Targets frequency response

[mikebg]

Signal flow diagram because of eddy currents

This is valid for each nonferomagnetic conductive object or each conductive soil or water.
The weighting coefficient kn of n-th time constant Tn depends on the shape of the object. If the object is annular (like bracelet), all harmonic coefficients are infinitesimal. The spherical shape of the target increases the weight of harmonic coefficients relative to fundamental coefficient k1.

[Qiaozhi]

Quote:
B^C07-14-2009, 12:26 AM: "Just wanted to compare findings with others who may have looked for & observed Specific Frequencies for Specific Targets--Eg--Au--Gold--etc.
Also I have read that some believe that higher Frequencies may be better for smaller targets...." Bruce^Crocodile

Hi Bruce,
:-) I could not figure out what is the difference between Au and Gold. May be Au means Aurum Australium. The browser does not find also the chemical element Eq, but it shows that EQ can mean EverQuest or Emotional Quotient :-)

It does not say "eq", it says "eg" which is a shorthand way of writing "for example". Au is the chemical symbol for gold, which means that the quote references gold twice. Clearly the person who made this quote either has a poor grasp of the English language and/or does not understand that Au and Gold are the same thing.

[mikebg]

Specific Frequencies for Gold, Eq and Au :-)

Quote:
B^C07-14-2009, 12:26 AM: "Just wanted to compare findings with others who may have looked for & observed Specific Frequencies for Specific Targets--Eg--Au--Gold--etc.
Also I have read that some believe that higher Frequencies may be better for smaller targets...." Bruce^Crocodile

Hi Bruce,
:-) I could not figure out what is the difference between Au and Gold. May be Au means Aurum Australium. The browser does not find also the chemical element Eq, but it shows that EQ can mean EverQuest or Emotional Quotient :-).
Now the serious issues they care about:
I can answer detailed via Email to all your questions, which I found in your Thread
"Specific Frequencies For Gold". May be Iknow too the answers and a lot of other
issues with your desire to construct a detector hypersensitive to gold. Send me
your questions via email, but please make them a short and clear as for a tabloid. Keep in mind that my knowledge of English Language and Australian terminology
is minimal, so I use machine translation. The machine gave far too meaningless translations of your posts in this forum, perhaps because you did not use the
correct terms. For example your term "circuit" in the above thread should be replaced
with any other term, such "loop" or something more appropriate (you should
specify how) to make the question intelligible.
Experience has shown that the brief explanation that I give on this forum are not sufficient for understanding the processes that formed signal in the sensing network at the presence of ferromagnetic and conductive mediums and subjects. To grasp the essence, I was provided for the Exercise 1, in which we will find the specific frequencies of an single eddy current loop using network analysis. The software for Exercise1 is called "NI_Multisim_Analog_Devices_Edition_10_0_1". I found it free dowloadable in 2007 in the Analog Devices website with 189MB size. Unfortunately I can not find it there, but surely it is somewhere on the Internet. We can continue this thread and doing exercises with other software.
As we will discover with this exercise, the higher frequencies really give a strong signal of the smaller targets and make it difficult the discrimination.. To use this effect, we should change the ground timeconstant so that it to differ significantly from timeconstant of target. However I do not know how the Australian soil increases the ground signal with increasing frequency. Did you made any measurements of Australian soils? Your Mike.

[Aziz]

G'day B^C,

welcome back. I am looking forward to your interesting findings. I am quite interested on your findings as I am now working on the PI and ground anomaly effects.

Aziz

[B^C]

Tinkerer,

Yes it is an interesting subject that's for sure, i will start sorting the data as soon as i have caught up with the family, been away for a while & need to catch up with all the gossip & spend some quality time with them.
I have over 36Gb of data to sort through & will be more than happy to post it once sorted. I'm happy to see some interest.

Bugwhiskers,

I registered on the link you gave & shall join in on the chat when i get a chance--already planning the next trip, can't tell the Mrs that though just at the moment. Might see you out there some time!

[bugwhiskers]

Hi All,

I have been out bush in the far north of Western Australia for nearly a year now prospecting with some good success. Just arrived back home last night, boy how things change when your away from civilization for long periods of time.

