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In article <616@haddock.UUCP> wolfgang@haddock.ISC.COM.UUCP (Wolfgang Rupprecht) writes:
>Here is a question that I never saw discussed anywhere before:
>	
>	What is the resistance of earth ground? Let's say we have two
>good connections to the earth, say two Chevettes :-), dropped into the
>ocean, one into the Pacific and the other Atlantic. With an ohm-meter
>connected between them, what kind of resistance can I expect to see?
>
	You will measure zero resistence.  Where do you think the term
"ground" came from?  Actually you will measure some impedance --
however this is actually an electrode impedance.

	The physics of the situation are classical and elementary.
If you put an electrode in the Earth the current spreads out spherically
with the potential falling off 1/r^2 if the Earth is homogenous.  With
two electrodes you can calculate the potential function for each one
and combine them additively.  The case where the Earth is not homogenous
is of considerable practical interest.  

	Earth resistivity measurement is a standard technique in
Geophysical exploration.  What you do is stick a pair of electrodes in
the Earth and measure potential differences on the surface in a lot if
places.  You then attempt to infer the underlying resistivity structure.
The general idea is that an underlying low resistivity body (e.g. a metal
ore deposit) will distort the observed surface potential function because
the body sucks up the current.

	The problem of calculating surface potentials for a given model
is an interesting one, albeit one of simple classical physics.  I was
working in this area ~15 years ago.  At that time one and two dimentsional
models were tractable but there were no good techniques for three dimensional
model, other than simple special cases.  The inversion problem (deducing
the underlying structure from observed data) is mathematically ill 
conditioned but one can do a good deal with it.

	Other techniques include magneto-telluric measurements and
gravity logging.  The oil industry mostly uses acoustic methods.  It's
been quite some time and I am not current, so I expect the techniques
and geophysics today are a great deal more sophisticated.
-- 

Richard Harter, SMDS Inc. [Disclaimers not permitted by company S!E#I1#I