Gold coins detection

[mikebg]

The frequencies shown above, seem to be roughly related to the TC of the coins. If you can show us how to translate the TC of a coin to the frequency, we can make a list for common coins.

The formula is
"Natural" frequency = 1/(2*pi*Tc)=0.159/Tc
Tinkerer, if you know Tc of common coins, please post them.

Reg, I can't find the term "natural" frequency for metal detected objects on the WEB.

The term "Natural frequency" of a coin is used for resonance at its mechanical vibrations:
http://me363.byu.edu/sites/me363.byu...tification.pdf

Synonymes used in electronics instead "natural frequency" are mentioned here:
http://www.geotech1.com/forums/showt...669#post101669

The term "-3dB frequency" is used here:
http://www.thunting.com/smf/index.ph....html#msg55033

The problem is: How in practice we can measure the "-3dB frequency"?

[Geo]

Hi Geo,

TID is short for Target ID. Many VLF detectors have a form of video readout, or in the case of older VLF's, a meter that is calibrated to give you a visual indication of different targets such as US coins.

My old Teknetics Mark 1 has a meter and one of the scales has the names of US coins across the top. They are placed according to their conductivity with the US silver dollar the highest. Here is a link of a picture of a Mark 1.

http://www.whiteriverprep.com/vintage/tek/tekMark1.jpg

On a different note, if there are no volcanoes nearby then you have a lot of clay and/or iron oxide in the soil. Both will affect the depth capability. Iron oxide usually can be determined by dragging a very strong magnet through the soil and seeing if a black fur ball is created on the magnet. The iron oxide ultimately transforms to maghemite which is what gives PI's problems. Intense heat from lightening strikes, fires, etc, helps transform iron oxides to maghemite, which is another iron oxide.

Ancient fireplaces or places that made pottery used an intense heat to glaze and harden the clay pottery and in doing so, would transform the ground and any clay bricks such that they would be detected easily with a PI or a VLF for that matter. I think this is what you refer to when you mention ceramics.

Reg

Hi Reg, thanks.
What to say!!!!, really I thought that the gold was at the upper scale (frequency) and the silver at down.
As you wrote at " Ancient fireplaces or places that made pottery used an intense heat to glaze and harden the clay pottery and in doing so, would transform the ground and any clay bricks such that they would be detected easily with a PI or a VLF for that matter", I mean this when i refer to ceramics.

Regards

[Tinkerer]

The formula is
"Natural" frequency = 1/(2*pi*Tc)=0.159/Tc
Tinkerer, if you know Tc of common coins, please post them.
Thank you for the formula. It will help me a lot.

Reg, I can't find the term "natural" frequency for metal detected objects on the WEB.

I think that Reg's explanation above, is very good. The frequencies correspond to the TC of the coins. To make a full list of TC's of a series of coins, for example, US coins, the date when the coins were struck, or minted, has to be taken into account. The reason is that the TC is very much influenced by the alloy used. A change of 1% in the alloy can make a very large difference in the TC of the coin.
Take a Nickle, US$ 5c that is a fairly large coin and it has a short TC, maybe 10us. A US$ 10c, or a Dime is much smaller in diameter and much thinner, yet it has a Tc of maybe 70us. When you look up the alloy these coins were made of, you find that the difference is minimal, maybe 1% to 2% only. I might have a list somewhere, but I might also have lost it since my backup storage hard drive crashed.
Anyway, the metal alloy of the coins can be found with Google.

This is actually a very interesting subject. I think we should start a different thread on it, not to hijack Geo's thread.
The term "Natural frequency" of a coin is used for resonance at its mechanical vibrations:
http://me363.byu.edu/sites/me363.byu...tification.pdf

We do have a recurring problem here. Most forum participants are not scientists. We do not use the correct names for things. This makes it difficult for scientific minded people to understand what we are talking about, specially after translations.
One thing that might help here, is to make a "sticky thread" with definitions, the scientific one and the common use ones, for the stuff we are talking about. (how do you like my word "stuff" in this context?)

Synonymes used in electronics instead "natural frequency" are mentioned here:
http://www.geotech1.com/forums/showt...669#post101669

The term "-3dB frequency" is used here:
http://www.thunting.com/smf/index.ph....html#msg55033

The problem is: How in practice we can measure the "-3dB frequency"?

