Taking a look at GROUND BALANCE on the bench

[Prospector_Al]

Hi Allan,

You are right - a housebrick does not come close to Oz material, but it is still useful for testing GB concepts. You can see the difference in susceptibilities on the next to last page of my report. Over 10,000 SI units for Wedderburn and 788 for a brick. These are 10gm samples of each material. The susceptibility has the effect of multiplying up the viscous signal. I have another 10gm piece of Oz ironstone with susceptibility of 30,000! I have some of the Arizona material (I think it it is from the same place near to a timbermill?). The problem there was magnetite with huge susceptibility but the viscosity was low.

Eric.

Yep, it's the same place! One may test concepts with bricks, but to design a detector that will work in the real world, one must know what the limits are. I was very fortunate in getting help from some guys in Oz who sent me samples of hot rocks. My assumption is that hot ground is nothing but weathered hot rocks. In fact, I pulverized a very hot rock and tested it again--the viscous signal had the same amplitude. Of course, the remanent magnetism disapperad. Since magnetite has such a short time constant, it's the viscous signal that presents a problem, owing to its ability to mimic metallic targets.

This is what all the fertile minds should focus on, instead of bickering about semantics. Just my opinion.

(The data published in some patent apllications are way off, so one must make one's own measurements.)

All the best,

Allan

[KahlesMilcon]

In my part of the ountry, North Carolina, the black sands as you call them are actually iron particles, many times I have gone panning and just pandown to the concentration of " black sand", take home the product and use a magnet to remove most of the "black sand".
The remainder is small sand particles ans small gold particlea on order of pin head size flatened out.

Just thought you might want to know.

Our mineral soil is ( I do not know for a fact) as strong as yours in the lower hemisphere, however we also have mineral rocks that are highly magnetic, some of these rocks are the size of grapefruit and when busted open contain powder of different colors, The locals and Indian tribes of North Carolina call them Paint rocks.

[mickstv]

Hi Eric, in the last couple of months you posted some CRO pictures showing the typical decay from a 1 gram nugget and decay from mineralization, I haven't been able to find the post again but maybe you could post the pictures again.


Cheers
Mick

Hi Eric, I found that post with the various target tests. link below post 281.

http://www.geotech1.com/forums/showthread.php?19895-Advanced-PI-coils&p=163735#post163735

[Aziz]

Haven't you anything more constructive to say? You seem to want to undermine all the research into soil magnetism that has ever been done. How would you know anyway what has been done regarding t, and what hasn't.

Eric.

Eric,

pity that we don't see what happens in the first 10 µs after switch-off. This is the region, where the magnetic viscosity effects should be significantly visible (if any there).

Anyway. All reactive parts and the demodulation of the signals (in a PI circuit) are highly time dependent.
A function f(t). One have to get rid of the variable t.
This is the most constructive part I can contribute to the discussion.
Aziz

[Ferric Toes]

Hi Eric,

I tried to make a comparison between a red brick and a sample hot rock from Oz. It seems to me that the viscous signal from a hot rock is about a thousand times stronger per unit weight than the signal form a house brick.

Using the latter to simulate ground gives one a much too optimistic assessment of a detector's capability to balance out the ground signal.

Some time ago I visited a claim in Arizona where neither PI nor VLF detectors worked because the dynamic range of the GB systems was exceeded.

Oz does not have monopoly on hot ground! I'm in the process of figuring out what to do to accommodate the hot ground in AZ.

Have you, by any chance made actual comparisons beween the Oz ground and a red brick?

In the land of OZ there was a yellow brick road--no iron oxides, it must be benign...

Allan

Hi Allan,

You are right - a housebrick does not come close to Oz material, but it is still useful for testing GB concepts. You can see the difference in susceptibilities on the next to last page of my report. Over 10,000 SI units for Wedderburn and 788 for a brick. These are 10gm samples of each material. The susceptibility has the effect of multiplying up the viscous signal. I have another 10gm piece of Oz ironstone with susceptibility of 30,000! I have some of the Arizona material (I think it it is from the same place near to a timbermill?). The problem there was magnetite with huge susceptibility but the viscosity was low.

