Would like an explanation?

[XT18000]

I think your trying to separate pepper from fly s--t !!!!!! I for one, have NEVER found any gold, sliver, iron, alum. or any thing
else in the wild with a detector up in the AIR! Every detector made and sold is for detecting metal in the ground not floating around
in the air. When metal starts floating in the air then we need to be concerned about how far it can be detected ( but then we won't
need detectors will we )

 

[Chris(SoCenWI)]

My gut feeling and less-than-scientific tests do not support this theory. Looks to me that the machine will never/seldom do better in ground than in airtest. I think there are MANY older machines that do way worse in the ground than in an air test because of ground mineralization. I think many of these were optimized for air tests as a sales tool. The newer machines with auto Ground balance might not do as well in an air test - and hence the idea they do better in the ground- but in reality they just don't lose as much depth in the ground as the older machines.

There may be a possiblility that in wet non-mineralized sand the moisture may help get greater depth. Haven't played with this but have heard credible reports of coins at great depths.

As far as the halo theory.... Saw that addressed by a thread somewhere recently that was an interview with a bunch of Metal Detector designers. Basically no halo what soever for gold or silver, a little for copper, and perhaps a bit for iron. Basically it is all chemistry and how prone a coin is react with the ground chemicals. I have seen places that have flooded repeated that every zinc cent gave a silver scream. But as is obvious zinc reacts hugely with certain soils.

 

[Chris(SoCenWI)]

n/t

 

[slingshot]

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

Thanks, this makes more sense and gives me a reference point. Technically, there may be no halo with gold or silver, but in the real world I have found LOTS of silver with "weak" machines at astounding depths-as much as 3-4 times their air test and normal in-ground depths.

 

[Larry (IL)]

The biggest problem with the "Nay-Sayers" on Halo's with Gold and Silver is that a very small percentage of the targets are in the perfect environment to have the possibility to even develop a halo. It is equivalent to me saying that there are no gold nuggets to be found because I have no gold nuggets in the ground where I hunt. (Central IL) But I do have an abundance of targets with halos.

For conditions to be right for a Nobel metal to have a halo takes four factors and the size of the halo is also affected by the four factors. You must have two dissimilar metals, one being an anode and the other a cathode, a dielectric and time. The metals are the target and usually mineralized ground. The dielectric is moisture, the saltier the better and time is a given. The "micro battery" must sit undisturbed for a long time, how long depends on how strong the dielectric is and how conductive the combination of the first three are. For me, it seems to be about 50 years for a gold target to develop a good halo, silver somewhat less. This is why some people will find targets in the ground that they thought was worked out after a rain. The moisture back in the ground will reactivate the "micro battery" with a new dielectric to complete the electrical circuit.

Keep in mind that a metal detector is used to induce an electrical current into a metal target and, "simply put", the detector measures how much current was induced and how fast that inducement decays in the target to give you an idea of what you have found. Now add the naturally made electrical activity, "Halo", to the equation and you will have a larger electrical image for the detector to evaluate resulting in/or a target that is much deeper in the ground or appears to be larger than the target really is.

The best example I can give you would be a 1927 men's Gold class ring I found near an old swimming hole just 10 feet from the lake that is still there. I had the target, mineralization, moisture and time, and my DFX indicated an Indian Head or wheat cent, VDI 72, at 8 inches. At 12 inches, I pulled out a beauty of a ring that now VDI'd a 55 out of the disturbed "halo" environment. A very obvious dark circle of soil was around the ring in the light grey clayish ground.

I don't think that the commonly known misnomer that the Gold or silver leaches into the ground is true at all, it is just a simple matter of electrons doing what they do. The process is known as Galvanic corrosion.

 

[nw1886]

The halo effect is real (Techies be damned!) Coins are only partially made of minimally or non-reactive metal and the copper/tin does leach out and CAN influence detectability/VDI response. That powdery/white coloration on silver coins is the result of this and can bet that it's pure silver with the copper/tin is out in the surrounding "halo" skewing response. (Other types of halo effect are tough to measure but that one is a slam dunk.)

 

[Shambler]

Looks to me that the machine will never/seldom do better in ground than in airtest.

