Iron masking/Silent Masking. Credit: Thomas Dankowski

Blackadder43
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This article was written by Thomas Dankowski (NasaTom) 20 years ago, but the information contained compared to recent machine technology is still the same and relevant

http://www.dankowskidetectors.com/behindthemask.htm

I posted this on someones topic, but it was decided it needed its own topic to be able to discuss it further :thumbsup:
So yer iz
I'm going to use some forum magic to transfer some of the replies to this thread, so as to start it off
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Blackadder43 wrote: Mon Dec 06, 2021 11:06 am I'm sure most folk are aware that when people speak of 'masking' targets by having iron near by, it doesnt always mean they are talking about horseshoes, or big rusting items
This article i am linking below was done by Thomas Dankowski or 'Nasa-Tom' as he is also known
He has worked for Nasa, and also for Fisher too and was instrumental in the development and fine tuning of the CZ-3D

Anyways, the man is a genius, and this article is well worth the read to get a better understanding of how tiny a flake of iron can be and still mask a coin

http://www.dankowskidetectors.com/behindthemask.htm
Very interesting article, and I for one did not fully appreciate how masking worked....it also struck me that when we talk about targets being brought up by the plough, in a lot of cases they are probably just being unmasked by the soil being moved/disturbed...
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Blackadder43 wrote: Mon Dec 06, 2021 11:06 am This article i am linking below was done by Thomas Dankowski.

http://www.dankowskidetectors.com/behindthemask.htm
Very interesting, thanks for the link, but I'm not sure what to do with the information. Do I now dig every iron signal just to make sure there's not treasure underneath or dig silent ground because a flake of out of reach iron may be masking deeper treasure :? It's also another variable in the machine debate. But it's a good article on why detectors aren't as smart as the ad men (and women) would have us believe.

I wonder if this also accounts for a couple of things I've seen both when I had the T2 (shouldn't have sold that) and the Nox. First, if I get a good signal that has a hint of iron (often older shottie or rotted out cartridge bases) the iron part can 'build' with successive sweeps. Second, good signal, take out the clod, now upside down, and no signal from the clod. Turn it back up the right way and the signal returns - it can't be due just to depth.
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DaveP wrote: Mon Dec 06, 2021 1:08 pm Very interesting, thanks for the link, but I'm not sure what to do with the information.
Glad I am not the only one pondering that!!

Just wondering if listening for nulls in the Threshold will tell us anything here???

That said, the more I read the article, Tom's results just don't seem to match what I get in a lot of cases...I wonder if air tests would demonstrate this masking effect or whether targets need to be in the soil?
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DaveP wrote: Mon Dec 06, 2021 1:08 pm Very interesting, thanks for the link, but I'm not sure what to do with the information. Do I now dig every iron signal just to make sure there's not treasure underneath or dig silent ground because a flake of out of reach iron may be masking deeper treasure :? It's also another variable in the machine debate. But it's a good article on why detectors aren't as smart as the ad men (and women) would have us believe.
As a hobby detectorist then this information is just overload, and can be read and pretty much ignored
For the more geeky of folk, then its interesting and causes you to think about a) whether it applies to your fields and b) can it be mitigated

I can think of 1 situation where this information would come into play
Lets say 1 of your fields has produced a really special find, something that you would expect to be in a hoard or burial type scenario
You know this field is noisy with iron, so you can safely assume there are more targets that are being masked, so you would relax your digging criteria to be able to start a)clearing the area of iron finds and b) hopefully unmasking the remains of your special find
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I can get my head around the concept if you are detecting with iron discriminated out, but if you are in all metal, getting the same results would be a bit mind blowing, at least to me....what I find especially interesting is that Tom says this also holds true for small non ferrous targets and quotes the example of a lead BB buried at too deep a depth to generate an audio response in the detector, but can still mask a coin a few inches deeper.....I suppose there must be a break point, as I can't imagine a BB masking a ali drinks can a few inches deeper for instances....
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I have one part of a field that is positively bursting with small iron. I have, when my sanity allowed, gone over it very, very slowly with the 9” HF elliptical coil and managed to winkle out two round half pennies, a cut quarter, a medieval casket key and a bit of a silver thimble. But by god it’s hard work and none of these signals were clean ‘good’ ones. They were all no more than 2-3cm deep either.

I am convinced there’s a treasure trove of other good finds there but there is no way I can hear anything below that top layer as they are all masked. It’s a small area - maybe 10 foot square. I’d love to dig it all up but it’s complicated by big tree roots and lots of pottery near the surface. Not sure if knowing more about masking makes me more or less inclined to bother to go over it again. I guess less inclined as there’s no real point trying to find something the machine can’t detect.

