Marine Sea Scan Manual page 11

A rock viewed from directly above
When viewed from an angle, the rocks produce shadows
A SONAR shadow
© 2013 Marine Sonic Technology, Ltd.
Depth sounders bounce their signals straight down, so the rock in
this image is viewed only from the top down.
However, just as the sun reveals shadows, SONAR shadows are
produced by the absence of sound.
This shadow example is an image of a bridge pylon. In
this image you can see the large base as a rounded
corner rectangle. In the center is the pillar. The shadow
shows that the base of the pylon's height can be
measured. The center pillar goes up out of the water,
thus the shadow extends to the end of the SONAR's
range.
Introduction to Side Scan SONAR
In the image to the left, the shadow area
has nothing there to return a sound. Thus
a shadow is created. Just as with a sun-
created shadow, the height of an object
determines the length of the shadow.
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