Sound bends at boundaries between water layers of different sound speed, so beams leaving the transducer at an angle curve on their way down. Processing traces each beam through the measured profile to place it correctly. When the profile is wrong, outer beams land too shallow or too deep while nadir stays nearly correct, giving cross-track curls that grow with beam angle. It is the classic multibeam artefact in estuaries and under summer thermoclines.
That's refraction, not seabed, the outer beams have been curling up on every line since lunch.
Refraction error is symmetric about nadir, while roll error tilts the whole swath one way; that difference separates the two in a cross-section.
Also heard as
- refraction error
- sound speed error
Related terms
SVP
Sound velocity profile: how the speed of sound changes with depth through the water, measured to correct sonar depths.
Smiles and frowns
Cross-track curling of multibeam swaths, up or down at the edges, caused by sound speed errors.
Surface sound speed
The speed of sound right at the sonar face, measured continuously so the multibeam can steer its beams.
Bust
A vertical or horizontal mismatch between overlapping lines or surveys that exposes an error somewhere.