The link below leads to a short paper about the effects of earth conductivity on groundwave fields in the AM broadcast band.
The thumbnail charts there are rather fuzzy, but clicking on them will open up a sharper view.
http://www.radioworld.com/article/am-coverage-frequency-vs-conductivity/23739
R. Fry
The thumbnail charts there are rather fuzzy, but clicking on them will open up a sharper view.
http://www.radioworld.com/article/am-coverage-frequency-vs-conductivity/23739
R. Fry