signal of flat coin vs. on end

[Tucson Dan]

How different will the signals be with the orientation of the coin in the ground? Is it possible the signal may not be repeatable if a coin is vertical?

 

[Frankn]

I think it is highly unlikely that you would find a coin on end in the ground. That said, how wood you know it was originally on end before you dug it up?
If you want an answer to your question, merely place a coin flat and another on end and check each with your detector. It is that simple. Frank...

 

[Tucson Dan]

Thanks. I need to get some coins in a test bed in different orientations, both clad and silver. Is it metal density or the circular shape that my detector knows what coin it "sees"? I'm still learning it, ACE350.

 

[TreasurePirate69]

As you know, your detector doesn't "see". It senses the electromagnetic signal returned from the target. This signal has magnitude and phase only (we will ignore frequency responses and such in order to keep this simple). It typically does not have any sort of directional component due to the use of a coil as the receiver and the design of the circuit. That doesn't mean that a coil isn't directional. It just means that your detector's circuit has no idea about direction of the target. The phase is mostly associated with the conductivity of the target which means that different metals can result in different phase shifts. But this is not something that can always be counted on to be correct (especially given the wide array of circuit designs).

Detectors that try to tell you how deep the target is and what type of object it is are really just "guessing" and should never be fully trusted. After all, a signal of 100mV and 120deg can be made from either a shallow coin or a deeper soda can. A coin on its edge might produce a lower magnitude than one sitting flat. So although the phase might be close the magnitude may not be which will potentially throw off your depth reading.

So the density and shape of the target alter the signal received. But since your detector only deals in magnitude and phase it can't really distinguish either attribute of the target. All it knows is that it received a signal of a certain magnitude and phase. The item that made that signal could be very dense and buried deep, or it could be much less dense and buried very shallow.

 

[Tucson Dan]

Thanks. That actually makes sense as I have a little electronic background. My detector is just guessing on depth as a large object deeper may seem the same as a smaller one buried shallow? With coins, the depth is pretty close though for the most part.

 

[TreasurePirate69]

Thanks. That actually makes sense as I have a little electronic background. My detector is just guessing on depth as a large object deeper may seem the same as a smaller one buried shallow? With coins, the depth is pretty close though for the most part.

Correct. Each detector can be different on how it is designed to guess. But it is nearly impossible to tell the difference between a very large object buried very deep and a smaller object buried very shallow. If the magnitudes and phases are the same, then the detector usually assumes they are the same type of object. This is a simplification and may not be true for all detector designs. But for the majority of detectors out there this is a relatively true statement.

For example, in order to tell the difference between a dime and a quarter (two objects having similar phase shifts), we could do relatively well using the following traits to aid in our guess. First, we know that a quarter is bigger than a dime and should result in a higher magnitude signal at similar depths. Second, we know from testing what the maximum detection depth is on a dime and on a quarter in some average type of soil. Third, we know that most coins are found in the first 5 inches of soil. Finally, we believe that the odds of finding a dime are higher than the odds of finding a quarter. So we can use all of these "facts" to create a decision table which would be right quite often if the signal falls within a certain set of values. We still can't tell the difference between a dime at 1" and a quarter at say 3". But we can know that any signal that looks like a dime or quarter at 16" probably isn't a dime or quarter. This type of guessing usually does a decent job but as you can see, that target ID and depth could be WAY off. It can easily be telling you something is junk when in fact it is a good target.

 

[63bkpkr]

Flat vs Horizontal coin target vs what I think my detector is doing

I will throw in my two cents worth here though it might only be worth half a cent.

There is phase, there is magnitude and then there is time. The time component would start as the detector sent the original signal and stop as the response (if any) was received. The amount of time coupled to the magnitude and the phase should all be in a mathematic algorithm that spits out the result we see on the screen/hear in the earphones and yes part of that algorithm is an estimate possibly based on types of metal response, a depth component as related to mass and distance and lots of other things I've NO idea about.

Electromagnetic fields as well as electricity do their own physics sort of stuff, electrons flow on the surface, amperage is carried through the mass of the wire and magnetic fields are modified by minerals and metals in various ways but diminish with distance for any given amount of mass. My coin detector does a good job at telling me how far down a coin is till it just does not reach any further into the soil. I started metal detecting in people's lawns with a probe and this allowed me to "sense" the position of the coins I was detecting. Probably 95% of my coins were flat or nearly flat when I first found them but the other 5% were at good angles and some possibly near vertical. Electricity entering a metal object will exit that object at its thinnest or smallest section/point. Watching magnetic lines of force working a pile of metal dust is interesting as it at least looks like it does something similar in that if you give it a chance it will focus on a point. So I would think, that is IMO, a coin on edge should have a different reaction to an electromegnetic field as the same coin at the same depth in the same soil would when it is flat. Again what I've said may only be worth about a half a cent. Please someone else that Knows this subject jump in here................63bkpkr

 

[Tucson Dan]

I have dug a "quarter signal at 4" only to find a crushed beer can at 8". I am starting to understand it more and more as I go. Is it simply how the electomagnetic lines are coupled and received back as to what the MD thinks it is? Is there a power factor to the signal received that dictates a depth guesstimate?

