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Tuning geomagnetic coil?

Discussion in 'Electronic Design' started by Jeff Sanderson, Aug 21, 2004.

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  1. Years ago, I was given two sensing coils from an Earth micropulsation
    monitoring station.

    each is 300mm long x 50mm diameter and has a 1m long core made of
    strips of annealed steel. I don't know the inductance, but the
    resistance of each is 18.75K.

    Can anyone offer their best guess on what parallel capacitance I
    should use on each to tune to the 3-20Hz range?

    Jeff
     
  2. Robert Baer

    Robert Baer Guest

    Gotta have an idea as to the inductance...
    Even with an air core, one would need to know the number of turns and
    how it was wound, in order to calculate indutance.
    Get some mylar caps: 0.1uF, 1uF, 10uF and try them out.
     
  3. James Meyer

    James Meyer Guest

    The frequency range you specified is almost a decade wide. You won't be
    able to "tune" an inductor to resonance over such a wide range with a single
    capacitor.

    I suggest that you simply use the coils without trying to tune them and
    then, if you need bandwidth restrictions, add an active filter to the system
    after you have obtained a usable signal level.

    Jim
     
  4. John Larkin

    John Larkin Guest

    An inductor with this much resistance probably can't be tuned usefully
    to such a low frequency; Q would be very low. You could just amplify
    its output and then run the signal through a bandpass amplifier.

    What is "Earth micropulsation"? Tiny wiggles in the magnetic field, or
    something mechanical?

    John
     
  5. Ken Smith

    Ken Smith Guest

    How many leads come out of these coils?

    I'm hoping the number is 4 or 5. Ideally the coil will have a sense
    winding and a feedback winding. An amplifier listening to the sense
    winding drives a current through the feedback winding. This flattens the
    frequency responce and takes the cores tempco out of the question.

    Your lower end frequency is a bit too high if you plan on watching micro
    pulsations with these coils. If you use a good amplifier you should be
    able to get about 50fT/sqrt(Hz) out of these coils.
     
  6. Ken Smith

    Ken Smith Guest

    Do you consider 1nT tiny?

    If you monitor the earths magnetic field, you will see an average value of
    about 50,000nT, a daily variation of about 200nT, and micropulsations at
    about 1nT.

    It is fairly routine to measure changes in the earths field down to about
    0.0005nT so 1nT is about mid span between the big number and the small
    one.
     
  7. Norm Dresner

    Norm Dresner Guest

    Does this have any practical value?

    Norm
     
  8. John Larkin

    John Larkin Guest

    Cool. I've just taken on a project to measure milligauss eddy current
    wiggles inside a 15T magnet. A milligauss is 100 nT, but my pickup
    coil has to be tiny, a couple of mm in diameter to be able to map the
    field locations. Of course, microscopic vibrations will be lethal
    inside this sort of static field. Wish I could have a coil that big,
    or a core!

    Where do the micropulsations come from? What can you learn from them?

    John
     
  9. Ken Smith

    Ken Smith Guest

    I almost have to say "not yet".

    A department of the US Navy uses that level of performance to detect UXO,
    but that is still more in the experimental area than something that is
    generally applied.

    Magnetocardiograms may prove to be a good idea too. There the field
    measurement can be done in a shielded room and at a fixed location so it
    is a lot easier to implement.
     
  10. Ken Smith

    Ken Smith Guest

    I believe the cause is the solar wind pushing around ions which create
    magnetic fields as they move.

    Micropulsations are low frequency EM radiation that is poured down on the
    earth from above. At these really low frequencies, they should go a long
    way into the earth. I suspect that this could be used to see what is very
    deep into the earth. Other than that they are more trouble than they are
    worth.
     
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