A common misconception: Target conductivity

Goldprospector

Junior Member
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Greetings one and all. Today I would like to clear up a common misconception.
Some metal detectorists think that certain coil frequencies are better than others for finding targets of a certain conductivity. For example, they might feel that to find gold, it is best to use a higher frequency, say around 18 kHz. They might also feel that there is a "best" frequency to use for any given conductivity. Nothing could be farther from the truth.

Let's take gold for example. What's the conductivity value of gold? 44, 2? Plug in your own value for the conductivity value of gold. That wont matter. Whatever the value of X is, this is the conductivity value of gold. That doesnt change. The only thing that changes is the size/weight of the gold.
A 45 KHz coil will only "see" very small, typically shallow gold nuggets. Even if honker size nuggets are below the very small nuggets.
A big 3 kHz coil will only "see" larger, deeper nuggets. It wont even see the very small nuggets that the 45 kHz coil can find.
Same metal, same conductivity value. The only thing that has changed is the size/weight of the nugget. Looking for very small nuggets/targets? Use a 45 kHz coil. Looking for honker size nuggets/targets? Use a (larger) 3 kHz coil. The metal itself has no correlation to any particular frequency. Same with any other target. It's all about the size/weight of the target.
If someone says, 18 kHz is the best frequency for finding gold, what they really mean is that 18 kHz is the best frequency for finding gold WEIGHING AROUND ONE GRAM. So they are not referring to the conductivity of gold, they are referring to the target weight.
 
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Goldprospector,

Your assertion flies in the face of actual results. Have you ever seen the signal response graph from the White's V3i ? It plots the received signal strength, from the target, at the three different frquencies that the detector uses. Same coil size, same target and same distance from coil to target.

In the first image, you see the response from the three frequencies to a quarter, which is a high conductance target.
analyze.jpg


Note how the signal produced in response to the 2.5KHz stimulus is much stronger than the other two, higher frequencies.

In the next image is the response to a US nickel, a low conductance target.
2_1_2.jpg


Note how, in the case of the low conductance, the response to the 22.5KHz stimulus is so much stronger than at the lower frequencies.

Conclusions:

  1. At relatively shallow depths, any frequency will likely detect the target.
  2. Alternatively, a large enough metallic target can be detected by any frequency.
  3. The sharply diffeent received signal strengths in response to target conductance of the target, translates to being able to detect a given target deeper, when using the frequency that ellicits the largest response and this is based on the target's conductivity.
  4. The target conductivity is a property of not just the intrinsic resistance, but also the amount of material and its shape.
  5. Although not shown by the above, everything else being equal, larger coils will detect deeper, up to about 18" diameter, where sensitivity to small targets begins to fall off primarily due to the inverse square law.
 
Interesting little discussion here.

I see the term weight being used here.

Take a look at this thread. Post #7.
Notice how the Minelab reads the gold coins especially the $20 dollar gold piece.
And notice how it reads a us quarter.
https://metaldetectingforum.com/showthread.php?t=191557

What is the weight difference between quarter and $20 gold piece,, and which would actually be considered the technical higher conductor here.
http://cointrackers.com/blog/44/how-much-does-a-quarter-weigh/
http://www.cmi-gold-silver.com/liberty-gold-coin/

And Rudy,,what in your opinion would the screen on V3i detector look like with both the quarter swept and the $20 gold piece??
 
I don't have a comment other than I just noticed Rudys avatar:lol::lol::lol:
That is HILARIOUS!! Thanks for the laugh,I'm going hunting now:lol:
 
In my testing of gold just lying on the ground with machines that have multiple freq options of hunting with only one freq at a time , the lower freq are without a doubt not as sensitive to gold chains, gold rings ranging from 10 to 24 k . I do not have any gold coins of any size unfortunately to test with. Also with lower freq, coil needed to be a lot closer to target to get a hit on it, with higher freq i can get coil significantly higher from target. Hope this helps some.
 
Read up on Eddy Currents, Skin Depth, and the Skin Effect.

Eddy currents is what lets us detect the target.

At the detector frequencies involved however, skin depth and by consequence skin effect, can be safely ignored.:D Though it never hurts to learn new things. Along those lines, funny things happen when you induce an Eddy current on a superconductor. Also very strange is the skin depth on a superconductor, until it reaches the Curie point. :shock: :yes:
 
Gold is a Cruel Mistress! Great subject!..all these subset skills within this Sport are intriguing and of interest to the Forum readership!

