field test unit no 001 "model T"

[Ferric Toes]

Yup, erotic serf, is another possibility but ferric toes is more appropriate having tramped around a few times on Australian ironstone. Have a look at http://www.wordsmith.org/anagram/advanced.html and have fun with names.

Qiaozhi, should be t3 = 150uS as you say. I drew in the part of fig.3 that was relevant to the sampling, and the other part of fig. 3 has to to with coil size/object distance. That will appear when I post the page on coil calculations, which will be soon.

Would it be best to start a new thread on PI History and Theory so that the older information is all in one place?

Ferric.

[sido]

Qiaozhi

"Lastly ... (perhaps this is generally known, and I'm just being slow) ... although everyone is aware that "ferric toes" is actually Eric Foster, I've only just noticed that it was also an anagram."

Great observation there, sure is a anagram.

Sid

[Qiaozhi]

Well this is what I have. Not by me but by John Alldred, employed by myself in the 1970's. He had a Masters Degree in physics and was one of those rare persons who could work out all the maths and theory and then build something that worked.

Hi Eric,

This is a very interesting extract from the document you posted:
"In the present application, high efficiency was required over a wide range of τ values, a self-conflicting requirement which was met in the experimental model by having two receiver channels operating in parallel per coil, with different values of t1 t2 and t3, and hence different values of optimum τ. Not only does this mean that an efficient response will be obtained over a wider range of object time constants, but also that an immediate indication of approximate τ is shown which gives an idea of object size."

Then it goes on to say:
"The relative signal output as a function of object time constant is shown in Fig. 3 for the examples (a) t1 = t2 = 50us, t2 = 150us, (b) t1 = 100us, t2 = 200us, t3 = 400us. The vertical scale is arbitrary, as the factor R/rT needs to be incorporated."

Please note that there is small error in the above, as it should read "(a) t1 = t2 = 50us, t3 = 150us"

However, the figure shown below this text only appears to be the first part of figure 3.
Would it be possible for you to scan and post the whole of figure 3?

Lastly ... (perhaps this is generally known, and I'm just being slow) ... although everyone is aware that "ferric toes" is actually Eric Foster, I've only just noticed that it was also an anagram.

[Aziz]

Hi all,

has anyone made the interesting proof yet?
You know, it will be only a theory until someone proves it to be true (or false).

I'm not going to bite the bullet...
Cheers,
Aziz

[dougAEGPF]

Thanks Eric ... the history is fascinating and I get the feeling some it will be lost if not already.

Regards,

Moodz

Another old but interesting one 1971
ON CALCULATING TRANSIENT ELECTROMAGNETIC FIELDS OF A SMALL CURRENT-CARRYING LOOP OVER A HOMOGENEOUS EARTH
James R. Wait and Randolph H. Ott
Institute for Telecommunication Sciences
Office of Telecommunications
U. S. Department of Commerce
Boulder, Colorado 80302

Contract No. PRO-Y-71-87Z
Project No. 5635
Task No. 563506

Work Unit No. 56350601
Scientific Report No. 53
July 14, 1971
http://australianelectronicgoldprospectingforum.com/electromagnetics-and-em-fields/on-calculating-transient-em-fields-of-a-small-current-carrying-loop/msg26520/?topicseen#msg26520
dougAEGPF

[Davor]

More, more ....

[Ferric Toes]

I have many of the historical documents still, and will post extracts from them when appropriate. The information may be useful to others who have more recently entered the PI camp and can apply cutting edge techniques to advance the technology further. Here is the cover page of Johnson's paper.

Eric.

[moodz]

Thanks Eric ... the history is fascinating and I get the feeling some it will be lost if not already.

Regards,

Moodz

[Ferric Toes]

Hi Eric ... was the work done by John ever in the public domain or was it under trade secrets or patents. Just curious.

thanks. Moodz.

The work was done as part of a contract we had in 1976 - 77 to develop a vehicle mounted UXO detector, to clear tracts of land in southern England that had been used for Army training. Much of the theoretical side came from a 1956 paper By F.B. Johnson entitled A Pulsed Bomb Locator. This was done at the Signals Research and Development Establishment of the Army. Again there was a previous paper by J. H. Wescott in 1955 which Johnson built on. Wescott was from Imperial College, London. I was given the latter two papers when I joined the Research Laboratory for Archaeology, Oxford, as groundwork for the PI method. After Johnson's paper nothing much happened as the Army considered the method too active in that magnetic fuzes could be triggered. Mind you, Johnson was advocating a peak current of 25A in a coil of 2 Henrys to detect a 100lb bomb at 45ft! I was given the latter two papers as groundwork for PI when I joined the Research Laboratory for Archaeology, Oxford, in 1966. Articles on PI were published in "Archaeometry", the journal of RLA between 1965 and 1970 by Colani and Aitken, myself, and J. Green who is now Curator of the Maritime Museum, Freemantle, WA. John Alldred was also at RLA working on Thermoluminescent Dating equipment having gained his Masters Degree researching fluxgate magnetometers. I left RLA in 1970 and started my own business, Geoelectronics Ltd, and snapped John up a year later when he left.

PI has a longer history than is generally realised although much of the earlier stuff is not easily accessed. I have traced it back at least to 1914 -18 War in a primitive form.

Eric.

[moodz]

Hi Eric ... was the work done by John ever in the public domain or was it under trade secrets or patents. Just curious.

thanks. Moodz.

[dougAEGPF]

I have 13 pages of links to papers on EM here:
http://australianelectronicgoldprosp...and-em-fields/
dougAEGPF

[Aziz]

Hi Eric,

thank you for your published interesting papers. I enjoyed reading them.

Aziz, it would be interesting if you could refine this and bring it up to date. Also to include extra samples for GB.

We aren't finished yet and I'm waiting for a math freak to solve the proof.
The GB is a whole different topic and a PhD work worth. One have to flip over between the time domain (TD) and the frequency domain (FD) to describe and understand it.

Aziz

[Ferric Toes]

We need to do what geophysics has done in TDEM for years. Bipolar pulses and logarithmically spaced sequential time gates. This gives good resolution at early time for fast decays and at late time maximises the signal. Samples the whole space between TX pulses and cancels EF too, as you say.

Eric.

[Davor]

Nice, thank you.
There are, however a few points that may be emphasized... vo can in fact be measured directly in case of a separate Rx coil placed in induction balance, and EF can in fact be cancelled by both bipolar pulsing and HPF.
Furthermore, I'd say keeping sampling periods equal may stand for easier math, but is certainly not good for S/N at later samples.
You may consider a period t-to to be a bandwidth limiting one, so in fact you are subtracting amplitude responses of ever so lower cut-off low pass filters, thus enhancing the high frequency content e.g. short tau. With 1/(t-to) being the boss, I can't see equal time spacing to be optimal. I also fail to comprehend why is a late sample taken after every pulse. Some kind of iambic pulsing would provide more punch per time, while keeping the goodies of the late sampling.

[Ferric Toes]

Well this is what I have. Not by me but by John Alldred, employed by myself in the 1970's. He had a Masters Degree in physics and was one of those rare persons who could work out all the maths and theory and then build something that worked. Sadly he died 10 years ago at age 61 of lung cancer.

Aziz, it would be interesting if you could refine this and bring it up to date. Also to include extra samples for GB.

There is data on coils but this will be posted later in Advanced Coils for PI thread.

Eric.