Here is a new scientist story about using the KamLAND detector to identify antineutrinos from the radioactive decay of semi-stable isotopes (such as U-238) inside the Earth.
http://www.newscientist.com/article.ns?id=...=mg18725103.700
The article points out that this is a first result, and that a lot of refinement is possible with future detectors, so that a more solid number for total power can be determined, and a map of the distribution can be made.
The article didn't indicate what the energy of these antineutrinos are, or how we know they aren't coming from another source, though I bet if we look up recent papers by Bill McDonough, we could find out those details.