Eric.

[Ferric Toes]

Is this the ultimate GB thread?
The end game?

Nah, forget it. You haven't eliminated the important variable in this game: the time t

Aziz

Haven't you anything more constructive to say? You seem to want to undermine all the research into soil magnetism that has ever been done. How would you know anyway what has been done regarding t, and what hasn't.

Eric.

[Aziz]

Is this the ultimate GB thread?
The end game?

Nah, forget it. You haven't eliminated the important variable in this game: the time t

Aziz

[Aziz]

...
Maybe I try to convert some into maghemite as Aziz suggested here:

http://www.geotech1.com/forums/showthread.php?14102-Let-s-made-a-PC-base-metal-detector-with-usb-interface-!!!/page59

"Unbedingt! Schmeiss den Grill an!!!"
(Absolutely! Turn on your BBQ grill!!!)

Hematite can be converted into maghemite.
Aziz

[Prospector_Al]

Hi all,

I've been away in France for a week in a place with no internet access, so it is good to see that there has been a lot of activity, and particularly a thread for ground balance has been started. Soil magnetism is one of my pet subjects and how to balance out its effects in a metal detector is a fascinating offshoot.
The reason that many ground types and rocks give a signal is a bit more complicated than one might first think, and it is important to get a bit of basic understanding if one is going to design an effective electronic GB system. A good starting point is to read the Hitchhiker's Guide to Magnetism which is available as a free .pdf download. The factors which come into play with soils and rocks are the minerals haematite, magnetite, maghemite, and their respective grain sizes. Alignment of domains, remanent magnetism, and conductivity have been mentioned in the above posts but are not the primary cause of the ground response. The primary culprits are superparamagnetic grains of <30um diameter. It is these that give rise to the 1/t^-1 decaying signal that we see in the offtime of a conventional PI. These grains are too small to accomodate a domain and there is no remanence or coercivity and they have an extremely thin hysteresis loop. In response to the applied field there will be a net statistical alignment of SPM grains with the foregoing growth and relaxation times. With larger nanometer size grains, single and multi domains can be supported which exhibit remanence and coercivity.

Australian rock, and housebrick, contains an assemblage of different grain sizes, so they can exhibit different behaviours side by side i.e. my Oz test rock has high susceptibility, high viscosity, and will strongly repel a compass needle. A housebrick, or piece of earthenware pottery, can do the same as a result of firing in a kiln and cooling within the earth's field. Larger multidomain grains get locked in, so to speak, and a net permanent field is created, but this does not affect the (important for GB) SPM grains.

The great thing is, that unlike metal objects, the decay law is practically the same for Oz rock, fired brick and pottery, volcanic rock, and much inland soil; in any quantity. It is only the amplitude that varies. Metal objects, ferrous and non-ferrous are all over the place in decay law and amplitude.

Ferric (not Ferrous). The Hitchhiker's guide will tell you why.

Hi Eric,

I tried to make a comparison between a red brick and a sample hot rock from Oz. It seems to me that the viscous signal from a hot rock is about a thousand times stronger per unit weight than the signal form a house brick.

Using the latter to simulate ground gives one a much too optimistic assessment of a detector's capability to balance out the ground signal.

Some time ago I visited a claim in Arizona where neither PI nor VLF detectors worked because the dynamic range of the GB systems was exceeded.

Oz does not have monopoly on hot ground! I'm in the process of figuring out what to do to accommodate the hot ground in AZ.

Have you, by any chance made actual comparisons beween the Oz ground and a red brick?

In the land of OZ there was a yellow brick road--no iron oxides, it must be benign...

