Any explanation for this?

[ElectroNovice]

I have been looking at the decay curves of some highly ferromagnetic rocks and soil samples from Victoria , Australia and the decay curves of a range of natural gold Victorian nuggets. To my great surprise the decay curves of all the nuggets very closely resemble a ferromagnetic response! Pure gold or other pure conductors have decay curves that are exponential in nature and easily distinguished from the ferromagnetic response. I find this strange behaviour with the natural nuggets at a variety of sample delays(10-50u secs) and variable transmit pulse lengths. What is going on i dont understand as the Fe content of the nuggets (from 4-330 grams) is i believe less than .5%. The decay curves have been done with a very small coil so i am wondering if this might be a significant problem particularly with big nuggets.The nugget height is adjusted so that there is no op amp saturation or other obvious electronic artifacts. Any thoughts appreciated.
ElectroNovice

[Ferric Toes]

Hi ElectroNovice,

You don't say how small your coil is, but that could be the problem. Ideally the object should be in a uniform and unidirectional field. If not, then other eddy current modes can be set up, so you get a sum of exponentials, some fast and some slower, depending also on the shape of the object.

Also, objects whose time constant is longer than the applied transmitter field duration can exhibit a decay that looks like magnetic mineralisation. Again, you are looking at a sum of different time constants that does not settle down to a single exponential until after 1 TC.

Eric.

[ElectroNovice]

Hi ElectroNovice,

You don't say how small your coil is, but that could be the problem. Ideally the object should be in a uniform and unidirectional field. If not, then other eddy current modes can be set up, so you get a sum of exponentials, some fast and some slower, depending also on the shape of the object.

Also, objects whose time constant is longer than the applied transmitter field duration can exhibit a decay that looks like magnetic mineralisation. Again, you are looking at a sum of different time constants that does not settle down to a single exponential until after 1 TC.

Eric.

Thank you very much Eric. the coil is a 4" diameter monoloop. I used such a small coil to help avoid shocking EM interferrence where I live. Some of gold the nuggets would of course be quite a significant size compared to the coil and many were very irregularly shaped.Coins and other non ferrous objects including a small loops of copper wire(wound in the form of a mini loop) all give a classic,pure exponential response,all the hot rocks and hot soils give a classic magnetic response. Would I be better to go to a place free of em and repeat the tests using a larger monoloop coil,say 18'' diam ?
Thanks again
ElectroNovice

[Ferric Toes]

Hi ElectroNovice,

One way of getting a uniform field is to make a long solenoid type coil. Say 6in diameter and 6in winding length. This is important for the transmitter, but if you want to get more coupling to the object, you can have separate bundle wound receiver coil mid way.

Noise is often a problem. It can be overcome by having a 2nd RX coil connected to the other opamp input, so that it is subtracting. This 2nd coil can be on the same tube but a bit further down from where you place your test object, otherwise that will cancel too.

Two good articles to look out for on the generation of eddy current in spheres and rods, due to a pulsed field -

Eddy Current Method for Measuring the Resistivity of Metals - Journal of Applied Physics Vol. 30 No.12 Dec. 1959

Appications of Transient Electromagnetic Techniques - Technical Note TN-7
Geonics Ltd of Ontario, Canada.

Discs and particularly rings, give a purer exponential all the way down.

Eric.

[ElectroNovice]

Thank you Eric for your advice and the references. However the "classic" exponential response for conductive objects may not be as simple as we think. Have a look at this this recent paper. I warn you that the Maths is horrendus! But the authors claim that for strongly permeable targets that the early time regime response following pulse termination separates into two distinct power law regimes and the ultimate response depends on the ratio of the object permeability to background permeability and other factors.
Physical Review Letters,Vol 93 No 2,8th July 2004
"Surface Modes and Multipower-Law Structure in the Early-Time Electromagnetic Response of Magnetic Targets".
ElectroNovice

[Unregistered]

Physics and PI

Hi Electronovice and Eric,
It is interesting to note that some multi-period or broad spectrum PI designs are being heavily experimented with in the field of de-mining technology and UXO detection. In arriving at my own PI design most of my research has centered on existing and proposed de-mining projects as there is a great deal of documented studies both theory and practiced to support further advancement in discriminating PI detector market. Coupled with Eric's long established works the future really holds some exciting possibilities for hybrid PI machines for both hobby/gold detectors but perhaps more importantly, de-mining technology.

Regards

Brian K

[Ferric Toes]

But the authors claim that for strongly permeable targets that the early time regime response following pulse termination separates into two distinct power law regimes and the ultimate response depends on the ratio of the object permeability to background permeability and other factors.
Physical Review Letters,Vol 93 No 2,8th July 2004
"Surface Modes and Multipower-Law Structure in the Early-Time Electromagnetic Response of Magnetic Targets".
ElectroNovice

I haven't had a look yet at the paper, but by "magnetic targets", I assume that they are refering to ferrous metals, to which I would agree that the response is more complex, and also depends very much on the orientation of the object in the TX field. The responses from non-ferrous metals are much easier to understand.

Eric.

[Unregistered]

I have been looking at the decay curves of some highly ferromagnetic rocks and soil samples from Victoria , Australia and the decay curves of a range of natural gold Victorian nuggets. To my great surprise the decay curves of all the nuggets very closely resemble a ferromagnetic response! Pure gold or other pure conductors have decay curves that are exponential in nature and easily distinguished from the ferromagnetic response. I find this strange behaviour with the natural nuggets at a variety of sample delays(10-50u secs) and variable transmit pulse lengths. What is going on i dont understand as the Fe content of the nuggets (from 4-330 grams) is i believe less than .5%. The decay curves have been done with a very small coil so i am wondering if this might be a significant problem particularly with big nuggets.The nugget height is adjusted so that there is no op amp saturation or other obvious electronic artifacts. Any thoughts appreciated.
ElectroNovice

That doesn't sound right. May I ask what your transmit current signal looks?

Paul