magnetic viscosity

[Anonymous]

For an excellent discussion of "magnetic viscosity" in the time domain, try the following link:
www.seg.org/publications/geoarchive/1993/mar/geo5803r0326.pdf
"The first mention of possible magnetic viscosity effects in archeological soils was in 1966 by Colani and Aitken where, using a metal detector, they observed that the instrument is sensitive to nonmetallic features such as filled in pits and buried earths. ....Confirmation that the observed response was due to magnetic viscosity was obtained by studying the time dependence of the voltage induced in the receiver coil."
Data from our friend Mr. Foster from 1968 is referenced on page 327b line 22.
--Dave J.

 

[Anonymous]

The following links on Carl's website,
www.thunting.com/geotech/metdet/pi/corbyn.pdf
and /corbyn2.pdf
will get you a copy of Corbyn's "Experimental system for overcoming magnetic viscosity effects" published in the March and April 1980 issues of Wireless World. This is a pulse induction metal detector he designed for finding gold (unsuccessfully, he reports) in Australia. In some respects the design is quite sophisticated, with a bipolar transmitter, separate transmit and receive coils (not induction balanced), and gated preamp.
I believe that nowadays it is customary to balance the two delays after they have been separately demodulated and low pass filtered. Corbyn's design is unusual in that the synchronously demodulated pulses are balanced prior to their being low pass filtered. Although I would not be inclined to do it that way, it does have the advantage of eliminating the need for matched low pass filters.
--Dave J.

 

[Anonymous]

Corbyn's work would seem to be one of Bruce Candy's works of reference. Candy actually disputes Corbyn's findings on magnetic viscosity in one of his insufferable patent write up's.
I remember when corbyn's article first appeared in the old Wireless world magazine in England in about 1978. (time goes by fast!)
A lot of people tried to build it. From what I heard at the time, they could not get the design to work. Looking at the design a few months ago, I could not see any reason for this

 

[Anonymous]

Know it, and them, well. Martin Aitken was the Deputy Director of the Research Laboratory for Archaeology in Oxford and Claus Colani was a German engineer with Siemens, who independantly developed a PI metal detector. He came over to Oxford for six months to investigate the archaeological applications, and I took over the development when he left.
Frank Skinner was studying for a degree at the Bradford University, Archaeological Sciences department and I built the bipolar multichannel PI for him to do his research. That must have been about 1984/5.
The 1968 reference was for an article on PI that I wrote for Prospezione Archeologiche, an Italian Journal devoted to prospecting methods.
Eric.

 

[Anonymous]

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