Reg... ironstone / magnetic viscosity explanation please...

[Jim Hemmingway]

Hello Reg… over on Nuggetshooter323’s thread ausgoldhunter observed that the TDI SL equipped with the “spider” coil… presumably meaning the 12” DF coil… has some difficulty with their ironstone. You mentioned that a mono loop might be less sensitive to ironstone. Is this related in any way to the magnetic viscosity encountered with some Australian ground conditions? Any explanations on the subject would be very much appreciated…

Jim.

 

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[Reg]

Hi Jim,

I wish I could give you a specific answer but I have never tested ironstone or the ground generally associated with it so I don't know what to expect exactly.

Now, with that said, the DF coil has two separate windings in series but separated far enough that the interaction between the two windings is not dramatic. However there is an interaction and in bad ground, this interaction causes a difficulty in the ability to get a really good ground balance.

From what Eric Foster has posted in the past about the ironstone and the associated bad ground, the simple explanation would be the ironstone and generally the associated ground has an intensity many times stronger that our worst ground. So, using common sense 101, I would expect this dual coil interaction would be amplified significantly causing much more serious problems with the ability to ground balance or stay in balance.

So, in simple terms, I can't give you a technical answer but my simplistic answer should help.

Keep in mind that each coil is generating its own field and any target including the ground or any form of ferrite oxide will distort the fields, thus upsetting any what I will call balancing by the detector to minimize signals. Intensify this field distortion dramatically can only severely affect the coil signal balance even more.

Since the mono coil is a single coil there is no interaction outside of the distortion caused by the elements, which will have an effect but not nearly like that between two windings.

Now, along related lines, a mono PI will or can generate a dual signal (double blip type signal) from a piece of elongated ferrous metal depending upon this metal's orientation. This is why we often get a dual response from a nail or wire. On a mono coil, a long piece of a non ferrous material does not respond the same and give only a solid tone for a similar size and shape piece of non ferrous metal regardless of orientation.

Eric Foster mentioned this characteristic quite some time back on his PI forum, so I had to do some testing and sure enough, I never could get a double blip from any non ferrous item I tried which included different size pieces of copper wire, brass junk, lead strips, or anything else I could think of at the time.

In the presence of ironstone and dramatically mineralized ground, I am not sure just what type of signal you will get from a DF coil when passing over an elongated piece of non ferrous metal. I suspect a pseudo double blip type signal is possible depending upon the conditions of the ground at that location. This is something I would like to investigate farther at some time in the future.

BTW, I have yet to encounter dramatically mineralized ground here in the US. It may seem bad when using a VLF but is quite mild with the PI's I have used and I have hunted across most of Colorado, New Mexico, and AZ. Also keep in mind a VLF has a lot of trouble with ferrite material such as black sand, while a PI ignores it but has trouble with maghemite contaminated ground.

Once again, I am sorry I can't give you a more technical answer.

Reg

 

[Jim Hemmingway]

Reg… thankyou very much… the answer you supplied is quite satisfactory to me. It is more curiosity than anything… a topic about which I’d like to know more. I hesitated to single you out for this question but felt comfortable that you wouldn’t mind. Heaven knows you’ve certainly helped me and others over the years to understand PI usage.

I guess what prompted the question Reg is that a year or two ago I read about magnetic viscosity as acting somehow like a “friction” or resistance by ground minerals to reacting to a magnetic field. According to the article, the author noted that a net result is that such iron minerals yield slower decay rates. If this is true… and I don’t know that it is true based on any personal experience… then I presumed that soil magnetic viscosity would only enhance the difficulty of acquiring a ground balance… a condition that ought to be somewhat alleviated by using a mono coil on the TDI as you’ve noted.

Well that’s it I guess, unless you have any further comment. Thanks for such a quick forthright reply… we’re lucky to have you here.

Jim.

 

[63bkpkr]

Simple Discussions about this and that

magnetic viscosity [mag′ned·ik vis′käs·əd·ē] <- that's the way I say it when I first wake up

(electromagnetism)

The existence of a time delay between a change in the magnetic field applied to a ferromagnetic material and the resulting change in magnetic induction which is too great to be explained by the existence of eddy currents.


(plasma physics) The effect, possessed by a magnetic field in the absence of sizable mechanical forces or electric fields, of damping motions of a conducting fluid perpendicular to the field similar to ordinary viscosity.

Come on Jim this topic has been covered about a million times here, it and "what detector should I buy" are so common, yawn.

He Dude heavy stuff! First I was thinking you were referencing the Gauss value of any location but it seems, as you indicated above, you are talking about a piece of ground that offers a resistance to a magnetic flux field, hmm. That would be a curious effect. Maybe it would be like being told "no not tonight honey I've a headache" and then pursuing it anyway? But I digress*, so the ground actually resists being affected by an outside magnetic field (metal detector coil) but under some conditions it might slowly respond to it, viscosity? Am I anywhere close??...................63bkpkr

* I had to throw that in as I'm sure "He" would have used it

 

[goldchaser3]

This is what the White's website states in their coil descriptions:

Dual Field - Ideal for soils with predominately high magnetic susceptibility (rapid signal decay rates) typical of North America.
Mono - Ideal for soils with predominately high magnetic viscosity (slow signal decay rates) typical of Australia.​

It has always perplexed me as to why White's doesn't offer the mono coils as a choice here in the US, given they seem to handle highly mineralized ground better/easier. Even though the ground here may not be as bad as in AUS, with all the hot rocks and (relatively) bad ground that we do have, a mono coil should work better here in the US too. Now, all I use are RB mono coils.

