The blank band down the middle of a side-scan record. Until the first seabed echo arrives the sonar hears only water, so the strip under the fish shows as a dark gap whose width is about twice the towfish altitude. Survey designs fill it either by overlapping the adjacent line far enough to see into the gap, or by running multibeam at the same time, since multibeam sees nadir best of all.
Contact sits right in the nadir gap on line 7, check the multibeam to see if it's real.
Also heard as
- nadir zone
- water column gap
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.
Nadir
The point on the seabed directly beneath the sonar, where the beam looks straight down.
Towfish
A streamlined body towed on a cable behind the vessel that carries side-scan or other sensors close to the seabed.
Coverage
The portion of the seabed actually measured or imaged by a survey, expressed as a percentage of the area.