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Anonymous
Guest
Here is one view of the site from which Chris recovered the nuggets shown in the post "Hakey's Gold" of Dec.18th.
Eric.
Eric.
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A
Anonymous
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Thanks Eric for posting the pic and thanks to Chuck for his 60 day free pic hosting service. The pic pretty well explains itself but there are a few points to mention. Just behind the Cat you'll notice a patch of dirt with not much grass on it. This is because others have used some sort of scraper pulled by a 4WD to flatten the grass to allow them to get their coils on the deck, but even that method didnt get the nuggets at the 10" - 14" level. When scraping with a big machine, the plan is to remove 3 inch layers as neatly as possible and detect each new layer. In this pic the Cat is taking off the 4th cut, down to about 12", but returns below that depth were negligible so most other cuts only went to the 9" level. The dirt has now been backfilled and within 12 mths, pending rainfall, it will be difficult to see where I've worked. Cheers, Chris Hake.
A
Anonymous
Guest
Hi Chris and all,
Here is another view of the nugget patch. It is worth doing some serious investigation as to why nuggets that should give a good signal are screened off from the detector by just a few inches of iron mineralised dirt. I've tried simulating it in the workshop without success, but, like the seawater problem, it must be the sheer volume of material that makes the difference. I'm going to see what I can find out theoretically, and from the geophysical literature where similar problems must occur, although on a larger scale. Any other ideas or suggestions are very welcome.
Eric.
Here is another view of the nugget patch. It is worth doing some serious investigation as to why nuggets that should give a good signal are screened off from the detector by just a few inches of iron mineralised dirt. I've tried simulating it in the workshop without success, but, like the seawater problem, it must be the sheer volume of material that makes the difference. I'm going to see what I can find out theoretically, and from the geophysical literature where similar problems must occur, although on a larger scale. Any other ideas or suggestions are very welcome.
Eric.
A
Anonymous
Guest
Eric,
Is this the same dirt you had in your shop? As I recall, it was easily detected. Looks the same as red Georgia clay, wonder how they differ?
- Carl
Is this the same dirt you had in your shop? As I recall, it was easily detected. Looks the same as red Georgia clay, wonder how they differ?
- Carl
A
Anonymous
Guest
Hi Carl,
The dirt I have comes from Victoria. I'm told the ground in Western Australia is much worse. Hope that Chris can send me a couple of kilos sometime and a few of the local hot rocks.
Eric.
The dirt I have comes from Victoria. I'm told the ground in Western Australia is much worse. Hope that Chris can send me a couple of kilos sometime and a few of the local hot rocks.
Eric.
A
Anonymous
Guest
[No message]
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Anonymous
Guest
Phew Eric, sending you a few kilos of OZ red clay is a bit scary !. What would HM Customs and Quarantine think of it !?. And you are right about the "mass" effect because once it gets broken up it is no longer magnetic and is easy to detect on. Somewhere in England I'll bet there is a red brick clay deposit that causes a compass to go haywire ?. That would make a good testing ground. Cheers, Chris Hake.
A
Anonymous
Guest
Kris, my CT 14"DD coil is not far away and I can change over to it in about 30 seconds. But I normally used the ML 11" mono for these reasons - 1. on my modded SD2000 the mono coils are much quieter than standard; 2. lighter weight; 3. faster pinpointing; 4. better sensitivity for small bits; 5. when scraping with a Cat you dont need depth, you need maximum sensitivity in the new 3" layer below you. So there is still some life left in the ML coils !. Cheers, Hakey.
A
Anonymous
Guest
There are three known causes of "mass effect", where you get ground noise from a spot (usually clay), but you break it up and the same amount of stuff doesn't give you near the grief.
CONDUCTIVITY
Clays hang onto both ions and moisture. This is especially true of the smectite group of silicate clays, less so of oxide clays which are common in the tropics. In order for electrical conductivity to happen, there has to be a good electrical path to form an eddy current loop. When you break up the dirt by digging in it, the electrical conductivity drops by an order of magnitude, and it no longer sounds off-- if that was the reason it was sounding off.