 

[Hard Prospector]

This is what the White's website states in their coil descriptions:

Dual Field - Ideal for soils with predominately high magnetic susceptibility (rapid signal decay rates) typical of North America.
Mono - Ideal for soils with predominately high magnetic viscosity (slow signal decay rates) typical of Australia.​

It has always perplexed me as to why White's doesn't offer the mono coils as a choice here in the US, given they seem to handle highly mineralized ground better/easier. Even though the ground here may not be as bad as in AUS, with all the hot rocks and (relatively) bad ground that we do have, a mono coil should work better here in the US too. Now, all I use are RB mono coils.

I couldn't agree more, there is some nasty stuff to swing over in our deserts.

 

[Reg]

I didn't go into the technical terms of magnetic viscosity because it most likely doesn't make much sense to most people. So, I tried to use less technical terms.

In my most simple terms, magnetic viscosity is the explanation as to why plain old dirt responds like a metal target. Metal objects have eddy currents created in them as the result of being subjected to a strong rapidly changing electronic current in an inductor (the coil). These eddy currents create their own fields which retransmits its own signal and this signal is detected by the search coil.

Plain dirt has no capability of maintaining eddy eddy currents but still creates an effect that mimics that of the eddy currents. Normally this happens in certain ferrite contaminated soils. The main problem here in the US normally comes from a form of maghemite which is simple terms is another ferrite material transformed by the presence of extreme heat. Lightening and fires are primary reasons for the transformation in many cases.

This is why when someone using a PI passes over ground that has been in a fire, they often will get responses that mimic a deeper target. Quite often people will assume they are simply detecting something like old burnt wood since it can be detected by a sensitive VLF. Unfortunately, that isn't true with a PI.

To verify a PI can't detect burnt wood, simply take a bag of charcoal and try to detect it.

Getting back to the ground found in OZ, the intensity of the viscosity action is dramatically more intense when compared to that found here in the US. Eric Foster measured the difference and those measurements can be found on the net. Right now, I don't have the exact figures on hand but I do remember the difference was extreme when the OZ ground was compared to samples sent from the worst ground found here in the US.

Reg

 

[63bkpkr]

Very Interesting Gentlemen! I suspect this Mag. Visc. anomaly could explain a few good signals that could never be found.

And Jim as I mentioned another time, I expect the more technical discussions from you and now Reg as well.

Good Hunting.................63bkpkr

 

[Reg]

Did you ever wonder?

We talk about magnetic viscosity and all the side effects like the need for ground balance and then we can take a PI to a saltwater beach and often to a running stream and have no ground effects. Yes, you can hunt with a PI with no GB. In fact, the most common pi's used on saltwater beaches are straight PI's.

Why is this? Well, the answer isn't that simple and to be honest, I can't give you an ironclad response except to say that what I have found is washed beaches (washed by the water) or washed streams are basically free or nearly free of any form of clay type materials.

I remember when I first began using a PI and how it was so sensitive to the typical ground. Then I went to a claim owned by the GPOC here in Colorado and there at or near the active stream bed, I could use that same PI and get no ground response. Move up the bank and the ground signal was back.

I found this to be true on other streams, so before I knew how to develop a ground balance feature, I spent a lot of time using my knowledge which allowed me to hunt areas where gold could be found.

Later I found that some drywashes displayed this same condition, so there was places where I could hunt even in AZ with no GB.

Once I figured how to build a ground balance system, I pretty well forgot about the ability to hunt certain areas without the GB feature.

Extensive studies have been made about clay type materials and even papers have been written on the subject as to how clay based materials cause responses on both the VLF and the PI. In fact, in one study by the the US government, they went so far as to state that clay type materials create opposing signals that basically fight back and limit the potential depth in such materials. I haven't seen that article in years, unfortunately, so posting a link is not available at this time.

On a little different note, there are many active streams and even some saltwater beaches that are not free of signals and thus require a GB feature. Generally, the reason is the beach or stream bed is comprised of a basalt (volcanic) base and this is what is causing the ground effect.

The point of this post is to point out that there are other less obvious reasons for ground signals.

Reg

PS: "A rather rare effect, but one which can occur,particularly in clayey soils, is that of induced polarization.
Rapid termination of the transmitter current can charge up
the minute electrical capacitors in the soil interfaces
(induced polarization). These capacitors subsequently
discharge, producing current flow similar to that shown in
Figure 4-27, but in the opposite direction. The net effect
is to reduce the amplitude of the transient response (thus
increasing the apparent resistivity) or even, where the
effect is very severe, to cause the transient response to
become negative over some range of the measurement
time. Since these sources of reverse current are localized
near the transmitter loop, using the offset configuration
usually reduces the errors caused by them to small values"

This is a quote from the article I mentioned that discussed what can happen within a clay environment. So, now induced polarization is a new item to review.