I would think that the -3db frequency can be measured by observing the response of a given coin, at different frequencies. Simple in theory, but a lot of work actually. But it should be possible to make a fairly close simulation

[Tinkerer]

Hi Reg, thanks.
What to say!!!!, really I thought that the gold was at the upper scale (frequency) and the silver at down.
As you wrote at " Ancient fireplaces or places that made pottery used an intense heat to glaze and harden the clay pottery and in doing so, would transform the ground and any clay bricks such that they would be detected easily with a PI or a VLF for that matter", I mean this when i refer to ceramics.

Regards

When we talk about the natural frequency or TC's of gold coins, we must remember that gold is not "gold". Gold coins are all made of an alloy. The old name for gold alloys, was carat. 100% was divided into 24 carats. therefore, a 22 carat gold coin contains about 91.6 % gold. The rest, or about 8.4% is made up of other metals. Looking at the color of the coins, copper makes the gold pinkish, silver makes the gold pale, nickel gives it's own nuance to the color of the gold.
Different metals are added to make the gold alloy harder. If just 1% of the alloy is different, the result in TC can be very big.
This even applies to gold nuggets. Gold nuggets come in different alloys too. Most of the time the natural alloys contain silver and copper, but sometimes also platinum or nickel. I live not far from a region where the platinum content reaches over 10%. And then, there is a patch in this region, where the nuggets have a green tinge. I wonder what that alloy could be.

To go back to your problem. Could you give us some information about the geological characteristics of your soil? What type of rock do you have nearby? Calcium carbonate?

The soil itself, in the backyard, does it have vegetation?
Have fertilizers been used? Is the coin container water proof?
Is it possible that water has penetrated to the coins?
Are there other metals nearby in the soil?
Are there a lot of ceramic shards in the soil?
How far to the nearest AC power grounding?
Is there any septic tank nearby?

The fact that your dowsing rod detects the gold, is classic. No need to delve further into that.

[Reg]

Mikebg,

I have never seen any reference to the term natural frequency except for the concept of that is the frequency that works best for the detection of that object. Now, in the case of a VLF, this occurs when the R component of that objects frequency is at maximum. So, I combined the information and referred to the frequency at which a target could be best detected.

In other words, the term "natural" frequency was defined by me in my previous thread as that frequency at which the R component signal is at maximum. This is the same as what George Payne defined as the -3db frequency. It is the frequency at which one will get the strongest R signal from that particular target.

Over the years, there has been almost nothing written or specifically defined on VLF' as a standard term that I know of. The same holds true for phase angle. Some engineers like George Payne refer to -180 degrees as that which is pure ferrite. Other engineers place the scale on the y axis and make 0 degrees as pure ferrite. Still others offset the ferrite at something some other angle. If you read a bunch of the patents you will see what I mean.

So, many of the terms have no standard meaning.

Reg

[Reg]

Mikebg, Tinkerer,

My information came from something told to me by George Payne. He also posted this information on a forum about 10 years ago. Here is a link to his post.

http://jb-ms.com/Baron/payne.htm

Now, for those of you who do not know who George Payne is, well, follow the patents and the introduction of new ideas in the metal detector history and one name will stand out and that is George Payne. He designed and patented the first consumer VLF, the first motion detector, the first motion detector with target ID, the first audio ID detector and the list goes on.

Because George developed so many features including those critical to the VLF development and advancement, I have complete confidence in what he says. He explained to me the technique he used to determine the ideal frequency for the various US coins but that was over 10 years ago and I can't find my notes from those conversations.

I suspect those notes went by the wayside when my earlier computer crashed.

George Payne said he wanted to write a book but has not done so that I know of. I wish he would because he is a wealth of knowledge.

For those interested in more information written by George Payne, he posted a lot of information on a forum and most of the important information was copied and posted on this site.

http://jb-ms.com/Baron/

Reg

[Skippy]

It was the few US coin frequencies quoted by George in one of those articles that I used as a reference for my own measurements. As I was able to reproduce with two methods George's figures, I felt confident I was doing my experiments correctly.

[Altra]

This is from page 15 of Minelabs "Metal Detector Basics Theory"
http://www.minelab.com/__files/f/110...D%20THEORY.pdf

"Definitions given below:

τ = L/r = mean metal target time constant, where L is the effective mean target series inductance and r the effective mean target series
resistance.

