gold nugget simulation?

[green]

Charted some aluminum targets.

[Chet]

Hi Green

The aluminium time constant does not appear to be a very good match to the gold nugget time constant.

Thank you again for your charts,
Chet

[detectormods]

I am surprised by the long decay on these targets yet they are exhibit the same conductivity as Australian nuggets from central Victoria. Maybe the targets require the surface to be sand blasted.

[baum7154]

Charted some aluminum targets.

-----------------------------------

Thanks Green,

My first impression of these targets is that they are very large and their size is more representative of very large gold nuggets not generally found by most detectorists. The smallest of these simulators is said to represent a 20 gram (300+ grain) nugget and is indeed a huge target. The sheer mass of these simulators results in the long T/Cs.

I lean toward simulators of much smaller targets in the 1/3 gram (5 grain) as it is far more probable to find these size nuggets in this area. A 0.7 grain 1/8"diameter cylindrical aluminum slug does a good job of emulating a 5 grain nugget in my testing and the T/C is much shorter as well.

Regards,

Dan

[green]

Some different clay and a hot rock from Ca. Wondered if a hot rock and ground had a similar decay. Looks close.

[green]

-----------------------------------

Thanks Green,

My first impression of these targets is that they are very large and their size is more representative of very large gold nuggets not generally found by most detectorists. The smallest of these simulators is said to represent a 20 gram (300+ grain) nugget and is indeed a huge target. The sheer mass of these simulators results in the long T/Cs.

I lean toward simulators of much smaller targets in the 1/3 gram (5 grain) as it is far more probable to find these size nuggets in this area. A 0.7 grain 1/8"diameter cylindrical aluminum slug does a good job of emulating a 5 grain nugget in my testing and the T/C is much shorter as well.

Regards,

Dan

I would agree. I think they show what to expect for large nuggets and what to consider when when ground balancing for large and small nuggets.

[crane]

Aluminum blocks and alloy test pieces are poor choices when attempting to model or simulate natural nuggets.

You can see why by presenting a large copper or silver coin to the coil at a distance where it just shows the late time curve at the right of the screen with the cro set to about 10 usecs per/div. The coin's decay curve should quickly settle down to a near exponential decay.

Now, fix the coin in this position with something suitable such as a large lump of play dough or hobby/modelling clay and then approach the coil with a piece of aluminum foil. The coin's early curve will change but the medium and late sections of the curve will remain the same. You can then add a medium TC object to the mix, and if it is all mounted in clay then you can rotate it and see how orientation affects the curve. This deliberate mix of short, medium and long TCs is reasonable because it's what we would expect with a large quartz/gold specimen and it clearly demonstrates why nuggets don't show an exponential decay.

A typical nugget is represented much the same because cracks, voids and contaminates break the eddy current path, which in turn breaks the curve into multiple TCs. This doesn't occur with aluminum blocks or Woody's detector test pieces because there is nothing to break the eddy current path and this makes these objects rather useless for modelling natural nuggets.

One thing that should be obvious from the above experiment is that the addition of the aluminum foil changes the ratio of the signal amplitude in the early and late GB samples. This can be sufficient to change the tone from low to high if using the TDI disc method. Most rusted iron objects also decay as a high mix of TCs, which is why the TDI tonal disc method isn't reliable when searching for natural nuggets and specimens.

A half ounce spoungy porous nugget might give a very poor signal in its natural state but you can improve the signal dramatically by hammering it into a solid lump, and you can lengthen a medium to large nugget's TC by melting it down down and turning it into an ingot. A solid lead object can have a longer TC than a similar size nugget so a metal's "conductivity" and purity can have little bearing on the object's TC or decay curve.

[detectormods]

To make an accurate analogue to natural gold nuggets would not be cost effective due to the anomalies in real nuggets, and never are any two nuggets the same. So to make accurate comparisons in regards to detector performance are we between a rock and a hard place?
The aluminium alloy used is as good as I can get it, maybe using a hydraulic press can flatten them and cause stress fractures to impede eddy current formation but then no two would be the same.

[crane]

To make an accurate analogue to natural gold nuggets would not be cost effective due to the anomalies in real nuggets, and never are any two nuggets the same. So to make accurate comparisons in regards to detector performance are we between a rock and a hard place?
The aluminium alloy used is as good as I can get it, maybe using a hydraulic press can flatten them and cause stress fractures to impede eddy current formation but then no two would be the same.

I think you may have missed the point. If you presented all of your test pieces to the coil at the same time then their combined decay would be closer to that of a natural 60 gm nugget than the 60 gm test piece on its own but the proximity and orientation of each piece relative to the other would still influence the result.

If a solid lead object can have a longer TC than a similar size porous gold nugget then it's obvious that "conductivity" or purity doesn't play a major part.

Note that a large solid gold ingot is classed as a good conductor but a small piece cut from the same ingot is a poor conductor?

