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HOT ROCKS - COLD ROCKS

E

Eric Foster

New member
While investigating electronic ground cancelling circuits, I recently noticed something that warranted further investigation. If I left a circuit on overnight, that had been accurately nulled on a lump of Australian ironstone, in the morning it would be slightly off null, but with the passage of time the null point would recover its accuracy. Nights here are cold at the moment, with the workshop temperature dropping to 8
 
Working with the most "Exotic Metals" and "Man Made products" such as Silcon Carbide and such in mechanicals seals for extreme chemical situations, there was always something new to learn, there was never a resting point where all was known... Mother Nature still has many secrets we do not know about her products....With all our education and all we have uncovered through experience...isn't it nice to know there is still lots to learn.

It makes tomorrow exciting for learning tomorrow's lessons. Frank
 
Hi Eric,

You're right on target (pun intended) with your observations about
"hot rocks". By the way, here in the Colonies, the term "cold rock"
refers to a rock that pulls a sine-wave detector in the opposite direction from metal. This action increases the threshold and makes the detector less sensitive.

(It is this unfortunate propensity that made the development of PI detectors a necessity.) In these parts, cooling a hot rock does not make it a cold rock--perhaps you should coin a new term. I have been aware of the fact that the metal-mimicking feature is mostly a mechanical phenomenon, and therefore subject to thermal influences. I tried to give a popular explanation on my web site:

www.dienco.com\rocks.htm

The energy absorption/release mechanism of magnetic domains in an inert matrix is also affected by external magnetic fields: If you a
approach a strong ceramic magnet to a hot rock, you will notice a change in the time constant and amplitude of the signal, since the external field exerts a braking effect on the rotation of the domains.

(A suitable magnet for this purpose would be a rare-earth magnet whose
coercive force is so strong that it itself is not affected by the relatively weak field from the detector coil.)

The subject of hot rocks is not well described in the scientific literature. People who are interested in paleomagnetic investigations related to continental drift are perhaps the only ones
who have gien the matter any attention.

It is my belief that a through understanding of the physical phenomena
in rocks is the key to the develoment of the "ultimate metal detector"
rather than devising new ways of displaying the mixture of target and
background signals.

If you try the "ceramic magnet experiment", please let me know what you find.

Prospector Al (You may remember me as the Moon Detector Man)
 
The logical question would be to ask if you will add some kind of circuit, or an offset switch to compensate for this phenomena.
 
Hi Prospector Al,

HOT ROCKS - COLD ROCKS was really a reference to the fact that I had heated a rock in an oven, and cooled it in a freezer, rather than as the normal detecting terminology. In the field of course, a rock that has a raised temperature, could well give a signal on an otherwise ground balanced detector, because of the shift in decay time constant, relative to its surroundings. As you say, this would affect PI and VLF detectors differently, although it is primarily the magnetic permiability that the VLF type responds to.

Interestingly, the rock I used, also has a high level of remanent magnetism. It has NS poles, and will deflect a pocket compass from 3in. This may well also affect its decay time, relative to an unmagnetised rock.

I tried the ferrite magnet test, mounting the magnet in a fixed position under the coil, and then waving the rock over it. The magnetic field causes the tone to shift low, indicating the the decay time has lengthened. This too, makes sense, in that the magnet's field has already aligned most of the fast switching domains in the rock, leaving the ones that take a longer time.

Eric.
 
Hello Eric,

An interesting adventure in this regard are saltwater black sand beaches with a mixture of cobble fields that transistion up to the surface. On the return tide, after the black sand and cobblestones have been warmed by the sun, and as the cold ocean water washes over them, there are great variations in temperature within a few feet of the waterline. Welcome to beach detecting on the coast of Maine, USA, and yes there are hot and cold rocks in the cobble fields.

Regards,
BarnacleBill
 
Hi Eric,

I didn't mean my comments about hot and cold as a criticism. The term "hot rock" is used in the vernacular of prospectors and even in patent applications. I am not aware of a scientific term for this phenomenon and I think one is needed.

I was suggesting that as a pioneer in the field, you could exercise you prerogative to coin a term that is less ambiguous and more descriptive.

I used the term "metal-mimicking mineral" as a catch-all term in a recent patent application, but I think that term is clumsy and I'd like to find another one for future use.

Any suggestions? Anyone?

Prospector Al
 
How about "Anomalous Rock" as in Anomalous Propagation, a radar term for a false return or false target. Or "Them Damn Noisy Stones" or TDNS for short. :):)
 
Hi Prospector Al,

Trouble is, that "hot rock" trips off the tongue easily, and, as you say, a more accurate scientific term will probably sound clumsy.

In archaeological circles it would likely be referred to as "anomalous artifact" or "anomalous rock", as Largesarge suggests. This could be shortened to "anorock", which is less of a mouthful.

