Rust

OK, I thought I could figure this out but have not had any luck. So I am going to ask what might be a stupid question. Why does rust give a high tone on most units? I am taking about the halo rust around an iron target.

Tom in SC

Excellent question. Can't wait for the answers.

Rust/oxidation increases the conductivity, I think even making targets especially iron highly conductive, tricking even the best of machines...you probably already know this though

I think Iron oxide is not conductive, but the moisture it hold is.

Tom in SC

The Halo leaches into the surrounding dirt creating a target that is much larger and it overdrives the detector ...Tricking the machine into thinking it's a good target...


small iron like the size of golf balls are bad about doing this till they are removed from the soil then they act like an iron target should....Larger iron that is not at great depth's like a axe head can sometimes be identified by just the huge sound it makes especially at a brisk sweep speed.....but at depth they sound like silver dollars...

I dig most of the iron false target in the medium size range at shallow to medium depth and larger iron at extreme depth...

but basically what happens is the detector becomes easily fooled by large target overload....Large target's just plain fool detector's ferrous or non ferrous...

Go outside and scan a square foot size of iron...I bet it read's high....Scan a 3" square of Iron and I bet you will see it start to i.d. iron...bury it in the ground for a few years and it will look like the square foot size of iron to the machine actually it would be more like a cubic foot of iron to the machine....And you will get the high conductance reading...

What is weird is the rust halo left in the ground after the target removal is unable to be heard yet is is still masking....


You will dig more chunky iron in undisturbed ground thinking it's a good target than in fields that get turned regular..


Just my take on what i have encountered ...

Keith



Edited 1 time(s). Last edit at 11/16/2011 10:58PM by Keith Southern.

I agree with Keith, I dig a lot of iron or rust targets, they sound to good, and 10% of then are good targets, meaning deep copper or silver, I think a large target or a deep target, the machine doesn't understand it so it throws it into a good target range, I hunted this place a couple of times and went over a target a few times, it was the same target and gave a tin can reading, after the 5 or 6 time going over it, I dug it and it was about 10 coins bunch together, I dig everything now

This is interesting...it always surprises me how small iron, such as a bent nail, can sound SO good in the ground (despite the fact that you have iron "disced out"), and then you dig it up and pull it out of the hole and wave it over the coil, and you hear nothing (as you would expect)...obviously, it would seem, a "halo" is at least part of the issue...

The only thing stranger than that, to me, is hitting a coin in the ground, and then removing a plug, and after checking the plug to find no target (coin still in the hole), you wave the coil over the hole, and -- no target! Then you stick in a pinpoint probe, and there it is!! I can't count the number of times hunting with an Explorer that I hit what I'm certain is a coin, and then dig a plug only to have the coin (still in the hole) give nothing but a threshold null. The pinpointer sees it, but the machine sees it as iron (threshold null). That is one I have a hard time explaining...

Steve

All I can add it sure adds to masking and perhaps finding copper coins much deeper than usual.

Once found a 2 cent piece that was still working ....wet with a green patina in the surrounding dirt....

I have read that acid rain will only increase the rate of a metal object breaking down. So it is only going to get worse as time goes by. Or better on those deep conductive targets.........

Some detectors will ID the HOLE once a deep plug is removed and ignores the targets. But i can say ive had it happen with the Explorer Steve. But then.... if it tells me its there ive come to believe it and just keep digging and probing. Thats where a good probe really helps. Ive noticed in fields with the SE you just cant get aways from the small rust... in cond those hot rocks sound good and hit good then in Ferr they hit in the lower right ... but sound good.

Dew

WHERE IS NASA TOM?

Ok here are my thoughts. Iron is conductive but will retain energy in the form of magnetic energy. This causes it to have a different footprint in forms of phase angle of the detector. Rust (iron Oxide) is not conductible but still has
a metallic image. Because rust does not conduct, then it can not have the same magnetic image that iron does. Thus reporting back to the detector as a metal but not a ferrous target. The concentration of rust will give
a stronger signal proportional to the amount of rust in that area. This is why you will get a high tone only in one direction due to the carry over of rust with the water or drainage away from the ferrous item.

OK, somebody set me straight......


Tom in SC

Whats odd is a deep bent nail seems to give a better high pitch than a straight one. Then you have round bolts. I also get more high tones from Hot Rocks in the summer than winter when its wetter.

Dew

You guys are hitting all around the answer. Recently..... I gave a long/full/definitive answer to exactly this topic.... in a very recent post. I think for SteveG. Time permitting....... I'll see if I can find it.

*****Why does rust give a high tone on most units? *****

In my opinion, it depends on the type and orientation of an iron target in regards to the detector response.

Picture a buried iron target, slightly elongated, that is orientated in such a way that the magnetic field lines at depth are parallel to the long axis of the iron target. There is very little eddy current generation (flux does not penetrate well) but there is the enhancement of the secondary magnetic field returned due to the ferromagnetic nature of iron. That is, under a magnetic field the mag polarity is rather easily aligned within the atomic structure of electrons in iron atoms (electrons are in unpaired orbits so there is no opposing spin to cancel out their reaction to the applied field. It is the electron's spin that creates the strong magnetic moments) . When the field is applied the domains begin to switch, the maximum is reached and then the field drops away as the primary coil polarity reverses. Between polarity shifts in the current of the primary field coil there is a space where the applied field falls to zero. In a high permeable target (iron) this is the opportunity for the many magnetic domains to relax and return to normal state of orientation. The relaxing of these secondary magnetic domains produces a secondary magnetic field sensed by the receive coil just as dissipating eddies produce a secondary magnetic field.

