Lowrance FishHunter 3D/PRO Operator's Manual page 20

Try to set the Surface Filter to a number that matches where
you see the surface noise stop in the Raw data view on your
screen.
In the example below, you would set the top sonar to 2 m (6.5
ft) and the bottom sonar to 0.5 m (1.5 ft) to give you the best
results.
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Menus and settings | FishHunter 3D/PRO Operator Manual