Eric.
 
Hi Largesarge and Eric,

I like the term "Anomalous Rocks". It says they behave in an unusal way, which they do. I have a collection of them and you can't tell a hot rock from country rock by looking at it. They come in all colors and they all pretend they are metal targets... TDNS is also appropriate for casual reference, but perhaps not for publication.

By the way, I was a RADAR technician in the service and we saw a number of anomalous reflections--we called them "ghosts". Sometimes
they were birds and at other times we suspected they were interfaces between masses of hot and cold air...

Hot rocks on the other hand are solid objects and a real nuisance.
I'm trying to design a detector that will reliably discriminate against them...

I have managed to make an artificial hot rock by pulverizing magnetite
and mixing the powder with plaster of Paris. In solid magnetite, the
domain walls are close and switching is fast. It appears that the magnetic viscosity effect appears when the coupling between the small grains of magnetite is weak...

Regards,

Prospector Al
 
Hi Prospector Al,

I have a 4oz lump of almost pure magnetite from the Kiruna mine in Sweden, which gives no signal on a straight PI. It is highly susceptible though, as it sticks to a magnet like a piece of metallic iron. It would be interesting to grind it up and mix it with plaster of Paris to see if it becomes magnetically viscous.

One researcher here used Cement Fondu (high alumina cement) as a sample to calibrate a PI for viscosity. However, I am puzzled as to why this should give a signal. He says it is a "homogenous material occasionally used in geophysical modelling". I once had a large bag of chocolate flavoured cattle lick to test for metal fragments. Surprisingly I found that this had a considerable viscous response, even though no metal was found in it.

Many of the hot rocks I have, are remanently magnetised. If the integrators in the receiver of a PI are not accurately balanced, these will give a signal purely due to the induced emf in the RX coil when it is scanned over them, even when the TX is switched off.

Eric.
 
Hi Eric,

It would be a shame to destroy that nice specimen of yours--I think you could use ordinary black sand for the plaster of Paris experiment.

Magnetized particles may elicit a stronger response though. A pulverized ferrite magnet would also work. In this context it's interesting to note that a "lode stone", which is chemically identical to Fe3O4, is permanently magnetized, with a very high coercive force. A lode stone does not respond at all to a metal detector, other than to induce an asynchronous voltage in the Rx coil.

I am speculating that this property was acquired under some unusual
circumstandes, such as a lightning strike, which caused a very high
magnetic field to permanently orient the domains in one direction.

My first contact with an anomalous rock was a chunk of lava from a
dormant volcano in the Mojave desert of California. That piece showed
some remanent magnetism, detectable with a pocket compass. I theorize
that as the rock cooled, the magnetic needles were locked in the alignment of the earth's field.

In an analogous process, an artificial AR could also be exposed to an
external field, while the plaster is hardening...

I don't know if all this speculation will lead to any improvements in
PI technology, but sometimes mere knowledge will suggest a new way to
overcome a difficulty...

Allan
 
Has anyone tried heating a hot rock in an oven until it is 'red-hot', then quenching it immediately, and at later times to find out if grain structure will influence its nature. I know that quenching steel in various states, from austenite, martensite, or ferrite can have a large impact on its physical qualities. I've just never taken it as far as to see if electromagnetic characteristics are changed.
Has anyone run into any problems with Tungsten in coastal areas while detecting?
 
Hi Vlad,

I have actually tried that, and I didn't see a difference. I also
pulverized a hot rock and determined that it gave the same response,
quantitatively, when in powder form.

That explains why you can find "hot soil" in some areas. When hot rocks erode, they don't lose the hot property--they become hot sand...

I'm curious about your comment about tungsten: Are you talking about
metallic tungsten? I received samples of "manganese nodules" from the continental shelf, but they seem inert...

Prospector Al
 
It just occurred to me that I should have tried the hot rock while it
was still hot--temperature-wise. Above the Curie temperature, the rock should revert to being just an ordinary rock. If one determined the exact point at which the magnetic properties returned, one could get a clue about the nature of the species responsible for the phenomenon...

Prospector Al
 
Hi Vlad,

Judging from the conductivity of tungsten, it should have a time constant that is about three times shorter than that of gold. Thus, when the signal gating pulse is appropriately delayed, tungsten shot should be "invisible" to a PI detector.

The same goes for bismuth and nichrome, whose time constants are about fifty times shorter...

Propsector Al
 
In principle, yes. However, the field strength required is so high that it could not be done with portable instrumentation.

Besides, the remanent magnetic field is only partially responsible for the anomalous behavior of the rock. It would remain "hot" even after being degaussed.

The response of a hot hot rock can be modified by imposing an external field, but again, to neutralize the effect would require a field strength that's beyond the capabilities of battery-operated equipment.

You can demonstrate the effect by moving a ceramic magnet close to a hot rock...
 
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