Oz Ironstone Plots

[Eric Foster]

For those interested in waveforms here are a couple of plots done with some new equipment I have designed.

[attachment 85037 Oz1000.jpg]
This is a linear plot and shows the rapid increase in ground signal strength at short delays. In fact I could not use 10uS because the instrument overloaded.


[attachment 85038 Oz1000.2.jpg]
This is the same curve but displayed on a log amplitude log time scale. It shows the classic linear response with a slope of -1.03.

Eric.

 

[mikebg]

Eric, I have 3 questions:
1. Does your ironstone excited by unidirectional TX current or by bidirectional?
2. What sensor are You using: monocoil or IB?
3. Can you plot the waveforms made with your new equipment without any target?
Yours Mike.

 

[Eric Foster]

Hi Mike,

Can't give too many details at this stage for proprietory reasons, but TX has unidirectional pulses. Bi directional would make no difference. Coil is a mono type, and with no target the readings would be a line of zeroes.

Eric.

 

[mikebg]

Thanks Eric that despite proprietory reasons , you answered to all my questions. Under

that circumstances, I won't ask you about:
Question No 4. Effect of TX pulse length on the OZ ferrite decay.
I'm trying to design a wideband metal detector - Meteorite Locator. Therefore, for me is

interesting to know frequency responses of all manner of meteorites, iron stones, hot

rocks, lossless and lossy ferrites and super-paramagnetic items. Since I can't find such

WEB resources, in February I started here a thread "Frequency response of a target".
Reg Sniff and Georg Payne helped me to reinvent the wheel. Now I'm able to find most

errors in "Bruce Candy's Gospel" and to post in forum transfer impedances of different

targets and soils in complex plane. Before to start posting in mentioned thread, in this

thread I should inform all for my important reinventions:
1. Mono-coil is the worst type sensor in 3 directions:
a) Target signal produced by mono-coil has least of all modulation index. An IB sensor is

better even if not precisely balanced, because increases the index.
b) Even at zero frequency, ferrous object produces signal in mono-coil like positive

imported inductance. IB sensor can't produce ferrite signal at zero frequency, because

the signal is proportional to frequency.
c) Ground signal is greatest at mono-coil. It is minimal at double balanced differential

sensor.
CONCLUSION: It is unfavourably to use mono-coil, especially for measurements.
2. The unidirectional TX current decreases ferrite signal. This is obvious from attached

figure - the signal is proportional of delta B.
Candy's machines will work better if not use bidirectional TX current.
CONCLUSION: The unidirectional TX current isn't suitable to search small ferrous

meteorites, but can make OZ soil more electromagnetic transparent.
3. In frequency domain, ferro-magnetism produces signal having phase lag 270 deg and

even more, independent on frequency. The term "Phase lag" means response delay in

time domain, and we will discuss and explain this in the forum.
CONCLUSION: If I have problems with ferrous signals, I should solve them sampling step

response.
As last conclusion, I should point out, that despite today is April Fool's Day, the present

my posting is joke-less. The term "Bruce Candy's Gospel" is copyright of Bulgarian ham

designers and relates to "White Paper No 8" published in Minelab's site (after I started

mentioned thread). The term "depth of modulation" is mine, because all my IB sensors,

even precisely balanced, produce voltage without target. Target only modulates it far

too shallow. That's all for today. Mike.

 

[Eric Foster]

Hi Mike,

As far as I can tell with my tests, the signal from Australian ironstone is independent of pulse width. This is also the case for other rocks and soils that I have tested. However there are certain conditions to be met, for this to be the case. Candy's TX waveforms do not fulfil these conditions.

I agree that mono coils are generally thought not the best for field use as their response to surface iron mineralisation is the strongest. However, my GS5 metal detector will work effectively on the worst Australian ground when using a mono coil.

The ferrimagnetic minerals that cause a response with a PI detector are superparamagnetic, and have a frequency dependent susceptibility. SP particles have no hysteresis and no coercivity, and the signal is the result of a temporary net alignment of particles with the TX field, which decays away logarithmically with time as a result of thermal energy.

Eric.

 

[Prospector Al]

Hi Eric,

I presume this is a plot of the resistive signal. Is it the response of an individual rock or the ground itself?

How does this compare with the magnetite sample I sent you?

Allan

 

[Eric Foster]

Hi Allan,

Yes this is a plot of the so called resistive signal i.e. the signal at a delay after the transmitter has switched off. It is a 10 gram sample in a 1.5in diameter coil. I will do a plot of 10g of the magnetite black sand and post it shortly. The black sand is different in that it has a higher susceptibility than the Oz rock, but a much lower "resistive" signal that seems to be slightly curved in a log log plot.

Eric.

 

[mikebg]

Thanks Eric. My odd idea, to make transparent OZ soil by DC magnetisation, drops off. Surely nobody won't seek meteorites in OZ (even with my future excellent broadband machine).
I started to design meteorite locator, having no meteorite. If you have any meteorites, please test them whether the step response seems like attached WEB information. I have ever more and more questions, but should keep them for late.
Now, despite my poor English, lack of knowledge on English grammar and technical terminology, I'm writing for the forum a lesson "Transfer impedance of conductivity and permeability". My goal is to involve you in realisation of an your idea known once upon a time as CLASSROOM. For beginning I,m posting in my thread the frequency response of your OZ ironstone. Respectfully yours, Mike.

 

[Eric Foster]

Hi Mike,

Finding meteorites in Oz would be difficult unless they are the nickel iron type. I have found that applying a steady magnetic field to Oz ironstone makes the ground signal worse, rather than better. However, you do get to a point where the ground signal does start to drop off, but I have only achieved this with a powerful ceramic magnet. Not practical in the field as all the loose ironstone would end up stuck to it. Rocks and soils from other places do immediately start to show a fall off in signal as a steady field is applied, so there must be something different in the Oz mineralogy.

I have seen the plot you posted. Was it to do with a Geonics PI detector? If so, it is the switch on waveform which is distorting the results. They show this in another picture. I can vary the TX pulse from 20uS to 300uS and the plot falls on a 1/t line all the way. If there is a small difference in slope i.e. -1.05, it is still constant through the range.

Unfortunately, I don't have any meteorites either, so we are a bit stuck in that respect.

Eric.

 

[mikebg]

The figure "Effect of Pulse Length on Ferrite Decay" was extracted from publication
http://www.eos.ubc.ca/ubcgif/uxo/PDF/magsoils_eudem2003.pdf
Mike