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PI Detector noises what do they tell you????


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We listen to a PI equipped with ground balance and we will hear different sounds. There are the wee-woo's, the woo-wee's or on a different PI, just the wee's or the woo's. Regardless, not all targets sound the same.

So, why is that and what is happening. The answer lies in how a PI ground balances and this is done by taking multiple samples and subtracting one or more from another. When a detector takes a sample soon enough, it can detect small gold. The sooner it can sample, the smaller the size of that gold it can detect.

Now, when the sample is taken soon so it can detect small gold, then part of the signal that is also amplified is the ground signal. That particular signal is a problem child that interferes with the ability to hear the deeper preferred objects. So, a mechanism had to be developed to try to eliminate or at least minimize that signal coming from the ground minerals.

Because of the design of a PI, it can't use the same techniques for ground balancing that is used on a VLF simply because all the signals are not there. So, a different procedure had to be used and that is to take a later sample, amplify it and subtract it from the main sample. The amount of amplification used is what is necessary that the ground signal in the second sample equals that of the main target sample. Once equal, then a simple subtraction process fundamentally eliminates the ground signal.

Now, this sound simple but because all signals are not linear, then exact subtraction isn't perfect. Thus in really bad ground, there is no way to fully eliminate the ground response. If this is the case, then quite often people resort to DD coil which have another secret weapon. Reduce the ultimate gain and in most cases that basic subtract process does a good enough job that the DD isn't required.

Now, this amplifying the second sample has a side effect and that is all signals are amplified which means the ground and any target responses will also increase in this secondary sample. Subtract the secondary sample and the ground signal may be minimized, but what happens to the extra signals also being subtracted.

Well, if the second samples have target signals that when amplified become greater than the signal from that same target in the main sample, the signal changes tone. This results in some targets heard are a wee-woo or a simple wee and others being a woo-wee or a simple woo.

On most ground balancing PI's, you can take a true silver coin such as a silver half and a small nugget and check the targets. You should get different tones. The reason is the silver half response decays very slowly and very little of the signal is gone at the time of the subtraction. On the other hand, a small gold target signal decays very quickly so most of the signal is gone in the subtract portion. Thus little target signal is lost.

Now, if we think about what is happening, then to change tones, the ground signal and the target signal have to have different decay characteristics at the time of the sample. But, what happens if the target signal sort of mimics the ground in how it responds. The audio may be louder but it is louder in both the main and subtract channels, so the result is quite similar to the ground which means the target signal ends up much like the ground signal and is minimized. Thus some target responses are dramatically reduced almost to the point they display very little depth of detection. On the ML's, this is more evident on the SD's, To be honest, I haven't spent enough time with the others to try to determine what is or which objects are changing from what tones to the other.

However, on the TDI, this procedure is quite consistent. Signals that are strong and haven't reduced much when the ground signal is taken, will become low tones, while signals that have reduced a lot will be a high tone. Now, because one can manually change the ground balance (GB) on the TDI, any target response close to the ground response just may change tones if that GB position is changed. So, adjust the GB control and you can cause certain targets to change from a low tone to a high tone. Stop right at the mid point or change point and the signal from that object is minimized just like what happens to the ground signal.

So, in simple terms, if you watched and listened to digger bob's ground balance adjustment, you saw how the ground signal can go from a high tone as the coil is lowered to a balance point with no tone and then to a low tone as the coil is lowered. If we decided to pick something else like maybe a nail, we will find out we can cause a tone change on that also.

On an automatic ground balance setup, the operator doesn't make the adjustment but the same situation is occurring. In other words, if the GB changes a certain amount, it might cause the signal from a good object to become louder or quieter and no one know why. So, one might check things with one coil and then another and because of slight coil variations, require different ground settings, and the result is a louder signal on one coil than on the other.

Add in the variations that can occur because of noise and the situation can vary even more. Thus, it is easy to walk over gold and never know it but just as important, not know why.


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I may be understanding what you are saying here as I had a hunting session in very hot ground last weekend and I fished out 2 pieces of gold from an original signal that sounded like ground mineralization.

After a couple more swings over the area and changing direction with them using the X pattern, I cleared some of the top soil to get another swing and hear if there was any difference.

There was a change and I moved more dirt and came to a rock outcroping that was just beneath the ground/soil . Then I proceeded to pick it apart and keep checking/swinging over it to hear any changes even if it was to disperse at least I would know that it was just ground minerals. It not only dispersed the sound but at the same time a definate "TARGET" signal came thru thats when I knew I had located a nugget and freed it from within the rocks.

Point being I was at first getting a signal while swinging that made me think it was just ground noise even by changing swings direction and speed of swing.

Then by removing some topsoil allowing the coil to get a bit closer and having the soil cleared away I started to hear a better response, more discernible than at first.

So I think I will be paying MORE ATTENTION to what I would have normally taken as a patch of ground mineralization and walked by. This particular day one of these patches that I would swear is just a mineralized patch produced 2 pieces of gold.

Puzzling signal sounds sometimes for sure.

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Reg that was a beautiful post.

Should be required reading...

There is a section in JP's new DVD that almost exactly mirrors what you describe.

He swings over a banded hot rock that has a rather large specimen sun baking right next to it.

He goes on to show which settings would allow you to pick up the target and which settings would overwhelm the signal

as it picked up the hot rock and why...

It is a fascinating sequence and worth watching a few times - the same way your post is worthy of multiple reads.


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