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Ground resistance penetration

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

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  1. 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:

    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:

    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

    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 Hobbs

    Phil Hobbs Guest

    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.


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

    160 North State Road #203
    Briarcliff Manor NY 10510

    hobbs at electrooptical dot net
  3. Syd Rumpo

    Syd Rumpo Guest


    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.

  4. 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
    more information about that please?

    (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
  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. has some downloadable software relating to this.

  6. amdx

    amdx Guest

    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.

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