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.
The amplitudes from all interacting sound waves are combined and form a new wave, according to the principle of superposition. The new wave generally has new properties such as changed frequency or amplitude. Any detected frequency that is different from what is emitted is interpreted as a Doppler shift by the instrument, meaning that the instrument will register it as water motion, and that affects the resulting velocity output. Figure 1 shows an example of how acoustic interference, caused by a nearby instrument, can appear in amplitude data.
Figure 1: An instrument experiencing acoustic interference from another instrument nearby.
There is no way in post-processing to automatically correct for the bias created by the acoustic interference, but there are some measures that can be performed to reduce the risk of it. 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 and bandwidth, beam angle, water depth, and scattering conditions. The broad-banded instruments are more sensitive to acoustic interference than narrow-banded ones because they operate within a broader frequency range. 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 will always be 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, staggering will be possible in most situations. 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 obstruction that possible can reflect signals.
Table 1: An example of how to stagger four instruments with average intervals of up to 2 minutes and profile intervals of 10 minutes.
|First profile interval||Second profile interval|
Even one instrument facing down and another facing up can interfere with each other's signal as the signals may be reflected at boundaries such as the surface and the seafloor. Figure 2 is an example of this.
Figure 2: Acoustic interference for two instruments in the same mooring, in which one is measuring upwards and the other downwards.