This shouldn't be true of any multi-freq machine and the science of harmonics (created by the square sine waves that are a byproduct of the two frequencies) makes it not open to debate other than to assume the scientific FACTS didn't translate to the end product. Too bad we can't get Dankowski to chime in (no pun intended!).

I also think some folks misinterpret the word conductivity in this context. It's less like a copper wire conducting electriity as mentioned in an earlier post and more like a radio antennae distinguishing signal from noise. You can only do so much with your squelch to hear the signal. At some point the noise and signal sound the same. Trying to compare a systems ability to recognize interference in a 3 - 50 Khz signal in the air doesn't make much sense. Heck if you're within 20 miles of an airport (or a NAVAID in a field) and air testing, you might get significant differences when you put the coil on the ground vs. in the air. If you're doing your airtesting on a hill, or your 4th story apartment then you're REALLY getting skewed results.

Come on, there's got to be an engineer or two reading these that can enlighten all of us one way or another...

 

[slingshot]

It had better be an engineer who can explain the increased depth and coloration of the surrounding matrix to convince me-the RESULTS are too obvious

 

[BarberBill]

Well, halo or not, I still haven't seen an explanation that would explain why supposedly some (but not all) detectors would test poorly in air, but do better in soil. Seems to me that all should test the same (better in soil or better in air) within the limits of their power output, frequency, and coil sizes unless some company comes up with a technology other than the electronic/magnetic signal.
BB

 

[Larry (IL)]

Sorry Bill, we got a little off of your subject. Some detectors have a preset ground balance while others need to ground balance to the ground for proper and best operation. The detectors with the preset GB seem to do better with air testing. In my opinion, air testing has limited usefulness like how does this target react to a particular coil or detector. Depth testing can vary widely from actual use.

 

[BarberBill]

Well, I've understood the difference in preset ground balance versus manual and true auto ground balance for a long time. However, my impression from things I've read implied that some machines just plain get more depth in ground than in the air. Maybe that's a mis-understanding on my part in what was written or implied . Certainly a properly ground balanced machine should get better depth in the ground than an improperly balanced detector, or a preset (especially if the preset is reading difficult ground), but I'm betting it'll be less inches than the air test, all else being as equal as reasonably possible. Even lightly mineralized ground has to offer more resistance to the signal than air, regardless of the make or model. However, I do agree that air testing is just a limited way to make any comparison as the actual depth in the field is what really matters in the end. If the snow would hurry up and melt here, I'd be out detecting and trying to prove that, rather than spending so much time on the forums. In any event, this turned out to be an interesting and lively thread.
HH
BB

 

[Jackpine Savage]

The quartz sand found on many Lake Michigan beaches saturated with hard water (calcium carbonate) can return a very strong signal on deep coins. It's called resistivity and changes due to differences in the size and shape of the sand particles and the overall density (compactness of the soil) affecting the amount of trapped moisture.

Tom

 

[Jackpine Savage]

deep coins don't need any mythical halo's to be seen either!

 

[UK Brian]

Why should you think a properly ground balanced machine would detect deeper ?
The ground balance doesn't affect how deep the detetector searches, the ground effect remains, it just appears to by reducing the ground noise that may be blocking out weak and deep signals.

Offset the ground balance markedly (positive) and you will start to hear every little bit of metal in the soil under the coil. If you have found or suspect a hoard then its better to have an incorrect balance and then make a judgement of the extra signals you can hear. This can be really effective but would only be used over small areas because the extra noise will start to desensitize your hearing and you will come to a point you are hearing less weak signals.
There's a few cases where you might want to use a negative "offset" ie to eliminate fragments of hot rocks but you don't want to offset the correct balance this way if it can be avoided due to lose of sensitivity. But a small positive offset is almost a must in general detecting.

 

[UK Brian]

This doesn't take into account the old non motion pulse machines that didn't test to well in air or dry ground but had very good performance in damp and wet sand. The difference between motion pulse and non motion doesn't seem as marked with modern P.I.'s but the wet ground still carries the signal further.

