Well, here's one of the threads which includes info on the 'shadow zone', but I think there are a couple more. This one was just the first one I could find with a quick search, but I'm sure you'll find the rest if you just search for "shadow" in this forum alone.
http://www.subsim.com/radioroom/show...ghlight=shadow
A very interesting and edifying thread BTW. Oh and yes, unless you are traveling at 7 kts or more, your TA will sink below your depth (the lower your speed the deeper it will dangle).