Hi Rik,
If one uses the four probe method of measuring ground resistivity, the
problem of contact resistance with the ground is overcome. Current is
injected into the soil through the outer two electrodes, and a voltage is
measured between the inner two electrodes. Use a constant current source on
the outer two probes, and a high impedance voltmeter on the inner pair. You
have to use a.c. because otherwise you get electrolytic effects. If there is
a poor connection the voltage on the constant current source rises to force
the same current through the soil. There is no volt-drop at the voltage
probes because there is negligible current here.
There are a variety of different arrangements of probes advocated for
different purposes, but basically the wider the spacing the deeper the
current flows in the soil. There is a diagram, and the equation to calculate
the resistivity at this location:
http://www.mines.edu/fs_home/tboyd/GP311/MODULES/RES/NOTES/2estres.html
Commercial equipment for measuring this (the sort of thing you see on time
team) uses about 1mA of current, and a frequency between 50 and 200 Hz that
avoids mains harmonics (I seem to have used 132Hz in my design).
Sometimes this sort of activity is called "geophys" - but I think if one
uses mains for the current source, one should call it "Geofizz" !
73
Hugh M0WYE
----- Original Message -----
From: "Rik Strobbe" <[email protected]>
To: <[email protected]>
Sent: Tuesday, December 17, 2002 1:19 PM
Subject: Re: LF: EARTH RESISTANCE
At 09:50 17/12/2002 +0000, you wrote:
>Has anyone measured earth resistance alone under different conditions? If
I
>recall correctly, this can be done with three earth stakes and Kirchoff's
>laws.
>
>Mike, G3XDV
I am wondering about that for years now.
Apart from the fact that the measurement will be frequency dependent I
would expect that it also depends on how deep the stakes are driven into
the soil (maybe unless you have a uniform soil).
Anyone ever tried this (with or without success) at frequencies of 136kHz
(or 180kHz) ?
73, Rik ON7YD
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