A sequence of colours is used to show the relative strengths of the echo returns. Several colour scale options are available.
Most of them use darker colours to indicate weak returns, and brighter colours to indicate stronger returns.
Bright spots in the image indicate strong sonar targets. Generally, bright spots indicate a hard, highly reflective surface.
Dark spots in the image indicate either areas of low reflectivity (soft areas), or possibly an acoustic shadow zone behind
a target.
Most targets will block the transmission of sound — either by reflecting it, or absorbing it. This will leave a shadowed area
behind the target that is not ensonified and therefore will not generate any echoes. This is very similar to the shadow formed
when an object is illuminated with a single light source. The shadow behind a target can often yield more information about
the target than the reflections from the target itself. The shadow will often reveal the shape of the target, but you must
remember that the shape will usually be distorted according to the position of the Sonar Head relative to the target and the
bottom, and according to the slope of the bottom. It is often possible to estimate the height of a bottom target based on
the length of the shadow and the known height of the Sonar Head.