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Old 05-19-2017, 04:23 PM
Goldprospector Goldprospector is offline
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Lightbulb A common misconception: Target conductivity

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.

Last edited by Goldprospector; 05-19-2017 at 05:04 PM. Reason: added text
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  #2  
Old 05-20-2017, 03:44 PM
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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.


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.


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.

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Old 05-20-2017, 04:26 PM
tnsharpshooter tnsharpshooter is online now
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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-...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??

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Old 05-21-2017, 01:35 AM
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Originally Posted by tnsharpshooter View post
And Rudy,,what in your opinion would the screen on V3i detector look like with both the quarter swept and the $20 gold piece??
The $20 gold piece would show a higher peak with the 22.5 KHz frequency.

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Old 05-21-2017, 07:08 AM
IDXMonster IDXMonster is online now
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I don't have a comment other than I just noticed Rudys avatar
That is HILARIOUS!! Thanks for the laugh,I'm going hunting now

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Old 05-21-2017, 11:16 PM
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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.
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Old 05-22-2017, 09:45 AM
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Read up on Eddy Currents, Skin Depth, and the Skin Effect.

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Old 05-22-2017, 12:09 PM
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Originally Posted by BottleCapKing View post
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. 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.

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Old 05-22-2017, 01:01 PM
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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!

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!

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Old 05-23-2017, 03:50 PM
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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.

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