In another way, the iron target can be thought of as having greater permeability (permeability is the ability of an object to concentrate magnetic flux). The primary field of the coil has a limited capability to affect the target based on the density of the field lines produced - which is a set amount for a particular coil/detector. The iron target will drain a portion of the energy by using the energy to realigning millions of magnetic moments - which leaves less energy available to generate eddy currents beneath the rust (eddies borrow their energy from the primary field too). The higher permeable targets (iron) tend to protect their inner-core better from eddy formation and therefore hide their conductive metal nature better. This is one reason why dual freq detector that utilizes a low frequency field can induce eddy currents deeper into the conductive layer of the target, which improves the discrimination of the ferrous/non-ferrous nature of the target rather than relying perhaps on only weaker surface eddies that may occur with a higher operating frequency.

So, despite weak eddy current generation there is stronger overriding returned magnetic field that interacts with the receive coil balance (permeability changes), and is interpreted as a ferro-signal - or iron signal (low grunt). Not all iron alloys are necessarily magnetizable to the same degree, so the variance in iron is really a variance in permeability. The higher the relative permeability the more positive the phase angle (more ferrous looking), so an apparently rusted target may provide a weak ferrous signature (high tone) because to the receive coil it is really more non-ferrous than ferrous (despite the outer layer of rust).

Back to the orientation issue. If on the other hand, we change the orientation of the coil so that the field lines now slice into the iron target and produce eddy currents beneath the oxidized outer layer, then with perpendicular field lines cutting the iron target the secondary field produced by the eddies appears to the receive coil as non-ferromagnetic and a metal signal is produced (high tone). That is, the eddies prevail over the ferro response.

The phase angle is going to vary with coil height changes, even minor ones, and the coil's axial offset - as field lines cut the target at different angles.

Also, a large ferrous target can also appear non-ferrous (high metal tone) because the greater surface area produces a larger eddy current that can override the weaker permeability change in the receive coil that would be due to the ferro-magnetic nature of the target alone.

THANKS for you taking the time!!!!!

That was a great read - thanks for posting it John, thanks for writing it Tom ;-)

Question - It was said by Tom that single frequency machines generally perform better in iron. And that true multiple frequency machines (Minelab, CZ's) are generally not as good in iron. But, if multiple frequencies penetrate into the iron better to induce eddy currents deeper (in particular with lower dual frequencies) which allows for better identification of the iron (see through?), then don't (lower) dual frequency machines actually perform better in iron (or is that only rusted iron?) than single frequency machines? I have heard from a few detectorists who have had both the V3i and the E-Trac say that the E-Trac is somewhat better in iron than the V3i, so it got me wondering - theory and practice. Again, I hope I got this right! Then again, the E-Trac is notorious for falsing on iron, which isn't that much of a problem for me V3i.

I also wonder about the ability to raise the transmit power (like on the V3i, Nautilus and a few other detectors) and how this might impact iron identification or see through better.

Thanks,
Albert

Albert...... the V3i is not a true multi-freq unit........ as........ all 3 of it's channels/freqs are ALWAYS kept independent. On the CZ/Minelab's... the freqs are merged in a comparator......... and cumulatively analyzed.

Simply the virtue of multi-frequency... is not what allows it to penetrate bad dirt to greater depths. The V3i... with its 3 freqs.... will ascertain the same depths as the multi-freq units....... BUT....... because the 3 independent freqs are NOT compared to each other....... less data can be assimilated...... so as to 'collectively' differentiate between 'dirt feedback'.......vs.......actual target. This is where the true multi-freq units excel.... by having the ability to KNOW the 'feedback signal' is merely bad dirt........ and not a real target. Single freq units (V3i included) can not do this.... as ..... they have nothing to compare 'to'.

Yes.... lower freqs do punch deeper. Higher freqs are attenuated/diminished by bad/mineralized dirt.

I have noticed what really fools me especially lately is the rusted screws especially with the round phillips heads. And this is in rich soil, not really wet and they sound real good, fools me a lot, way more that nails, especially w the CZ.

Yes Tom, I remember you telling me that the V3i is not a true multifrequency machine. (That is why I didn't list it with the CZ and E-Trac - I'm learning!)

So, do true multi frequency machines perform better or worse in iron, that is where my confusion lies as I thought you said they do worse but by your "comparison of frequency examples" I thought they would do better. I don't think the CZ's do well in iron (from what I have heard). I do understand that true multi frequency machines perform better in bad ground - Thx. Oh, the V3i (and I'm not pro V3i, it's just the machine I'm using) can use correlate to compare the frequencies in iron or bad ground, so it does have that ability and it quiets the machine down in iron at least (I have noticed). But yes, it doesn't combine the frequencies as the true multi frequency machines do.

Anyone feel free to jump in.

Learning, Thanks for your time,
Albert

Yes..... multi-freq units will do better in bad dirt....... but do not handle iron that well. Single freq units, in general.... will handle (ID) iron better.