 

[BarberBill]

Brian, I've come across the statement several times that a poorly balanced manual GB machine will give poor performance. Haven't , till now, had any reason to question that. However, if it is set so far off as to be falsing or giving erratic signals (at the user end) in my mind it doesn't matter how deep it gets as the user can't utilize it well, anyway. As far as ground balance only canceling the noise from the ground effect, that's true, but extreme mineralization will reduce practical effective depth as far as the user is concerned so theoretical depth past that is a moot point. Anyway, I'm digging as deep as I care to most of the time, so I'm happy to live with what we have.LOL
HH
BB

 

[UK Brian]

I detect for a living so need an edge. Best performance is always with the true "ground balance" point being offset. As I said in the post further up the page there's times when you need to put up with the extra noise which can add as much as three/four inches to the inground depth but you would not expect to detect more than a limited area at these settings.
My reply was concerning the depth Pulse v VLF( or IB/TR) where the wet salt improves performance of the P.I. design. The pulse have no ground balance control what so ever in the case of most machines until recent times. Even with the newer P.I.'s best performance on the beach is with the G.B. turned off.

 

[Wayne/TX]

Brian, all you say is true, IMO. It's certainly also true that ground
moisture plays a role in depth penetration of the EM field in most
soils as opposed to very dry ground, just ask any gold nugget
hunter in ironized gold bearing soils. And it's logically true that
an AIR test does NOT require any ground balance setting other
than a pre-determined in-ground "neutral" setting of the GB
control just for "stability" of the circuit's transmitted signal in
a non-mineralized medium such as air, even though such
effect would not be crucial for sensitivity tests.

It's also true that certain conductive salts or crystalline
"saturation" effects can increase the penetration of the
field within a Moisturized matrix. But we must always be
cognizant of the fact that GB is only necessary for
decreasing mineral effects on the hearing of eddy current
effects on the receive circuit. And the inclusion of a manual
or "auto" threshold control will always yield the best audio
response to deep targets, especially in all-metal or pinpoint mode.
So the logical conclusion should be that Barber Bill's question
is moot in most cases and Air tests will generally be the most
indicative of a detector circuit's sensitivity to any metallic target,
regardless of what soil matrix is involved.
..W


With the advent of VLF waves, it of course became possible to
more easily detect metals in an iron rich soil matrix as opposed to
earlier high frequency TR circuits, thus revolutionizing the search
for all metals within such media, almost equalizing an air medium.
Thank you George Payne!

 

[Mike Hillis]

Have you ever dug a hole, plopped a coin in the bottom of it, covered it up, ran your detector over it and not get a signal? But yet have often dug that same type of coin at greater depths and you stand there scratching you head wondering whats going on?

That happens because the detector electronics are comparing two signals: The ground signal and the target signal. One of these measurements is the "rate of change" of these two signals.

The ground signal is very strong and fairly steady. It normally changes slowly over distance. The target signal changes rapidly over distance. Your detector is designed to compare the "rate of change" of these two signals. In the example above in which a hole was dug and a coin dropped in, disturbing the ground changed the rate of change of the ground signal. Another words the ground became the target signal which in turn hid or "masked" the coin signal at that particular depth because the ground signal was tuned or balanced out. The coin signal's strength and rate of change wasn't great enough to overcome the rate of change and signal strength of the disturbed ground. In real hunt conditions the buried coin lies in undisturbed soil and the rate of change is enough to raise the coin signal above the ground signal. The rate of change of the coin actually allows it to spike high enough above the slower changing ground signal to be picked up and reported on.

The designers and engineers create detectors for different purposes. Some detectors are designed to work very well in freshly disturbed soil, like ploughed fields. Others are designed to work very well in dry ground conditions, and so on. Chances are your favorite detector is your favorite detector because it's design works very well in your ground conditions for the targets you most often seek. But you may hate using it in different ground conditions or when looking for a different type of target. Most often that is why we have multiple machines.

The detector's electronics do not measure air. Air doesn't give a signal to compare to. Air tests only reflect the rate of change of the coin without the needed ground measurement. Kind of like driving with only one eye. Yes you can still see but you have no depth perception and and you can't see whats there on the far side of your nose

Hope that helps.

HH

Mike

 

[jim tn]

n/t