Calculating what frequencies a dielectric can be considered perfect
A material is a perfect dielectric if the conductivity is zero, in which case the AC resistance would approach infinity. My initial guess without doing any research would be that you have to find the frequency at which the skin depth of the earth becomes effectively zero. The skin depth is given using the following formula:

http://www.wolframalpha.com/input/?i=skin+depth+formula

There is a simplified version of the equation but I can't remember it off the top of my head.