Minimum electrical resistivity of contact material for human protection from lightning
The usual method of lightning protection is to convince the lightning strike to follow a less harmful path than through the item being protected. Hence, lightning rods and lightning arrestors. 

Ohm's law resistivity assumes linear behavior- this is not what happens when you have enormous voltages and currents at play. Air is supposed to be a pretty good insulator, but lightning reaches through it. Many meters of air. A thin bit of polyethylene film is an almost perfect insulator, but a few thousand volts will punch a hole through it. At high current it will carbonize and become a pretty good conductor. A lightning strike is hundreds of thousands of volts and tens of thousands of amperes typically. 

That said, there may be information available on what ground conditions are like. Geophysical surveys looking for natural resources sometimes use EM methods to find ground conductivity such as this map of Electromagnetic Ground Conductivity. It will vary greatly depending on what is under the ground.