buried antenna communication
I did a long study once. Burried under 4" of dirt is not much of a problem for short range communications (30 feet or so). The problem is more in the composition of the soil. If it is going to get wet, it is going to be more of a problem. Also, there are problems with the antenna. You can use something like a standard quarterwave antenna, but you need to enclose it in something to keep the dirt/moisture away, as they will detune the antenna's resonant frequency and cause real trouble.
As you might expect, lower frequencies would work much better. Using an ISM frequency, like 13.56 MHz, would penetrate much better.
There is some recent work on "magnetic" communications that would do better underground, you might want to look into it.
Rich