Any physical object located along one or more beams will affect the data. When a transmitted sound pulse reaches an object and gets reflected, the registered Doppler frequency shift represents the velocity of that object and not the water. This means that all data influenced by blockages should be discarded. If the blockage only concerns one beam and the data from the other beams are reasonable, it can be considered to exclude one beam in the post-processing of the data.
There are many things that can block the beams, such as rocks, ropes, buoys, trawl balls, and so on. Keeping the measurement area free of known obstacles is something to be aware of when planning a deployment setup. This includes measures such as placing a release buoy far enough away from the instrument or making sure ropes do not interfere with the transducers.
Figure 1 shows an illustration of beam blockage, where one beam is pointing towards a stone wall and another towards a chain. Data from the measurement cells marked with red are affected by the physical objects and should be discarded. Sound waves don't just stop when they encounter an obstacle. Instead, they can propagate behind this and we can get valid velocity measurements further out in the profile. In these situations, it is essential to assess the data quality behind the obstacle and assure that the amplitude and correlation are good enough. If obstacles reflect or absorb much energy leading to the sound wave's energy further along the beam being substantially reduced, the blockage can reduce the effective profiling range as well. Looking at Figure 1 again, we can see that there might be good data beyond the chain, but once the left beam meets the stone wall the rest of the profile is blocked. One more note to gather from Figure 1; none of the obstructions are located directly above the instrument. The beam angles make sure that the area of measurement gets increasingly wider the further away from the instrument, so objects to the side of the instrument can still interfere. Keep the beam's angles in mind when considering possible blockages.
A blockage is visible in the amplitude. An example of this is shown in Figure 2. The amplitude readings in space and time in Figure 2.a present a blockage in the first half of the measurement period, and this seems to disappear in the second half. Most objects are stronger reflectors than the passive particles we make use of to calculate currents. The amplitude profile will thus deviate from the Sonar equation (Figure 1.c) and have spikes at the location of the obstacle, which can be seen in Figure 2.b. It is also possible to notice an obstacle in the velocity measurements. For instance, if the obstacle is stationary, the velocity in the area covered by the obstacle will be zero, even though the velocities before and after are not. Due to flow disturbances caused by the physical obstruction, the velocities reading near it can also be different than expected.
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