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Metal detectors

Started by Don Y October 12, 2021
I caught the tail end (last 2-3 minutes) of a TV show which depicted
a couple of guys hunting for gold deposits with the sorts of metal
detectors you'd look for trinkets on a sandy beach.

And, apparently, *finding* same!  (gold just lying around in
surface rock outcroppings?  who'd guessed!)

But, they each used detectors with small diameter (10"?) coils.

Wouldn't a larger diameter give greater soil penetration?
Are tehre other reasons why a small coil might be prefered
(easier to carry?)
On 12/10/2021 12:53, Don Y wrote:
> I caught the tail end (last 2-3 minutes) of a TV show which depicted > a couple of guys hunting for gold deposits with the sorts of metal > detectors you'd look for trinkets on a sandy beach. > > And, apparently, *finding* same!  (gold just lying around in > surface rock outcroppings?  who'd guessed!) > > But, they each used detectors with small diameter (10"?) coils. > > Wouldn't a larger diameter give greater soil penetration? > Are tehre other reasons why a small coil might be prefered > (easier to carry?)
Idle speculation on my part, but I'd think that a larger coil would encompass a larger search volume and would require a larger nugget to give the same response as a smaller coil with a smaller nugget, percentage wise. The detectorists probably know the likely size range of nuggets. Small deep ones can't be detected, and large deep ones don't occur. -- Cheers Clive
On 10/12/2021 14:53, Don Y wrote:
> I caught the tail end (last 2-3 minutes) of a TV show which depicted > a couple of guys hunting for gold deposits with the sorts of metal > detectors you'd look for trinkets on a sandy beach. > > And, apparently, *finding* same!  (gold just lying around in > surface rock outcroppings?  who'd guessed!) > > But, they each used detectors with small diameter (10"?) coils. > > Wouldn't a larger diameter give greater soil penetration? > Are tehre other reasons why a small coil might be prefered > (easier to carry?)
Not that I know much if anything about metal detectors, never been a gold digger (friends have come to me but I have laughed them away). But I have a story to tell - it was not a friend, it was someone I casually talked to a few times during my daily walks/bike rides uphill to Lucy's for coffee and tea. He asked me what I was into, I told him "electronics" and he immediately got started about treasures he knew where to look for, metal detectors, once he asked me if I knew French so I could write a letter to the French president on his behalf about a treasure France must somehow be involved/interested into.... I did not want to spoil his dreams, he had little else, so I did not just laugh him away like I usually do; but I did joke once, I told him being in nuclear spectrometry the way I would go looking for a treasure would be using neutron activation analysis. You run around with a portable HPGe detector and one of our MCAs and there you are. Of course you would first have to activate the metals you are looking for, no problem if you had a neutron bomb at hand to throw in the area. We parted after I told him that and I thought we had finished the subject. Well, next time we met he asked me how much my machinery would cost... Dimiter ====================================================== Dimiter Popoff, TGI http://www.tgi-sci.com ====================================================== http://www.flickr.com/photos/didi_tgi/
On a sunny day (Tue, 12 Oct 2021 04:53:31 -0700) it happened Don Y
<blockedofcourse@foo.invalid> wrote in <sk3t02$csg$1@dont-email.me>:

