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Physical Logic Behind Frequency Interrogation Methods?

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Greetings fellow detectorists, I'm hoping someone with a deeper understanding of the technological methods of the Equinox series might be able to shed some light on why certain Interrogation methods work they do. I think we are all aware of the little trick whereby a potential target in the mid conductor range in multi frequency can be examined in 10hz; a sudden jump from the teens to the twenties indicates a likely bottle cap. My questions are two fold; first, what is responsible for this phenomenon? Why does a change in frequency potentially change the vdi of a target, and what determines when a change occurs?  Second, is there a potential logic here that one can use to devise further Interrogation methods using similar principles? Looking forward to your insights!

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Shinto --

Briefly, different "classes" of targets (low conductor, mid conductor, etc.) respond/react differently to different frequencies.  Essentially, as the transmitted electromagnetic energy emanating from the detector's coil encounters a metal target, the transmitted electrical field moving across the target as you sweep the coil induces electrical currents in (actually on the surface of, for the most part) the metal objects.  And the characteristics of the induced electrical currents are then "received" by the coil, and analyzed by the machine. 

One of the things that affects the characteristics of the electrical "eddy" currents induced in a given target (in addition to the target's shape, size, depth, and metallic composition, etc.) is what frequency of electromagnetic energy (as transmitted by the coil) the target is exposed to.  Certain metal objects are more "resonant" to certain frequencies than others, and so by changing your transmit frequency, you are changing the character of the electrical currents that are induced in the target, by the transmitted energy. 

As the electrical field induced in the target is received by, and then analyzed by, the algorithms in the detector, these observed, induced electrical field differences (as related to changes in the transmit frequency) manifest as changes in the machine's reported VDI.  

While there are "clues" which can be gleaned from these differences -- i.e. how a given target responds to different frequencies, it takes alot of experimentation (switching between the various frequencies, and observing resulting responses of many targets over time) to begin to glean any meaningful/helpful information.  And, of course, there's still a certain amount of "subjectiveness" there, due to how many different types and orientations of targets exist in the ground.  

Gleaning these details, is what a good detector algorithm tries to do FOR you.  This is what detector designers try to incorporate into their VDI algorithms, of course.  That's not at all to say that there are not things you can glean from the information at hand, that can enhance or even "outdo" the detector algorithms; of course, that's what we are ALL trying to do.

Anyway, hopefully that helps to partially address some of your questions.


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Well stated Steve.

We have a few algorithms in our brains too.


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Nice explanation, Steve, both the theoretical and the practical views.  Mitchel made a good point which I expound upon.  Even the most sophisticated detetors today don't take advantage of all the information at hand, leaving our brains to do some of the 'heavy lifting', too.  With time as more powerful processing (hardware, maybe, but software for sure) gets implemented it's likely the disparity between the best, most experienced detectorists and the less experienced ones diminishes.  As to how much or when, we need an even better crystal ball than is currently used in the months leading up to a new detector release.  😏

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Fantastic discussion folks, and many thanks for the explanations given as they fit well into my own understanding. While there are obviously many gray areas in target interrogation methods and results will be highly dependent on target composition, depth, environmental context, etc, I would be interested to hear what other phenomenon folks have noticed and whether they've been able to turn them into general rules. The 10hz check would be one example, or pumping a coil to detect for iron tones. 

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If you are hunting in  multi iq and you get a hit in the nickel range and then  switch to 5kz. and get a coin  hit beware.The shallow  sounding high hit is  probably a bottle cap but if its a   weak sounding high  hit it could be a deep nickel or gold ring.You have to consider up  averaging in this situation.

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12 hours ago, ShintoSunrise said:

Hey all, while researching this question myself I came across a technical write-up from Minelab I hadn't seen before; sharing here in case others found it interesting! https://www.minelab.com/__files/f/11043/KBA_METAL_DETECTOR_BASICS_&_THEORY.pdf

Yeah, that's a pretty well known and informative article to those who've deep dived this site.  That document was also linked in the "more to be found here" post and thread that Steve provided above.  Specifically, it is included in Steve's Metal Detecting and Prospecting Library.  There are other similarly informative artcles on detecting principles and theory there as well.  Steve basically makes it easy by consolidating the information and links on his site so you don't have to go down the Google rabbit hole as much.  That's why if Steve responds to a question of mine with a link, I usually dig into it like I would if I just got a coin spill signal on my detector.  It is usually worth the click.  :smile:

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