It varies. I started out using small woodland creatures. We all know how well THAT turned out. Not one of my greatest brainwaves, I'll admit. I then stole the concept of Chaos Drives from GUN after Evil Plot #210: Adventure 2. Modified them to work better, of course. Seriously, GUN's scientists need to do better. The current power source is a classified piece of information. For obvious reasons. Nice try though.
If I didn't know any better, I'd say the same thing, and 'Scourge' just put the Master Emerald bollocks out to cover for it, but no, I can detect emerald signatures across dimensions (bloody things are just too powerful), and he did it all right. Still no match for Sonic, but there you are.
EDIT: Incidentally, that's also why it's impossible to find the Chaos Emeralds using regular sensor arrays. The power signature is so ridiculously massive it's a bit like trying to detect the sun from it's core. It's undoable.