How Does a Metal Detector Work?

How Does a Metal Detector Work?

“I’ve got another confession to make.  I’m your fool.  Everyone’s got their chains to break, Holdin’ you.”   – Foo Fighters, Best of You

Confession Time:  I hunted for twenty years before a conversation with an engineer in the detecting manufacturing industry enlightened me on how detectors actually worked.  Twenty years.    Twenty years of operating a metal detector without clearly understanding how it worked.   Twenty years of setting up my machines, making adjustments, and deciding what to dig without a clear understanding of the basic principles of the device I used for hours at a time.   I’m thinking twenty years of some missed targets and missed opportunities.

I assumed that the detector sent electromagnetic energy into the ground, which is correct, but that this energy bounced off of metal objects kind of like sonar, and dudes with thick rimmed glasses somewhere figured out how to parse the return signal into a depth and conductivity guesstimate that manifested itself on the detector’s display and through its audio.   Sure I was kind of close, but no cigar.  Now in my defense, I didn’t really care.  I probably made that initial assumption when I was in 7th grade, and never really spent time thinking about it.     With a ton of trial and error, I knew how to set my detector up in a way that worked for me, and I stuck with those lucky settings, unknowingly limiting myself severely.

So How Does a Metal Detector Work?
In general, the principles behind how a detector operates is very simple..    According to Minelab’s website:

1) The detector’s search coil transmits an electromagnetic field into the ground.

2) This electromagnetic field energizes metal targets to enable them to be detected.

3) A new electromagnetic field is generated from energized metal targets.

4) The search coil receives the return electromagnetic field from a target.

The basics are that simple.    For more detail, check out the full MineLab Knowledgebase Article.

In addition, many detectors transmit multiple electromagnetic fields at different frequencies at the same time, and compare the resulting readings against each other in real-time to help provide more accurate and stable readings.

What about Double D vs Concentric Search coils?

Another much argued and misunderstood topic is whether to use Manual Sensitivity vs the detector’s Auto/Recommended Sensitivity.    This article by Metal Detecting World sums it all up and keeps me from having to take sides!

We have plenty to learn outside of the confines of our detector’s manual.

SO WHAT?
In all fairness, I bet you won’t find, for example, a lot of professional basketball players that can explain the complex physics behind the basketball itself.  You can certainly be extremely good at something – even an expert – without understanding completely how your equipment and tools work.   You have enough experience that you “just know”.   Just like many expert detectorists.  They’ve worked hard enough to “just know”.

THE VOODOO OF THE DIALS

When I get a new detector, I find an expert user of that detector, and I generally set my detector up like he does, or work hard to get to a point where I can do so.   What I’ve found over the years, however, is that many detectorists, especially experts, tend to treat certain individual detector settings with pure superstition.     They are successful with a particular setting, and that setting is always the same – just like having a lucky detecting hat.   Hey – it’s not crazy if it works.

A great example of these settings are the settings on the “Expert Menu” on the e-Trac: Recovery Deep, Recovery Fast, Trash Density, and Ground.   Other examples are certain frequencies, audio settings, and discrimination patterns.

I call this phenomenon, which I am guilty of participating in all of the time, the Voodoo of the Dials (back in my day metal detectors had dials – apologies to Fisher F75 engineers).   I’ll be the first person to tell myself that this isn’t good enough.  If I do not understand any setting on my detector, I need to RTFM (Read the Manual), consult the forums, talk to other users, and even get on the phone with the manufacturer.   “Because it works” just isn’t good enough.    I need to be able to understand the function, and better than that, understand why and when I should adjust the setting.   Else changing the setting shouldn’t be an option and I shouldn’t be aware of it.

INTERPRETATION

A metal detector is, obviously , very different and more complex than a simple basketball.   A basketball gives you direct feedback to your hands that your brain interprets.  The metal detector, on the other hand, does not give you direct feedback.    Though you might not realize it, the audio and visual feedback you get from the detector are interpreted.   The detector’s computer takes the physical feedback it receives, and the wizards in the detector labs have worked hard to program the computer to give you sound through your headphones and display information on your screen that interprets the raw feedback received by the search coil.  Interprets.   In the same way as if the search coil might as well receive Ukrainian words and Tchaikovsky’s music and the control box is interpreting such into the bars and/or numbers you see and the sounds that you hear.

When I came to this realization of the obvious – that I was hearing and seeing interpreted feedback, I immediately wondered “What’s left out?”    What is lost in the translation between the raw signals received by the search coil, and what we see and hear?   In other words, if I could hook my brain up directly to the search coil and my brain could comprehend the raw signal, what useful information could I receive that I am not getting now.

Just like with our hypothetical Ukrainian signal above, the nature of interpretation, or translation, is that by nature some things do not translate very well.  For example, “А дело бывало — и коза волка съедала” is generally translated to English as “Pigs might fly”.  But go ahead, run it through Google translate.   It really says something closer to “And it happened – and the wolf ate the goat.”    It is only interpreted as the English equivalent of “Pigs might fly” because it is used in similar instances to convey a similar thought – but not the exact one.  Just consider the statistical probability of a wolf eating a goat versus that of a pig flying.   So at best, what you see on your detector’s display is an approximation of the underlying electromagnetic signal received – hopefully a very accurate approximation – but an approximation nonetheless.    The detectors I use do a great job at interpreting the obvious signals – a quarter at 4″ lying flat all alone with no trash – but somewhat lacking in instances where the signal is not straightforward.

