Sidelobe interference
Your measurement area will be limited by sidelobe interference when measuring close to boundaries. Even though most of the acoustic energy transmitted from the beams is focused in the center of each beam, a small amount of energy will leak out in other directions - this is sidelobes. When these low-energy signals strike a boundary before the main lobe, the echoes from the leaked energy can be so strong that they dominate and contaminate the received signals - this is when you have sidelobe interference. There is no way in post-processing to filter out the effects of sidelobe interference. All cells affected should be discarded.
Roughly speaking, we often say that sidelobe interference can affect up to approximately 10% of the velocity profile between the instrument and the boundary for slanted beams. Vertical beams (Signature 500/1000) will not experience sidelobe interference since they point directly to the surface so that \( \alpha = 0^\circ\) and hence \( R=D\). However, this applies to instruments that are leveled. The impact of sidelobes increases with tilt.
Even though there is no way in post-processing to separate the bias effect from the sidelobes, there are some measures that can be taken in advance of a deployment to reduce its impact. One action is to move the instrument closer to the boundary (10% of a short profile is less than 10% for a long profile). Reduction in cell size can also be positive, as this increases the spatial resolution. Keep all objects on the rig out of the measurement area. Also, make sure to keep the instrument as leveled as possible.
For further theoretical background on this topic, please refer to this article.
Beam obstruction
Any physical obstructions within an instrument’s acoustic beams can introduce visible data errors, potentially compromising measurement accuracy. Common sources of interference include nearby instruments, marine growth on the seabed, mooring components, or buoys. When an obstruction reflects part of the acoustic signal, it often appears in the data as a constant line at a speed different from the surrounding measurements. To minimize the risk of such errors, careful deployment planning is essential. Before deployment, assess the surrounding environment to identify potential obstructions and ensure that no part of the mooring rig interferes with the acoustic pings.
It is also important to consider that as an acoustic beam travels farther from the instrument, its width increases. This expansion is influenced by blanking distance, cell size, and beam geometry. As the beam grows wider, the chances of it encountering unintended obstructions increase, making it even more critical to ensure a clear measurement path.
To be sure to avoid contamination of your signal by blockage, you should ensure that you have at least a 15 degree clearance to each side of the main beam.
Acoustic interference between multiple instruments
Acoustic interference occurs when sound waves from different sources interact with each other. An ADCP can experience acoustic interference by detecting sound waves from external sources, other instruments on the same rig or nearby, and reflected signals that originate from the instrument itself. The last is acoustic interference with itself. Signals can be reflected from all sorts of nearby objects and structures. Even if only one beam is reflected, all beams can experience interference because the reflected signal can propagate into the measurement area of all beams as it is not necessarily reflected in the same direction as it came.
Regarding multiple instruments in an area, one measure is to keep them so far apart that the measurement areas don't overlap. "How far apart do they need to be to avoid interference?" is, however, a question without an exact answer. It depends on several factors, such as the instrument frequency, beam angle, broadband, and scattering conditions. Broad-banded instruments are, for instance, more sensitive to acoustic interference than narrow-banded ones. If one broad-banded instrument and one narrow-banded instrument measure in the same area, it is possible that the instrument with broadband detects signals from the instrument with narrowband, but not the other way around. Regardless, the safest option is always to alternate the sampling regimes, which is called staggering. This means that only one instrument measures at a time, while other instruments are in sleep mode. Since ADCPs usually don't ping constantly, this will be possible for many measurement series. One example of staggering is shown in Table 1, by giving the start times for four different instruments. They start at two-minute intervals, which means that the average interval can be up to 120 seconds and still avoid interference. The second profile interval starts 10 minutes after the first, giving all instruments time to complete the first averaging interval in advance (with this setup, instrument 4 can measure for four minutes, or if all measures for a maximum of two minutes, a fifth instrument could be included as well, without increasing the risk of interference). When it comes to external sources, such as echosounder, hydrophones, sediment sensors, etc., it is wise to get an overview of all known acoustic instruments nearby and preferably stagger them as well. Familiarizing yourself with the measurement area is also beneficial when it comes to awareness of obstructions that possibly can reflect signals.
Please keep in mind that even one instrument facing down and another one facing up may interfere with each other's signal as the signals may be reflected at boundaries such as the surface and the seafloor.
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