Why do side-scan sonar images seem to show shadows?

Question

I have been browsing various side-scan sonar images of sunken objects, mostly shipwrecks. I understand sonar imagery to be drawn completely by soundwaves, so if that were the case why does it appear there are shadows in some of the images?

Answer 1

The objects in the images block the sound waves, creating shadows.  The mathematics of acoustic shadows are identical to that for optical shadows.