Would like an explanation?

[BarberBill]

We've all read on the forums and sometimes elsewhere that air testing a detector is not a good measure of its actual performance as some machines test poorly in the air, but do much better on buried targets. I'd like to see a logical or tested explanation of how that works. Many targets do not leach into the soil as does rusty iron, so the so called "halo effect" can't account for every instance, especially with silver or gold that remains inert in the soil. Also, how can a machine recognize a target through soil or another medium, yet do poorly thru the air which would seem to have little or no blocking effect at all? If anyone can pass on a solid, logical explanation to this, I'd appreciate it. Always trying to learn more.
HH
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[Elton]

Or the detectors ability to ignore the ground. The Minelab Sov., and explorer series would tend to back up this theory.

 

[Nauti]

The main reason that air tests are not to accurate is because soil conditions will prevent the detector reaching the maximum depths it can attain in air.....not the other way round.If u do an air test in perfect conditions(no interference etc) and your detector picks up a silver or gold coin at say 10" that will be it's maximum depth full stop....it will not go any deeper in any type of soil.A detector creates a magnetic field that has a peak distance from the coil....the distance of this magnetic field will not increase in soil...if anything it will decrease even in the most mild of soil conditions.People who claim their detector performs better in soil than in air have usually made the mistake of air testing their detector indoors where there is a lot of electrical interference which effects the performance of the detector.Outside in the field where there is generally a lot less interference they get better results hence why they think the detector is better in the ground than in air.I don't know enough about the so called halo effect on copper or iron objects leaching into the soil...this may help a detector to pick up targets that are deeper...but even if this is true it's not down to the detector going deeper it's due to the object becoming bigger and getting closer to the peak on the detectors magnetic field.So if u do an air test on an inert object such as silver or gold the maximum depth u get in air(under ideal conditions)will be the maximum your machine will detect in ground.I'm sure people will disagree on this point but all u have to do is ask an electronics engineer who specialises in electro magnetics and they will say exactly the same.

 

[BarberBill]

Thanks, that makes sense to me. I know I have one detector that AIR tests better, out , away from home where there is no possible interference. The statement that in totally mild ground the detector should test pretty well equally on the same target makes sense to me. It apparently isn't a matter of detecting thru X amount of ground as much as how much mineralization is the signal trying to deal with. Thanks for the clarification.
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[ToddB64]

Dear friends,

Seems to me that moisture content would play a big roll in AIR testing versus IN-THE-GROUND. It is common knowledge that conductivity is enhanced by water. So, in order to conduct a valid comparative test, one would want to use an inert metal like gold of high purity as the target, to exclude the possibility of "halo" for the in-the-ground test. The moisture content would need to be controlled via test setups, i.e. the air in the air test would need to have the same moisture level as the soil in the in-the-ground test. Furthermore, the soil used for the in-the-ground test should be clean, i.e. free of extraneous metal objects and any minerals known to be disruptive to the magnetic fields created by the metal detector and the gold target. In addition, the tests would need to be conducted in an area free from electrical and magnetic interference. In short, laboratory type testing under controlled conditions, to insure "apples to apples" comparison and valid test results. Without this degree of control, test results would be questionable. If I missed any other important factors, these would also need to be considered and controlled during the tests.

An explanation based solely on theoretical analysis is interesting reading, but physical tests as above would yield the final proof.

Todd

 

[Shambler]

The main reason that air tests are not to accurate is because soil conditions will prevent the detector reaching the maximum depths it can attain in air.....not the other way round.

This isn't true for multi-freq units.

 

[slingshot]

I don't care what the EXPERTS or TECHIES say, there IS a halo effect under certain conditions. In the 1980's I had an AH Pro, and if anyone remembers you could max out the threshold and get a penny at 5", if you listened hard. I had the Pro set at PULLTABS because I was in a field LOADED with them. All at once I got a HUGE signal and my immediate reaction was aluminum can. I raised the coil 4-5" and sure enough I got a wide signal still. I took my hunting knive and plunged it into the ground expecting to drive it through a can and pull it out of the ground-NOTHING. Digging an 8" hole, I plunged the knife all the way to the hilt and heard the sickening "clack" that meant coin-or at least something hard. It was a Silver dollar from the 1800's with a nice gash in it from my knife. Since I was lifting the coil and Still getting a signal, that means that technically the detector was "seeing" the coin at 14-18" Here is another oddity: immediately after a rain, the depth decreases, but a day or two later, it increases in the same area after it has soaked in the ground. Also, back in the old days, old timers used to take hand-crank telephones with wires attached to rods and "electrify" the ground to find the deeper coins by the magneto in the phone. Also, they would hunt a day or two after the soaking rain.

 

[Nauti]

The main reason that air tests are not to accurate is because soil conditions will prevent the detector reaching the maximum depths it can attain in air.....not the other way round.

This isn't true for multi-freq units.