The above added info came from;

http://en.openei.org/wiki/Time-Domain_Electromagnetics

Just drop down to the bottom of the page and select 6 under the references. The above info is located at or near page 39.

 

[Steve Herschbach]

"It has always perplexed me as to why White's doesn't offer the mono coils as a choice here in the US, given they seem to handle highly mineralized ground better/easier."

You can get the mono coils from White's dealers or the White's website. I am also pretty sure you can order a TDI and get it with a mono coil if you want it that way.

 

[Reg]

Hi Steve,

The answer to your question why White's doesn't make the mono coil standard or even easier may be more complex than one realizes and have multiple answers.

First of all, the DF coil is a basic stroke on genius in the fact it offers the best of small coil and large coil capabilities without losing much in the design. We can thank the late Dan Geyer for this design.

Now, most people know the 12" DF provides greater sensitivity to small gold than a conventional 12" mono coil. What isn't known is I believe the DF also provides another feature and that is a faster coil, which allows the size to increase and still work at the shorter delays.

Take a serious look at Eric Foster's designs and you find the 10" or the 11" coil to be the max size available and then it depends upon the detector.

For some reason the 10 usec is the barrier that is extremely difficult to break and along with that fact is the 11" mono size is about the maximum that is easy to build and still work at the minimal delay.

Advance the delay a little and now larger coils can work fine. Unfortunately, people think they want a larger coil and still detect small gold. Well, the two don't go hand in hand. Yes, people can find small gold with a large coil but usually, it isn't using the typical techniques. Instead, what is used is the edge sensitivity which is enhanced for short depth increases. In other words, in simple terms, the field around the coil windings is concentrated and this concentration is effective a few inches out from the winding. So, detection of very small gold is often if not most of the time done by this edge effect when using very large coils including those 12" and larger.

To compound matters, the TDI SL has a little trick built into it that further enhances small gold but in the process, makes it even more difficult to use larger coils including even some 12" mono's at the minimal delay. If a large 12" or larger coil works at 10 usec, it will be on the ragged edge.

In other words, there is a typical size limitation that can be used at 10 usec for mono coils and this can be smaller size than that of a DF designed coil. At least, that is what I have found from my experiments.

Now, Dan Geyer used to make Jimmy Sierra's special coils such as the Jimmy. Since Dan's death, I don't know if anyone has taken over or not.

What I feel White's should do is add a least two mono coils to the PI line and they should be the Jimmy elliptical size mono and a 10" mono.

What White's TDI owners should learn is if they want to use coils larger than the 12" DF, then they will most likely have to advance the delay to make the coil work properly.

One final note along the lines of which coil might work best, Eric Foster conducted a lot of experiments and what was determined is a general rule and that is, if a target can only be detected to a depth of half the diameter of a coil,then using a larger coil will usually result in a depth loss. An object that can be easily detected at a distance of much greater than half the diameter, then increasing the size of a coil's diameter will most likely result in a greater depth.

Obviously, there are factors that will influence the final results such as the noise factor. A larger coil is more susceptible to noise simply because it is a bigger antenna and can pick up more noise.

Reg

 

[Hard Prospector]

Reg, I too have noticed that when swinging my TDI SL through some creeks and desert washes I can often reduce the GB or even turn it completely off on rare occasions. My question for you is; under these circumstances(GB reduced or off) is the machine's depth and sensitivity increased much?
Thanks,

 

[Reg]

HP,

The answer to the question of whether there is much depth increase with the GB off, is it is quite possible the increase can be significant, but generally, is noticeable. The actual answer lies in just what the target is and how it responds with respect to the GB setting.

If the target is close to its balance point then the depth loss because of the GB can be quite dramatic and can approach 50% depth loss. Turn off the GB and the depth increase is very noticeable.

On the other hand, on small gold, the depth loss may be quite minimal but still occur. A post by Geo on the Geotech1 forum give some idea of what happens if you adjust the GB or the delay to different targets. Hopefully, the link attached will direct you to the chart where you can analyze what happens for yourself.

Geotech Forums

Reg

PS: if you just follow the gold coin depth chart you will notice the depth can vary from 30 cm down to 10.5 cm when at 10 usec. Other objects if checked by yourself (not on the chart) will not display this dramatic of a depth loss, but there will be some.

 

[Hard Prospector]

I appreciate your time answering my question Reg.

 

[goldchaser3]

Reg, thank you for re-introducing that chart. I had forgotten about that one. What it tells me is that if I am looking for gold I would, ideally, like my GB at 5 or lower.
My personal experience has been that low conductors like most gold are detectable at much greater distances with GB off, as confirmed in this table.
The bummer is that at 10us delay and most ground in the Gold Country, the GB balances around 8. That shows a response of 11 versus 31 with GB off. That's a big difference!!!