Re: the rib-like image, what you see is called refraction. It typically sits on the perimeter of an image and is the sonar wave bouncing off the thermocline-the different layer(s) of water at different temperatures that present a boundary, if you will, like a wall, that deflects/reflects, the sound wave instead of letting it pass thus generating a useful image. Refraction can be a huge problem for sonar operators.
It is a well known topic amongst submarine captains, who use the thermocline to hide their subs, knowing that a sonar wave bouncing off a thermocline sometimes won't reach them thus giving away their presence.
If I recall correctly the concept of thermoclines was used in the movie The Hunt For Red October, when they were playing cat and mouse game using them at one point to hide one sub from another or a surface vessel.
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