Hunting Nuggets has got to be some tough duty! Heck, any gold hunting is one big bummer that takes a while to figure out...Inland jewelry, water, it dont matter...All gold hunters have a tough deal! Trying to discern any tricks/patterns to allow a guy to replicate some form of quantifyable success...

I try to read a huge amount of posts from our Members that seem to be successful in its capture to distinguish a repeatable pattern, gear involved, location focus, etc....Then, just when I think I got it figured out, some complete Noob will knock down a huge Gold find just diddling around in a totter!:laughing:

Anyway, I'd sure like to find a nice Nugget someday...It would be a cool goal and hunt! Hopefully the size of a pumpkin to make up for all this trouble!:laughing:
 
The ability to pick up gold nuggets depends on effective (better for electromagnetic penetration) surface they have. If they are flat and even thin (like hammered coins), they could be easily picked up at great depths, but should they be compact like a water drop, it's a tricky thing to pick up them - that is a job for high frequency devices mostly.

High frequency coils (12 kHz... 20 kHz etc) produce 4 times smaller signal on high conductors than low frequency coils (3 kHz) of the same size on the same high conductor.

High frequency coils (12 kHz... 20 kHz etc) produce several time larger signal on low conductors than low frequency coils (3 kHz) of the same size on the same low conductor.
 
Goldprospector,

Your assertion flies in the face of actual results. Have you ever seen the signal response graph from the White's V3i ? It plots the received signal strength, from the target, at the three different frquencies that the detector uses. Same coil size, same target and same distance from coil to target.

In the first image, you see the response from the three frequencies to a quarter, which is a high conductance target.
analyze.jpg


Note how the signal produced in response to the 2.5KHz stimulus is much stronger than the other two, higher frequencies.

In the next image is the response to a US nickel, a low conductance target.
2_1_2.jpg


Note how, in the case of the low conductance, the response to the 22.5KHz stimulus is so much stronger than at the lower frequencies.

Conclusions:

  1. At relatively shallow depths, any frequency will likely detect the target.
  2. Alternatively, a large enough metallic target can be detected by any frequency.
  3. The sharply diffeent received signal strengths in response to target conductance of the target, translates to being able to detect a given target deeper, when using the frequency that ellicits the largest response and this is based on the target's conductivity.
  4. The target conductivity is a property of not just the intrinsic resistance, but also the amount of material and its shape.
  5. Although not shown by the above, everything else being equal, larger coils will detect deeper, up to about 18" diameter, where sensitivity to small targets begins to fall off primarily due to the inverse square law.

Rudy,

I am getting into metal detecting. I have some questions:

1. In this case, does conductivity refer to how easily/not so easily a metal can conduct electricity, or does conductivity take on a different meaning in this case?

2. A high frequency is better a picking up on low conductivity metals. Why is this the case?

3. A low frequency is better a picking up on high conductivity metals. Why is this the case?

4. When people say a stronger signal what does that mean? Does that mean a higher volume or does that mean something else?

5. Where can I see additional graphs like the one you posted?
 
Rudy,

I am getting into metal detecting. I have some questions:

1. In this case, does conductivity refer to how easily/not so easily a metal can conduct electricity, or does conductivity take on a different meaning in this case?

2. A high frequency is better a picking up on low conductivity metals. Why is this the case?

3. A low frequency is better a picking up on high conductivity metals. Why is this the case?

4. When people say a stronger signal what does that mean? Does that mean a higher volume or does that mean something else?

5. Where can I see additional graphs like the one you posted?

1. Yes, conductivity is the inverse of resistance.

2. & 3. Needs more elaboration before I can answer.

4. It means the strength of the target's signal response arriving at the receive coil. In the case of a detector with modulated audio, it also means a higher volume.

5. I got those from White's Electronics.

To get back to 2. & 3. We in the hobby use the term conductivity, but this is somewhat inaccurate. Conductivity is the Real part of a complex number known as the target's Admittance. The Imaginary part of the Admittance is a reactive component (usually inductive) and the two components, together, form the Admittance vector quantity. You can think of the vector as having a magnitude and a phase angle.This phase angle is a time delay between the applied transmit coil magnetic field pulse and the target's response to the applied field.

The above phase angle difference by the way is the effect that the detector uses to discriminate targets.

