Ground effects

[Unregistered]

Hello all,

I've seen the following explanation from George Payne about ground effects in the loop of a metal detector:
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A pure ground is a soil condition that reacts like it was pure ferrite -- in other words, a perfect magnetic condition where no electrical conduction (eddy currents) takes place. We can think of this as a soil that produces a signal in the detector with zero phase shift relative to the transmitted signal. This is considered our reference signal of zero phase to which all other signals can be referenced to. Of course, the only real life object that produces this type of signal is pure ferrite. So ferrite becomes our reference target and produces what we call a pure "X" reactive signal.
Real ground conditions do not behave like pure ferrite. When subjected to a detector's magnetic field, small currents begin to flow in the soil. This will cause the soil signal to be displaced slightly from that of pure ferrite. We call this difference a phase shift and define it to have an angle in degrees negative relative to pure ferrite. In addition, this phase shift produces a new signal in the detector which we call the "R" component signal. We can carry this analysis one step further. Using trigonometry, the ratio of the X signal to the R signal can be shown to be the actual measured phase of the ground.

All grounds have varying amounts of magnetic and conductive properties. Therefore, the ratio of the X (magnetic signal) and R (conductive signal) will vary from one location to another. However, the phase produced by this characteristic will always be negative relative to zero (the phase of pure ferrite).

From my experience, most grounds produce a phase that falls somewhere between zero (ferrite) and a -5 degrees. Some highly magnetic soils can have a phase that is quite low, but it can never be zero. Once the phase exceeds several degrees the ground characteristics begin to fall into an area where it becomes more saline. This doesn't mean that it's not magnetic. It's just that the R (conductive component) of the ground becomes stronger in relation to the magnetic portion. Thus, the phase becomes greater.
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Fantastic. I´m sure George is a md Guru and I'm an idiot (it is not an irony), because I can't see what ground phase is named, is it the delay in the ground received signal with regard to ferrite received signal?, what does it happen with the amplitude of the signal?, is the phase the ratio of X to R? what really are R an X really? what is the influence of real and imaginary parts of the loop impedance? What is ground conductivity? Could any of you provide an explanation for idiots as I? Could be indicated with graphics and mathematical expresssions?

I am very sorry because there are a lot of questions before. I have a mathemathical and electronics background, but I am not able to undestand (see) the ground influence and perhaps the discrimination phenomenon. Could anybody give a clear (for me ) information?

Thank you

Frank

[Reg]

Frank,

First, yes, George Payne is a MD guru to say the least. For the record, he developed the first consumer useable VLF while working at Whites, the first motion discriminating detector while at Bounty Hunter, the first with target ID while at Teknetics, the first with audio ID, the first depth blanking, the first with notch, and designed the first automatic ground balance, all while at Teknetics. Now, this is just some of his accomplishments.

Most of his patents can be found on the Geotech forum.

As for answering your questions, George tried to describe the target phasing found with targets as they apply to a VLF. This foundation sets the basis for developing a detector that can not only discriminate, but display targets based upon their fundamental conductivity properties.

At one extreme, you have pure ferrite or a pure x component (pure reactive). At the other extreme, 180 degrees later, you have a pure conductor, something better than silver. For the purpose of clarity, George labeled the pure ferrite as 0 degrees. This makes the pure conductive object as -180 degrees by his definition.

It just so happens that saltwater occurs at -90 degrees. Ferrous objects will range from maybe -20 degrees to approaching -90 degrees. The ground signals are generally between 0 and -8 degrees. Non ferrous targets such as gold and silver objects will fall somewhere between -90 degrees and -180 degrees. A nickel, I believe, at -135 degrees.

Now, any real target that is detected by a VLF can be described as something having both conductive and reactive properties and as such that component falls within the two extremes mentioned above.

Now, to try to answer your question more specifically, yes, the ground signal phasing is the phasing as seen on a scope that occurs to the response as the result of a sample entering the field of the coil, and yes, it is the phasing when referenced to the ferrite signal.