I't's true whether your machine is multi frequency or single frequency.Also the adverts stating that multi frequency machines go deeper than single frequency machines is hype.In fact i think c-scope proved beyond a shadow of a doubt that a good single frequency machine is as good or in some cases better than a multi frequency machine in most situations except wet salt sand.In fact i believe that c-scope actually managed to get an advert for multi frequency machines removed from a certain publication for claiming multi frequency was deeper than single frequency.Manafacturers of multi frequency machines will always state that their machines find older,deeper targets but none will now state they go deeper than a single frequency machine.

 

[thejetman]

Has someone e-mailed Mythbusters yet?
Seriously, I think there needs to be a test, with a bunch of different machines, and some sort of variable. Being an aircraft mechanic, I can tell you this, everything corrodes. And, I know that the "halo" effect is real, if you have two dissimilar metals that are in contact with each other, they will eat each other. Add a dielectric, like water, and it speeds up the process. All metals want to go back to their natural state. The minerals, and oxides that leech out from coins, wont go very far around the coin, but it may make a difference. When my 3D comes back from service I might whip up a test to see.

Just my .02

 

[Shambler]

SILLY TECHIES using that dern science and stuff !!

For one, look at the square sine wave of a multifreq unit and how harmonics effect it AND what happens to that wave/harmonics in tightly packed dirt (or dirt bound with water after a rain). Even when you consider noise to signal ratio an air test could easily fall flat on its face based on EMI. You're imagining light or sound and figuring the air to be "clean" while the ground is "dirty" (or really hard to get through).

Also, I don't remember seeing any OEM marketing saying that BECAUSE a machine was multi-freq it was deeper. VLF technology is going to tank at around 12" for a coin and Fisher attained that depth 25+ years ago. The atmospheric noise becomes indistinguishable from a signal at around that point (at least in affordable and portable technology - meaning powered by light weight and cheap batteries). The multi-freq machine will most likely be slower but will ID better. It's a trade off. The F75/F70 is still faster than the e-trac but if the e-trac has you digging half of the trash, which is really faster to use?

On the halo effect, it would seem like halo would occur more often in naturally poorly conductive material which is not what you want your detector to see anyway. Or do I have this backwards based on the misnomer of conductivity in detecting? It would be interesting to look at the signal from a degraded piece of iron and see if its "softer" and allows the nearby conductive metal to be more easily "seen". Hmmmm Mythbusters might be the key after all - or at least NASA Tom.

 

[BarberBill]

For what it's worth, I believe we see a halo effect with iron and copper etc., but I don't buy into that on silver and gold coins. If that were the case, older silver or gold coins would be pitted and corroded when they come out of the ground and they're not. I seem to have opened a lively subject here which may never get everybody to agree with, but gives us all something to think about.
HH
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[Leslie(nova scotia)]

From the land of the Bluenose.....that is a good point and not a true test of a machine four shore me bouy as so many other factors come into play such as battery strength, moisture and the atmosphere.

 

[Nauti]

I agree with what you're saying about the halo effect on silver and gold BarberBill...there is'nt one.If the metal is non corrosive it cannot produce a halo effect.

 

[BarberBill]

Well it seems to me there really are a good number of factors. Another one would be if the comparison in the ground was in fairly mineralized ground using a DD coil versus a concentric as well as the size of the coil(s) being tested..
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[Nauti]

Thats another complicated issue and one that's not as clear cut as people may think.

 

[Larry (IL)]

In my 25 years of hunting, I have seen the Gold and Silver halo effect hundreds of times. With less Nobel metal's like copper and iron the electron depletion will change the iron and copper to oxides. With more Nobel metals like Silver and Gold the electron loss is reversed, (Anode-Cathode reversal) and it is the surrounding ground that will have the change in chemistry due to electron loss. This is sometimes visual in the right ground, often resulting as a darker soil "halo" around the target in lighter colored ground.

For more info on Galvanic corrosion or Sacrificial anodes, Google or click here:

http://en.wikipedia.org/wiki/Sacrificial_anode

 

[thejetman]

In my 25 years of hunting, I have seen the Gold and Silver halo effect hundreds of times. With less Nobel metal's like copper and iron the electron depletion will change the iron and copper to oxides. With more Nobel metals like Silver and Gold the electron loss is reversed, (Anode-Cathode reversal) and it is the surrounding ground that will have the change in chemistry due to electron loss. This is sometimes visual in the right ground, often resulting as a darker soil "halo" around the target in lighter colored ground.

For more info on Galvanic corrosion or Sacrificial anodes, Google or click here:

http://en.wikipedia.org/wiki/Sacrificial_anode

thats what i was sayin

 

[BarberBill]

Interesting and informative link.
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[pmike]

What about such variable as the conductivity of soil? By that I am wondering if there are certain conditions in which the soil could act as a conductor in a manner akin to a copper wire that is able to conduct current from one point to another. In other words if a coin that was beyond the normal range of a detectors depth is in a highly conductive soil, could that soil enable the detector to react to the coin because of the soil???

Mike

 

[slingshot]

Even Karl Von Mueller, one of my favorites, believed that things such as the humidity and other weather conditions affected the depth and performance of metal detectors.