There have been many improvements in high impedance low current voltage measurement devices, but I have virtually no expertise in this area. FET and CMOS devices can provide exceptionally high input impedances but I cannot state what level of impedance is necessary to avoid large errors in measurement values when attempting to measure potentials at the earth-atmosphere interface.
First, we do not know what potential differences exist over underground voids as compared to a non-void area. Also, we do not know whether or not specific atmospheric conditions will effect the potentials at the surface over voids. All we have are the various published statements, "caves and mines are dangerous during thunderstorms". Does a void increase the potential difference? The article by the caver suggests that potentials over voids are different.
We know that golfers are ushered off of golf courses when there are nearby thunderstorms, as it is known earth-to-air potentials at the surface can approach breakdown levels and any object protruding above the surface can influence the breakdown level; the term "point discharge object" is relevant.
Here is another site that provides info on building a field mill.
http://freespace.virgin.net/paul.z/E...meter%20MK%202
This device provides a methodology for determining polarity, which would be important for the determination of earthquake induced surface charge levels. Earthquake induced charges provide a short term polarity reversal at the surface relative to the atmosphere.
