Ground Balance Theory

[Ferric Toes]

Hi Eric,

Man-made ferrites can exhibit strange behaviour not found in nature. I had a rod that showed resonance. It looked like a travelling wave going back and forth...

Allan
P.S. I just discovered you on Facebook.

Hi Allan,

Yes, I have some rods that ring. This appears to be due to magnetostriction where the rod gets slightly longer when magnetized, then springs back at the end of the pulse. Undamped, it translates to a mechanical ringing which then appears as an electronic signal in the coil around the ferrite. Squeeze the rod end to end with your fingers and you can see the damping effect. Rods tend to ring if you drive the coil with lots of amps - maybe reaching saturation point.

Eric.

I hardly use Facebook as it is just another thing that sucks up time. Two or three detector forums is enough for me.

[Prospector_Al]

Hi Eric,

Man-made ferrites can exhibit strange behaviour not found in nature. I had a rod that showed resonance. It looked like a travelling wave going back and forth...

Allan
P.S. I just discovered you on Facebook.

[Ferric Toes]

I have just found a ferrite rod that exhibits a good viscous decay. However, when I plotted the graph it was a t^-1.7 decay. So far I have not found a soil or rock that is greater than -1.07 on my measuring system, so I shan't be using it as a substitute for the real thing. Nice straight line though in log log plot - pity about the slope.

Eric.

[Ferric Toes]

Hi Eric,

I have performed similar test of the temperature dependance of the viscous signal. In my case it was not the result of independent thinking but rather knowledge of the practice of "thermal cleaning" in the study of paleomagnetism.

Knowledge of this phenomenon helped me absolve the detector from suspicion of thermal drift, but I couldn't find any other practical use of this knowledge.

Are you investigating this matter to satisfy your curiosity or do you have a practical application in mind?

Curioous in Georgetown

Hi Allan,

I must admit that I find the subject of soil and rock magnetism very interesting in itself but there is a practical side to it as well. Unusual phenomena are sometimes reported by detectorists and sometimes the popular explanation is an incorrect guess. This is certainly so with signals from rocks and so called mineralised ground. This is a very big subject in the demining field, as you know, and the more information we can find out about the origin and nature of the ground response, the better designs will be to minimise it. I am working on this too. I have found today that a temperature rise of just 10degC will change the amplitude of the viscosity signal of Red Hill soil by about 5%.

Eric.

[Prospector_Al]

Hi Eric,

I have performed similar test of the temperature dependance of the viscous signal. In my case it was not the result of independent thinking but rather knowledge of the practice of "thermal cleaning" in the study of paleomagnetism.

Knowledge of this phenomenon helped me absolve the detector from suspicion of thermal drift, but I couldn't find any other practical use of this knowledge.

Are you investigating this matter to satisfy your curiosity or do you have a practical application in mind?

Curioous in Georgetown

[Ferric Toes]

There is another explanation for temperature dependence of soil properties:
Some OZ dirts contain semiconducting ore minerals.
The electrical conductivity of a semiconductor increases exponentially with an increase in absolute temperature T.

The graph of equation for conductivity (sigma) vs temperature T is given below as log vs. 1/T axes to get a linear plot.

Changes in magnetic viscosity amplitude plus small changes in t^-x exponent are the favoured explanation. In the small volumes of soil that a metal detector energises, dispersed mineral conductivity, or semiconductivity, does not play a part. At least not with the detectors I have tested. I have proved by practical tests that base susceptibility and frequency dependent susceptibility change with temperatures that would be experienced in the Australian goldfields. On a hot sunny day ground surface temperatures are heterogeneous for lighter and darker materials and whether shaded or not. This results in a spread of exponents of which some fall on the edges of the GB notch. I can hold a piece of ground balanced ironstone in my hand and due to that small rise in temperature the GB point is slightly altered. During the night, the ground temperature differences will even out and the spread of exponents reduced considerably, with the result that the detector runs quieter.

