To What Extent Does Iron Shield Target Detection?

[CliveHamy]

Now I think it is widely accepted amongst experienced detectorists that iron next to a non-ferrous target can shield or even hide that target. It depends on their relative positions and the sweep of the coil. - Somewhere in the Equinox (or Manticore) threads there are some great examples. If the iron is slightly to one side and a smaller enough coil is used, and a sweep is made at the right angle, it is possible that a detectorist will pick up that there is a chance that a non-ferrous target present amongst the iron. I came across an article dated back to 2010 from Nasa Tom that suggested that even a small office iron staple close to a non-ferrous target can shield that target and make it extremely hard to notice. What was more shocking was that Tom explained that he found that if the tiny iron staple was placed above the non-ferrous target but too deep for his machine to detect, then no signal of anything was detected. That was before more advanced smf machines like the NOX were available. - I'd love to know whether NASA Tom or others think that with the current machines this is much less of a problem?

Now there are times (working in Archaeology) I want to be sure that I have cleared an area of all targets. I know if is grid one way and then at 90% and investigate all targets I will have covered the whole area, but this has got me wondering how many I have missed because they have been shielded by iron signals that I have ignored, or not even heard at all. It was implied in the 2010 paper that a large % will be shielded and missed. - What's your opinion folks and what evidence is there to back it up?

Cheers Clive

[snakejim]

If you are hunting with a detector in a true all metal mode, there should be no metal left where you have detected unless it is too small or too deep for the technology you are using.  A lot of newer technology has been developed in the last 15 years to deal with the iron masking issue. There are a lot of videos on YouTube and elsewhere. I believe if you search this forum there as several discussions on this matter.

[Wild Bill]

Hi Clive, 

you might want to test the detector you’re using by actually putting coins in the ground with iron like a nail, screw or a little piece of sheet metal next to and covering a coin or several of different sizes.  See how your detector reacts.  The other day I was out with a buddy on the wagon train trail in Northern Nevada.  Had a similar situation with lots of rusty iron particles, pieces and chunks in distinct patches.  I believe these are the remains of wagons that didn’t make it.  That’s the history in this location.  An epic disaster with thousands of dead and dying livestock and hundreds of broken down wagons.  A desolate unforgiving land.  Somewhat of a spiritual experience when you see it.  Anyway I was using a Garrett AT max and stock coil 8.5x11 DD I think.  Highly mineralized ground as well.  All metal setting was just too much so I used Zero and Custom.  In the iron patches I set iron discrimination at full (44) with iron audio on.  I wanted to listen to it, all of it.  Lots of crackling , grunts and pings on what little conductivity was left in these iron pieces.  I still got distinct tones that stood out.  Some bullets, shells, mostly modern and one tiny brass percussion cap at six inches with iron particles everywhere.  I thought that was pretty cool that it was an obvious signal.   Couple of bullets are very old and a couple buckles that are real rusty.  One was in mud and has leather still attached.  I can’t speak for all detectors, just the ones I’ve used.  That’s my suggestion… test it if possible.  Best of luck and enjoy! 

[Fugio]

As snakejim said, if you're using a machine in all metal mode (with true all metal), it should clear out all targets. If you're trying to pick out non-ferrous targets out of the ferrous with any discrimination, you will miss nf targets. You might get 90%+, but some will be masked. No machine will unmask everything with discrimination. Besides, in an archeological site I would assume the ferrous/non-ferrous targets are equally important. Sure, I pick out all the easy non-ferrous targets in my early sites first, but eventually I also clear out the ferrous and have made some great finds doing so. 

[CliveHamy]

14 hours ago, snakejim said:

If you are hunting with a detector in a true all metal mode, there should be no metal left where you have detected unless it is too small or too deep for the technology you are using.  A lot of newer technology has been developed in the last 15 years to deal with the iron masking issue. There are a lot of videos on YouTube and elsewhere. I believe if you search this forum there as several discussions on this matter.

Hi Snakejim, I think you have missed the point I'm making. Reading Nasa Tom's article of 2000 https://www.dankowskidetectors.com/behindthemask.htm he outlines the danger that iron may shield a good target and in itself not be detectable. i.e. you get a quiet area. If that is the case how do you know that there is no metal left? Now it is a no brainer that to clear an area of everything (including iron possibly shielding good targets) you should detect in "All metal mode" (and not have any discrimination, and to sweep the area from multiple directions, and to change the sensitivity to push the boundaries of mineralisation). Yes we've got a lot better technology since his 2000 article, and yes I've monitored this forum for years on this issue. But what I'm doing is raising this issue to alert people to the possibility that what they assume is clear of targets (how many times do people say on this forum they've cleared an area, but gone back with a different machine and found some more?) may not be the case.