Anyway, i have conducted many extensive tests over the past year with some very good results. Real time out in the goldfields seems the only way i can get the answers i need.

One test leads to more questions & more questions lead to more & more tests.
I will sort through all my test data in the coming weeks & post what i have discovered, there are some very interesting findings that may be usefull to others.

No more so than the extensive tests done on Ground Conductivity Vs Target Response which i am very excited about & seems to open new doors & will explain a lot about the way Targets respond in varying ground conditions. I have found this very interesting indeed & have spent countless hours trying to understand it & test it with real numbers.

I have forgotten how good a home cooked meal is & the comfort of a good woman & a good bed really is, so i'm off to test this out at the moment--no test data on this subject though!

Hi B^C,

You may find the following link an interesting read.

http://goldprospecting.invisionplus.net/?mforum=goldprospecting&showtopic=3244

regards
bugwhiskers

[Tinkerer]

B~C,

thanks for coming back to this most interesting thread. We are curious to see your findings.

Tinkerer

[B^C]

Hi All,

I have been out bush in the far north of Western Australia for nearly a year now prospecting with some good success. Just arrived back home last night, boy how things change when your away from civilization for long periods of time.

Anyway, i have conducted many extensive tests over the past year with some very good results. Real time out in the goldfields seems the only way i can get the answers i need.

One test leads to more questions & more questions lead to more & more tests.
I will sort through all my test data in the coming weeks & post what i have discovered, there are some very interesting findings that may be usefull to others.

No more so than the extensive tests done on Ground Conductivity Vs Target Response which i am very excited about & seems to open new doors & will explain a lot about the way Targets respond in varying ground conditions. I have found this very interesting indeed & have spent countless hours trying to understand it & test it with real numbers.

I have forgotten how good a home cooked meal is & the comfort of a good woman & a good bed really is, so i'm off to test this out at the moment--no test data on this subject though!

[mikebg]

Step-down response in frequency domain

The attached figure is suitable for visual analysis only, but it shows methods for elimination signals of mineralized conductive soil and ferrous metal objects.
There is difference between Pulse Induction and Step-down response. At PI the TX pulse has width, but at Step down the TX width is infinitely.

[Tinkerer]

Mikebg,

thanks for the detailed explanation. Could you explain to me how to correlate frequency to transient signals?
Can I use the slew rate as an approximation?

Tinkerer

[mikebg]

Combination magnetizm & conductivity

Transfer impedance of conductive permeability

Curved line 3 represents transfer impedance form in common case when soil or metal object possesses conductivity as well as permeability. The half-cardioide 3 is obtained by summing pairs points – one of line 1 (impedance of ferrite) and another of line 2 (impedance of nonferrous conductivity) having equal frequency.
The line 3 can't be normalized exactly because the line of ferromagnetic impedance 1 can't be normalized at all. Besides, the line 1 is not exactly straight and represents nonlinear properties – hysteresis, saturation and magnetic viscosity. Complex plane can represent correct only linear properties. Despite all we can show shape of impedance form and make several conclusions:
1. If a soil or a metal object have both conductivity and permeability, transfer impedance seems like half-cardioide line 3 laying in first and fourth quadrant intersecting Re axis. The intersecting point Q represents resonance frequency. The shape of half-cardioide can differ from shown form because depends on permeability/conductivity ratio. After rain conductivity of soil increases and changes impedance form of ground signal.
2. We can define three characteristic frequencies on half-cardioide 3:
- Resonance frequency (point Q), at which phase lag is zero (the actual phase angle is minus 360 deg),
- Quasi (pseudo) cutoff frequency (point C), at which the phase lag is 45 deg (the actual phase angle is minus 405 deg) and
- Mirror cutoff frequency (point D), at which phase lead is 45 deg (the actual phase angle is minus 315 deg).
3. If an object is made of ferrous metal and is excited by relative high TX frequencies having impedance points in 4-th (conductive) quadrant, we can't determine whether it is ferrous. Only at relative low frequencies we can identify ferrous objects by impedance points situated in first (ferrous) quadrant.

