question on eddy current

[wyndham]

I understand that a PI induces an eddy current into the target, but is there already a current or some field in the target before the induction and the induction disturbs this field that is within the ground and the target?
Have you every looked at water so calm that the reflections of the sky and water seem to be the same until you throw a stone into it and the wavelets show where the water realy is?
The reason for the question is that the earths mag field should have already produced a field and an eddy current on its own, right/wrong?
I'm just trying to understand what's going on with the interaction between the target and the earths constant mag field.
Last week with the Calif earthquakes, people were reporting a lot of interference on there PI machines at the time of the quake.
I understand that the quakes produce electrical discharges so the interference. Thanks for the time Wyndham

[Carl-NC]

Hi Wyndham,

Long time no hear.

Eddy currents are generated by change... either the magnetic field must be changing relative to a still metal target, or the target must be moving through a static magnetic field. Interestingly, the Earth's molten metal core does move around in the Earth's magnetic field, thereby generating it's own eddy currents. And these eddy currents give rise to slight variations in the Earth's magnetic field, but not enough to induce reasonable eddy currents in small metal targets.

A PI starts out by turning on the coil, which produces a small turn-on eddy current in the target until the magnetic field of the coil stabilizes. Then the coil is abruptly turned off, and it's that fast change in the magnetic field -- from fully on to abruptly off -- that creates the desired eddy currents in the target. The Earth's (rather) static field has no effect.

Still would like to get down to your shop. Maybe later this summer.

- Carl

[wyndham]

Hi Carl, yea it's been awhile. Gold bug seems to get me in the spring and summer holds on til fall but lets go in the winter
I like the new forum, looks good.
I see that Finds forum has changed their software also. Tried to post on Eric's PI forum but can't seem to to get on. I have cookies set for the site but who knows, sent an email,just waiting to get back on.
Any thoughts on Eric's new Goldscan 5, I thinks that their just now getting them over here.
Thanks also for the field info. The thought I had, was that the target had a charge and that the induction either amped it or nulled it. What it is therefore is fairly neutral and the PI is the major signal factor. Thanks again and let ne know when you're able to come by. Wyndham

[Cossaro]

Reply to Carl-NC...

>The Earth's static field has no effect...

This is not true (not exact), the effect is important and very strong but a good PI detector can easy subtract to the eddy-field generated by target, like the ground effect, using a delta-signal rather an absolute signal discriminator.

If the "noise" is a constant in your reading, you can always consider the delta in that signal to discover the target. This is what happen in a detector.

[Ferric Toes]

Cossaro

Could you explain how the earth's field would have an effect. I put a coin in the middle of a coil and looked at the decay curve on a scope - amplitude and time. Put a powerful ferrite magnet on the coin. No change in either parameter. Tried a nail and there it does make a change. The amplitude reduces dramatically, presumably because the nail is saturating.

Earth's field does change throughout the day, but this is too slow of a change. Superimposed on this slow change are micro-pulsations, but these are very low level and make little contribution to the other noise picked up on the coil.

Ferric Toes

[Cossaro]

Reply to Ferric T.

what you are doing is a static test, and you don't "cut" the earths magnetic field (or something like this one you've produced).

When you're searchin on field, you're swing the coil and this movement produce an energy like a coil moved across a mag-field.

This is like the gound effect of the soil

[Ferric Toes]

Hi Cossaro,

I agree that moving the search coil in the earth's magnetic field will induce a voltage, but this is not the same as ground effect. Most PI's have a dual sampling system that cancels out the low frequency signal due to movement in the earth's field. An early sample picks up the wanted signal plus the induced field signal, and a late sample picks up the induced field signal only. The sample spacing is such that even power line frequencies are cancelled to a large degree by subtractive integration.

Ground effect is due to magnetic minerals in the soil being energised by the transmitter pulse. The induced magnetism decays rather slowly at the end of each pulse, and is a phenomenon known as magnetic viscosity, or lag. This signal is quite different from the induced field signal, as it decays logarithmically with time after each TX pulse. This needs different measures to cancel it out.

Ferric Toes

[Cossaro]

reply to Ferric Toes

>Could you explain how the earth's field would have an effect

…on the first time this was your question and i post a simple reply to help you understand.

>I agree that moving the search coil in the earth's magnetic field will induce a voltage, but this is not the same as ground effect.

It’s clear that is not the same, if you have the answer why are you doing the question?

>This needs different measures to cancel it out.

A compass will hel you ;-) ?

In a PI the ground and the E-mag shift the signal about by the same frequency (below 1Hz), the dual-sample you describe (is a classic method) reduce the effect of each one because this kind of noise is about a constant, compared to the target signal.

[Carl-NC]

>The Earth's static field has no effect...

This is not true (not exact), the effect is important and very strong but a good PI detector can easy subtract to the eddy-field generated by target, like the ground effect, using a delta-signal rather an absolute signal discriminator.

Yup, but the original question was whether the Earth's relatively static magnetic field would induce eddy currents in a metal target. It does not, ergo for buried metal, the Earth's static field has no effect.

- Carl