As far as ground penetrating microwaves — microwaves are absorbed by water, so ground moisture is the enemy of this technique. I believe, the desert environment ought to be the best place to deploy this stuff.

Seismology should work as well. I have been puzzled why the Isrealis have not gone down the border with Egypt and blown off dynamite every quarter mile and caught the echos on a grid of microphones, and then back calculated the tunnel locations. The border is only 10 miles long.