Sidelobe interference is an interference phenomenon that prevents us from measuring currents close to a remote boundary. Even though most of the acoustic energy 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.
Sidelobes are always present when measuring with ADCPs, but because their energy is so much less than the main lobe's, they are only of importance when approaching a boundary that reflects much stronger than the suspended particles in the water. Strong reflections occur when there are large differences in the speed of sound between two mediums, one being the water. The sea bed is a strong reflector, and the sea surface provides an almost perfect reflection. Other boundaries that can cause sidelobe interference are physical objects, such as underwater structures, buoys, and so on. Boundary conditions thus play a crucial role in determining the impact on velocity measurements. So does the scattering strength from the water and the acoustic properties of the transducers. Sidelobe interference may be unimportant with strong backscatter. It all comes down to how strong the signals from the sidelobes are compared to those from the main lobes.
Figure 1: Sidelobe interference for a leveled instrument. The sidelobe interference layer starts at the same distance along the beam as the distance from the instrument to a boundary.
Figure 1 shows an instrument measuring the surface and illustrates how much of the profile can be affected by the interference. If the vertical (and shortest) distance to the surface is D, then the contamination of the current measurement begins at the same distance D along the slanted beams. The velocity data are contaminated from this distance and onwards to the water surface. The same principle applies to other boundaries as well. If sidelobe interference extends partly into one cell, the whole cell should be discarded, because we cannot distinguish where in the cell it applies to. The relation between the effective range R (area unaffected by sidelobe interference), the distance from the instrument to the boundary D, and the transducer angle α can be described by the following trigonometric identity:
R = D * cos(α)
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 ( α=0° → R=D). However, this applies to instruments that are leveled. The impact of sidelobes increases with tilt, as illustrated in Figure 2. In such situations, the effective range decreases according to the tilt θ with the following formula:
R = D * cos(α+θ)
Figure 2: Sidelobe interference for a tilted instrument. The sidelobe interference layer starts at the same distance along the beam as the distance from the instrument to a boundary.
Regarding the velocity data, sidelobe interference will typically appear as bias toward the velocity of the interfering boundary. This is a bias towards zero for the sea bed (unless there is a moving bottom). For the surface, the bias will depend on the sea state or surface wind conditions. Sidelobe interference has in many cases been noticed by high velocities in the upper layer. This comes from the sidelobes detecting movement of the surface. A tip when analyzing data is to check the vertical velocity (Up or Z) extra carefully in these areas. It should typically read close to zero. If not, it might be an effect of interference.
Sidelobe interference can also be spotted in amplitude profiles, as shown in Figure 3. The profile to the left is not affected by sidelobe interference, and then the amplitude just decreases until it reaches the noise floor. But the profile to the right shows an example of sidelobe interference. After a gradual decrease in amplitude, the signal strength increases as the signal approaches a boundary (here the sea surface), which is represented by the peak. Sidelobes interfere in the area of increase.
Figure 3: Typical amplitude behavior along a profiling range when not measuring any boundaries (left) and when measuring the sea surface (right).
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.