Richard Harrison wrote:

The FCC`s ground wave intensity charts assume that if you have ground
conductivity such as sea water, a certain power and antenna efficiency
deliver 100 millivolts per meter at a distance of one mile from the
antenna. Field intensity in millivolts declines linearly with distance
so that at 10 miles you might have 10 mv/m if you have 100 mv/m at one
mile. . . .

The attenuation of the ground wave is the same as the free space
attenuation?

Roy Lewallen, W7EL