Certain metals detectable by ground scan? 4

[bennylava]

Hi all. I don't get by here much but I wanted to ask a question that I've been curious about, since my father inlaw has decided to move to Alaska to mine for gold. I'm no metallurgist, I am but a lowly engineering student that is still seeking my associates degree. But it does seem to me, with my limited knowledge of such things, that one should be able to at least tell what type of metal is under the ground (or laying on your shop floor) with a scan of some type.

For example, doesn't every metal give off a certain, measurable magnetic field? If so, wouldn't some instruments or scanners be able to pick that up, and then be able to tell you what type of metal was there? I was wondering, because the method he's using to try to get some of the gold seems... archaic at best. Simply letting water flow through a sieving machine, and collecting the particles. Well of course anyone who thinks about it, would realize that there are probably much larger sized chunks of gold, at various depths beneath the bottom of the river/stream that he is mining in. And the water is only 1 foot deep (or less) in some places.

Of course the trick is locating the larger chunks, as the river/stream is miles and miles long. Where do you dig? How deep? Is there some way to pick up the weak magnetic field that gold has? Or am I barking up the wrong tree here? I haven't gotten far in my engineering studies, but I remember hearing somewhere that all metals have a magnetic field which surrounds them, but of course this will vary from metal to metal. Thus, gold would have its own, and once you calibrated your equipment or had sensitive enough equipment, you may be able to tune it to detect only gold.

Is this possible? If not, how far off base am I?

 

[IRstuff]

But it does seem to me, with my limited knowledge of such things, that one should be able to at least tell what type of metal is under the ground (or laying on your shop floor) with a scan of some type.

>> No.

For example, doesn't every metal give off a certain, measurable magnetic field? If so, wouldn't some instruments or scanners be able to pick that up, and then be able to tell you what type of metal was there?

>> No. Where did you get that piece of erroneous information?

Is this possible? If not, how far off base am I?

>> No. You are pretty far off base. Even if you knew nothing about the physics, do you really think that you're the only one that's ever asked this question? IF such methods existed, don't you think that every accessible location would have already been mined out and that your father would be completely wasting his time?


TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[EdStainless]

Large deposits can be found in the same way that submarines can be detected. You use the earths magnetic field and a very sensitive magnetometer and you look for distortions in the field. The problem with doing this in rock is that every kind of rock offers some distortion, so figuring out which ones are important is the issue. It really comes down to signal-to-noise, and there is no system that can clearly identify 'how much' of 'what' in a natural mixed setting.

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P.E. Metallurgy, Plymouth Tube

 

[ornerynorsk]

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[bennylava]

Whoops, accidentally gave the wrong guy a "Great post!". That was meant for Edstainless.

Regardless, thanks ed for the helpful reply. Not so much to the other two. Must not actually be engineers.

Anyway Ed, it sounds like you're on the same line of thought I'm headed down. I wonder if the noise you speak of, could be filtered out in some form or fashion. They do have ground penetrating radar that yields some different colors on screen for different densities.

 

[metengr]

bennylava;
The other two are indeed engineers and valuable contributors to ET. I was the one that gave the purple star to ornerynorsk because of his wit.

 

[1gibson]

I imagine sifting for gold is like fishing. It's not necessarily about finding the most gold possible.

 

[btrueblood]

Metal detectors can differentiate non-ferrous and ferrous metals fairly well. A colleague does a lot of mining/prospecting, and they use one to find small nuggets hidden in cracks/crevices.

 

[ornerynorsk]

bennylava, I did pass along the star to EdStainless. Don't write off IRstuff, either, he is extremely knowledgeable, as evidenced by his many posts on subjects far, far above my level.
Research "proton magnetometer". However, to be entirely practical, you may find yourself with a pan doing "stream surveys" after all. Some of the best minds on the planet have been on the case for the easy track to gold for centuries, and no magic solution has appeared.
Thanks metengr!

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[EdStainless]

Metal detectors work by setting their own fields up, not just using the earths field.
The benefit is that you get much better signals, but you can only 'see' a few inches.
Mining companies have truck mounted units that can penetrate a few feet.
The best method is still simply being able to identify minerals and follow the deposits.