 

[TreasurePirate69]

Yes, time is important and without it we don't really have phase. For a single receiving coil detector we really only have magnitude and time. Phase is obtained by looking at the magnitude over time. I wanted to keep things simple and have not gone into depth on things like frequency analysis and other time based techniques because these are rarely used in the lower end detectors since simply looking at magnitude and phase usually gives acceptable results.

You are absolutely correct in your discussion on electromagnetic fields. They are very complex and the soil as well as the target can add to the resulting response. With a single receiving coil detector all of these signals add together creating a final single resulting voltage or current that our detector then processes over time. Electromagnetic signals are also directional in nature so the orientation of the target in the ground plays a part in determining the final signal seen by our detector. At any given point in time our detector sees a single voltage or current that is the sum of all electromagnetic fields flowing through the coil. Spread out over time, that signal can be very complex and have multiple frequency components.

You touched on another important topic and that is how electromagnetic signals decay exponentially. This is another rule that we can use to guide our detector in target identification. A coin sitting within the first couple of inches of ground is likely to overload a detector while a coin sitting at 4" might only have 1/3 the magnitude. It is not linear. Our example detector also doesn't have to be linear in its readout. Finally, people are very forgiving in their interpretation of the results. So for example, if my detector gets a very strong signal that overloads the circuit I can report a coin in the 1-2" range. If the signal is much weaker I can report a coin in the 4-6" range. If the detector reports 1-2" and the coin is at 3", most people will say that it is "close enough" and claim that the detector is usually right on the money. In reality, since most coins are found in 1-6" of soil then there is plenty of wiggle room in being considered "right". I could just have easily used an on/off LED called "shallow/deep" that would have been just as accurate as indicating exact depth measurements. After all, if a detector reports 4" and the coin is at 5", who is really going to complain? How many of us go out in the field with a ruler and judge the accuracy of our machines? So just being able to tell the difference between a shallow coin and a deep coin is close enough for most people to say that their detectors are "good" with depth detection. There's nothing wrong with that. Just something to think about when putting a lot of trust into a detector's readouts.

 

[TreasurePirate69]

I will throw in my two cents worth here though it might only be worth half a cent.

There is phase, there is magnitude and then there is time. The time component would start as the detector sent the original signal and stop as the response (if any) was received. The amount of time coupled to the magnitude and the phase should all be in a mathematic algorithm that spits out the result we see on the screen/hear in the earphones and yes part of that algorithm is an estimate possibly based on types of metal response, a depth component as related to mass and distance and lots of other things I've NO idea about.

Electromagnetic fields as well as electricity do their own physics sort of stuff, electrons flow on the surface, amperage is carried through the mass of the wire and magnetic fields are modified by minerals and metals in various ways but diminish with distance for any given amount of mass. My coin detector does a good job at telling me how far down a coin is till it just does not reach any further into the soil. I started metal detecting in people's lawns with a probe and this allowed me to "sense" the position of the coins I was detecting. Probably 95% of my coins were flat or nearly flat when I first found them but the other 5% were at good angles and some possibly near vertical. Electricity entering a metal object will exit that object at its thinnest or smallest section/point. Watching magnetic lines of force working a pile of metal dust is interesting as it at least looks like it does something similar in that if you give it a chance it will focus on a point. So I would think, that is IMO, a coin on edge should have a different reaction to an electromegnetic field as the same coin at the same depth in the same soil would when it is flat. Again what I've said may only be worth about a half a cent. Please someone else that Knows this subject jump in here................63bkpkr

Exactly. As I said in my last post, all of the electromagnetic signal lines that flow through the coil are added together. Electromagnetic signals decay in a non-linear fashion. So you would not expect a quarter at 8" to have half of the signal power as a quarter at 4". It might be more like 1/10th. That beer can is bigger so it makes up for the reduced signal power due to greater depth.

There is also the issue of phase shift. That aluminum beer can should give you a phase shift in the aluminum range instead of in the clad/silver quarter range. Why does this happen? Well, as 63bkpkr pointed out, the shape and orientation of the object can impact its magnetic field. A crushed can may have many sharp corners which change the signal properties. The signals coming off of all of these edges add together and can cancel each other out to the point where the phase appears to shift. The whole concept of phase shift being an indicator of the conductivity (and thus type) of metal is based on the idea of a uniform target and single response field. Put a whole bunch of electromagnetic response fields together going in different directions and all bets are off as to what the target is. It's not as bad as I make it out to be. But it isn't a perfect world either.