Interesting article :thumbsup:
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Blackadder43 wrote: Mon Dec 06, 2021 2:07 pm This article was written by Thomas Dankowski (NasaTom) 20 years ago, but the information contained compared to recent machine technology is still the same and relevant.
An interesting article for sure, it makes me want to do some similar tests. I have only gridded my back lawn with the Nox 15" coil so far and there is variable iron about so maybe will give it a go over with the 6" coil. Though not sure if I would want to dig all with my PI, but then again it's not a big lawn and could have some interesting results, or not! :Thinking:
The more recent detectors have better target separation and often better depth than a 20 year old detector but iron masking principles likely don't change. Though I'm not convinced that a poxy iron staple would block out a non-ferrous target from the Nox, I best check that too! :o :thumbsup:
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Blackadder43
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Easylife wrote: Mon Dec 06, 2021 8:28 pm Though I'm not convinced that a poxy iron staple would block out a non-ferrous target from the Nox, I best check that too! :o :thumbsup:
Very interested to see your results easylife if you do recreate this experiment with the Nox :thumbsup:
I'm not sure if the physics of the detecting signals has changed in the last 20 years though from this analysis:
Here is what's happening: The electromagnetic energy from the coil punches into the ground and hits the staple producing an electromagnetic "halo" around the staple. The signal strength is dramatically reduced, and IF any energy does manage to make it down to the dime (very doubtful) - that signal must then return back to the coil - but will fail on the return path when it hits the staple "halo" again. (Metal detector coils operate under the principle of inductive coupling).
I also have an experiment i want to conduct based on the principle of Dankowskis writings here, but i need a nice rolled field and dryish conditions to set it up
I will video it for future useage
Any takers within 100 miles of me? :ugeek:
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For what ever reason, I think the article over simplifies the situation and omits a lot of background information.
The first thing that struck me is that although the ferrous metal staple is physically quite small, because of its electrical properties, it probably gives a much stronger response than we detectorists realise....

I would therefore suggest the first part of any experiment would to adjust the iron volume settings on the detector to match those of the non ferrous volume settings, and then compare the audio strength of first the staple and then the coin, from the same distance.......I have a hunch the volume of the ferrous audio signal will be surprisingly high...

If so, this could perhaps explains how such a small ferrous target could mask a some what larger one in certain situations...
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Blackadder43 wrote: Mon Dec 06, 2021 8:53 pm Very interested to see your results easylife if you do recreate this experiment with the Nox. :thumbsup:
I'll let you know if I have any different results. :thumbsup:
But do we know whether Tom was using a concentric or DD coil? Even the 15" Nox DD seems to give quite decent target separation though basically just a longer blade of sorts!
I expect that lots of us have best intentions of experimenting in the field though rarely do as no doubt keen to find something of interest?
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Here’s a visual representation of the same thing curtesy of XP.
CA389841-AC15-4DC0-BA4B-62EA0E81C1A8.jpeg
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Blackadder43
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Easylife wrote: Mon Dec 06, 2021 9:52 pm I'll let you know if I have any different results. :thumbsup:
But do we know whether Tom was using a concentric or DD coil? Even the 15" Nox DD seems to give quite decent target separation though basically just a longer blade of sorts!
The machine he used in this experiment back then was "Fisher CZ-6a metal detector. "
So i assume there was only 1 type of coil for that machine?

The repeated experiments was done with the again the "Fisher CZ-6a metal detector. " followed by "My White's 6000 Di Pro and Minelab Excalibur 800 equally failed. "
Not sure of stock coils for these machines
(invariably on a machine test he will use stock coils, unless otherwise stated)

Easylife wrote: Mon Dec 06, 2021 9:52 pm I expect that lots of us have best intentions of experimenting in the field though rarely do as no doubt keen to find something of interest?

The experiment i want to do is related to this in a way, but not signal based or even using a detector :ugeek:
Blackadder43
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Oxgirl wrote: Mon Dec 06, 2021 9:54 pm Here’s a visual representation of the same thing curtesy of XP.

CA389841-AC15-4DC0-BA4B-62EA0E81C1A8.jpeg
Thats assuming an iron object 'close by'

Dankowski experiment was 'directly over'
Reactivity wont make any difference in dankowski scenario

also as the machine sees the staple irrispective of whether its discrimed out, notched or any other form of hiding iron, and the iron whatever it might be is directly over the good find
The machine still see's it, usually first and its presence affects the signal going down and the same signal returning to your machine
Therein lays the issue he is pointing out

Technology has not made a solution for this scenario as yet :thumbdown:
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Blackadder43 wrote: Mon Dec 06, 2021 10:08 pm The experiment i want to do is related to this in a way, but not signal based or even using a detector :ugeek:
Watch out, it looks like Bruce has his divining rods out! :o :lol: :thumbsup:
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