Allan

[PiTec]

Hi Eric,

The great thing is, that unlike metal objects, the decay law is practically the same for Oz rock, fired brick and pottery, volcanic rock, and much inland soil; in any quantity.

This confirms what I have read in several documents about magnetic viscosity. However, among the many samples I tested there are some with slightly different decays. How similar should they be?

Below are some decay curves. Test setup is a small cylindrical mono coil, samples are in film containers inside the coil, standard PI waveform with 50µs on and 2µs off. Decay curves start approx. 5µs after the 2µs pulse. The TX coil decay curve was removed in the oscilloscope by subtraction, i.e. the plots show the pure decays of the samples. All initial amplitudes are equal so that the curves can be superimposed as in plots D) and F).

I actually made these to show how close the viscosity decay can be to that of metallic targets. It is very easy to get almost the same decay curve with two rings with different TCs by simply varying their distance and angle. This simulates the problem with the detection holes in GB systems that subtract a later sample from an earlier one.

Samples:
A) Hematite powder (alpha-Fe2O3, could also contain some amount of gamma-Fe2O3/maghemite):

http://www.kremer-pigmente.com/en/pigments/iron-oxide-pigments/iron-oxide-red-110-m--light-48100.html

The data sheet says the predominant particle size is 90nm

http://www.kremer-pigmente.com/media/files_public/48100e.pdf

Maybe I try to convert some into maghemite as Aziz suggested here:

http://www.geotech1.com/forums/showthread.php?14102-Let-s-made-a-PC-base-metal-detector-with-usb-interface-!!!/page59

B) Magnetic viscosity soil from my area (concentrated with a magnet)

C) A thin gold ring with one TC of 13.5µs

E) A thick silver ring with at least two TCs of approx. 15µs and 120µs

Thomas

[mickstv]

Hi Eric, in the last couple of months you posted some CRO pictures showing the typical decay from a 1 gram nugget and decay from mineralization, I haven't been able to find the post again but maybe you could post the pictures again.


Cheers
Mick

[Ferric Toes]

The attached report includes a comparison between housebrick and Wedderburn ironstone from Oz., plus other materials. All have similar slope on the log log graphs.

Eric.

Comparison Samples002.pdf

[Ferric Toes]

Eric, thank you for your help.

So, basically my ground model was wrong. Bricks are OK. Ferrite is OK. Cast iron is NOT OK.

Start over again.

Next try: use several pieces of ferrite together with the bricks, to produce a ground signal that is of greater amplitude than the largest target.

To make the PIVOTS easier to see, chose a target with a TC that decays to the baseline before the end of the cycle.

Tinkerer

Bricks are OK but vary a lot in the amplitude department. You want about three layers with a small gap, say 1/10in, between layers to insert coins etc. The area should be at least twice the search coil area to be reasonably realistic. Ferrite material is not necessarily useful. Soft ferrites are not (transformer cores, many pot cores) as they are coarse grained and have very little viscous signal. Some fine grained hard ferrites, such as used for radio antennas have a strong viscous response. Soft and hard refer to magnetic properties rather than physical hardness. I used some very heavy "bricks" from an old electric storage heater at one time. These had a reasonable viscosity signal. Cast iron is definitely out.

Eric.

[bklein]

I have a Schoenstadt survey pin finder that is awesome for this and meteorites...

[Prospector_Al]

You want a magnetometer for this.

Hi Barry,

Carl is right, you need a magnetometer. You can build one yourself. Popular Mechanics had plans for one, some time ago.

Here are some facts about these devices:

http://perso.infonie.be/j.g.delannoy/BAT/IntroductiontoMagnetometerTechnology.pdf

"Traps" in bedrock are full of black sand and nuggets. Owing to their specific gravity compared to the country rock, gold and blacks san are found together.

If you are looking for bigger nuggets, the correlation between magnetic mineral content and prevalence of nuggets may not be so good.

All the best,

Prospector_Al