As for the amplitude, one can think of this as the strength of the signal. So, the ground signal can have both phase and strength or "intensity". So, the ground signal can have a phase and vary in signal strength simply because of the nature of the ground.

Generally, the ground conductivity is generally thought of as the salts in the ground, but should include the effects generally referred to as viscosity. Red clay generally has a magnetic viscosity.

If you read some of the patents that can be found on the Geotech forum, some do have phasing diagrams trying to display what is being described.

Unfortunately, there are no books that I know of that actually delve into this area of study. I do know that George is working on one and I have offered to pay for it in advance in hopes of getting the first one, but so far, there he hasn't agreed. So, the recommendation is to read as many of the patents as possible since they are generally the best source of more information.

Now, X and R are the mathematical components for the reactance and resistance of any object. When you took control systems, if you did, you learned that all objects can be simulated by mathematical components. In the case of control systems, we had friction as resistance, coils as inductances and shock absorbing components represented by capacitors. So, most any physical object could be represented by a mathematical formula.

Now, when we deal with objects such as those found with a metal detector, such objects can also be represented by inductors and resistors. The combination is such that it generates the phase the object appears to generate.

If you have used a TID machine, you will notice objects go from iron to silver. This display is basically showing how the targets fall on the 0 to -180 phasing display mentioned by George. The difference is, the display isn't in the specific numbers he mentioned, but the reference has been based a little different for simplicity. However, the principles are the same.

Now, TID signal will display the phase, which is the ratio of X/R. The amplitude or target signal strength will be heard in the all metal mode. So, if the target is close to the coil, the signal strength will be great and the all metal mode will have a very loud signal. If the target is deep, the object should still read the same on the TID, but the audio signal strength as heard in the all metal mode will be weaker. This same concept is true for the ground signals also.

I recommend you read the few other articles written by George, since that is really all the technical information I am aware of outside of the information found in the patents.

Overall, the concepts mentioned are confusing to someone who has not spent a lot of time working with detectors.

I have decided to add a link to one of the patents that does display a phasing diagram. In this case, the patent is from Tesoro and Jack started from a different reference point. However, his phasing is also 180 degrees from one extreme to the other. His does show where some coins and object fall.

Here is the link:
http://geotech.thunting.com/pages/me.../US4486713.pdf

I hope I have answered your questions.

Reg

[Reg]

Frank,

I have decided to add just a little more. Since many of the newer units use a form of digital readout and number systems instead of the 180 degree chart mentioned by George Payne, I recommend you look at the White's patent for their digital unit.

See patent number:

http://geotech.thunting.com/pages/me.../US4868910.pdf

Now, when you look at figure 6 on page 8, you will see numbers ranging from 95 at one extreme to -96 at the other. If you look at the fundamental layout, you will see it is again a phasing diagram that spans 180 degrees.

Fortunately, this pic shows how the digital numbers can correspond to what George said. The -96 is the same as George's 0 degrees and the + 95 correspondes to his -180 degrees.

Now, what isn't generally known is the step from +95 to -96, isn't 180 as it appears, but is really 1 degree.

Now, that doesn't make much sense unless you know how detectors work internally.

So, what am I trying to say? Well, because of the mechanics involved, the step from +95 to - 96 is minimal. So, on this particular metal detector, if you turn on the most negative numbers, you can detect objects deeper than when those numbers are blanked.

The reason has to do with how the ground signal affects the actual target signal. George does explain that condition in his discussions.

So, to add to the confusion, when a target is deeper, the signal becomes distorted by the ground signal. Once it reaches the maximum depth of detection, the ground signal shifts the signal a little and the target appears to flip over to the ferrite side of the chart. Now, turn on the last 5 negative numbers or so, and suddenly, you can detect coins and other objects that will not TID accurately, but are good targets. Since the inclusion is only certain numbers that are for ground conditions, then about the only signals that will generate a response are the good targets.

