Taking a look at GROUND BALANCE on the bench

[Tinkerer]

I open this thread to discuss the matter of GROUND BALANCE as seen during bench testing and simulations.
Difficult ground is the one that gives a resistive response and also a reactive response.

The resistive response is due to the conductivity of the ground.
The reactive response is due to the magnetic permeability of the ground.
The combination of the 2 responses in a wide variety of proportions, amounts to the generalized HOT GROUND.

We would like to totally eliminate this ground response, so that we can better see the desired targets.

The best material I found so far, for simulating hot ground in the lab, is to use a pile of red bricks under the coil. The red bricks have their color from the iron oxide, generated by the firing at high temperature of the bricks.

The firing also leaves a remanent magnetism by aligning the magnetic particles in the brick to the earth's field. A red brick therefore has a slight magnetic field surrounding it, in the shape of a magnetic dipole. This can easily be verified with a magnetometer.

When we take a few bricks and pile them up, without verifying the pole sign of the dipole, most likely the bricks end up with the poles in all different directions so the various fields partly cancel themselves.

When we subject the bricks to the TX field, some of the magnetic particles align themselves to the TX field. When we remove our TX field, these particles realign with the Earth's field.

So far, we seem to have a fair similarity with HOT GROUND.

Let's look at some signal traces, to see if we can discern changes with the proximity of the Bricks to the coil.

For a start, I propose to take some screen shots with brick ground and without.

I would much appreciate, if somebody would superpose the pictures by GIMP or Photoshop, to make the differences easier to see.

Then I would like to see some suggestions of how to improve on this ground model and how to reproduce the wide variety we find in the field, in a controlled and repeatable manner.

Let's do it!!!!!

Tinkerer

[golfnut]

There is always other approaches Im sure were all aware.


I often find, in my day job getting simulations to tie in with real life is a perennial theme.


In order to not mimic real life, I want to be able to record real life with my detector.

A recorder with the ability to playback exactly the same on the bench - this would give me repeatable gnd signals I could feed into a detector - in order to then reduce its effects in experiments.

Not sure about implementation though!!

S

[Davor]

The bricks idea is fine. You can also go a step forward with "pottery pigments" which are in fact finely ground metal oxides, such as red and yellow iron oxide, and also some more exotic ones, say cobalt or chromium. Packs are usually 1kg and they cost ~15USD or more. So you can mix it with soil in a small patch in your garden, or in a box, as our friend WM6 did.

See 1kg iron oxide:

[dfbowers]

One issue I have seen with using the "Brick test" is just how many bricks are used in the test. You can use two scenarios - One is a complete sidewalk made of bricks and the other is brick fragments. In either case the answer is probably just a smaller coil ..

[Aziz]

The bricks idea is fine. You can also go a step forward with "pottery pigments" which are in fact finely ground metal oxides, such as red and yellow iron oxide, and also some more exotic ones, say cobalt or chromium. Packs are usually 1kg and they cost ~15USD or more. So you can mix it with soil in a small patch in your garden, or in a box, as our friend WM6 did.

See 1kg iron oxide:

Hi Davor,

some (less magnetic susceptible) iron oxides need reactivation to make it highly susceptible.
Add charcoal powder (50:50) to the iron oxide and some moisture (a bit water) and put it onto a BBQ grill in a pot.
At 500-600 °C, the reactivation process starts. Stirr it gently from time to time and let cool down the mixture (leave it alone).
Then try the magnet test.

Aziz

[Tinkerer]

Hi Davor,

some (less magnetic susceptible) iron oxides need reactivation to make it highly susceptible.
Add charcoal powder (50:50) to the iron oxide and some moisture (a bit water) and put it onto a BBQ grill in a pot.
At 500-600 °C, the reactivation process starts. Stirr it gently from time to time and let cool down the mixture (leave it alone).
Then try the magnet test.

Aziz

I have a few patches of ground on my property, where I can not go closer to the ground than 1 foot, with a traditional PI coil not to saturate the input. These are patches where large trees burned and baked the clay that contains iron ore or iron oxides. Fire obviously has a strong influence.

Reactivation of iron oxides to produce very HOT GROUND samples makes sense.

The problem I have with a bag of iron oxide, is that it is not conductive. What makes hot ground so difficult to balance, is the combination of conductivity, magnetic susceptibility and magnetic viscosity.

If we use a ferrite, we have no conductivity.

We might have to use a combination of different targets to obtain the right combination.

About the pictures I was going to post: I can not put the full wave form on the scope screen with a gain greater than 10, with this coil. At this gain, the hollow bricks I have at hand, are not visible. Massive bricks give a stronger response, but I have none available right now.

Of course, I can sample anywhere along this wave form, but to compare about 20 or 50 stacked sample amplitudes with each other does not fit into my schedule right now.

So we need new and better ideas.

[Tinkerer]

One issue I have seen with using the "Brick test" is just how many bricks are used in the test. You can use two scenarios - One is a complete sidewalk made of bricks and the other is brick fragments. In either case the answer is probably just a smaller coil ..

I like the idea of a larger or smaller mass of brick. It shows a relative response amplitude. A smaller or larger coil may do the same.
When we ground balance, we want to control the ground response amplitude. We want to obtain the same response amplitude at 1 inch from the ground as at one foot from the ground.

[Tinkerer]

The bricks idea is fine. You can also go a step forward with "pottery pigments" which are in fact finely ground metal oxides, such as red and yellow iron oxide, and also some more exotic ones, say cobalt or chromium. Packs are usually 1kg and they cost ~15USD or more. So you can mix it with soil in a small patch in your garden, or in a box, as our friend WM6 did.

