Does anybody know specifications for the new border "fence"?

Okay, so President Bush has signed legislation to build a "fence" between the United States and Mexico. Silly, but there it is.

I'm hoping readers can help me: Has anybody seen actual specifications for what this fence will look like? Texas Senator John Cornyn said it might be a "virtual fence," but I'm certain some physical barrier will be built someplace, if only for politicians to have their picture taken beside it for campaign commercials.

So assuming a real fence is built, how will it physically manifest itself on the ground? I've Googled and searched the blogs but can't find any detail. Here it's called a "double layered fence," but what does that mean? How tall will it be? What materials will it be made of? Will there be barbed wire on top? Will they embed concrete several feet down to deter tunneling? What will they do to stop people from just cutting a hole in it? In Texas it will be built in a river basin; what will be done to prevent the predictable erosion?

If you've seen any specific information about that sort of thing, I'd appreciate readers letting me know in the comments.

UPDATE: Here are a few preliminary answers from the bill language itself.