Below is a plot for powdered Yucca Mountain Tuff, which is an accepted standard for a natural magnetic material, showing how the viscosity changes with temperature. The changes are more dramatic for Oz ironstone because of the very much greater susceptibilities. I haven't yet plotted these but I have seen the effect by putting a 10gm piece in the freezer, then in the oven.

Yucca Mountain Tuff peaks at around room temperature as it has a very narrow range of SPM particle sizes. Other soils and rocks, with a larger particle size range, will peak at somewhat different temperatures no doubt.

Eric.

[mikebg]

.... One thing I have to be careful of is ambient temperature as this affects the viscosity reading. Theory predicts this as borderline SPM grains become progressively blocked as temperature is lowered. This is why the ground in Australia becomes quieter at nightime when the ground cools down.

Eric.

There is another explanation for temperature dependence of soil properties:
Some OZ dirts contain semiconducting ore minerals.
The electrical conductivity of a semiconductor increases exponentially with an increase in absolute temperature T.

The graph of equation for conductivity (sigma) vs temperature T is given below as log vs. 1/T axes to get a linear plot.

[Ferric Toes]

Very interesting issue. I suspected something similar looking at graph. However in this case energy delivered per pulse is not constant for different delays. This is also done with relatively long pulses and not too different in length. I noticed before that constant pulse width can produce anomalous decay curve, not 1\t, but rather something like multiple 1\t decays with different t superimposed, however measured under improvised conditions time ago so cant claim anything. What is more interesting for me is response to very short pulses, less than target TC, say down to1us, in comparison with some usual value, say 50uS, delivered with same amount of millijoules per pulse, response should be slightly different. I just assembled test setup to try this, and make measurements at 1-2-5-10-20-50uS sequence but unable to operate it yet due to truly stupid reason, faulty variac in HV PSU needed for TX. Hope to fix it soon, but then:


??? ONE VERY IMPORTANT QUESTION ???



I don’t have any chance to field test this, also lacking larger quantities of proper, known soil samples, considering improvised and noisy conditions (EMI), this will probably end up with serious noise issues, what can be the first aid? What kind of material i can use for substitute, at least to calibrate setup, can large ferrite block be relevant?


( Lovely, after all equipment in place i just need bag of dirt. Ignoring Murphy law: if you have any chance to get 3 different values from 3 measurements, measure only once.)

Hi Tepco,

The TX pulses do vary from 30uS at 10uS delay, to 300uS at 100uS delay. All are flat top with same final current at switch off. Earlier than 10uS you may see the decay departing from the later response, but this is an unknown area at the moment. TX pulse shape may make a difference, but I have made my current step from a non varying value to zero so as to approach the theoretical conditions as closely as possible.

A block of ferrite is not the best idea as its constituents are not the same as soil or rock. Crushed red housebrick would be better.

The result I get from 3 measurements is the same +- one digit i.e. 100 may read 99 or 101, 25 may read 24 or 26. System noise does not get worse at late times. One thing I have to be careful of is ambient temperature as this affects the viscosity reading. Theory predicts this as borderline SPM grains become progressively blocked as temperature is lowered. This is why the ground in Australia becomes quieter at nightime when the ground cools down.

Eric.

[Tepco]

Advice needed

Very interesting issue. I suspected something similar looking at graph. However in this case energy delivered per pulse is not constant for different delays. This is also done with relatively long pulses and not too different in length. I noticed before that constant pulse width can produce anomalous decay curve, not 1\t, but rather something like multiple 1\t decays with different t superimposed, however measured under improvised conditions time ago so cant claim anything. What is more interesting for me is response to very short pulses, less than target TC, say down to1us, in comparison with some usual value, say 50uS, delivered with same amount of millijoules per pulse, response should be slightly different. I just assembled test setup to try this, and make measurements at 1-2-5-10-20-50uS sequence but unable to operate it yet due to truly stupid reason, faulty variac in HV PSU needed for TX. Hope to fix it soon, but then:


??? ONE VERY IMPORTANT QUESTION ???