See 1kg iron oxide:

One thing I have often wondered about, is the magnetic viscosity of a brick. With fine powder, we have very small magnetic particles or domains, randomly orientated.
With a baked brick, we have many of these magnetic domains orientated in the same direction, that is, aligned to the environmental magnetic field, usually the earth's field at the time of baking.

When we expose these aligned domains to a TX pulse of 10 times the earth's field strength we might expect some of these domains to align to the coil field.
Will they return to their previous alignment when the coil field collapses?
Could this be the cause of the magnetic viscosity that has been observed?

Tinkerer

[Davor]

Most probably the olden bricks are even hotter than the new ones as the traditional method included straw in mixture of the bricks' body. Add fire ...

Regarding combination terrain with lots of resistivity too, I got it. It is red clay (terra rossa) soaked with salt water. Surprisingly it is not a problem for my VLF. Ground balance needs a bit o' good ol' bobin' and potentiometer turnin' but that's about it. I did not notice much depth loss, but frankly I did not dig much either - it is also full of rocks, and red clay cements them together. Only once I dug a real deep hole as there was some serious Non-Fe object ... which turned to be a big chunk of lead. That was a let-down.

[mickstv]

Hi Tinkerer, when you post the pictures I'll superpose them using gimp.


Cheers
Mick

[Midas]

One thing I have often wondered about, is the magnetic viscosity of a brick. With fine powder, we have very small magnetic particles or domains, randomly orientated.
With a baked brick, we have many of these magnetic domains orientated in the same direction, that is, aligned to the environmental magnetic field, usually the earth's field at the time of baking.

When we expose these aligned domains to a TX pulse of 10 times the earth's field strength we might expect some of these domains to align to the coil field.
Will they return to their previous alignment when the coil field collapses?
Could this be the cause of the magnetic viscosity that has been observed?

Tinkerer

That's not necessarily true, if the powder is baked in place by heat from say a bushfire it would have been created with domains aligned with the earths magnetic field and since Australian dirt is often well packed, hard as buggery and too dry for worm activity it probably still contains a lot of order even decades later.

Perhaps a good approximation of this type of hot ground would be, find a patch of neutral ground, at least few square meters, throw down some iron oxide pigment and an assortment of different sized brick fragments for some hot rocks. Insert a few targets, pack it all down and light a fire on top. Let it cool down, maybe walk on it a bit to throw in a bit of surface disorder.
I'm not sure if this fits into your definition of 'bench testing', its more 'simulated field testing' but should be far more useful. What your trying simulate is a target in a changing matrix, waving the same piece of matrix under the coil doesn't simulate that well.


Midas

[Tinkerer]

That's not necessarily true, if the powder is baked in place by heat from say a bushfire it would have been created with domains aligned with the earths magnetic field and since Australian dirt is often well packed, hard as buggery and too dry for worm activity it probably still contains a lot of order even decades later.

Perhaps a good approximation of this type of hot ground would be, find a patch of neutral ground, at least few square meters, throw down some iron oxide pigment and an assortment of different sized brick fragments for some hot rocks. Insert a few targets, pack it all down and light a fire on top. Let it cool down, maybe walk on it a bit to throw in a bit of surface disorder.
I'm not sure if this fits into your definition of 'bench testing', its more 'simulated field testing' but should be far more useful. What your trying simulate is a target in a changing matrix, waving the same piece of matrix under the coil doesn't simulate that well.


Midas

I agree on your description of the hot ground.

The question is to look at the changes in the signal wave form on the oscilloscope, generated by the hot ground. To do that in the field is too complicated.

[Tinkerer]

Hi Tinkerer, when you post the pictures I'll superpose them using gimp.


Cheers
Mick

With a gain of only 10, the bricks alone could not be seen, while keeping a full wave form on the screen. Therefore I added an antique 6 pound cast iron cannon ball to simulate a very hot ground.
I set the distance of the ground target to be such as to give a somewhat similar target response amplitude as the silver dollar at the center of the coil, that is, close to saturation of the pre-amp, to represent the maximum dynamic range.

The vertical setting is 1V/div.

The horizontal setting is not calibrated, but the full cycle is about 100us, for 10,000 pulses per second.
Here are some screen shots:

NO ground_1, transient only
NO ground_2, full cycle
With ground_1 transient only
With ground_2 full cycle

Your offer of superposing the pictures is much appreciated. I am going to wait, to see the superposed pictures, before taking more pictures, as we may want to make changes to the setup.

All the best

Tinkerer

[mickstv]

Hi Tinkerer, here you go.

[Tinkerer]

Thank you mickstv.

So what do we see here?

1) We see the dynamic range near maximum. With +/-5V supply, the pre-amp saturates at about 4V. If we were to use +/-12V, we could increase the gain considerably. No need to increase the dynamic range, this is already extreme.

2) We see 2 spots along the cycle time, where there is no signal change. If we sample at the place where the traces crosses over, or at the end of TX, the changes are minimal.

We can also look at the crossover as the pivot. If we sample before the crossover, the signal change is negative relative to the NO ground signal, after the crossover, it is positive.

3) The biggest change in the signal is at the first positive swing of the signal, at about 6us. If we sample there, taking a 5us sample window, we get the highest amplitude.

Now, please suggestions for more tests.

I could think of adding a strong ceramic magnet to the cannon ball. This should increase the reactive response, without increasing the resistive response.

We could also try to add a short TC resistive target. I can see some change with a 1" square alu foil, that has a TC of about 10us, however, at the gain of 10, the change is very little and probably difficult to see on the picture.

Suggestions???????????

Tinkerer