Detecting in IRON

[Cody]

We can detect good targets mixed in with iron. If we set our detector to IM-16 and sweep the coil over targets, both good and bad, we see that all the targets are detected. If we are detecting in iron and have a constant null or sound then we need to make adjustments which can be settings or using a different coil.

One adjustment that is very helpful is to adjust the sensitivity if the problem due to detecting more than one target at a time. Lower sensitivity will decrease the number of faint targets that are presented to the user. It appears to the user as if the electromagnetic footprint of the coil is being reduced. Depending on the size of the target the fringe of the footprint is where fainter targets are located or at extreme depth.

Another very helpful adjustment is to use Ferrous Sounds. Since the tone does not depend on conductivity we get a good indication of targets based on ferrous content. It does not matter how conductive the metal is as the scale used on the Explorer is from iron, the most ferrous, to silver the least ferrous. If we keep a threshold and enhance the sounds with Ferrous Sounds, reduce sensitivity and use manual, turn fast recovery ON, we can go a long way towards detecting in the heaviest iron infested ground.

If we still have difficult then a smaller coil is going to make a big difference. We can consider the above adjustments with a smaller coil. We might look at a concentric coil instead of a double d. A 5

 

[Captain Kirk]

Cody,

You stated:
"Lower sensitivity will decrease the number of faint targets that are presented to the user."

I think that what you are saying is clear to most, but I would add a clarification. I would first add the qualifier that we are assuming that the targets are distributed equally at all depths. For that case, then reducing the sensitivity will reduce the TOTAL number of targets, but not necessarily reduce the number of faint targets. The reason is that the previously very faint targets will disappear, but will be replaced by a new set of faint targets that were previously louder.

I would also add a caution when using the FERROUS mode. When hunting in the CONDUCTIVE mode the silver coins produce a high pitched tone the same as rusty nails (although the rusty nail is pitched somewhat higher). This is true even if there is a nail co-located with a silver coin. When using the FERROUS mode the nail and silver coin are easy to identify if they are not very close together. But if they are very close together, then you will get an intermediate tone pitch. This will take some getting used to.

HH,
Glenn

 

[Cody]

Thanks, I always look forward to your post and learn much from you. I seem to be on the edge of how much to say and how little. My primary point may have been lost in the post but it was that we can work in very heavy iron and other trash if we are willing to make the necessary adjustments to do so. I don't recall any situation where I could not do that but have found some I prefer not to mess with. I have hunted some sites loaded with hot rocks, clinkers, and that was what I thought was the hardest sites to hunt.

You have so much knowledge to share so hope you do so often. What is your thinking on how the electronics selects which signals to reject and to accept when in discrimination and there are many targets under the coil at the same instant in time? Also, I have never been at a site where ground minerals caused a null. We have some very heavy minerals around the mines in Oklahoma that would null other detectors I have used. I have never seen this with the Explorer but instead will hear an abrupt change in the minerals as a high long faint zip sound. I am fairly sure I can hear the circuits update and compensate so have come to recognize the sound.

Have a good one,

 

[Captain Kirk]

Cody,

You asked:
"What is your thinking on how the electronics selects which signals to reject and to accept when in discrimination and there are many targets under the coil at the same instant in time?"

I have not studied the Minelab patents in great detail. As a result I do not know the details of the Explorer design. I have some basic thoughts that I have expressed before on how the ferrous and conductive properties effect the magnetic field. I am sure you already know all of this, but I will summarize as follows:


#1 THE RELATIONSHIP BETWEEN A LOOP OF WIRE (COIL) AND THE ENCLOSED MAGNETIC FIELD
Consider a single loop of wire with the ends slightly separated. The voltage between the ends of the loop is directly proportional to the time rate of the total flux enclosed in the loop. It does not make any difference whether:
* The flux results from connecting a voltage source between the ends of the loop or
* The voltage is the result of some externally generated flux pattern.


#2 THE BEHAVIOR OF COUPLING BETWEEN A TRANSMITTING AND THE RECEIVING COIL

#2.1 A rectangular voltage waveform is impressed on the transmitting coil. This induces a voltage on the receiving coil. The voltage induced on the receiving coil is a function of the turns ratio between the two coils and the coupling coefficient. If there are no conductive objects in the field region, then the induced voltage in the receiver coil has exactly the same shape as the voltage across the transmitting coil.


#3 CONSIDER HOW TWO LIGHTLY COUPLED COILS BEHAVE
For lightly coupled coils the flux seen by the receiving coil is only a portion of that generated by the transmitting coil

#3.1 When a ferrous target is placed in the magnetic field, then the magnetic field is distorted by the increase in permeability in the target region. The magnetic field is "sucked" into the ferrous target. This tends to increase the coupling coefficient between the two coils. The result is an increase in the voltage across the receiving coil. The voltage across the receiving coil remains exactly the same shape as that across the transmitting coil, but greater than without the ferrous target.

#3.2 When a conductive target is placed in the magnetic field, then the effect is completely different than for a ferrous target. In this case the magnetic field is distorted not by a change in permeability, but rather by the build up of eddy currents in the conductive target. The build up of eddy currents is time dependent and depends upon the resistivity of the target and the inductance presented to that current. We have in effect an L/R time constant that can be measured. This build up in eddy currents results in a dynamic change in the effective coupling coefficient. This produces a voltage in the receiver coil that looks different than that of the transmitting coil.

#3.3 When a target that has both ferrous and conductive properties, then we get a combination of results. We can examine the initial step in the receiving coil voltage to determine the magnitude of the ferrous component. We can examine the dynamics of the receiving coil voltage to determine the conductive component.

