# Ground resistance penetration

Discussion in 'Electronic Design' started by Adrian Tuddenham, Sep 6, 2013.

I am trying to take 4-terminal ground resistance measurements in such a
way as to distinguish between shallow and deep anomalies. The basic
idea is shown at:

http://www.poppyrecords.co.uk/other/images/Electrodes3.gif

By spacing the electrodes further apart, the depth of penetration is
increased. In theory, by comparing the 'short' spacing reading with the
'long' one (proportionately weighted), the differences due to the deeper
penetration should be measurable. (There ought not to be a difference if
the ground is entirely homogenous, so anomalies will indicate the
presence of deeply buried objects.)

In practice the system is made a lot easier if five electrodes are used,
with the connections being changed as appropriate:

http://www.poppyrecords.co.uk/other/images/Electrodes4.gif

However, this raises a question: If the current path is the same for
both the 'short' and 'long' readings, the sum of the short readings
ought to equal the long one as long as the electrodes aren't disturbed
when changing over the connections. The current doesn't know where the
voltage is going to be measured and the rule of summation of voltages
should apply. In practice this does not happen and there is usually a
difference.

I have not yet tried exchanging the current and voltage leads to give
different short and long current paths with fixed voltage measurement.
One theory says this should give a different current penetration and
show up deeply buried objects - but the the theory of reprocity suggests
that there should be no overall difference between this and the
previous set-up.

I'm getting in a tangle with this, is there someone who can see the wood
for the trees and explain what is really going on?

[All measurements were made with A.C. at about 850c/s to avoid electrode
polarisation. The current was around 60mA with a bridge-type
measurement system to balance out variation in the current and a
synchronous detector to reject mains hum. Voltmeter input impedance was
around 9 Megohms with screened electrode leads using bootstrapped
screens to avoid capacitance effects]

2. ### Phil HobbsGuest

In addition to the path resistance, there are at least two effects that
will give rise to differences: surface resistance of the oxide layer on
the ground rods, and spreading resistance. Those will look like an
additional resistance in series with each tap. As a SWAG you might take
the end-to-end resistance, subtract the sum of the N individual section
resistances, and divide by N-1.
(That's counting N sections, N+1 ground rods.)

That won't give the right answer if the current paths aren't nearly the
same, of course.

I recently built a transcutaneous spectrometer that needed careful
control of the source/detector spacing to trade off penetration depth
vs. optical efficiency. Those sorts of things are always hard to do
really well, because you're fighting Laplace's equation, and that makes
it exponentially hard as you go deeper.

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

3. ### Syd RumpoGuest

<snipped>

You probably know this, but searching on 'geophysics electrical
resistivity' will bring up a host of information.

I did a lot of this a long time ago. Thousands of readings in fields on
a grid, then enter it into the PDP11 for plotting. To be honest, it
seldom showed anything of interest, although we once detected a buried
barbed-wire fence. At least it showed the technique worked to some extent.

We were mostly looking for solution cavities, and often the survey would
be a small part of some larger site investigation contract.

Cheers

Have I misunderstood something?

The current electrodes are driven from a more-or-less constant current
source and the voltage electrodes are monitored by a high impedance
measurement system. How would a small change in the resistance of the
electrode surfaces (or even quite a large one) make any significant
difference to a 4-terminal measurement?

The spreading resistance is a different matter. Could you give me a bit

(To give some idea of the results: with electrodes about 1ft deep and
1ft apart, the soil resistance is usually somewhere between 200 ohms and
2k.)

5. ### Guest

You might also try searching on hydrology earth resistance. I had a neighbor who was a hydrologist and he had a program in basic to take earth resistance measurements and come up with a model of how layers of earth at different resistances would produce those results.

The same type of measurements are used to determine if lightning protectionsystem are properly grounded. Biddle makes meters for doing this. So youmight search on Biddle too. BiddleMegger.com has some downloadable software relating to this.

Dan

6. ### amdxGuest

About thirty plus years ago I did a grid pattern in a very large
excavation dug for a garbage dump. The idea was to make sure there was a
thick continuous clay layer across the bottom of the excavation. This
was my first time in the field doing this type of measurement. When I
ran into an area that had substantially different measurements then the
rest of the hole, I did a second set of measurements and recorded them
on a separate log.
A few weeks after turning in all the information I ask the person in
charge of the job what they did with the problem area. He hesitated
a bit and then said we dug out the area and refilled it with clay.
To this day, I think they changed my records and ignored the
measurement anomaly.
Somebodies groundwater may be polluted today because of that.

Mike