Response of various materials to EM waves

[rube the noob]

Does anyone have a link to books or studies which show the spectrum of response of different materials subjected to various frequencies of EM waves?

It would be nice to know stuff like:

What frequency induces the greatest response in a gold, iron, etc target?
What frequency, if any, produces a response with the greatest duration?
Are there frequencies that produce a bimodal, or multiple frequencies of large output?
What frequencies pass through rock, soil, etc?

I have found some sites which, for example, cite a resonant frequency of gold as 1.729Mhz, but I don't know if the site is legit or quackery. It would be nice to have some comprehensive info.

Thanks,
Rube

[99thpercentile]

These questions all can be addressed with an understanding of electromagnetic theory. Looking at these issues from the point of view of electromagnetic induction (EMI), which is the operating theory behind metal detectors, the important physical properties are electrical conductivity and magnetic susceptibility. What is important is the constrast in these physical properties between the target and the surrounding soil. Frequency is most important in a concept known as skin depth which tells you how far the energy will penetrate until it has attenuated to 1/e of its original value. The transmitted energy, whether a time domain pulse or a continuous wave in the frequency domain, generates a primary field in the coil. The primary field induces a secondary field in the target which is measured in the receiver coil. All frequencies of electromagnetic energy pass through rock and soil, but lower frequencies go farther than high frequencies.

Some of your questions are mixing electromagneti induction phenomena, which is a diffusion process, with electromagnetic wave phenomena. If you have more questions about this, let me know.

[Rudy]

Does anyone have a link to books or studies which show the spectrum of response of different materials subjected to various frequencies of EM waves?

It would be nice to know stuff like:

What frequency induces the greatest response in a gold, iron, etc target?
What frequency, if any, produces a response with the greatest duration?
Are there frequencies that produce a bimodal, or multiple frequencies of large output?
What frequencies pass through rock, soil, etc?

I have found some sites which, for example, cite a resonant frequency of gold as 1.729Mhz, but I don't know if the site is legit or quackery. It would be nice to have some comprehensive info.

Thanks,
Rube

I would also add to what has been said by the previous poster that any resoances will be largely determined by the target geometry (shape) and not by the intrinsic metal composition.

In addition, as long as the target's resistance is linear and bilateral, no harmonic frequencies would be generated by the target. In other words,
if you "illuminate" the target with a pure sinusoidal frequency, the response will be created by an opposing field (ie. the Eddy current) at the identical sinusoidal frequency. That is, no harmonics would be generated.

Now, if you "illuminate" the target with a electromagnetic pulse (which by definition contains many harmonics, but of decreasing energy) and analyze the response in the frequency domain, the response at each frequency will be somewhat different. Minelab, for example, uses three different harmonic frequencies for analysis.

[rube the noob]

Thanks for the replies.

I am trying to get an understanding of all this, and I have many, many questions, so if I say something silly, go easy on me.

I should say for this particular question, I am more interested in EM waves across the spectrum, not so much in metal detectors specifically. In my imagination, I envision a magical frequency that would be reflected at metals but transparent or translucent to dirt, rocks, etc. Or, a frequency in which metals would absorb waves, and emit at some other particular frequency or frequencies that one could tune to. I am thinking about something similar to spectroscopy, but I realize a solid behaves differently. In spectroscopy, particular frequencies of light are emitted from a gas, but solids have other interactions besides electrons changing energy levels, so the spectrum isn't the same. But are there still peaks of frequency?

Extrapolating from our visual experience, it would be like "color" in the domain of longer wavelengths.

Does reality work this way or am I mistaken?

Thanks, Rube

[Rudy]

Thanks for the replies.

I am trying to get an understanding of all this, and I have many, many questions, so if I say something silly, go easy on me.

I should say for this particular question, I am more interested in EM waves across the spectrum, not so much in metal detectors specifically. In my imagination, I envision a magical frequency that would be reflected at metals but transparent or translucent to dirt, rocks, etc. Or, a frequency in which metals would absorb waves, and emit at some other particular frequency or frequencies that one could tune to. I am thinking about something similar to spectroscopy, but I realize a solid behaves differently. In spectroscopy, particular frequencies of light are emitted from a gas, but solids have other interactions besides electrons changing energy levels, so the spectrum isn't the same. But are there still peaks of frequency?

Extrapolating from our visual experience, it would be like "color" in the domain of longer wavelengths.

Does reality work this way or am I mistaken?

Thanks, Rube

I assume you are imagining something you can use in the field, so nuclear magnetic resonance (NMR) is out of the question (temperature, high intensity magnetic field, ....). Here are some things to keep in mind:

For equal power output, the higher the frequency, the less ground penetration capability.

Dirt and rocks have significant mineral content in them, so a frequency that is reactive to a precious metal target and not reactive to the dirt and rocks is .....

Penetration of metals by EM waves is very limited (read up on "skin effect"). That is what keeps you safe when lighting strikes your car or the airplane you are traveling in.

As I mentioned before, a metal is electrically linear and bilateral (the current through it is a linear function of it resistance and applied voltage, and flows equally well in any direction), therefore, it doesn't respond at a different frequency from what is used to stimulate it.

Now, if you move into frequencies beyond the RF range into the higher energies, then you are dealing into "black body" radiation effects where you could detect emissions at different frequencies due to quantum changes in electron orbits. Problem is, these EM waves are in a region where lots of energy is required and their ground penetration abilities are less. It is how astronomers can tell what a star is made of. In short, I think this path would be impractical for field use.