Am I saying your detector’s display should be able to tell you straight up: “There’s a lot of nails here but I think there is also something good here.”    Probably.  If you are good enough using it perhaps it does, but that’s a discussion for another time.  What I am saying is that you are relying on your detector’s control box/computer, as an intermediary, to translate the signals the detector receives via the search coil, and that you do not have access to the signal itself.   And I’m saying this matters.

It follows that the sounds you hear are interpreted as well  After all, the search coil is not receiving sound from the ground.      And as a result, feedback potentially as useful to an expert detectorist as the beautiful nuances heard in Tchaikovsky’s scores are probably lost in the translation of the signal to a sound that we can hear. It’s fair to note that many good detectors provide detailed and flexible audio to help maximize the quality and detail of the audio interpretation of the electromagnetic signal.

Manufacturers interpret the detector’s electromagnetic feedback for us the best they can – some clearly much better than others.   In addition, different models of detectors take into account different types of users and different experience levels.   A “turn on and go” detector like a Garrett Ace, for example, has a lot more interpretation for a beginning or casual user, than say, Minelab’s juggernaut, the e-Trac:

detector-displays-fish

This is not to compare two of my own detectors , which are both great, but to say the amount of interpretation you see is aimed at different levels of users.   The Ace’s display (left) has plenty of plain English and icons for common finds – and no display numbers, just bars.  It’s trying to tell you “hey I think you are about to dig a dime.”

The e-Trac, on the other hand, might as well be the control module for a cruise missile (or a fish finder).  Same goes for the Fisher F75, another one in my personal arsenal.   They perform less interpretation in order to provide more feedback to an expert user.   They are trying to say “Here’s the most information we can provide to you.  It’s up to you to how effectively you use it.”

As a result, though an e-Trac or F75 can be frustrating to a beginning detectorist, it is, at least theoretically, more powerful in the hands of an expert user that is committed to the mastering it.

So how can we use this knowledge to become better detectorists?

GET CLOSER TO THE SEARCH COIL
Discrimination and other various detector settings can add even more interpretation to what you see and hear.  With discrimination, for example, you are relying on the control box to accurately omit signals that are likely undesirable so you can focus on signals that have a better probability of being whatever it is you are looking for.    Not to knock discrimination – I use it all the time, else a constant barrage of signals would drive me mad.     But what is it going to hurt to see and hear a few iron signals in a field where signals are scarce at all?  As you become a more experienced detectorist, you can reduce your discrimination to try and see and hear more of what the detector has to tell you, and make more of your own decisions versus the detector making them for you resulting in picking up extra good targets that were discriminated out due to things like depth, orientation, and/or proximity to trash.    With less discrimination you are getting closer to the real signal, getting “closer to the search coil”.

CLOSE” YOUR EYES
The Audio is the most important feedback you receive from your detector.   Any of the top experts, such as Terry “Goes4Ever”, will straight up tell you so.  In fact, many high-end metal detectors, like the Blisstool (which I have never used – maybe they will send me one) do not have a display at all.

Why?   Because sound is generally the least interpreted feedback that you are receiving from your detector, especially while using minimal discrimination.   The sound isn’t necessarily “trying” to tell you anything.   Sure some detectors have a “cha-ching” cash register noise that sounds on a signal in the “coin range”, but in general, once you learn what to listen for, you can learn to rely on the display less and less.

Experienced e-Trac users know the famous MineLab silver squeal and are very likely to dig when they hear it, regardless of what the display is showing.   And I love the staccato sounds of nails in a trashy site.  Why?  Because with my detector setup properly with almost no discrimination, I can pick out good targets in the nails by hearing faint high chirps in between the low tones of the nails – and many of them can be difficult or impossible to see on the display.

I used to allow my detector’s display to dictate to me what I dig.  Many deep signals may not display “correctly” on the display for aforementioned reasons. Many signals don’t show up at all on the display, or are illegible because of a machine gun barrage of constant signals in trash.   Is your reliance on your detector’s display handicapping you?

Final Thoughts:  A METAL DETECTOR IS LIKE A RACE CAR

Take two Indy drivers with identical levels of driving experience and knowledge.   The driver that knows his equipment – his car – better has a distinct advantage over the other.   And the same hold true for two detectorists with the same experience and skill level on the same machine.

Whether you are interested in detecting, a beginner, or an extremely experienced, taking the time to understand the principles of how our metal detector basically  operate is an integral part of maximizing your knowledge of your machine.   False beliefs or assumptions about how your metal detectors functions will adversely affect your thinking and decisions.   Conversely, the confidence of clearly understanding how your machine basically works will lead to mastering your detector faster, better decision-making and more treasure.

We hunt too long and too hard to miss good targets:   “Because it works” just isn’t good enough.

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Photo Credits
Detecting365
Fishfinder adapted from Amazon.com

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There are 4 comments for this article
  1. Detecting Diva at 3:17 pm

    Okay, I’m guilty–You lost me at the word “electromagnetic”, and my machine is set to “I just know”, but thats not to say I haven’t spend hours reading that darn e-trac manual and perusing forums for a better understanding. Thanks for the confession and great article!

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