Esteban, no need to use static needles. One bar magnet close to the needle would produce a null point depending on the position related to the earth's magnetic field. The null point is a point where the resultant magnetic field is zero.
It means that when they are in equilibrium, any other magnetic source nearby will affect the needle position.
Actually a simple magnetic source depending on its nature and distance, might be stronger than the earth's own and will make the needle turn.
And you might be right. The device might be even simpler than a magnetic anomaly detector as it could be simply employing a bar magnet and a stabilizer circuit to do the job.
I will look in the net to see if I find a good text about magnetic nulling for you to read.
Regards.
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Maybe I'll available jan-feb 2010...
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OK. I will email you with details. Let's see if things work fine and we can go.