ω = 2π/τ = frequency of target resistive component (R) peak value (characteristic frequency).
t = time variable."

[mikebg]

This is from page 15 of Minelabs "Metal Detector Basics Theory"
http://www.minelab.com/__files/f/110...D%20THEORY.pdf

"Definitions given below:

τ = L/r = mean metal target time constant, where L is the effective mean target series inductance and r the effective mean target series
resistance.

ω = 2π/τ = frequency of target resistive component (R) peak value (characteristic frequency).
t = time variable."

Altra, please don't use as truth something written by Bruce Candy. As his "Basics theory", as the most his patents are rich on errors:
http://www.geotech1.com/forums/showt...9855#post69855
ωc = 1/Tc, where ωc is characteristic angle frequency measured in radians pro second and Tc is virtual timeconstant of target. The angle frequency is
ω = 2πf, where f is periodic frequency masured in Hz (Hertz has dimension periods or cycles pro second). That's why cutoff (characteristic, corner) frequency is
fc=1/(2πTc), where Tc is L/r or RC (for first order circuits).
http://en.wikipedia.org/wiki/Time_constant
http://en.wikipedia.org/wiki/RC_time_constant
http://www.sengpielaudio.com/calcula...meconstant.htm

[mikebg]

Re: RESONANCE FREQUENCY

Ferrous coins (for example 1, 2 and 5 eurocent coins) have RESONANCE frequency (electrical resonance). This was described in details here:
http://www.geotech1.com/forums/showt...2375#post72375
http://www.geotech1.com/forums/showt...2415#post72415
When Skippy reveal his method for measurement, we will estimate it.

[Geo]

When we talk about the natural frequency or TC's of gold coins, we must remember that gold is not "gold". Gold coins are all made of an alloy. The old name for gold alloys, was carat. 100% was divided into 24 carats. therefore, a 22 carat gold coin contains about 91.6 % gold. The rest, or about 8.4% is made up of other metals. Looking at the color of the coins, copper makes the gold pinkish, silver makes the gold pale, nickel gives it's own nuance to the color of the gold.
Different metals are added to make the gold alloy harder. If just 1% of the alloy is different, the result in TC can be very big.
This even applies to gold nuggets. Gold nuggets come in different alloys too. Most of the time the natural alloys contain silver and copper, but sometimes also platinum or nickel. I live not far from a region where the platinum content reaches over 10%. And then, there is a patch in this region, where the nuggets have a green tinge. I wonder what that alloy could be.

To go back to your problem. Could you give us some information about the geological characteristics of your soil? What type of rock do you have nearby? Calcium carbonate?

The soil itself, in the backyard, does it have vegetation?
Have fertilizers been used? Is the coin container water proof?
Is it possible that water has penetrated to the coins?
Are there other metals nearby in the soil?
Are there a lot of ceramic shards in the soil?
How far to the nearest AC power grounding?
Is there any septic tank nearby?

The fact that your dowsing rod detects the gold, is classic. No need to delve further into that.

Hi Tinkerer, thanks for answer.
The place with the buried golden coins is a garden with rich vegetation? The garden is at a mountain, perhaps the soil fell after.
I don't know if has been used fertilizers. The coins are wrapped 3 times with a car inner tube, so i don't know if they are 100% water proof.
Is it possible that water has penetrated to the coins? Yes, maybe
""Are there other metals nearby in the soil?"" Yes
""Are there a lot of ceramic shards in the soil?"" No.
""How far to the nearest AC power grounding? "" 10...20m (230V/50Hz).
""Is there any septic tank nearby?""" Yes, very near (about 2...3 meters)
""The fact that your dowsing rod detects the gold, is classic. No need to delve further into that"". ... I did not speak for rod but for electronic LRL


Regards

[Geo]

Mikebg, Tinkerer,

My information came from something told to me by George Payne. He also posted this information on a forum about 10 years ago. Here is a link to his post.

http://jb-ms.com/Baron/payne.htm

Now, for those of you who do not know who George Payne is, well, follow the patents and the introduction of new ideas in the metal detector history and one name will stand out and that is George Payne. He designed and patented the first consumer VLF, the first motion detector, the first motion detector with target ID, the first audio ID detector and the list goes on.

