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PI 2-box detector

A

Anonymous

Guest
Was wondering how come no one has made a 2-box PI detector? The depth increase would be incredible (I realize the pinpointing would be broad like current T/R 2-boxes of which I presently own). What say ye Eric/Dave Emery/MR. Bill?
Randy Seden
 
Hi Randy, I thing that this idea is a dead end. The double box systems is based on the ELMG field distortion, so imagine that you send a pulse to exite Eddy s current and try to monitor it by a crossed coil. To increase the depth is easier via increasing the draw current, using sectional or tandem coils. Nice day Sid
 
Sid,
This idea has been used successfully in airborne operations by Anthony Barringer inventor of the INPUT system in the early 1960's. In that situation you have the xmit coil wound around the wings and the receive coil (aka the bird) towed behind and with this system his company made some of the biggest discoveries of any Candian Exploration company.
Would be good to adapt this same technology to a smaller scale (i.e. handheld), including the recording of the signatures of the variuos ores.
Do a patent search for Anthony Barringer,the man's a genius.
Randy Seden
 
Hi Randy,
I once built a PI two box, but never really investigated its full potential. There is no need to have the two coils at right angles, as in the conventional type, as nulling is not required when the receive sampling is taken after the TX switch off. Having the two coils in the same plane, but separated by some distance is good, as any ground signal is dramatically reduced, as is the response to small near surface metal objects. I doubt if the detection depth is any more than would be the case for a single coil of the same size, but the reduction in ground signal and small trash would be of benefit to this type of searching.
Eric.
 
Randy,
The reason I built the CC or coaxial coplanar coil (see posting on this forum on November 18th) was because of my curiosity as to how well one would work on a PI. I had never seen a PI which used a CC.
I was very pleasantly surprised to find that it went very deep on small targets. Indeed I believe that the CC works better on a PI than it does on a VLF.
My point is that the only way to find out how well a two box configuration works is simply to build one. It is too difficult to predict what advantages or disadvantages a PI two box configuration may or may not have when used as a PI.
If you do build one, make sure that you make the spacing between the coils variable. This will effect the detection depth to different types and sizes of targets.
As Eric stated, for a regular PI there is no reason to do more than seperate the coils from each other as there is no advantage in nulling them, Dave. * * *
 
What if you put the two coils in series to produce a huge linked field, one coil switchable in phase to modify the pattern every few milliseconds, syncronised . Maybe switch the front horizontal coil into recieve circuit for pinpointing. Looks ok on paper to me. Any thoughts.
Cheers , Allan.
 
Allan,
Wow!!! I think that your idea would benefit from a bit of computer modeling. Very nice idea to say the least, Dave. * * *
 
Very good,looks like I'm going to give the CC coil a go.
Randy Seden
 
I agree with Sid the 2 box PI config, is largely a "dead end" for metal detection. If anything increasing the TX/RX seperation will result in greater negative returns.
The misconception that TX/RX separation governs depth probably springs from the geophysical DC resistivity method (Wenner etc...) where the current path does extend with increased electrode separation. Other geophysical exploration methods like seismic function use separation for different reasons. I would surmise that the towed airborne RX in the Barringer system is primarily a way to reduce RX saturation.
Just a couple of important points re, Dave Emery's excellent concentric coil post. If you wind the bucking coil directly on the receive winding THE END OF THE RECEIVE WIRE NEAREST THE BUCKING COIL MUST BE CONNECTED TO THE COIL SHIELD AND THE CIRCUIT GROUND. Failure to do this will result in excessive capacitive coupling and will cause an " excessive R null component" which may overdrive a TR type detector (not so important in a PI) And again, the bucking coil must be connected to the cold or earth side of the TX loop (George Payne's tips).
This generally accepted concentric config was the subject of an old Whites coil patent?? and the mail claimed advantages were less knock sensitivity and better thermal stability. Incidently, the bucking coil technique will cost you about 15% of your TX current.
I haven't posted for years. Thanks
Jim Stewart
 
Robert and David Crone published plans in the British 'E.T.I.' magazine in September 1989 for a Twin (Dual)loop P.I. unfortunately I haven't got that particular issue but I was told that details are on the Geotech forum.
 
Hi Brian,
here are the links. Both are the same PI projects. The PI project at Gary's web page locks a bit modified (maybe better?). Check it out!
http://www3.telus.net/chemelec/Projects/Metal/Metal.htm
http://www.thunting.com/cgi-bin/geotech/pages/common/index.pl?page=metdet&file=/projects/twinloop/index.dat
Chris
 
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