For protection of sensitive electronic equipment, a ground resistance of 5-ohms or less is recommended (NEC, IEEE, etc). To achieve this value in some soils, multiple ground rods may be required.
The two relevant concepts are equipotential and conductivity. Lower resistance is better. But we never get resistance low enough. So we also need equipotential. But we never get that good enough. So we increase conductivity.
One best earthing is an Ufer ground. Rebar in concrete footings means the entire house is surrounded by an earth ground. IOW one big single point earth ground. Obviously it was not done when surge protection is best installed - when the footing are poured. So some surround their house with a buried ground loop. Both solutions provide best equipotential. And superb conductivity.
They could not install best earthing on this mountain. Described is how they did it and how successful it was:
http://scott-inc.com/html/ufer.htm
Getting to five ohms is a goal to strive for. But if earth is not conductive, then simply many earth ground rods are still a major improvement. A first rod is the most important. Each additional rod adds diminishing improvement.
In another example, a couple suffered direct lightniing strikes to one wall. They installed lightning rods. Lightning still struck that wall. An investigation determined lightning was striking bathroom pipes that connected to deeper limestone. Their lightning rod grounds were only in sand. So longer ground rods were driven down into limestone. Then lightning stopped striking that bathroom wall. Appreciate how subjective this is and where an investigation starts should anything suffer damage.
Make earth ground so much better that is remains a best path for a surge. If soil is not conductive and you still do not achieve 5 ohms, well, that earthing still remains a best and therefore sufficient earth ground. It provides better equipotential and conductivity.
Commercial sites use expensive equipment to measure ground resistance. Human safety codes simplify it. If less than 25 ohms cannot be achieved, then a facility only needs two ground rods. Most electricians do not have nor learn what that equipment measures. So electricians automatically install two 8+ foot copper clad steel rods.
Some important features are found in this utility's good, bad, and ugly (preferred, wrong, and right) solutions.
http://www.duke-energy.com/indiana-business/products/power-quality/tech-tip-08.asp
Note that any interconnecting wire is buried. To increase conductivity and equipotential. Code defines how thick and deep that buried wire must be.
Protectors are simple science. The 'art' is earthing.