>I caught the tail end (last 2-3 minutes) of a TV show which depicted >a couple of guys hunting for gold deposits with the sorts of metal >detectors you'd look for trinkets on a sandy beach. > >And, apparently, *finding* same! (gold just lying around in >surface rock outcroppings? who'd guessed!) > >But, they each used detectors with small diameter (10"?) coils. > >Wouldn't a larger diameter give greater soil penetration? >Are tehre other reasons why a small coil might be prefered >(easier to carry?)
The dig for gold down under is a show that runs here practically every day on one free to air satellite channel for example on DMAX. I reached saturation point watching long time ago, now I only see repeats. But anyways to go deeper they indeed use bigger coils. You can use ground penetrating radar too to find gold veins. If you are into that watch a few more of those series, that also shows the hardship. Of course us 'tronics tinkerers could make our own super detector...
On 12/10/2021 12:53, Don Y wrote:
> I caught the tail end (last 2-3 minutes) of a TV show which depicted > a couple of guys hunting for gold deposits with the sorts of metal > detectors you'd look for trinkets on a sandy beach. > > And, apparently, *finding* same!&nbsp; (gold just lying around in > surface rock outcroppings?&nbsp; who'd guessed!) > > But, they each used detectors with small diameter (10"?) coils.
8-10" is about the sweet spot for a coil that is easy to work with and sensitive enough. Modern metal detectors can distinguish ferrous from non-ferrous metals which is a big help finding precious metals. I have yet to see one that can recognise and ignore coke can ring pulls.
> Wouldn't a larger diameter give greater soil penetration? > Are tehre other reasons why a small coil might be prefered > (easier to carry?)
That and being able to get a better fix on the target. The sensitivity to objects in the ground extends a couple of coil diameters outwards in all directions. How deep in depends also on how wet the soil is. -- Regards, Martin Brown
On Tue, 12 Oct 2021 14:15:33 +0100, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>On 12/10/2021 12:53, Don Y wrote: >> I caught the tail end (last 2-3 minutes) of a TV show which depicted >> a couple of guys hunting for gold deposits with the sorts of metal >> detectors you'd look for trinkets on a sandy beach. >> >> And, apparently, *finding* same!&#2013266080; (gold just lying around in >> surface rock outcroppings?&#2013266080; who'd guessed!) >> >> But, they each used detectors with small diameter (10"?) coils. > >8-10" is about the sweet spot for a coil that is easy to work with and >sensitive enough. Modern metal detectors can distinguish ferrous from >non-ferrous metals which is a big help finding precious metals. > >I have yet to see one that can recognise and ignore coke can ring pulls. > >> Wouldn't a larger diameter give greater soil penetration? >> Are tehre other reasons why a small coil might be prefered >> (easier to carry?) > >That and being able to get a better fix on the target. The sensitivity >to objects in the ground extends a couple of coil diameters outwards in >all directions. How deep in depends also on how wet the soil is.
Yes. For those that want to dig deeper, so to say, google on "pulse induction metal detector" without the quotes. The circuits are pretty simple and easily built, and many people do just that. For what companies sell, the patent literature is best, and often the only source of technical data. But the best widely published overviews of the technology of metal detecting is provided by academics looking into how best to find and disarm unexploded ordinance, and especially minefields in the 3rd world, where whole areas are too dangerous to farm. Joe Gwinn (who passed through a metal-detector phase)
On 10/12/2021 5:11 AM, Clive Arthur wrote:
> On 12/10/2021 12:53, Don Y wrote: >> I caught the tail end (last 2-3 minutes) of a TV show which depicted >> a couple of guys hunting for gold deposits with the sorts of metal >> detectors you'd look for trinkets on a sandy beach. >> >> And, apparently, *finding* same! (gold just lying around in >> surface rock outcroppings? who'd guessed!) >> >> But, they each used detectors with small diameter (10"?) coils. >> >> Wouldn't a larger diameter give greater soil penetration? >> Are tehre other reasons why a small coil might be prefered >> (easier to carry?) > > Idle speculation on my part, but I'd think that a larger coil would encompass a > larger search volume and would require a larger nugget to give the same > response as a smaller coil with a smaller nugget, percentage wise.
Yes, but you could tweek the rest of the electronics, accordingly, to improve the sensitivity (?)
> The detectorists probably know the likely size range of nuggets. Small deep > ones can't be detected, and large deep ones don't occur.
I'm not sure small deep ones *cant* be detected. That's the point. OTOH, your point wrt the hunters knowing the likely sizes of stuff they will find is probably true -- for their locale. (the whole idea of gold being accessible so close to the surface was interesting... what other ores can be similarly located?)
On 10/12/2021 5:27 AM, Dimiter_Popoff wrote:
> But I have a story to tell - it was not a friend, it was someone I > casually talked to a few times during my daily walks/bike rides > uphill to Lucy's for coffee and tea. > He asked me what I was into, I told him "electronics" and he > immediately got started about treasures he knew where to look for, > metal detectors, once he asked me if I knew French so I could > write a letter to the French president on his behalf about a > treasure France must somehow be involved/interested into....
<rolls eyes>
> I did not want to spoil his dreams, he had little else, so > I did not just laugh him away like I usually do; but I did joke once, > I told him being in nuclear spectrometry the way I would go > looking for a treasure would be using neutron activation > analysis. You run around with a portable HPGe detector and > one of our MCAs and there you are. > Of course you would first have to activate the metals you are > looking for, no problem if you had a neutron bomb at hand > to throw in the area.
Trifling detail...
> We parted after I told him that and I thought we had finished > the subject. > Well, next time we met he asked me how much my machinery would > cost...
Too funny. Had you been less scrupulous, you could have "salted" a nearby field with gold painted flecks of stone and given him a "demonstration" to cinch the deal!
On 10/12/2021 6:15 AM, Martin Brown wrote:
> On 12/10/2021 12:53, Don Y wrote: >> I caught the tail end (last 2-3 minutes) of a TV show which depicted >> a couple of guys hunting for gold deposits with the sorts of metal >> detectors you'd look for trinkets on a sandy beach. >> >> And, apparently, *finding* same! (gold just lying around in >> surface rock outcroppings? who'd guessed!) >> >> But, they each used detectors with small diameter (10"?) coils. > > 8-10" is about the sweet spot for a coil that is easy to work with and > sensitive enough. Modern metal detectors can distinguish ferrous from > non-ferrous metals which is a big help finding precious metals. > > I have yet to see one that can recognise and ignore coke can ring pulls.
What originally caught my attention (I had just turned on the TV in order to watch a movie/DVD and hadn't yet switched the video source) was that they were out in the wilderness -- grass, trees, shrubs, etc. I am used to seeing folks combing beaches for lost wedding rings, etc. "What the hell are they looking for out there??" I would imagine they got fewer false positives simply because it didn't appear that there was any sign of "civilization", nearby. [But, I only saw the last 2-3 minutes when they found a nugget and congratulated themselves]
>> Wouldn't a larger diameter give greater soil penetration? >> Are tehre other reasons why a small coil might be prefered >> (easier to carry?) > > That and being able to get a better fix on the target. The sensitivity to > objects in the ground extends a couple of coil diameters outwards in all > directions. How deep in depends also on how wet the soil is.
Surely, one could carry a smaller, "wand" style detector for that sort of effort. A local firm builds these VERY large coils for geophysical exploration. But, I think they are looking for geological features, not "lost wedding rings". (think oil exploration, acquifers, etc.)
On 10/12/2021 9:23 AM, Joe Gwinn wrote:
> On Tue, 12 Oct 2021 14:15:33 +0100, Martin Brown > <'''newspam'''@nonad.co.uk> wrote: > >> On 12/10/2021 12:53, Don Y wrote: >>> I caught the tail end (last 2-3 minutes) of a TV show which depicted >>> a couple of guys hunting for gold deposits with the sorts of metal >>> detectors you'd look for trinkets on a sandy beach. >>> >>> And, apparently, *finding* same! (gold just lying around in >>> surface rock outcroppings? who'd guessed!) >>> >>> But, they each used detectors with small diameter (10"?) coils. >> >> 8-10" is about the sweet spot for a coil that is easy to work with and >> sensitive enough. Modern metal detectors can distinguish ferrous from >> non-ferrous metals which is a big help finding precious metals. >> >> I have yet to see one that can recognise and ignore coke can ring pulls. >> >>> Wouldn't a larger diameter give greater soil penetration? >>> Are tehre other reasons why a small coil might be prefered >>> (easier to carry?) >> >> That and being able to get a better fix on the target. The sensitivity >> to objects in the ground extends a couple of coil diameters outwards in >> all directions. How deep in depends also on how wet the soil is. > > Yes. > > For those that want to dig deeper, so to say, google on "pulse > induction metal detector" without the quotes.
The question has more to do with the apparent lack of a *need* to sense deeper. That gold could be found within inches of the surface. Gold mines that I've visited were deep underground...
> The circuits are pretty simple and easily built, and many people do > just that. > > For what companies sell, the patent literature is best, and often the > only source of technical data. > > But the best widely published overviews of the technology of metal > detecting is provided by academics looking into how best to find and > disarm unexploded ordinance, and especially minefields in the 3rd > world, where whole areas are too dangerous to farm.