wave propogation in overhead wires with ground return

mrmojorizing

Hi, there's a well known paper called "wave propagation in overhead wires with ground return" you can get it at the link below.

I'm trying to understand this paper, but on the second page I'm not clear where equation 1 is from. Can someone point me to a book (name and pg numbers please) or website where it explains why the form of the wave equation is what's presented in the integral in equaiton 1, and why the E field in the ground must be a solution of this? I think i'm just missing something basic, it's been a while since i've done this. Also what does mu stand for in the equation? The author doesn't define it.

Thanks

paper link:

http://www.alcatel-lucent.com/bstj/v...stj5-4-539.pdf