Because George developed so many features including those critical to the VLF development and advancement, I have complete confidence in what he says. He explained to me the technique he used to determine the ideal frequency for the various US coins but that was over 10 years ago and I can't find my notes from those conversations.

I suspect those notes went by the wayside when my earlier computer crashed.

George Payne said he wanted to write a book but has not done so that I know of. I wish he would because he is a wealth of knowledge.

For those interested in more information written by George Payne, he posted a lot of information on a forum and most of the important information was copied and posted on this site.

http://jb-ms.com/Baron/

Reg

Hi Reg, thanks for the sites.
I give a photo from George (the only that i have found before years).

Regards

[Reg]

Hi Geo,

A couple of years ago Eric Foster and I spent a week at Whites. We met at the airport in Portland and rode down to Whites together. When it was time to leave, Eric and I stopped by and talked to George who lived in a city on the way back to the airport. It was fun watching the meeting of the minds of Eric and George. Both are extremely knowledgeable.

I met George back in the 80's and we became good friends. Since then, I have relied on him for a lot of technical information about VLF detectors. When I was writing for a treasure magazine, I used to consult George a lot to make sure what I wrote was correct.

BTW, George has gained quite a bit of weight and his hair is thinner and straight now. Outside of that, he sure seems that he is as sharp as he ever was.

Reg

[Tinkerer]

Hi Tinkerer, thanks for answer.
The place with the buried golden coins is a garden with rich vegetation? The garden is at a mountain, perhaps the soil fell after.
I don't know if has been used fertilizers. The coins are wrapped 3 times with a car inner tube, so i don't know if they are 100% water proof.
Is it possible that water has penetrated to the coins? Yes, maybe
""Are there other metals nearby in the soil?"" Yes
""Are there a lot of ceramic shards in the soil?"" No.
""How far to the nearest AC power grounding? "" 10...20m (230V/50Hz).
""Is there any septic tank nearby?""" Yes, very near (about 2...3 meters)
""The fact that your dowsing rod detects the gold, is classic. No need to delve further into that"". ... I did not speak for rod but for electronic LRL


Regards

The reason why I asked about other metals, humidity, and AC power is to determine if there is any possibility of electrically conductive ground. This seems to be the case. With conductive ground and grounded AC power, it is well possible that some ground fault induces currents in the soil. Under such circumstances, all sort of strange things can happen. For example, once I plugged in an old refrigerator, standing on the wet soil, out in the garden. It rattled a bit but seemed to be working OK. However, it drove all earthworms within meters, out of the soil within minutes. Funny to see the soil becoming alive. Touching the refrigerator did not give any electric shocks, but there must have been some stray current into the moist soil that made the worms want to leave their home in a hurry.

The gold coins are the most noble metal in the ground. If the humidity penetrated to the gold coins, there is an electrolyte connecting the other metals nearby. This produces a voltaic cell and a magnetic field surrounding the current flowing between the more noble metal and the less noble metal. This could possibly be some zinc coated iron water pipe. The voltage of this cell could be over 2 volts.
Many times in old houses, the "AC Earth" was connected to such iron water pipes, being the best available AC Earth available. Any electrical ground fault in the house would then generate currents in the pipe.
With the gold coins recently buried and dry, there would be no connection.
After some time, when the electrically conductive water has penetrated to the gold coins. The situation has become dramatically different.
This is just speculation, but, if I was there, it would be the start for trouble shooting.

I have never seen a LRL that did not work on the dowsing principle. If it does exist, I would be very interested to see a schematic. If you have a schematic, please post it in the LRL forum section, as I would love to build it.

[Geo]

Hi Geo,

A couple of years ago Eric Foster and I spent a week at Whites. We met at the airport in Portland and rode down to Whites together. When it was time to leave, Eric and I stopped by and talked to George who lived in a city on the way back to the airport. It was fun watching the meeting of the minds of Eric and George. Both are extremely knowledgeable.

I met George back in the 80's and we became good friends. Since then, I have relied on him for a lot of technical information about VLF detectors. When I was writing for a treasure magazine, I used to consult George a lot to make sure what I wrote was correct.

BTW, George has gained quite a bit of weight and his hair is thinner and straight now. Outside of that, he sure seems that he is as sharp as he ever was.

Reg

Hi Reg, thanks for info.
Before years i was thin as George and with same hair.
Now i am the double weight and without hair