A gap in the side-scan record where an object stops the sound, leaving the seabed behind it unlit. Shadow length, together with towfish height and the range to the object, gives the object's height by similar-triangle geometry. A long, crisp shadow marks a standing object worth developing; a soft or missing shadow suggests a low feature or simply a change in seabed texture.
Shadow's 9 metres long at 36 metres range with the fish 10 off the bottom, so that thing's standing about 2 metres proud.
Also heard as
- shadow
Related terms
Side-scan sonar
A sonar that images the seabed to either side of its track, revealing objects and textures by their echoes and shadows.
Contact
A target picked out of side-scan or multibeam data that may be an object worth investigating.
Towfish
A streamlined body towed on a cable behind the vessel that carries side-scan or other sensors close to the seabed.
Least depth
The shallowest depth over a wreck, rock or other feature, which is the value that gets charted.