#3.4 It does not make any difference whether the ferrous and conductive components are the result of a signal target or from two different targets. We can still measure each component separately.

#3.4.1 For example a nickel has both components homogeneously mixed into a single target. A nickel laying right on top of (but not touching) a silver dime will look much like a different homogeneous mixture that is neither a nickel or a silver dime.

#3.4.2 If you separate the nickel and silver dime slightly in the horizontal plane, then the target no longer appears homogeneous but rather two separate targets that can be seen separately. But even in this second case the two targets will tend to blend together in such a manner that it does not look like a nickel and a silver dime. The reason is that the nickel's presence effects the dime ID and vice versa.

#3.4.3 If you increase the separation between the nickel and the silver dime, then the targets begin to look more like a nickel and a silver dime.


#4 SUMMARY

#4.1 As multiple targets are moved closer together, then they begin to take on a composite identity and lose the individual identity.

#4.2 When the targets take on a composite identity, then there is a single composite target ID. If it falls within the accepted region, only then you will hear the composite target tone.

#4.3 When two different types of targets are close together, then you will get a component ID for each target that is not really accurate for either one because they are interacting with one another. You hear either two, one or no target ID depending upon the discrimination setup.

#4.3.1 It is also possible that a slow sweep will show one target and reject the other. A fast sweep may show a target in one direction but not in the other direction because of the detector recovery time.

I hope you can follow this line of reasoning.

HH,
Glenn

 

[Cody]

Excellent and well explained! I understand what you are saying and don't disagree at all. You explain things much better than I do which reflects your engineering background and nice logical thinking.

These discussion are of more interest than we may think to a lot of people. They do encourage other to express their thoughts. I always enjoyed a good debate on campus as it caused a lot of thinking to take place.

Have you given much thought to the how soil minerals acts as a bond to "alloy" targets? I am thinking of a piece of iron and silver in heavy minerals and salts vs those target in light minerals. I am still playing with using the learn function to sample, accept and reject, targets in an actual matrix and construct a program instead of using samples and the X1 and air the air as the constant. Somewhere way in the back of my mine is the idea of a variable metal core transformer and air. The soil matrix with the targets is the variable core vs air as a core. I think we can construct a pretty good discrimination pattern from what I am seeing if we use the actual site we are going to hunt to do so.

My assumption is if you hunt the beach you mostly use IM-16 with fairly high audio gain and sensitivity? I am doing a lot of trash hunting in parks and some relics in clean and heavy trash.

 

[Chris(SoCenWI)]

Glenn, Cody, and anyone else geeky enough to care,

There was a discussion on this a month or so ago that kind of turned on a lightbulb. So far it hasn't been a very bright or useful lightbulb but.....

If changing sensitivity determines how small a signal gets processed by the rest of the electronics, it should mean as you lower sensitivity you would start loosing fainter signals unless the detector rescales or you mess with the gain.

Say wide open sensitivity processes the full range of signals, lets say just for kicks it ranges from 0 to 20 MV. I realize from your discussion farther down the thread that this may not be a simple voltage but rather the change in the waveform (phase, amplitude,?) from the xmtr coil to the receiver coil. However there will be some minimum change in signal that is processed, anything less is considered noise. Increasing sens decreases this minimum, decreasing sensitivity increases it.

Say we dropped the sensitivity so now we are only processing signals from 5-20 MV, anything less is ignored. If gain is evenly applied to the full range of signals, where a 1 MV signal would be a faint peep, now we now longer have signals in the 1-5 MV range so the quietest signal that should turn up in your earphones should be considerably louder than if you were running at max sensitivity.

Follow what I am saying? I realize that perhaps the machine could adjust the gain so it still spans a certain dynamic range but don't think this is happening.

I've tried a couple tests using sophisticated test equipment (my ears) and it does seem that as you decrease sens you do get a louder minimum signal that you hear. Unfortunately it seems that anytime you are on the cusp of getting a signal because of sens or perhaps also because of discrimination you get sort of a broken signal that has some lowtones and noise built in. Think this could be verified with a scope pretty easily.

Long story short I'm wondering if lowering sens would make the previously louder iron signals fainter or would you just loose the faint ones?

I think this is what auto sens does in high trash, pretty soon it gets mighty quiet and only the biggest signals get through. There was also a post not too long ago I think by Cody talking to one of the dealers that said in auto lift the coil of the ground, let it recover sens(much like pinpoint mode) and then it will get the same depth. My gut feeling is that it does not recover that quickly, that if you are in auto in lots of trash it will detune and maybe take a week and a day to get back up to higher sensitivities once you clear the trash.

Am I missing something?

Chris

 

[Cody]

"Long story short I'm wondering if lowering sens would make the previously louder iron signals fainter or would you just loose the faint ones?"

I don't think the previous louder signals are fainter but we lose some of the faint ones. You will notice when detecting that if the gain is set to 5 or less the audio is modulated pretty good. If it is higher than that then we here most signals pretty loud. It seems like using 8 to 10 is the way a lot of guys like to run that function so everything sounds loud. A large target on the fringe of the footprint can at first be a fairly faint hit. As we pinpoint the hit and get the coil closer the signal increases in relationship to the hot strip of the coil as a reference point for sake of the discussion.

Decreasing sensitivity makes it a little easier to work the coil but is not the best answer in my opinion for the reason you state. An 8" or 5" is ideal in my opinion and what I use most of the time.

 

[Captain Kirk]

Chris,

I believe that I jumped to a conclusion that was probably in error. What you say makes sense to me. It does seem reasonable that lowering the sensitivity should just make the faint signals disappear and not have any effect on the stronger signals (such as making them sound fainter).

Thanks for your input.
Glenn