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There may be Another Explanation...

A

Anonymous

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why larger nuggets give poor ranges in iron mineralised ground. Not having any 2oz nuggets to hand, I cut a 2oz lump of lead, which would not be too far removed from gold both in size and conductivity. I then looked at, and recorded, the decay curve of the signal on one of my detectors. Next, I took a tray of Australian iron mineralised ground and had a look at the signal from that. The two curves were almost a match in decay time. This means that a detector that is ground balanced for iron minerals will virtually tune out the 2oz metallic lump. Hence poor range. I don
 
Just found a Goldscan electronics under the bench with the ground balance circuitry. When balanced on the iron rocks, the 2oz piece of lead detected OK and indicated a slightly longer decay time than the iron minerals. I cut down another piece of lead until the signal disappeared at the GB point. This weighed in at just under 20gms. Much will depend on TX pulse and sample timing as to the size of the metal that will tune out at the GB point, also as it is a fairly broad notch, there will be a range of sizes either side whose detection performance will be compromised.
The SD's have two channels which can either be used separately or together. This, I think is to reduce the chance of notching out a particular size nugget, how effective it is, I am not sure. Pity I don't still have my SD2200. However, I suspect the 2100 would be better for tests as it has manual GB. Have to look out for a second hand one <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT=":)">
Eric.
 
Eric,
Let's make a call on all the SD owners who go out in the goldfields to make the following test.
Ground balance your detector and run it over different size pieces of lead placed on the ground and watch to see if any get tuned out. Don't buy the lead at a fishing tackle shop as much of the weights used are no longer lead.
Post the results so we can continue with our speculation as to what is happening.
 
Hi Eric and Dave
Minelab have always admitted privately that the SD's can miss certain nuggets of a certain size and shape. I will see if I can hunt up some old information that is stashed away somewhere in the shed and will report back.
Cheers
Steve D
 
Hi Steve,
You are right, Bruce Candy wrote an article for the Australian Gold Gem & Treasure magazine about the SD 2000 where he stated the SD could give a poor response to some nuggets. Unfortunately, I have a copy of the article and that copy does not show a date.
In that article Candy stated; "Also, there are a small percentage of nuggets (1% or so) which can give disappointingly poor response. These rare nuggets normally lie in the 1/4 to 1 oz range and give poor response because by coincidence they happen to give particular signals which the SD almost interprets as soils signals because of the particular way the SD measures the magnetic field."
Reg
 
Thanks Reg
You have saved me turning ny shed upside down.
Cheers Steve D
 
Hi Eric
How about a peice of aluminium, for conductivity,what would be the results for that.
 
Hi to all
Today I was testing a 2ft DD with a friend,on some extremely HOT ground I had it running in channel 1 on boost [2200D] found a signal clearly and called my friend with coilteck 14dd,which works extremely well,he could not hear the signal so we removed 1 inch of the top soil and the signal come in,the target turned out to be a brass stud down about 4 inches,which normally would be eaten by 14 inch DD so the mineralization is definitely masking the signal
 
Looks like Eric was right on the money! I will be doing some clay tests here as many areas here have bright orange volcanic clay with no topsoil at all. We also have a lot of local gold mines (now closed) so my guess is that our clay here is similar to that found in other gold bearing areas of the world. This stuff sticks to your shovel when it's wet. You almost have to cut it off your boots! It also murders a lot of VLF machines.
I have access to a buried 2 inch plastic pipe. The pipe is buried in orange clay at a depth of about one foot and has been there for years. The end of the pipe bends at 90 degrees and comes to the surface where it was capped to keep the rain out of it. I am going to place a small pick up coil inside the pipe with a built on amplifier. I am then going to fire a PI at it from the surface and see how much signal I pick up. I dug down on one end of the pipe to be sure that the clay was the same as clay that had never been dug. It was perfect and was without any gaps etc. It looked like the stuff you would make a pot from. The rain had long ago caused it to ooze into a solid mass. In the summer when it is dry, this clay is almost impossible to dig through. Someone said their pic bounced off it. This really sounds like the same stuff to me. Dave. * * *
 
Hi Dave,
Can you also get some metal objects down the pipe and see what you can detect? Compare with air ranges?
Eric.
 
Eric,
Thats a good idea. I can indeed get some different objects inside the pipe, Dave. * * *
 
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