Tuning the CARIS implementation of CUBE for Patagonian Waters.pdf

In extreme seabed geometry, with steep slopes and smooth surfaces, for example,
sidelobe echoes can dominate. Thus the main lobe echo could be masked and presented
weaker than sidelobe due to a weak reflection of the bottom. First arrivals from the
sidelobe tend to produce confusion in sea bottom tracking. Although multibeam systems
apply sidelobe suppression, extreme seafloor morphology could produce first arrival from
sidelobe, which if also combined with a high contrast in backscatter strength will produce
a higher specular return compared to the desired signal. For any MBES these situations
will make the bottom detection a very complex task (Figure 2.3), in which case its data
will be corrupted and the seafloor will be mistracked. Independently of the automated
depth estimator (e.g., CUBE), such situations will represent difficulties to define where
the true bottom was. This noise-data affecting the downhill side will be difficult for
cleaning for any algorithm and require operator intervention since outliers will appear to
cluster strongly in space and occur where data (representing the true depth) is poorly
defined.
That scenario is a very common in Patagonian fjords where survey lines have
been planned to run parallel to the shoreline. One side of the sonar faces towards high
backscatter signal (from steep slope walls), while the other side of the sonar faces toward
very low backscatter signal (from the flat-bottom of the fjord). The presence of high
contrast in backscatter strength (25dB), between the clay and gravel (Figure 2.4), can
cause the bottom detection to fail. According to the designed sidelobe level for the
beamforming (~23dB) the sonar recognizes the low signal to noise level, thus beams are
either rejected or mistracked where this problem occurs [Hughes Clarke, 2006a].
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