I don’t have any chance to field test this, also lacking larger quantities of proper, known soil samples, considering improvised and noisy conditions (EMI), this will probably end up with serious noise issues, what can be the first aid? What kind of material i can use for substitute, at least to calibrate setup, can large ferrite block be relevant?


( Lovely, after all equipment in place i just need bag of dirt. Ignoring Murphy law: if you have any chance to get 3 different values from 3 measurements, measure only once.)

[Ferric Toes]

You wouldn't know it, but the answer is in the graph. Explain tomorrow as it is bedtime here.

Eric.

The graph points plotted at delays of 20,30,40 etc are using progressively longer TX pulses; also sample widths are changed in proportion. For example the TX width at 20uS delay is 60uS, at 30uS delay Tx is 90uS...and so on. That way you always get proper excitation of the target right through the full decay till it disappears into the noise. This is particularly important with viscosity measurement where the power law decay lingers on. However late I measure it, it still obeys the 1/t law, while fixed length TX pulse method causes the decay to deviate at late times.

Eric.

[Ferric Toes]

One interesting question (i'm trying to find out some answers) is: How this response varies (shape and magnitude) vs. excitation pulse width in different materials? Should be different for, say, 50 to 1 TX pulse width change, generated with same amount of energy released, can this be utilized for ground balance?

You wouldn't know it, but the answer is in the graph. Explain tomorrow as it is bedtime here.

Eric.

[Tepco]

One interesting question (i'm trying to find out some answers) is: How this response varies (shape and magnitude) vs. excitation pulse width in different materials? Should be different for, say, 50 to 1 TX pulse width change, generated with same amount of energy released, can this be utilized for ground balance?

[Ferric Toes]

I've just done a couple of plots and superimposed them. Linear plot this time and shows the decay of a piece of Oz ironstone compared to that of a 10gm nugget. Both were initially set at an arbitrary 1000 units amplitude at 20uS. It looks as though the nugget starts at 950, but it does curve up to 1000 due to early time skin effect. Curve fitting software cannot fit mixed exponentials. Optimum nugget/ground SN seems to be 30uS

Eric.

[Ferric Toes]

Good-bye civilization, back to analog scopes. Just improvised one simple LOG amp setup to try few tests. This is temporary solution, signal displayed directly from log amp video output. Serious noise problems prevented DSO use, also this is short of at least 30-40dB additional dynamic range available but still can be interesting and informative to someone. With different design (sampled, integrated and time resolved video output) i want to build test rig specifically to test different soil responses, over 80-90dB dynamic range and various pulse widths, down to 1us or less. Lot of fun with log amps, also lot of observations and conclusions to be done. Last time i tried this, big fat coil and big fat pulse prevented me to make useful measurements. On picture below, horizontal scale is 200uS (20uS\div), vertical not calibrated, around 50dB, ignore top 1 division. Analog display is better for noisy signals, tried averaging on DSO but in fact DSO monitor itself produce significant interference, so dusty old 1740 did the job.

Looks like pictures of that meteorite that came down over Russia . Can't beat a good ol' analog scope at times. It's good that you are seeing some differences now.

Eric.

[Tepco]

Good-bye civilization

Good-bye civilization, back to analog scopes. Just improvised one simple LOG amp setup to try few tests. This is temporary solution, signal displayed directly from log amp video output. Serious noise problems prevented DSO use, also this is short of at least 30-40dB additional dynamic range available but still can be interesting and informative to someone. With different design (sampled, integrated and time resolved video output) i want to build test rig specifically to test different soil responses, over 80-90dB dynamic range and various pulse widths, down to 1us or less. Lot of fun with log amps, also lot of observations and conclusions to be done. Last time i tried this, big fat coil and big fat pulse prevented me to make useful measurements. On picture below, horizontal scale is 200uS (20uS\div), vertical not calibrated, around 50dB, ignore top 1 division. Analog display is better for noisy signals, tried averaging on DSO but in fact DSO monitor itself produce significant interference, so dusty old 1740 did the job.