Gold coins detection

[TH Jim]

If it's any help, British gold full sovereigns (8 grams) are 22 carat (0.917 fine) gold. Their 'natural frequency' is about 7.4KHz based on (90+45)=135 degrees phase lag.

Hi Geo & Skippy,

Thanks for the photo, those are indeed gold soveriegns! Thanks for the disc info, I am suprised by the frequency, much lower than i expected.

Cheers all,

Jim.

[Geo]

If it's any help, British gold full sovereigns (8 grams) are 22 carat (0.917 fine) gold. Their 'natural frequency' is about 7.4KHz based on (90+45)=135 degrees phase lag.

What do you mean with "natural frequency"??

[Skippy]

I specifically mean the frequency at which the phase lag is 135 degrees. I used the term 'natural freq' rather than 'resonant freq' (as many do) because there is no resonance (as far as I can determine). For analogue detectors, it is commonly considered that a detector frequency slightly higher than the targets 'natural frequency' gives the 'best' detection - in this case, a machine running at perhaps 9KHz to 12KHz would be suited. For reference, other common coins 'natural' frequencies are US dime = 2.8KHz, and US nickel = 16.5KHz. I mentioned the frequency because in one of your posts you said you thought these coins should be similar to silver ones (eg. US quarters).

[Geo]

I believe that for gold detection any frequency between 14...17Khz is better than any lower "natural frequency"

[Skippy]

You cannot possibly group ALL gold items together. You have to consider the size, mass, carat-rating and (in the case of nuggets) the irregularity and inclusions. A US 20 dollar coin (approx 30 grams of 0.900 fine gold) is vastly different to a 0.2 gram nugget. Best detector frequency for the 20 dollar is probably as low as 3KHz, the nugget would be better found with a 30KHz machine.

[mikebg]

Hi Skippy,
Can you explain how to measure the 'natural frequency' of different coins? Are these frequencies WEB published?

[Tinkerer]

Hi Skippy,
Can you explain how to measure the 'natural frequency' of different coins? Are these frequencies WEB published?

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.

[Altra]

Tinkerer

See if this makes any sense? frequency = 1/TC or TC = 1/freq

[Davor]

Only marginally. It is OK to have a detector frequency within a decade of such a "natural frequency", but it is far from being ideal. For example, when Russian Alpha transmits at 14.8kHz, and your rig plays it's tune near that frequency, you may forget about any deep coins that day.

[Skippy]

There's hardly anything on the web about target frequencies, so I have gone to some trouble to find out the information myself. I will publish the info in the New Year.

[Geo]

There's hardly anything on the web about target frequencies, so I have gone to some trouble to find out the information myself. I will publish the info in the New Year.

Ok, but why all metal detector companies when wants to make a gold detector tune the detector between 14 and 17 Khz????

[Funfinder]

Because detectable gold-stuff likely is rather small and for small stuff higher frequency suits better.

What does this semi-unscientifcal talk here?

Please first show us repeatable scientifical double blind test results @ reprovable conditions and second talk about long range detectors and stuff.


The most obviously reason why the coins now are harder detectable is that the soil has started to compress
or if the soil is weak and there is enough rain the coins even could start to go slowly deeper into the ground
because those have alot weight compared to the usual ground so they start to sink.

[Reg]

Geo,

All coins have a "natural" frequency at which they will generate a maximum R signal on a VLF metal detector. George Payne analyzed this many years ago and determined that some of the US coins had the following results. A dime and penny were about 2.7khz, while a silver dollar was down at about 800 hz and a nickel was at a frequency of about 17khz. Fine gold and thin gold rings had a natural frequency that was quite often higher than the nickel. I believe George was using one of his 6.59khz units when he made his determinations.

Based upon George's calculations and the above mentioned coins, the gold coin you were using should read maybe in the screw cap range. Since I don't have one of the coins, I tried a 1/4 oz nugget I know is quite pure and it read in the screw cap range.

If you have one of the old TID analog machines and look at the meter, you will see the natural frequency basically relates to the TID readings with the high frequency objects indicating near zero and the low natural frequency at the upper end. This makes sense when you think about it. Take a small gold nugget and it will quite often read well below a nickel and often in the low foil region.

Someone with different values of gold coins might want to test them and see what they read on a detector. Of course it helps if it is an old analog meter so the values as they relate to US coins will be displayed.

Now, even though each coin has a natural frequency at which it generates a maximum R signal, it doesn't mean one can't get a decent signal from a detector operating at a different frequency.

Changing the subject to your TDI, you will notice that the gold coin transitions tones at a value of 7 or so. Right at the transition point, the signal will be minimal so you will have very little depth of detection. The farther you are away from that transition point either way, the stronger the signal. That is what your chart should say for all targets that do transition tones. What this also tells you is there will be certain objects that will have a minimal signal at the GB setting where the ground is ignored. If you know this is going to happen, other techniques should be used.

Changing the subject again, you indicate you have strong ground signals, which indicates that you might live in an area where there was previous volcanic activity. Do you live in an area where there is or was ancient volcanic activity? If so, you are going to have difficulties with any type of detector, especially if trying to use the disc mode on a VLF.

Reg

[Geo]

Hi Reg.
I don't live in a area where there is or was ancient volcanic activity. But there is much moisture and the ground is "heavy", so there are strong signals from earth.
What are you mean with the TID machines???

Regards

[Reg]

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