[Tinkerer]

Mikebg wrote:
<In time domain, resonance point Q can be achieved by appropriate sample delay.>

Thanks for the input.
Your figure explains a lot. I wish I knew how to do the mathematical analysis of what I do with the circuit.
In general the discriminating PI, (Time Domain) has to overcome the problem as you mention above. The steel disk test mentioned above is an easy way to try to coax a specific response.
A much more difficult target to differentiate is a clad coin. That is a coin with a FE core and a copper, silver or other non magnetic alloy outer cover.
The problem with the decay times is that different size\mass targets will produce different decay times even if they are of the same metal or alloy. Talking about core eddy currents.
Now, when you look at skin effect eddy currents, it is rather the surface area that is important. Example, a thin aluminum candy wrapper, gives a large response in the skin effect time domain.
If we lump all different responses into one single output, we will know very little about the target's identity.

Therefore we need to spread the responses into several times so that we can sample each window that can give us the desired information.

At present I use 6 samples and I can differentiate quite a few different metals and alloys. But, there is still a lot to learn. For instance, I still don't quite understand what causes the different readings, why the readings change when the target is very near the coil or why a gold leaf shows a negative response like FE, but a gold coin or ring is of opposite polarity.

Oh, by the way, I can differentiate a clad coin from a FE disk or a copper disk.

Tinkerer

[mikebg]

Resonance Frequency of soil and ferrous target

"Resonance" is term of frequency domain. It means that there is a frequency, at which the phase lag of response is zero because the imaginery part of complex transfer function is zero. Since EMI sensor excitates environment with magnetic field, induced EMF in RX coil has the same phase angle as TX current at resonance frequency.
Visual analysis in complex plane shows that there is resonance frequency of soil or target if it possesses both conductivity and magnetizm. This is shown in attached figure. Magnetizm pulls working point upwards in ferro quadrant, but conductivity pulls it downwards in conductive quadrant. In this case, the curved line of transfer impedance intersects Re axis at resonance frequency.
In reality, the phase lag of induced in RX coil voltage can not be zero at resonance frequency. It is 360 deg phase delay, because the response is causal. Conductivity generates signal ON at resonance frequency. The phase angle of point N is negative. Ferromagnetizm generates signal OF at resonance frequency. The phase lag of this signal is more than 270 deg because is shown lossy ferrite.
The vector sum of both generated signals OF and ON is the resultant received signal OQ. In frequency domain, point Q coincidents visual with the axis of excitation Re. In time domain, resonance point Q can be achieved by appropriate sample delay.
There is an comprehensive explanation of term "Electrical resonance" in Wikipedia:
"Electrical resonance occurs in an electric circuit at a particular resonance frequency when the impedance between the input and output of the circuit is at a minimum (or when the transfer function is at a maximum). Often this happens when the impedance between the input and output of the circuit is zero and when the transfer function equals one."

[Tinkerer]

Fully discriminating PI

Tinkerer,

A fully discriminating PI has been everybody's dream for some years.
Would you care to tell us how you accomplish that?

Monolith

Monolith,

I have been posting bits and pieces in different posts on the Forum. If people show interest, I will start a dedicated thread and explain the methods and show the circuit.
Tinkerer

[Monolith]

Fully Discriminating PI

The target response is composed of several different components.

The resistive component
The reactive component
The skin effect eddy currents
The core eddy currents
And maybe a few more.

To visualize or read a specific component, the processing of the signal has to be specifically designed for that specific component.

For example: to read the response of the skin effect, the sample has to be taken very early, meaning after 5 to 10 us after the transient.
The skin effect relates more to the surface area presented to the field.

The effect of the core eddy currents last much longer, they are related to the TX time and represent mostly the TC of the target.

If You want to see the reactive response of FE targets, you need to know where and how to look for them.

A tip from Eric Foster: If you present a thin, FE disk the flat way to the coil, you see mostly the resistive component of the target.
If you turn the same disk on edge, you see mostly the reactive component of the target.
Try this and compare by doing the same test with a thin disk of similar dimensions of non magnetic material. The results will prove without doubt.

Starting with this good piece of advise, I developed a fully discriminating PI platform. Thanks Eric.

Tinkerer

Tinkerer,

A fully discriminating PI has been everybody's dream for some years.
Would you care to tell us how you accomplish that?

Monolith