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Coil Guru

A

Anonymous

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I am working on a PI probe,as it gets more and more
sensitive it detects hot rock farther away.
Tryed shilding no good.To much shield stops it from
working.
My PI detects hot rock at coil,small nuget 1/4".
Probe,same hot rock about 2" small nuget 1-1/4".
Question.
Could the problem be in my PI detector.
Can you cut out hot rocks by shielding.
 
No, shielding won't help against hot rocks. Sone hot rocks can be discriminated against owing to their magnetic properties, but conductive hot rocks act like metal targets and have to be lived with.
For more info on hot rocks, see www.dienco.com/rocks.htm
 
Wasn't this one of Eric's points in a thread below when you was comparing the SD2200 to the GoldScan 5?
" but differences may lie in other areas such as immunity to em interference and less detection of hotrocks"
So my interpretation of this is the amount of "hotrock" detection is not the same for all PIs.
George
 
Yes, indeed, there is a great deal of variation between the different PI detectors.
The traditional "all metal" PI can not tell the difference between good targets and hot rocks. The ability to discriminate between hot rocks and good targets depends on the hot rock's content of magnetite. Some have a lot--others less.
Even detectors that claim to have ferrous/non-ferrous discrimination don't extend that ability to the limit of the maximal detection range. The GP-3000, e.g. discriminates to about 1/4 of its max. detection depth.
 
Anybody who uses a discriminator while detecting for Gold has rocks in their head. Gold nuggets have been known to be coated with a layer of IronStone so be careful about using a discriminator.
 
Thank's.
All of you answered my question.
Now I can keep make-ing my probe more sensitive
until I max it out, to see what it will do.
 
Hi Frank,
No Guru, but your post got me thinking.
When I made my first DD coil I was playing around with the overlap and found that I could tune out some hot rocks, and reduce the signal from others, with only a slight loss of sensitivity. I wonder if a probe could be made to have similar properties as the DD?
By having the overlap variable and calibrated, one might be able to make a form of crude discrimination.
I'm not sure how one would go about construction, torroidial cores would allow overlap but I think the fields wouldn't be focused. Perhaps if one wound around the outside of the torroid rather than around and through, or maybe flat disks of ferrite? Maybe only one core would be required?
I often use Reg's technique in the field, but this doesn't work with all types of rocks, I
suspect conductive ones will still need to be dug. When you locate a target you raise the
coil/probe and observe how quickly the signal reduces over distance, a rock will fall off
much more rapidly than a metallic object. Then place the coil/probe dead centre over the
target and wait for the detector to tune it out, and then lift the coil, if there is a signal it is
likely metallic, or one of those pesky rock types, if on the other hand the detector goes quiet
when lifting the coil/probe, it's probably mineral.
You likely know all this Frank, and it's probably way off your original question, but it might help some.
Cheers
Kev.
 
Kev,
I am in FL,US.only check E-mail 2 or 3 times a week
There not many place's to try different technique
on nugget,with a PI probe.You said that you were
going to make one.
I need some test nuggets and pure black sand.
I will trade you one of my first PI probe slugs for
the nuggets and black sand.if you are in US.Slug is
about 3".it will detect a nuget.needs a cable and
put in pvc pipe and connector for your detector.
Its for a CS-6, might work on a goldquest.
 
Hi Alan and all,
There are four main types of hotrock with, some types being a combination. Firstly there are rocks, such as basalts, that contain a high concentration of maghemite. These give rise, when using a PI, to a signal which is the result of a magnetic lag after each pulse is terminated. Other rocks have a high concentration of magnetite, which has no lag, and does not give a signal on a PI, but will give a strong response on an induction balance detector. The third type is a rock that is sufficiently conductive to give an eddy current response, so that it responds as a metallic target. Fourthly, a rock that is magnetised, so that it behaves as a small permanent magnet.
A PI with no ground balance will respond to the viscosity signal, and if sensitive enough i.e. short delay, to any conductivity signal. With ground balance, you can eliminate the first, but not the second, unless the delay is lengthened.
A good PI, should not respond to magnetic field variations and therefore not pick up a magnetised rock. However, the degree of balance in the usual differential sampling and integrating circuits, has a marked effect on this.
Many Australian hotrocks that I have tested, are quite highly magnetic, as well have having a strong viscosity signal. The magnetism can be demonstrated by the way in which they deflect a compass needle. If a PI's integrator is not accurately balanced, the signal from a magnetic rock will still be there even if the transmitter is switched off.
Eric.
 
Hi Eric,
In CS-6, Which is the integrator.
UB4-A or B.
UB5-A or B, Thank's
Frank.
 
Hi Eric, I'm surprised that there is such a "hot rock" expert in England. I have hot rocks from China, South Africa, Chile, Canada and Australia, but not a single one from Europe. I didn't know you had them there. Have you made any measurements of the characteristics of the endemic hot rocks? is there anything unique about the European hot rocks?
Do your detectors ignore them or identify them?
Best Regards,
Allan Westersten
 
Hi Allan,
We have our share of hotrocks in the UK as it is a geologically diverse country. Many parts of the country have basaltic rocks, which can give as strong a response as material that I have from Australia. Here it generally does not cause detectorists much of a problem, because little hunting is done in those areas, as they are mostly devoid of gold.
In the early days of PI, I was involved in archaeological applications, which included studies of magnetic mineralisation in soils and rocks. In our department, we also did magnetic dating of kilns and basalts, which is based on the strength and direction of the remanent magnetisation. All this was good groundwork for understanding how rocks give signals on gold hunting PI's.
One favourite area for Europeans to go holidaying are the Canary Islands. These are volcanic islands with many black sand beaches. Detecting on these beaches is all but impossible, unless you have a good ground balancing unit. Even on "normal" sand beaches, the buried stones and lumps of lava sound off like metal targets.
Fortunately the signal from iron mineralisation does not vary much in its decay rate wherever you are. It is just the amplitude that changes. You can ground balance on a red housebrick in England, and the setting will be little different on Australian laterite, provided the dynamic range of the detector can accomodate the amplitude difference.
Eric.
 
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