Finally, the magnitude of the signal can effect the phase calculation. Phase is a 360 degree phenomenon meaning that it wraps around onto itself (repeats every 360 degrees). A very large iron object can overload the detector throwing off the phase calculation. It can make that iron look like a quarter because the circuit may not be designed to handle these types of overloads. But all detectors are different and some may do a better job than others at rejecting large iron signals.

 

[Tucson Dan]

I have been very happy with my ACE350 in that it seems to be correct, for the most part, most of the time with target ID and depth. Allowing an inch or three obviously. In some respects, figuring out how the MD functions is almost as fun as the hunt!

 

[TreasurePirate69]

I have been very happy with my ACE350 in that it seems to be correct, for the most part, most of the time with target ID and depth. Allowing an inch or three obviously. In some respects, figuring out how the MD functions is almost as fun as the hunt!

I agree.

 

[Tucson Dan]

I hunted woods in WV last week and thus had my discrimination at zero. I got a quarter/dime signal that turned out to be a broken horseshoe. Very interesting in that the iron created a signal that was completely was opposite to what the detector thought it was.

 

[cudamark]

Lots of good info. Keep in mind that the detectors we buy here in the U.S. are calibrated to U.S. size coins when it comes to depth readings. Other objects and foreign coins can be all over the board but generally the same size object made of a similarly conductive metal at the same depth will read about the same. So will a relic of different size and metal at a different depth. Size can generally be determined by sweeping the coil and finding the "edges" of the target. Depth can usually be accurately guessed at by the sound you hear......how sharp or loud it is. As for the OP's question, I've found coins on edge that read different than ones flat. Sometimes the ones on edge will only give a good signal sweeping in one direction and a broken or non-existent signal the other direction. The way I know they were on edge is that they were still in the plug when they were removed from the ground and once the plug was split in two, you could see the angle at which the coin was laying in the ground.

 

[wingmaster]

I've dug coins on edge before with a good signal one direction of swing and the other direction it was coming up as iron, the only reason I dug it was it had a VDI just like a silver dime. If you hit the flat side of the coin you'll get a good signal and the edge might come up as iron or not at all if its really deep, I bought a ultimate 13" detech coil looking for more depth which didn't give more depth but did give a better enough signal with some coins on edge that I dug about 4 more deep silvers on edge where an old baseball diamond. HH

 

[63bkpkr]

Thank you knowledgeable advisers!!!!!!!!!!!!!!!!! I love the extra detail and input on wiggle room!

Tucson Dan ,
YES! Figuring out how the detector works adds some amazing insight into hunting. I pay homage to those kind folks who post tested/reliable menus or directions for using machines as they so help those of us, me, who do not have all that much time or knowledge to put into demystifying this thing in our hand that goes beep/zip zip and makes us giggle and dance. Have more fun everyone..............63bkpkr

 

[blaylock]

I have dug a "quarter signal at 4" only to find a crushed beer can at 8". I am starting to understand it more and more as I go. Been there done that..lmao

 

[Jackalope]

All lines of force are parallel they never intersect and always return to the source, that is, the detector coil. Like the imaginary magnetic field lines around a horseshoe magnet. When the magnetic field lines generated by the coil intersect a metal object they create a voltage which causes current to flow, which in turn causes another weaker magnetic field to be generated, which if strong enough causes the inductively balanced transmit and receive coils to no longer maintain equilibrium and again generate a voltage which drives a current which is amplified into a beep.

But, the greater the number of magnetic field lines passing through an object the greater the induced return signal caused by the eddy currents. So, a coin near the surface will react with a stronger secondary field than one deeper. Generally the field loss obeys the 6th power law, that is a cubed loss into the dirt and a cubed loss back to the coil. Thus, a target twice as deep will be 1/64th weaker at the coil (1/2^6). But also, if the position of the target is altered so that a coin is on edge, the field lines may not "cut" the target as well, being more parallel now. This results in a weaker secondary signal. It may be in this case that it is the outer edge of the coil that signals first because the field lines in the ground are turning and are more perpendicular to the coin surface. As the coil sweeps past the target the signal is lost (more parallel lines into the ground along the coil center line or Tx/Rx portion of a DD) and the coin now TIDs as iron. You can also get a double beep on coins on edge - once from the right-to-left sweep and upon reverse - each hitting opposite sides of the coin.

