Hi Willy
I can do the Fluxgate heads. They are delicate to build but straightforward.
Basically a small tube with 3 layers of bifilar wound 40swg wire of 200 turns per layer.
Key part is the high permeability material in the middle.
Try these guys:- MetGlas - Magnetic Alloy 2714A (Cobalt-based) DC Permeability of 1,000,000. Works very well.
Driver is basically a Hartley oscillator, though the better this is built the better. Receiver is a rectifier filter at 2nd harmonic.
This is a well documented technique. You will have to adjust the dynamics / freq / drive etc to match the coil properties.
We used to drive a fast and accurte counter with an MCU to produce very sensitive and stable data.
Hard part is temperature sensitivity which is why for Earth Field Magnetometery, it's better to have the unit burried below ground to improve thermal stability.
Careful design minimised this risk and its supprising how stable you can make them.
Calibration is not so bad. Use the Mag a a single unit and look up the magnetic index for your geographic position on the net. Do this over a number of days to average out. Make the output of the Mag the basis of that value, then do the same for the other Mag and balance to match.
Plot the response over time of your mags with data from institutions on the Web and see if they track. They should. If you want to get absolute values then professional Calibration is expensive.
The primary purpose of a prospection Gradiometer is to detect change and not to measure it as an absolute, only the variation. It depends on what you are looking to acheive.
In a Gradiometer, temperature stability is not such a problem providing the Mags are not too far apart, so there is a trade off between stability and differential sensitivity between top and bottom units
Hope this helps
Regards
G
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