Both 2D sonar and DI require "some" movement ot get a true reading. You can see fish swim thru while sitting still. I don't think you have to be going a certain minimum speed to get a reading, just moving. The key here is, match your scroll speed to your boat speed. If you are just drifting or slow trolling, you need to slow the scroll to get the best readings. Go to Youtube and type in Humminbird Advanced Sonar. There are 7 parts that give the best instructions, explanations, and tips I have ever found. If you don't have side Imaging, you can skip those parts.