This is not a solution ;-)

Why would anyone use tags on an oil pipe and hope for the tags to remain
there for years (one uses poinsons to press letters and symbols into the
soft-walled pipes with a small hammer. This lasts over 300 years,
guaranteed). Next, one draws a schematic of what was done and places
copies thereof in suitable places.

If none of this was done then I think that injecting current into the
pipes (as far as possible from both ends, or at least as far as possible
from the machine end) will likely cause an unnoticeable current vs. the
existing ground currents and noise. Injecting a fairly high powered single
frequency signal (probably as high as 1-2MHz) and looking for it with
narrowband probes may have a fighting chance imho.

If I'd do this I'd set up a test oscillator with AM30% 2kHz and carrier
somewhere in the AM band (unused frequency) and couple it directly into
the pipe vs. ground at the machine, and go hunting with a small AM radio
tuned to it along the pipes. The generator should output 0.5 to 10W or so.

I know that someone is upset because of the 'illegal' broadcast in the
AM band. Rest assured that the plant around you already fills that
frequency with much more than your 10W, and probably the whole rest of the
spectrum too, for several miles around, and up (and down). At least the AM
radio has a 0.5-2% bandwidth and this should put the noise down 20dB or
more vs. dc only with the same power. If a second filter is used on the
2kHz then even more noise attenuation is possible. Even so getting a
decent reading is going to be interesting, if the plants are as I think
they are, i.e. not designed with ground current and EMI/RFI suppression in
mind.

I have always likened using sensitive (RF) devices in an industrial plant
to doing DX on the AM broadcast band in the middle of a thunderstorm,
while standing under a 110kV HV power line ;-)

good luck,

Peter

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