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Detecting nuggets under Red Clay

A

Anonymous

Guest
Hi All,
Chris Hake, a gold prospector in Western Australia, recently posted a problem on the Finders.com.au forum. This was the inability of his detectors (SD
 
Hello Eric,
Do you think maybe a high end multi frequency vlf detector would be able to detect into or through this clay layer for the larger gold nuggets.
DAVID Di
 
Eric,
As long as your contemplating laying a 5m coil on the ground you would be better putting rods in the ground and electroconductively charge the nugget but still receive the signal with a loop. This method is known as Magnetic Induced Polariztion and has been used successfully down under for many decades. Chris Hake has worked with a mining company that used this technique, his only gripe is that you could be chasing a sulphide deposit instead of a nugget. Then there's the TURAM System that puts a continous AC signal into the ground but uses dual loops for receive (one loop is 90degrees out of phase to the other I believe) and it looks at the phase changes.
I think eventually with the ground so worked over in the past were going to have to go to ground rods to get the depth we need and eliminate the collapsing field problem of the TX loop. So that would leave us with working on perfecting the identification of the receive circuit so were not digging for junk.
My 2 cents worth.
Randy Seden
 
Hi Eric and all
On my last trip to the central Victorian goldfields I was using a large magnet screwed to the pick to pick up bits of ferrous rubish and some interesting results were obtained.
Result one when the magnet was rested on an unusually hot patch of ground the signal became wishy washy and gave an over-run type of signal rather than the previous strong signal???
Result two When the magnet was rested on the ground for a few seconds in a spot where there was no previous signal from the detector On swinging the coil over the spot after the magnet was removed a definate "nugget" signal was obtained???
How long these effects persisted was not tested for.
There must be some way to demagnetize the ground and allow the detectors to achieve their full depth.
I will prepare a post for the geotech forum site shortly.
A magnetometer would go a long way towards solving some of the magnetic ground questions but I need more info on mags and how they react in hot ground
Cheers
Steve D
 
Hi Steve,
Your observations are interesting. A couple of thoughts have come to mind like maybe the magnet wasn't free of metal at that time and a small piece of metal got crushed when you set the magnet down and was deposited the second time. This is is quite possible, since I have done it myself.
When I first started using a magnet on my digging tool, I found I was chasing trash that I had picked up earlier. It seems that no matter how strong the magnet is, some piece of metal will fly off when picking at the ground. Now, I immediately take all metal off my magnet and put in my trash pouch I wear.
Did you try to repeat the conditions? If so, did they repeat? If they were repeatable, then I think you are on to something. Just what the solution is, I don't know, but I am sure something will crop up.
Reg
 
hi Eric and All,
As a person who has hunted in some really strange red clay, I would like to say that one issue we learned came to mind.
We had an area on Rich Hill in AZ that had some deep red clay bordered by a strange greenish clay, which was bordered by a much darker almost charcoal colored clay. The area was being scraped with a large piece of mobile equipment, and then detected. All three ground areas gave distinctively different ground balancing conditions.
We found we could hunt the area extensively and seemingly pound every square inch of the area until we were sure everything had been recovered. Then we could wait a few days for the clay to dry and then go back and find more nuggets. As for the effects in the different clay, it appeared the ground signals changed as the ground dried out.
Most of the nuggets were found in the red clay, both before and after the drying. Because so few nuggets were found in the other clay areas both before and after, it was hard to tell if the drying really helped those areas.
We were all using VLF's at the time. Noboby, including myself was really sure what was happening but we knew this condition existed.
The point I am trying to make is, it appears that moisture can seriously alter the red clay ground response, and the ability to detect nuggets in it. At least it did when using a VLF at this location and I would expect similar changes would have occurred on a PI also.
Reg
 
Hello Reg,
I know exactly what you are talking about. However, there is a few trade-offs. I have noticed that the ground is more noisier when the red clay is wet or at least moist. However, the trade-off is that the moist ground elliminates a lot of the static build up which causes a lot of false signals.
I know exactly where your talking about since I have hunted most of those areas for several years. Without a doubt, the VLF experience more ground noise in red clays, laterites, and even iron gossans or "iron hats or blankets." However, I have found that the SD's and even GP's work fairly well even in wet or moist conditions with a mono search coil. Although, were talking US red soils, not the AU red soils which are far more extreme!!
Chris Hake is highly respected, and anything he states should not be taken like a grain of salt!! His experience and knowledge is highly sought after...
Talk care,
Rob A.
http://www.nuggethunting.com
azplacers@qwest.net
 
Hi Rob,
Boy, you used some words that I was not familiar with. Had to look up Jim Strait's article in the Nov 2000 of ICMJ to fill in the blanks. Jim is a great writer with tremendous knowledge of geology. He is also a great nugget hunter.
Google worked well to look up the article. I just searched for iron gossan and up popped this site: www.icmj2.com/OtherRecentArticles/ResidualLateriticGossan.htm
Reg
 
Hi Randy,
Is there any hard evidence that gold nuggets can be detected by induced polarisation? I would have though that, being an inert metal, it would not polarise.
Eric.
 
