This table of ground constants provides some description:
Earth Diel Cond Ground
Surface Const mS/m Quality
-------------------------------------- ----- ---- ---------
Salt water 81 5000 Excellent
Fresh water 80 1
Pastoral, low hills, rich soil 20 30 Very Good
(Dallas TX to Lincoln NE)
Pastoral, low hills, rich soil 14 10
(OH and IL)
Flat country, marshy, densely wooded 12 7.5
(LA near Mississippi River)
Pastoral, medium hills and forestation 13 6
(MD, PA, NY, except mtns & coastline)
Pastoral, medium hills and forestation, 13 5 Average
heavy clay soil (central VA)
Rocky soil, steep hills, mountainous 12-14 2 Poor
Sandy, dry, flat, coastal 10 2
Cities, industrial areas 5 1 Very Poor
Desert (southwest ID) 3 0.55
Cities, heavy indust areas, high bldgs 3 0.1 Extr Poor
Brian