Magnetic Fields and Hot Rocks

[Anonymous]

Here is another copy of a post I made on the Finders.com.au forum.
This magnetism thing may be red herring. Hot rocks can certainly be magnetised. I have one here from Australia that will attract small bits of metal. I

 

[Anonymous]

Hi Eric,
Thanks for this post. It does clear up why I got a strong signal from a typical hotrock when I was experimenting with the PI kit I built. This kit doesn't have any circuitry to eliminate the effects of the earth's magnetic field. As such, it would give a very loud distinct signal off a typical magnetite hotrock. So, when I increased the gain of the circuitry, the problem got worse.
My solution was to reduce the the pulse duration and this did basically eliminated the false signal from a hotrock. I wasn't sure why except that the longer pulse would magnetize the rock better and this brief magnetization was the culprit. The shorter duration did not do anything for the signal one gets from the earth's field. So that still remains and will until additional circuitry is added.
Your explanation now clears up why your detectors do not see this same condition I was experiencing with a PI kit.
I felt I should post this since I am sure there are other kit builders out there that are experiencing what I experienced. Maybe this will help some of the kit builders who read this forum.
Reg

 

[Anonymous]

Hi Eric
I agree that magnetic rocks may not be a problem but the sad fact is that magnetic shielding is a big problem. At a test bed constructed by some of the local prospectors in Pigeon Gully near Dunolly they have drilled a number of holes deep into an eroded creek bank. These holes are at various depths from about 6 in to 3 feet below the surface. This test bed is constructed in old alluvial beds that have several layers visible in the vertical section. Unfortunately I didn

 

[Anonymous]

Hi Reg,
Yes, many kits only take a single sample of the signal, with the result that the signal produced by moving the coil in the earth's field is not cancelled. This is not too much of a problem provided you keep the coil parallel to the ground throughout the sweep. i.e. the field is constant for the duration of the sweep. If you swing it in an arc, then you will get false signals at each end of the sweep. As you say, though, you will pick up magnetised rocks with a single sample system. Having a second, late sample, which is subtracted from the first, overcomes these problems and that is the reason for the dual sampling gate and differential integrator that is commonly used in commercial PI's.
Eric.