoid higher ranges unless required. 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. However, a side-looking probe will have exactly the same problem if there is a wall or other vertical obstruction facing the probe.
For the standard Vectrino, the corresponding numbers are:
|Velocity Range [m/s]||Weak spot [cm]|
|4||2 and 5|
|2.5||3 and 10|
|1||5 and 12|
|0.3||10 and 23|
|0.1||23 and 45|
|0.03||38 and 75|
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 when selecting one of the higher velocity ranges. Also, the numbers are subject to change since they are configured with the Vectrino software. The above values refer to Vectrino software version 1.05.
For the Vectrino Profiler, it is more difficult to define problematic distances based on velocity range, since this is a profiling instrument. However, the Vectrino Profiler uses Adaptive ping interval algorithms to help alleviate these issues. To accomplish this task the instrument attempts to measure the channel impulse response between the transmit transducer and all four receive transducers by taking deep profiles down each receiver beam. These profiles are then scanned to determine the temporal position of the relevant energy in the backscatter. In environments that exhibit large amounts of acoustic interference, ping rates are chosen that are long enough to avoid all reflections by constraining the ping rates to values larger than the duration of the channel impulse response.
In environments that exhibit less acoustic interference, a more sophisticated approach can be employed to avoid weak spots while at the same time allowing fast ping rates for the measurement of faster and more turbulent flows. In this case, the instrument predicts the temporal position of all relevant interferers for a large number of ping intervals through use of the convolution operator. A minimum ping rate is then selected that satisfies the conditions of range, ambiguity velocity and weak spots. In this case, every attempt is made to place the profiles between relevant reflections rather than after all of them.
So with the basis above in mind, there are two approaches for avoiding weak spots:
- Adjust the velocity range (small steps on the Max Interval ping algorithm is one method)
- Utilize the Adaptive ping interval algorithm and have it check once at the start of data collection
The simplest method for dealing with weak spots is to use the Adaptive ping interval algorithm. Adjusting the velocity range manually will move the weak spot around. If the Adaptive ping interval algorithm cannot eliminate the weak spot, manually adjusting its position to the least important part of the profile is a good option.
Set the Adaptive check interval to an appropriate value based on measurement conditions. Over a hard boundary where the Vectrino Profiler position is fixed, Once will work well. In moving probe or boundary measurements, select an interval reflecting expected time scales for changes in the probe or boundary position.
We received a question asking whether it is possible to tilt the probe (so that it is 15, 30 or 45 degrees from the vertical) to avoid the interference of weak spots. The issue then is that the problem of boundary interference is felt for each individual receiver element. If you reduce the effect in one beam (and remember they are all tilted about 30 degrees relative to the vertical), then it is likely to show up stronger in one of the others. Also, for down-looking probes you need all three receivers to correctly measure the velocity.
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