Keep in mind that magnetic materials, iron in particular, produces a much stronger signal at the coil because it not only may produce weak eddies it also causes magnetic moments to align with the primary field briefly and then relax. This movement creates an reinforced secondary field that can overpower the weak coin signal. So, you only get an iron audio or TID or perhaps a chirp of high tone associated with the non-ferrous coin. The more iron buried between the coin and the coil the more the coin TID is down averaged so that the E-Trac will show deep coins near 25-27Fe instead of 12-14Fe when recovered. You may just walk on by thinking it is a nail. Plus, the E-Trac can reject the coin and null and never register its weak eddy signal - you won't know the coin even exists at all.

If you are still reading ... Iron tends to concentrate field lines and non-magnetic metals tend to deflect them around away. This has to do with the way the secondary field interacts with the primary - it cancels it. But no so much with the iron - which has a permeability above one, which makes it easier for the field lines to pass through. The net effect is that magnetic metals, like iron nails and maghematite and magnetite or rust flakes will pull or distort the field lines away from the vertical and bow out the pattern, causing a loss of depth. The coin may still be detectable at 6" but with concentrated iron the field lines spread outward and very little lines cut the coin to generate a secondary. Thus, you'd have to dig out the iron to get the coins underneath - the iron was "masking" the coin.

A bigger coil will drive more field lines into the ground and generate a better response from the coin - but also will drive a much much larger response from the iron - negating the gain. In other words, a small coil has less volume seen under the coil which decreases the effect iron has and increases the sensitivity to smaller targets. So, in iron we use smaller coils and get more depth - an oxymoron - smaller gets better depth than larger. The DD coil is preferred since it has less volume than the concentric and can wedge between the iron to hopefully zap the coin masked between.

One thing worthy of note: Iron in the ground responds via momentary magnetism of its iron atoms and is very fickle to the energy source (its orientation sensitive). Moving the coil slightly will cause a wildly changing response returned to the coil. This causes the TID to jump about. When a coin is nearby, if the field lines are altered slightly the iron response may not give the ferro response as strongly and the coin squeaks out through the iron. Thus, to make the eddy current response prevail you have to turn around the suspected deep coin target and every so slightly sweep the coil. The goal is to listen for that repeatable high chirp. It may be repeatable only in a very discrete and narrow sweep and perhaps only one way (i.e., left to right but not right to left). But that is a deep coin signal and needs to be dug. You may never get a two-way repeatable clear high tone in iron ridden soil. While the E-Trac maintains TID better than most at depth, the TID will fail as the depth increases, become more wild, due to the weakened signal. But the audio will still reveal the target as a coin - so place the highest priority on the sound and less importance on where it falls on the TID, especially if the depth meter is pegged below 6-7" deep.

Large deep low-conducting iron, though producing weak eddies on its rough, poorly conducting surface will still in total energy produce a strong eddy response due to its large surface area (such as a buried iron hinge or iron plate). The coil has no idea and assumes it is a weaker eddy from a nearer non-ferrous coin-sized target (to which it is scaled). You dig to 3" and no coin, then 6" and still the target is deeper, finally you find the worthless iron at 12" or more. Likewise, very deep non-ferrous (like soda cans) produce immense eddies on its surface which produces a relatively strong high tone audio and the depth meter says it should be a shallow coin too. Much digging later and you have your mangled soda can at 15". Note that if you are digging past the reasonable maximum depth of detection for a coin sized object - it ain't a coin. If you hear hoof beats think horses not zebras. Most large deep ferrous or non-ferrous have a wide dispersed or broad audio when swept rather than the more isolated and centered audio of a coin-sized object.

One other thing, elongated iron responds strongest at the extreme ends and along bends - these are points that eddies are more easily generated in the surface. A horizontal nail at 10" will produce a strong signal that jumps around, but you think you are getting a high chirp from a non-ferrous coin buried nearby - it parrots a masked coin. After you find a few nails and isolate the nail tip as the culprit you will recognize the rather truncated chirp that elongated iron gives from the masked coin chirp (which is fuller).

Since the nail is elongated the center point of the target will often jump from where you first located it in DISC - a sign you are dealing with iron (the eddy producing tip is offset from the iron shaft ferrous signal by a few inches causing the center of the target to jump around when pinpointing with the coil). As you dig the signal will often be strongest off to the sidewall of the hole where the ferrous is emanating (the tip only sounds off strongly to the coil not the pinpointer. You centered on the tip and now the pinpointer is finding the nail shaft). The combination of being off-center when pinpointing with the coil and off-center in the hole is a common nail signature. Note: the nail can phase shift wrap so that the 35Fe bottom signal is pushed so it appears at the 01Fe top on the E-Trac - don't be fooled into thinking you have a silver dollar, if it's erratic in the TID like this, it's just a nail.

 

[cudamark]

Nice explanation, Thanks!

 

[swantek]

Now that was some "deep" reading. Thanks jack.