In the early 80s I had been invited to visit and hunt for nuggets at an old operating placer mine out of Fairbanks. The miner was using a trommel and a dragline to mine the ground and he was plagued with a layer of greyish sticky clay and this material was extremely hot. The trommel had lift bars inside to really churn this stuff and break it up. Detecting this material was a nitemare as the unit I was using at the time would see it as as a positive signal, you could not balance it out, yet you could take it and pan it out and all you would find would be pieces of magnetic black sand etc. I have since run across this material in old dredge tailings and the stuff has long since dried out and is easily detected. This is one of the things that I look for up here in the tailings as it seems most of the dredges just spewed this material out and you can see patches of it in patterns across the tailings and it is just dirt now growing trees and brush. When wet, the stuff was unmanageable....Geo
 
Hi Reg
The results were repeatable and no there were no bits of metal that fell off the magnet I was careful not to contaminate the test sites.
What is needed is some method to demagnetize the hot spots and allow them to be detected. I would like to find a nugget in one of these hot spots and then do some experiments. Could prove very interesting.
Cheers
Steve D
 
Hi Steve,
Boy, I don't envy anyone who has to hunt in that stuff. I am not sure any present type of metal detector is going to work well.
By nature, all detectors induce a magnetic field. VLF's have a varying field and most PI's a brief repeating pulse. If the ground retains a significant amount of the magnetization, then this is going to present a real problem.
Did you notice how long the distortion caused by a strong magnet would remain? In other words, mark the spot that you were getting a false signal and then go back, say 5 minutes or longer later to see if the pseudo target was still there?
I don't profess to have any great knowledge of the distortion or how something like this could affect how a detector would work, but just playing with magnets and knowing a little magnetic theory, I would think it would be a real problem and inhibit any depth.
By nature, common poles of magnets repel each other. Also, I know if a magnet becomes covered with magnetic particles like black sand, pieces of metal, etc, the field strength diminishes tremendously.
If we add the fact that magnetic lines of flux are lazy (like me) and take the path of least resistance trying to return to the opposite pole, then the guys idea of the field dispersing laterally in hot ground makes some sense.
Obviously, the technology will have to make some changes before significant depth can be obtained.
I suspect that some form of reversing the polarity of the pulse and varying pulse durations and time between them will have the best chance.
Reg
 
I know of no multi-frequency unit that is sensitive enough to detect nuggets unless they are of larger sizes. Certainly you can
 
If one brand and technology of detector is not achieveing the desired results in a particular enviorement, as has been indicated at the begining of this post string, then can anyone positively say, from their own trials and not on the theory based asumptions that they have tried to achieve these results using another brand and/or technology of detector.
I do realise the chances of good results are low, but I "cut my teeth" on searching for gold nuggets with Garret ADS Deepseeker and Ground Hog VLF technology back in the late seventies in the West Aust gold fields and I am not about to let anyone tell me it aint' possible to detect nuggets with VLF technology. I did come across "HOT" spots and hot rocks and I look back and think about why I could not find much in certain "patches". Maybe I could not detect as small nuggets as P.I. technology can in those really difficult soil conditions can but, even the gt1600 VLF and the XT17000 VLF and the XT18000 VLF are reputed, to this day, as very good GOLD NUGGET HUNTING detectors.
I am not here to knock P.I.s ability or anybody elses ability or knowledge.
If that means that asking whether can the BBS of FBS technology such as the Sovereign and especially the Explorer XS achieve these results of being able to push in and through this type of soil,to detect nuggets, which I have been led to believe they can and quite beyond some P.I.s in some cases.
The conditions that nuggets are to be found in are somewhat difficult, as we have been told, but beleive me that the Explorer XS , from my own experience, can certainly detect small naturally occuring nuggets.
The question begs to be asked.
Has anyone tried using this technology in the place of the that what is emloyed by P.I. /MPS for SD Series or the GP/DVT detector? Maybe even a DEEPERS6 Metal detector
(http://www.metaldetection.net/english/index.htm)
might just have it.
I do not know for sure that is whay I am here ........asking.
I think it is a simple enough question and I am as interested to know as much as the next bloke.
 
HI Reg
We need something to demagnetize the ground and allow full penetration to occur.
There are several possible ways of attempting this such as a large coil with a powerful alternating current applied or possibly hammering the ground may upset the magnetic field. I remember that hammering a bar magnet weakened its field.
Something to consider
Perhaps the D8 dozer option might work best
Are you following Chris and his posts on the Finders Forum??
Cheers
Steve D
 
Hi
My prospecting mate once had a Minelab Sovereign multi frequency detector and it failed to detect a gram nugget lying on the surface
This detector disappeared frm our prospecting arsenal a long time ago
Cheers
Steve D
 
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