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Date Posted: 22:09:14 08/03/02 Sat
Author: Reg
Subject: Re: Curious......
In reply to: Ralph 's message, "Curious......" on 20:21:36 08/03/02 Sat

Hi Ralph,

Yes, I did a lot of testing and observing the signals using a scope.

What I found was this, a hotrock signal, especially those that responded like magnetite, produced a tremendous signal when compared to a nugget signal. In fact, it took a very large nugget to even be seen (in the disc mode) sitting right up against a hotrock and then it would only be seen as it was approached from the side the nugget was on.

If you approached the rock/nugget combination with the nugget on the opposite side, the nugget would be ignored. In many cases, this was true even in the all metal mode.

Again, looking at the scope signals, it was clear why the nugget would be ignored. That was because the very strong "rock" signal simply overwhelmed any nugget signal.

After observing the scope signals, I cannot see any conventional way a VLF can overcome this limitation, simply because of how it works and the signals generated by the rocks. I can see how it might be possible to "cheat" if the signals are analyzed in software rather than in hardware. By this I mean I think the detector could be programmed to detect or sense a nugget or any metallic object under a rock, but I am not sure it would really distinguish an iron object from a nugget.

Since I was hunting the Rich Hill area at the time I was experimenting, I only concentrated on experimenting with rocks containing large quantities of magnetite type materials (ones that will readily be attracted to a magnet). These are the rocks that give a strong negative response when a typical VLF is ground balanced to the normal soil.

To put it perspective, it has been stated that the signal from mineralized soil can be 40 times stronger than the signal from a buried nugget. Well, it seemed like the magnetite hotrock signal was about 40 times stronger than the soil signal.

Since those experiments and with experiments with a PI, plus some info from one of the more noted designers, I have figured out there are techniques now used to reduce some of the secondary effects of the ground and hotrocks. However, they cannot eliminate them. Also, most of the present gold machines are using these techniques already, so I really don't see a simple solution.

A simple evaluation of the signals with a scope will display just how intense those rock signals are. Fortunately, a scope will also show how fast those "rock" signals fall off (reduce) as the distance between the rock and the coil changes.

One can experiment with a simple rare earth magnet and tell that the field strengh is intense when an iron object is real close to the magnet. However, move the object away just a little and the magnetic strengh reduces significantly.

Well, the signal strenth of a hotrock or ground for that matter follows a similar pattern. When the rock is extremely close to the coil, the received signal is usually so big that it overloads the circuitry. Move the coil away a little and the signal reduces significantly.

In your experiments on hotrock/nugget combinations using the disc mode, you might try varying the coil heighth to see if it makes a difference. I never really did much of that at the time.

Good luck with the experimenting and let us know what you find. Unfortunately, my time has been dedicated to refining a simple PI so it ground balances, and can detect nuggets down to the 2 to 3 grain range. It is a great learning experience.

Reg

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