I have used a stud finder set for high sensitivity to locate floor joists through carpeting. Once the joist is located, you can use a 10 or 16 penny twist shank casing nail to nail down the flooring. (the same type of nail to secure brick molding)
I have never heard if berber carpeting unraveling from nailing through it, but if the other poster has had problems with it, I would approach it very carefully.
Good luck!
Dodgeman