High conductivity targets, nee, high admittance targets have large phase angles, compared to lower admittance targets. What this means is that it takes a longer time for a high conductivity target (to use our hobby's jargon), to build up the Eddy current field, in response to the applied filed. What this means is that a high frequency is not very effective because the target can not respond fast enough. In essence, the transmit field decays back to zero before the target's Eddy current has time to build up to a maximum so it doesn't produce the strongest signal it could. Conversely, a lower frequency transmit field allows the target's Eddy current to build up to its maximum and thus generate a stronger signal.

Low conductance (low admittance) targets on the other hand, have no trouble following a higher frequency transmit field and their Eddy current builds up faster.

The above is also the reason why a PI detector needs to be able to have a short sampling delay to be good at detecting small gold jewelry.

Hope the above helped.
 
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Too technical for me, I'm Polish. I just want to know if my headphones beep if I swing over it. I'll leave all the technical stuff up the the engineers who design these things. :lol:
 
A shorter answer for you is, the best frequency depends upon what the vdi of your ground is. As the ground vdi approaches -95, lower frequencies are better because they are less likely for a larger target to wrap into the negatives.


So, if you ground is -95, a large silver target will wrap into the negatives at 18-22khz. That same target at 2.5khz will not wrap into the negatives.


It is also true that higher frequencies are better for really small targets compared to lower frequencies.
 
A shorter answer for you is, the best frequency depends upon what the vdi of your ground is. As the ground vdi approaches -95, lower frequencies are better because they are less likely for a larger target to wrap into the negatives.


So, if you ground is -95, a large silver target will wrap into the negatives at 18-22khz. That same target at 2.5khz will not wrap into the negatives.


It is also true that higher frequencies are better for really small targets compared to lower frequencies.

Phase shifting, or wrapping, is also a problem, but it is of a different nature than the question that was asked. However, as you say, highly mineralized ground does present the added difficulty you described.

Anybody that has detected Golden Gate Park in San Francisco, looking for the deep coins, has experienced the above, in spades.
 
Re: A common misconception: Target conductivity.

Hi MetalNovice !

This reply is late and possibly passè since a previous reply on page 2 by Admin. Rudy was dated 9-30-2019. Nevertheless, for what it might be worth here's my reply.

In regard to your post #11, question #1, Rudy's reply of "Yes, conductivity is the inverse of resistance." certainly capsulated the answer to your question in a concise statement that "hit the nail on the head". Also, the part of your question, i.e. "In this case, does conductivity refer to how easily/not so easily a metal can conduct electricity, " was an excellent way of expressing and attesting to your correct understanding.

However, if I may elaborate, it is of course understood that when we say "conductivity is the inverse of resistance" we are indicating that metals with low conductivity resist the flow of electricity and the inverse or reverse effect of this is that metals with higher conductivity, depending on their level of conductivity, will allow electricity to flow at a higher amperage rate, (Amperage being the term used to describe "the number of free electrons moving past a fixed point in a conductor in one second.").

For example, with a copper metal conductor the outer electrons of it's atoms are shielded by the inner shells of electrons and therefore the outer electrons are free to move easily between and around the copper atoms when voltage is applied. It's the movement of these excess outer electrons that causes an electrical current to flow in the copper conductor and is measured in Amperes and the abbreviated form more often used is Amps..

On the other hand, a resistor to the flow of electricity is any material that has atoms with strong bonds to the outer electrons of each atom, so when voltage is applied nothing happens as the atoms aren't providing anything for electrons to flow. BTW, silicon and oxygen atoms bonded together in gaseous form also makes a good insulator. ;)

Perhaps this will add to your understanding; that's my hope anyway!

ToddB64
 
This is a very open ended question because there are many factors to being able to detect a gold target. The most obvious being the density of the object and its electron affinity. These variables change the size of the magnetic field of the target in question.

TLDR:That being said, frequency response does play a role in the detection of metals but is not the only deciding factor.

If you want more info I suggest picking up a book on electro magnetism and brushing up on your differential equations.
 
Hi Dflan83 :tiphat:

Sorry for my ignorance; I'm guessing "TLDR" in your post is an Internet slang acronym, or maybe an abbreviation for something, but my curiosity gets the better of me as I've never come across that one and so have to ask; what does it stand for/represent ?

Thanks !

ToddB64
 
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