Below is a link to a "HOW TO" site you might find interesting.
http://www.ehow.com/how_7215160_out-...ductivity.html
Also if you do a "GOOGLE" search for ground conductivity map you will find several maps that have a "BALL PARK" rating of conductivity in the U.S. .
Soil conductivity will depend on several things among those are , level of water table, type of soil, salinity content, mineral content, there is quite a string of variables that need to be considered.
73, John