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Minelab Inverted response???


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Hi all!

I am in need of help trying to figure out why an object (a meteorite) that is detected by a Minelab (GPX 5000) is giving an inverted signal when normally other meteorites do not.

I am guessing it has something to do with frequency/coil size/ mass size, harmonics??, etc. I am sure there is an exact explanation for this but I am not a minelab user so it has me interested and curious.

Any ideas?

Jim

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Don, I believe Jim is referring to inverted meaning LOW===HI instead of HI=LOW

As when you swing over like a 1/2 oz nugget and larger.

Hi Frank on the later GPX we can change the response from Hi---Low to Low----High by flicking a switch is what Don was referring to. When switched all small shallow targets give the low---high and deep targets will give a high---low signal to help better ID deep targets.

As far as the question I have also had some targets give a low high when I expected the standard Hi low signal when out of the ground and even a nugget I found did that once and not sure why that would be. I think maybe someone with knowledge of the inner workings of a PI detector could explain in more detail like Reg, but I sure do not have an answer for you.

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Hi Frank on the later GPX we can change the response from Hi---Low to Low----High by flicking a switch is what Don was referring to. When switched all small shallow targets give the low---high and deep targets will give a high---low signal to help better ID deep targets.

As far as the question I have also had some targets give a low high when I expected the standard Hi low signal when out of the ground and even a nugget I found did that once and not sure why that would be. I think maybe someone with knowledge of the inner workings of a PI detector could explain in more detail like Reg, but I sure do not have an answer for you.

Right now, all I know is it is in the salt/gold mode when it does it on one meteorite. It's Keith Semanko's detector that is doing it and he has not seen this before on many meteorites. It's a 279g stone about palm sized. I suspect it's size had a relationship to the frequency, coil size, etc. Maybe a chunk or metal in it or??? Sure is interesting....gotta be a technical explanation!

Jim

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  • 2 months later...

Hi Bill,

PI's have basically one way to ground balance and that is to take different samples and subtract one or more from the main sample such that the subtraction signal equals the main signal for the object you want to ignore.

Normally, this adjustment is done to ignore the ground signal. So, in the case of the TDI and the SD's, a later sample was taken, amplified and then that signal was subtracted from the main sample to eliminate the ground signal.

When this technique is used, some objects will create a high-low signal while other objects will cause a low-high response. Just which signal occurs has to do with the relationship of the object and the ground signal decay response.

On the TDI, if the object that is being detected has a longer decay time than the ground adjustment position than the response will be a low tone. Smaller objects or materials that have a short decay time will create a high tone. Now, usually the ground adjustment is set to ignore the ground, so when that is the case then any object having a longer decay time than the ground will create a low tone and objects that have signals that decay quickly like small gold will have a high tone.

On the ML's, expecially the old SD 2100 or earlier, this was also the case. Later ML's probably used a very similar technique but altered another parameter or two such as just how long after the main sample is taken they take the second sample used in the ground balance circuitry. Change the time delay between the two samples and you will change the relationship of the ground signal and the target signal. What this amounts to is at one setting, maybe a target will give a low-high response, but on a different timing setup, that same target will give a high-low response.

ML owners might want to do some testing to see just what happens when different settings are used and should do this with a wide range of targets just to see if some of those targets change tones as the detector is adjusted.

One can think of this ground balance thing or target tone change thing as simply two signals that are the result of two different samples and those samples are amplified at different amounts and subsequently one signal subtracted from the other. So, if we have signal A and signal B with signal B subtracted from A, and signal A gives a high tone and B gives a low tone, then if A is bigger than B, the summation will be a high tone or on the ML, a high-low response. If signal B is greater, then the subtraction process will create a low tone on the TDI but maybe a low-high on the ML.

Now, keep in mind that ML may have an inverted signal already as well as the ability to invert signals, so in some cases, what is a High tone on the TDI, maybe a Low-high tone on the ML. Personally, I have not had one of the later models of ML's in my hands long enough to really know for sure just which way it is.

Now, with the above info, we know if two signals are subtracted from each other. the tone will be either a low-high or a high -low response. The exception will be when the two signals are exactly equal and at that setting, just like what happens to the ground signal, one can have a good target not generate any or much of a response.

Now, getting back to the question as to why a meteorite might not have a common tone, well that is simple, the meteorite is clearly different in its characteristics as far as the detector is concerned. Since something as simple as a stony meteorite can vary dramatically which is noted in the L or H numbers, meaning the amount of nickel can be scarce or it could have a large amount of this metal. The results would be one would clearly have a much stronger signal than the other. The more nickel the stronger the response, the longer the decay. If the concenteration of nickel is sufficient, then it could easily change tones.

So, the chemistry and characteristics of the meteorite as well as the size clearly will play an important part in how the space rock sounds off.

Now, for those of you who have a ML and also have a wide range of gold nuggets, you might test to see which common size creates a high-low tone and which creates a low-high tone. Then if you have a bunch of different timings or settings on the detector test each nugget with all of the settings. I suspect you will find certain nuggets just might change tones as the detector is adjusted.

Believe it or not, knowing this and just what is going on when the detector's controls are adjusted is very important if you want to get the greatest depth out of your detector. What complicates matters is the fact that nuggets vary dramatically, so you may have two almost identical size nuggets that act totally different and this be normal. The variable is the nugget itself.

So, in very simple terms, the detector is telling you something about the target with the tone you hear. Just what it is telling you can only be known by knowing your detector thoroughly as well as the gold characteristics.

On the TDI, I found I could easily tell Rich Hill gold from that found over at Model Creek just by the sound even if I wasn't able to see the gold but knew the approximate size of the nugget. In other words, two identical weight and size nuggets with one from Rich Hill and the other from Model Creek would respond totally different. Again, this is completely normal.

Reg

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