This is a little trick many people do not know about. BTW, I did write about it many years ago when I field tested White's Eagle for LT. So, the info isn't a complete secret.

Reg

[Carl-NC]

Yes, an ideal conductor (superconductor) should have exactly a 180-degree phase shift. That's because magnetic fields cannot penetrate a superconductor, so the boundary conditions at the surface demand that the magnetic field -> zero. This only happens if the apparent target field is 180 degrees.

Reg's explanation is pretty complete. Target phase response is relative to whatever "nominal" phase response the ground creates. So detectors have a ground balance to zero out the phase of the ground signal.

- Carl

[Delbert grady]

Phase

I am trying hard to get my head around this.

There are two sine waves going from left to right on the scope.
One is leading the other by 45 deg. So is the one that is leading further to the right or further to the left on the screen.

[Reg]

The one leading is the one that starts first. So it is the one to the left.

[Unregistered]

Ground efects

Hello to all again,

First of all, I have to say that English is not my first lenguage, and I didn't want to offend to anybody with my last mail. Perhaps I expressed bad my ideas. I know that George is one of the md gurus, as some of you are. I wanted to emphasize that you are speaking about a physical phenomenon, that are very well known by all of you that are very experimented md tecnicians, but it is very difficult to understad for people that are out of the md technics. I am a beginer in md world, but I have experience in other electronic areas.

The only thing that I wanted was an easy explanation of the whole phenomenon, for people that are new in md. Something like "you transmit a signal that is: A sen (wt)

In the receiver loop you received a signal: A' sen (wt+x), and the x is positive for ...."

Something like this but starting from the beginig of the phenomenon. And saying what are conductive and not conductive materials etc.

Thanks to all of you Reg, Carl and the others for the answers and help. I'll read the patents, and I'll try to understand you are speaking about. I don't know things like X/R bt in the future I'll do.

Regards

Frank

[Reg]

Hi Frank,

I am sorry if I came across wrong also. My intent was to let people know when George Payne speaks, he knows what he is talking about. Many beginners are not aware of his accomplishments or abilities and as such have a tendency to question his facts.

Now, I have to admire you Frank because you expressed yourself very well in english. I wouldn't have known it was not your primary language. I am still struggling with one and that is english. So, you are way ahead of me in that area.

Personally, I wish George Payne would finish his book. This would help greatly since he planned on discussing detectors from the basics to the most complex features.

Now, as for your request, it is very difficult to explain how a detector works without writing a book, especially a VLF.

Basically what happens is a coil is built such that the receive winding is nulled or positioned such that there is no or almost no signal in it. This may require an additional winding to cancel any transmit signal.

Now, one can connect a scope to the receive winding and not see anything or almost nothing at all. Now, once some object enters the field of the coil and that object has some magnetic or conductive properties, that object will cause the receive signal to change and increase. Also, there will be a corresponding change in the phase of the signal.

In other words, the receive signal will get larger and will shift with respect to the transmit signal. By analyzing the shift, we can determine much about the object.

Patents help because many of them do explain the principles of what is happening. However, some are quite confusing and a few are inaccurate in some respects. This is why it is a good idea to read several.

Now, in my previous postings, I mentioned X and R. I also mentioned that any target (any object that is detected) can be simulated by an inductor and a resistor. Generally, they are shown in parallel. Now, when we analyze an inductor and resistor, we know there will be some phase shift since a pure inductor will have a phase shift of 90 degrees, while a pure resistor will have no phase shift. Now, if we let X equal the inductance characteristics, and let R equal the resistance characteristics, then the the ratio of X/R is the tangent of the particular phase. It is this tangent that provides the signal used for target ID (TID).

So, if you have taken electronics and learned about the phase angles and what happens when a resistor and an inductor connected to a circuit, then you have a better idea of just what is happening.

In the case of the objects detected, the better the conductivity, the higher the R value and lower the X value.

