Humminbird Wide View Operation Manual page 25

The transmitted sonar signal travels downward, and is reflected back toward the surface
by the bottom or other objects for display on
screen.
The signal does not stop there - it is reflected downward again by the surface of the
water, and a weak "second return" is usually visible if the depth range is sufficient to see it. A
second return is shown in the figure at the bottom right. Some users use this second return as an
indicator when setting the sensitivity bias.
If a target is detected between the surface and the bottom, it is displayed as a fish
symbol. Depending on the strength of the signal reflected from the object, one of three different
size symbols is used. These reflected signals are "normalized" for depth, so that a small fish does
not appear to be a large fish if it is close to the boat.
Since some species of fish tend to be better reflectors of sonar than others, the strength
of return is not always an accurate indicator of fish size, however, typically, the larger the fish, the
larger the signal return.
Additionally, since the Wide View uses a dual beam transducer, each of the two beams is
evaluated independently, and some directional indication is possible. If a target is seen in the