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.
Sidelobes are always present when measuring with ADCPs. However, as their energy is much less than that of the main lobe, they only become significant when approaching a boundary that reflects much more strongly 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 shows an instrument measuring towards 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. If sidelobe interference extends partly into one cell, the whole cell should be discarded, because one 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 \( \alpha\) can be described by the following trigonometric identity:
| \( R = D \times \cos{(\alpha)} \) | (1) | |||
| \(R\) | - | effective range \( [m]\) | ||
| \( D\) | - | distance to surface \( [m]\) | ||
| \( \alpha\) | - | transducer angle \( [^\circ]\) | ||
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, as illustrated in Figure 2. In such situations, the effective range decreases according to the tilt \(\Theta\) with the following formula:
| \( R = D \times \cos{(\alpha + \Theta)} \) | (2) | |||
| \(R\) | - | effective range \( [m]\) | ||
| \( D\) | - | distance to surface \( [m]\) | ||
| \( \alpha\) | - | transducer angle \( [^\circ]\) | ||
| \( \Theta\) | - | tilt of the instrument \( [^\circ]\) | ||
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