Now, the coil and associated circiutry can be thought of as a transformer and the target as another secondary windings. I have a poor pic I copied from a patent that sort of shows what I mean and attached that pic.

Because the target has both inductance and resistance, it will cause a signal shift accordingly. Now, once the signal is amplified, then it can be analyzed with respect to the transmit signal to see if there are any phase shifts as well as amplitude changes.

The rest of the circuitry basically does this type of analysis. Just how it does it, is quite complex and not easy to explain. If you go back and read some of the earlier patents, you will see it takes several pages of information just to explain how the sample is taken to create the all metal mode on a VLF.

Basically, what is done is to take a sample that is in sync with the transmit pulse. Now, by adjusting the phase shift between the two as to when the sample is taken, we can get a different type of information. If the phase shift is taken properly, we can obtain a signal that ignores the ground signal and only the target response is heard. This signal is commonly called the R signal or all metal signal. Actually, not all of the ground is ever eliminated, but most is.

Now, if we take a second sample 90 degrees or so with respect to the main sample, this signal is sometimes called the X signal. Now, by combining the R signal and the X signal properly by taking the ratio of the two, we can now analyze the target itself. Unfortunately, when we take this second sample, there will be a lot of ground signal to deal with and that ground signal will inhibit any logical analysis unless something else is done.

One thing to note is we do not have to be exact on this second sample. In other words, it doesn't have to be exactly 90 degrees, but does have to differ sufficiently from the main sample. On some detectors, this second sample is or can be up to 135 degrees or so different.

So, what is done is to filter out the ground signal, or at least try to. The reason this works has to do with the coil design and the signals themselves. Generally speaking the ground signal is a very slow changing signal that varies because we do not keep the coil level. However, this is only true when passing the coil over flat ground such as commonly found in parks or yards where people look for coins.

Anyway, on a motion detector, there are filter circuits built in that are there to remove this ground signal. The number and type of filters will determine just how well that is done.

Because a coin or other metal object is small and very different than the surrounding material, the coin (target) will produce a sharp pulse type signal. This sharp pulse type signal has harmonics that can be filtered out. So, by using a bandpass filter that rejects slow signals (ground signals) and amplifies faster signals (target signals), the target signal can be separated from any ground response.

Now, this filtered signal is then compared to the all metal signal which will go through identical filters to assure the timing is the same.

The final filtered signals are then analyzed to determine the target.

Unfortunately, that is about as good as I can explain it without writing a book to cover each aspect in a much finer detail.

Hopefully, this simple explanation will make it easier to understand some of the patents.

Reg

[Unregistered]

Ground efects

Hello again to all,

Thank you Reg for the excellent explanation, so for your effot doing it.

I start to see more clearly the things. At the end I will understand, I hope.

One of the things that are confusing for me is that depending on materials, phase can be at the maximums - 90 and + 90 degrees. In the transformer coupling equivalent circuit wou draw, with only inductance and resistance, I can only reach from 0 to 90 degrees.

Other thing I have not information about is the possible detected materials. A scale with conductivity and magnetic properties of some of them, and the response to them produced in a VLF md, could be useful to understand what are speaking about.

But, for the moment, I have bothered enough. I have to read more and learn by myshelf.

Thank you again

Regards

Frank

[Unregistered]

Ground effects

With errata correction. Sorry

Frank

Hello again to all,

Thank you Reg for the excellent explanation, so for your effort doing it.

I start to see more clearly the things. At the end I will understand, I hope.

One of the things that are confusing for me is that depending on materials, phase can be at the maximums - 90 and + 90 degrees. In the transformer coupling equivalent circuit you draw, with only inductance and resistance, I can only reach from 0 to 90 degrees.

Other thing I have not information about is the possible detected materials. A scale with conductivity and magnetic properties of some of them, and the response to them produced in a VLF md, could be useful to understand what are speaking about.

But, for the moment, I have bothered enough. I have to read more and learn by myself.

Thank you again

Regards

Frank