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. As for the latter, 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 range 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.
There is no way in post-processing to separate the bias from acoustic interference, but there are some measures that can be performed in order to reduce the risk of it. These are outlined in the following article: Performance limitations.
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. This is the case of the data in Figure 2.
Updated