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P.E. Metallurgy, Plymouth Tube

 

[IRstuff]

Magnetometers can only detect innate magnetism, which is typically limited to iron-bearing soils and minerals, but unless they're extruded in place, the magnetic domains will tend to be randomly oriented. Gold is not a ferromagnetic metal and has no innate magnetism. Were it surrounded by magnetized ferromagnetic minerals, you might possibly detect a drop in magnetism, but, typically, gold veins are relatively small.

Most other methods described above can detect differences in the materials, but not what specific materials they are, so you'd have to dig, which is expensive, compared to the potential return. In fact, even a GPR would likely have a poor ROI. Gold miners of the 1849 variety, when gold was more commonplace, seldom found even gold to pay for the services and goods they consume while they were mining for gold. Interestingly, most of them didn't actually "mine," they "panned" for the gold.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[moltenmetal]

Do a little Youtube searching and you'll find lots of clowns running around with metal detectors looking for nuggets- in rock traps near streams, in road drains...

Others just placer mine whole areas, like the deposits in the road drains and the rock traps. The density difference between gold and other minerals is so large that it doesn't take much- just a big pump and a properly designed flume, and of course a shovel and having no care about flooding a stream with sediments. We tend to try not to do the latter when building stuff, so why it should be OK to disturb and re-entrain the sediments along the sides or bottoms of streams is beyond me.

 

[berkshire]

Molten Metal ,
I think you will find that a great many states now have laws in place to deter people from carrying out the activity you describe. I believe Alaska is one of the few states that allows it, as long as you do not mess up the salmon spawning grounds.
B.E.

You are judged not by what you know, but by what you can do.

 

[mrfailure]

On your original post: In case you are interested, there is a good way to identify the type of metal laying on your shop floor - a portable X-ray diffraction (XRD) unit, which can identify elements and relative quantities (Niton XRD analyzers are common). As I read through this thread, I wondered how it would do on a piece of unpolished rock in detecting if gold is present...

 

[metman]

I don't know the current magnetic technology, but 50 years ago it was believed that ferromagnetism was the result of unbalanced electron spin. Some years later, a non-metallurgist friend said that Aluminum (non-ferromagnetic metal)

Design for RELIABILITY, manufacturability, and maintainability

 

[metman]

Ooops - I hit the wrong key and left a truncated post

I don't know the current magnetic technology, but 50 years ago it was believed that ferromagnetism was the result of unbalance electron spin. Some years later, a non-metallurgist friend said that Aluminum (non-ferromagnetic metal)will become magnetic in a moving electrical field. I accepted this at the time because I know electric current produces a magnetic field but I don't understand how that works with a non-ferromagnetic metal.

Is it true that...
Aluminum (non-ferromagnetic metal)will become magnetic in a moving electrical field??

If so, what is the mechanism?

Design for RELIABILITY, manufacturability, and maintainability

 

[EdStainless]

Any electrically conductive material will generate Eddy Currents when moved in a magnetic field.
The result of these eddy currents is that they have a magnetic field associated with them (like all current flow does).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[metman]

Thanks Ed. I knew that but had forgotten. It is why they insulate layers of sheet steel in motor rotors to interrupt eddy currents

Design for RELIABILITY, manufacturability, and maintainability

 

[IRstuff]

re: XRD, see:

http://www.olympus-ims.com/en/xrf-xrd/mobile-benchtop-xrd/terra/

Only 15mg required, i.e., sample chamber is TINY, and sample must be small enough to fit the chamber

alternative systems require access to a thin sample:

http://www.icdd.com/resources/axa/vol52/v52_23.pdf

http://www.eng.uc.edu/~beaucag/Classes/Analysis/Chapter7.pdf

In either case, bulk testing of large quantities or areas is not feasible, and the depth of penetration of x-rays limits its ability to probe; for hydrocarbons, which have low absorptivity, optimum thicknesses are on the order of millimeters, which means that for metals and rocks, the thickness limit is even thinner.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[mrfailure]

I wrote the wrong thing in my question - what I meant to ask about was feasibility of identification of gold using handheld X-ray fluorescens (XRF). XRD would not really make sense.