The term "conductor" in MD speak can be very misleading.

[Tinkerer]

Aluminum blocks and alloy test pieces are poor choices when attempting to model or simulate natural nuggets.

You can see why by presenting a large copper or silver coin to the coil at a distance where it just shows the late time curve at the right of the screen with the cro set to about 10 usecs per/div. The coin's decay curve should quickly settle down to a near exponential decay.

Now, fix the coin in this position with something suitable such as a large lump of play dough or hobby/modelling clay and then approach the coil with a piece of aluminum foil. The coin's early curve will change but the medium and late sections of the curve will remain the same. You can then add a medium TC object to the mix, and if it is all mounted in clay then you can rotate it and see how orientation affects the curve. This deliberate mix of short, medium and long TCs is reasonable because it's what we would expect with a large quartz/gold specimen and it clearly demonstrates why nuggets don't show an exponential decay.

A typical nugget is represented much the same because cracks, voids and contaminates break the eddy current path, which in turn breaks the curve into multiple TCs. This doesn't occur with aluminum blocks or Woody's detector test pieces because there is nothing to break the eddy current path and this makes these objects rather useless for modelling natural nuggets.

One thing that should be obvious from the above experiment is that the addition of the aluminum foil changes the ratio of the signal amplitude in the early and late GB samples. This can be sufficient to change the tone from low to high if using the TDI disc method. Most rusted iron objects also decay as a high mix of TCs, which is why the TDI tonal disc method isn't reliable when searching for natural nuggets and specimens.

A half ounce spoungy porous nugget might give a very poor signal in its natural state but you can improve the signal dramatically by hammering it into a solid lump, and you can lengthen a medium to large nugget's TC by melting it down down and turning it into an ingot. A solid lead object can have a longer TC than a similar size nugget so a metal's "conductivity" and purity can have little bearing on the object's TC or decay curve.

I totally agree. This is the best description or explanation I have seen until now.

[green]

[I think you may have missed the point. If you presented all of your test pieces to the coil at the same time then their combined decay would be closer to that of a natural 60 gm nugget than the 60 gm test piece on its own but the proximity and orientation of each piece relative to the other would still influence the result.]

Wondered why that would be true when the three test pieces charted the same decay slope. Charted the 2 and 2,4,6 together. They charted the same. Agree, the nugget shape has a lot to do with the TC. When charting the 10 and 18 grain nuggets the 18 grain had a shorter TC than the 10 grain.


[You can see why by presenting a large copper or silver coin to the coil at a distance where it just shows the late time curve at the right of the screen with the cro set to about 10 usecs per/div. The coin's decay curve should quickly settle down to a near exponential decay.

Charted some other targets including a 1 oz .999% copper coin. Added the clay decay curve to the log-log charts. I've been trying to pick the best delay and sample times to GB. Looking at the decay curves, a longer delay between the target and GB sample would make more sense than what I've read. Coil on time was 80 usec for the tests. On time does effect the ground and longer TC slopes. The chart time scale is usec., with zero being gate off.

[green]

[Looking at the decay curves, a longer delay between the target and GB sample would make more sense than what I've read.]

Not as long a delay as I was thinking. Control sequence for small gold. My best guess looking at recorded data. May be all wrong or won't work.

[Chet]

Hi Green,


Those are good views on ground balancing. Moving the ground balance sample time out past small gold time constants makes sense.


Somewhere in the Posts it is stated that increasing the transmit power is counterproductive. The concern is that increased power saturates particles in the ground which causes problems with seeing small nuggets.


Is there a way to chart nuggets simultaneously with a large ground sample at different power levels that would demonstrate the level at which this saturation would greatly effect or mask the detection of small nuggets?


Along the same line of thought is it possible to chart the eddy current saturation levels of nuggets (with no ground background) at some selected distances with increased power levels?


On both of these thoughts consider the effective collapsing flyback power injected into the target material. Disregard the opposing eddy currents built up in the target material during the slow coil charge time. The eddy currents created during the wide transmit pulse will be overcome by the faster and stronger collapsing field.


Thank you for your thoughts on this,
Chet

[green]

Hi Chet, Some things to try. I'll give it some thought.

[green]

[Is there a way to chart nuggets simultaneously with a large ground sample at different power levels that would demonstrate the level at which this saturation would greatly effect or mask the detection of small nuggets?]

I think Dan posted awhile back about putting the coil on ground. I tried the 6 inch coil on two 10 x 10 inch sip lock bags with Ca clay, total height 1 3/4 inches. The clay caused a reading 8.5 times the 10 grain nugget at 1 inch. The signal change for the nugget at different distances was the same as no clay. I would try to put the clay between the coil and nugget if someone thinks it would be different. Harder to get a good distance measurement without building something. I can try the different power levels with the coil on the bags if that makes sense.