I'm hoping that someone has done further experiments to test for instance situations of silence when they know iron and a target is present (cause they put it there!) and that NASA Tom or another expert comments on how valid that original 2000 still is for machines of today. 

Cheers Clive

[CliveHamy]

10 hours ago, Fugio said:

As snakejim said, if you're using a machine in all metal mode (with true all metal), it should clear out all targets. If you're trying to pick out non-ferrous targets out of the ferrous with any discrimination, you will miss nf targets. You might get 90%+, but some will be masked. No machine will unmask everything with discrimination. Besides, in an archeological site I would assume the ferrous/non-ferrous targets are equally important. Sure, I pick out all the easy non-ferrous targets in my early sites first, but eventually I also clear out the ferrous and have made some great finds doing so. 

Hi Fugio, it's a given that All metal is used, that there is no discrimination and that as an archaeologist all signals are of merit. (unless you are on a battlefield or area of high mineralisation, where there are so many iron fragments that there is not enough time and resources to dig everything and so a planned sampling/discrimination approach is the only practical approach)

[CliveHamy]

13 hours ago, Wild Bill said:

Hi Clive, 

you might want to test the detector you’re using by actually putting coins in the ground with iron like a nail, screw or a little piece of sheet metal next to and covering a coin or several of different sizes.  See how your detector reacts.  The other day I was out with a buddy on the wagon train trail in Northern Nevada.  Had a similar situation with lots of rusty iron particles, pieces and chunks in distinct patches.  I believe these are the remains of wagons that didn’t make it.  That’s the history in this location.  An epic disaster with thousands of dead and dying livestock and hundreds of broken down wagons.  A desolate unforgiving land.  Somewhat of a spiritual experience when you see it.  Anyway I was using a Garrett AT max and stock coil 8.5x11 DD I think.  Highly mineralized ground as well.  All metal setting was just too much so I used Zero and Custom.  In the iron patches I set iron discrimination at full (44) with iron audio on.  I wanted to listen to it, all of it.  Lots of crackling , grunts and pings on what little conductivity was left in these iron pieces.  I still got distinct tones that stood out.  Some bullets, shells, mostly modern and one tiny brass percussion cap at six inches with iron particles everywhere.  I thought that was pretty cool that it was an obvious signal.   Couple of bullets are very old and a couple buckles that are real rusty.  One was in mud and has leather still attached.  I can’t speak for all detectors, just the ones I’ve used.  That’s my suggestion… test it if possible.  Best of luck and enjoy! 

Thanks Wild Bill, yes I've done my own experiments and followed the threads on others. I was looking for further feedback to see how valid Nasa Toms 2000 article still is. I agree that highly mineralised and ground with lots of Iron in it makes it very challenging to dig, let alone investigate every iron signal in case it is shielding something. But what about the quiet areas with no signals, are they really devoid of targets, or are they being silently shielded as suggested by Tom's  https://www.dankowskidetectors.com/behindthemask.htm. ?

[snakejim]

Thomas J. Dankowski stated "Are there remaining items (including coins) in the baseball diamond that all three detectors missed? ABSOLUTELY! Awaiting future technology!"

That is an article from the year 2000. I rest my case!

[Geotech]

5 hours ago, CliveHamy said:

Now it is a no brainer that to clear an area of everything (including iron possibly shielding good targets) you should detect in "All metal mode" (and not have any discrimination...

Tom's experiment was valid but his conclusion -- that the staple altered the TX field so much that it could not energize the dime -- was not. He even more-or-less proved it by using the SD2200 to find more targets in a "cleaned-out" area. If iron was suppressing the TX field, it would still be happening with the SD2200. Also, his comment that an 8" Double D coil will generate a signal (hypothetically) 8" long by 1" wide is also not true; the TX field of a DD looks about the same as that of a concentric.

The culprit that causes the masking is the motion filters used for ground balance. It's not good enough to hunt in all-metal or with Disc set to zero because, for many detectors, the motion filters are still fully engaged. Instead, you need to use a static all-metal mode. Some detectors used to include a full static AM mode but for any that still do it's implemented as a static pinpoint mode for which you have to continuously hold down the pinpoint button. Anyway, when you test a dime with a staple above it you will then find that the staple has practically no effect on how deep the dime can be detected.

If your detector doesn't have a static AM mode then the next best thing is an AM mode with a single derivative filter. This is commonly how VLF gold detectors operate or MF detectors with a prospecting mode. Not as good as a true static AM mode but better than a double-derivative VLF-motion mode. The SD2200 effectively operates this way and is even closer to a static AM operation than a VLF gold detector.