Targets frequency response

[mikebg]

Are there published anywhere frequency responses of metal detected targets: different coins, rings, hot rocks, junk etc.? I can,t find in WEB even one frequency characterstic of target in complex (impedance) plane! Mike ***

[Max]

Are there published anywhere frequency responses of metal detected targets: different coins, rings, hot rocks, junk etc.? I can,t find in WEB even one frequency characterstic of target in complex (impedance) plane! Mike ***

Hi,
Frequency ?

Usual MDs of today (VLF/IB discriminators) use phase shifts to say which kind of metal and similar stuff, not frequency.

There are characteristic frequencies you can read in e.g. magnetic nuclear resonance but this is another story... and doesn't apply to MDs or other TH devices.

If you see "frequency" identification... you better start having dubts on device (could be a fraud thing) unless you are interested in old BFO stuff where , in some cases, was possible discriminating metals using frequency shifts (so frequency differences) induced by the presence of particular metals etc

Best regards,
Max

[Carl-NC]

Max, even in phase discriminators, targets can have a "frequency response". That is, their phase response can vary over frequency, and you can plot this as "phase vs frequency."

Mike, to answer your question, no, I've not seen this information anywhere. Might be found in a Geophysics journal somewhere. I have intended to do these measurements myself, but haven't had time.

- Carl

[Seeker]

Hi guys,
ten years ago I try analyse some characteristics of finds and I found for some multimetal alloys and copper coins with CuO and Cu2O cover semiconductor effect. If that finds are in field of two basic frequencies they produced two different frequencies f1-f2=f' ; f1+f2=f" and can register they on narrowband receiver. I made narrowband filter myself on 32 kHz using clock quartz .

[mikebg]

Hi Seeker,
Your experiment shows nonlinear properties of conductive object. I,m obtaining nonlinearity by nonconductive meteorites because of its ferromagnetic properties.
To discriminate hot rocks from meteorites, I need information in wide frequency band.
In my city Plovdiv - BG, there is experts on processing targets spectral information , but I dont know them. They have produced a cheap project "Pulsediscriminator PD3200", where spectral information is calculated as "color coefficients" of target. Unfortunately, last year, when I start to design nonmetal detector "meteorite locator", they were disappeared from WEB.
Thanks for information, Mike ***

[mikebg]

Carl: "I have intended to do these measurements myself, but haven't had time".
Carl, Sean Goddard knows method to measure frenquency response for no time using only one pulse. Please make him to post in the forum! Mike. ***

[mikebg]

Wrongful terminology and formulas in the Bruce Candy's Gospel
"Metal Detector Basics and Theory"
http://www.minelab.com/consumer/xstandard/files/ML_08_007WhitePaper8.pdf

[mikebg]

Lesson "Target Properties in Frequency Domain"

There is the lesson:

[Aziz]

This is also very interesting to me and would bring more accurate modelling results in sensitivity map creation of a coil arrangement. As I noted, modelling a target isn't easy.
But it can be modelled by simplification: short circuited small coils, having complex resistance Z = R+jXL. Also the skin-effect would affect the resistance Z. Maybe looking more in detail to transformator equations would bring more success. Target and TX coils are more or less complex coupled transformators.

Any hints will be appreciated.
Aziz

[gwzd]

...having complex resistance Z = R+jXL.

Z is called impedance and is the sum of the resistance R and the reactance X

Also the skin-effect would affect the resistance Z.

skin effect at 20 to 40 kHz and tiny currents? you should be more worry about proximity effect, skin effect is more like for isolated wires but you are talking about coils.

Maybe looking more in detail to transformator equations

good idea, I thought you said you are a software engineer.

would bring more success. Target and TX coils are more or less complex coupled transformators.

Care to elaborate that?

Any hints will be appreciated.
Aziz

See above,
regards

[bbsailor]

Technical Link

Mikebg,

Look at the web link below. It is a very detailed (178 page) document that covers metal detectors (UXO) in technical terms. This is a good read for all. http://www.serdp.org/Research/upload/UX-1122-FR-01.pdf

Pay attention to the three stages of eddy current decay. This relates to surface eddy currents, deep eddy currents and total target TC.

bbsailor

[gwzd]

Mikebg,
... covers metal detectors (UXO) in technical terms.

UXO stands for unexploded ordnance, buried bombs and mines. EMI methods are absolutely useless for finding plastic mines. So, before going into the field with one of those I'd think twice
Regards

[Aziz]

Mikebg,

Look at the web link below. It is a very detailed (178 page) document that covers metal detectors (UXO) in technical terms. This is a good read for all. http://www.serdp.org/Research/upload/UX-1122-FR-01.pdf

Pay attention to the three stages of eddy current decay. This relates to surface eddy currents, deep eddy currents and total target TC.

bbsailor

Hi bbsailor,

thanks very much for your hint. There is much usefull information therein, that takes me some time for dealing with.
Aziz

[mikebg]

To gwzd,
Boy, I have posted in an other thread for Aziz the description of a sensor used to detect metallic and nonmetallic UXO. It can detect by EMI and plastic mines if they are buried. The sensor creates powerful ground signal, which changes in the place where is the nonmetalic object. Read patents citted in that posting.

[gwzd]

To gwzd,
Boy, I have posted in an other thread for Aziz the description of a sensor used to detect metallic and nonmetallic UXO. ...

Hi mikebg,
I'm really bad at searching stuff in forums. I couldn't find your post, could you please post the link again.

Essentially, what you are saying is that you can induce an electromagnetic field in a piece of plastic, am I wrong? or this is something in the same league as the real part of the impedance being dependent on frequency? or to put it bluntly, next time I'll use a resistor should I check first the operating frequency of the circuit? am I wrong again?

The metal content in plastic mines is so minimal that you would have really hard time discriminating it from the minerals in the earth. The closest we have got to the moment is using GPR combined with metal detectors and I can tell you is far from being perfect or 100% reliable.

There people here who really wants to learn, please stop typing your nonsense and confusing people, specially in a subject so serious like UXO detection.

Waiting for your answer

RA