A spot can be electrically conductive for reasons that are not obvious-- unless you happened to see a cow piss on that very spot the day before yesterday.
NATURAL MAGNETIC "FABRICS"
Almost everyone is familiar with how black sand or iron filings line up with each other in the presence of a magnetic field. When particles are lined up end-to-end in filamentary structures rather than just being oriented randomly, the magnetic susceptibility of the bulk material increases. This makes the bulk material containing the magnetic particles more magnetic than it would otherwise be. Furthermore, when exposed to a magnetic field, the ends of the particles are forced through a larger hysteresis loop, increasing their magnetic loss angle.
When magnetic mineral grains are deposited in water or allowed to reorient themselves when soil becomes liquefied, they line up with the earth's magnetic field, and tend to attach to each other forming an anisotropic arrangement. This is well known in those branches of geology which are concerned with dating soils and lake sediments according to measurement of their magnetic properties.
If the magnetic particles in a soil have come into an anisotropic configuration, they are more likely to "sound off" on a metal detector. When they are disturbed by digging, they become like the surrounding (presumably previously disturbed) soil. Oxide clays, which are common in the tropics but which can be found in some temperate zone areas as well, often contain several percent or more gamma iron oxides, which gives metal detectors fits not only because of their magnetic susceptibility, but because of their relatively high magnetic loss angle. (High magnetic loss angle is what makes dirt "positive", in contrast to magnetite, a naturally occurring ferrite which has a low loss angle and is therefore "negative".)
RUSTED IRON METAL (ESP. NAILS & WIRE)
When iron metal rusts in place, you get a little deposit of iron oxides which are usually of the magnetic, high loss angle type. What's worse, if the object which rusted was a nail or piece of wire, the agglomeration of rust is linear, which is to say highly anisotropic. It's gonna sound off.
Now, you dig, and break it up. The particles get distributed into the soil and reoriented at random. No more "target".
--------------------------
--Dave J.
CONDUCTIVITY
Clays hang onto both ions and moisture. This is especially true of the smectite group of silicate clays, less so of oxide clays which are common in the tropics. In order for electrical conductivity to happen, there has to be a good electrical path to form an eddy current loop. When you break up the dirt by digging in it, the electrical conductivity drops by an order of magnitude, and it no longer sounds off-- if that was the reason it was sounding off.
A spot can be electrically conductive for reasons that are not obvious-- unless you happened to see a cow piss on that very spot the day before yesterday.
NATURAL MAGNETIC "FABRICS"
Almost everyone is familiar with how black sand or iron filings line up with each other in the presence of a magnetic field. When particles are lined up end-to-end in filamentary structures rather than just being oriented randomly, the magnetic susceptibility of the bulk material increases. This makes the bulk material containing the magnetic particles more magnetic than it would otherwise be. Furthermore, when exposed to a magnetic field, the ends of the particles are forced through a larger hysteresis loop, increasing their magnetic loss angle.
When magnetic mineral grains are deposited in water or allowed to reorient themselves when soil becomes liquefied, they line up with the earth's magnetic field, and tend to attach to each other forming an anisotropic arrangement. This is well known in those branches of geology which are concerned with dating soils and lake sediments according to measurement of their magnetic properties.
If the magnetic particles in a soil have come into an anisotropic configuration, they are more likely to "sound off" on a metal detector. When they are disturbed by digging, they become like the surrounding (presumably previously disturbed) soil. Oxide clays, which are common in the tropics but which can be found in some temperate zone areas as well, often contain several percent or more gamma iron oxides, which gives metal detectors fits not only because of their magnetic susceptibility, but because of their relatively high magnetic loss angle. (High magnetic loss angle is what makes dirt "positive", in contrast to magnetite, a naturally occurring ferrite which has a low loss angle and is therefore "negative".)
RUSTED IRON METAL (ESP. NAILS & WIRE)
When iron metal rusts in place, you get a little deposit of iron oxides which are usually of the magnetic, high loss angle type. What's worse, if the object which rusted was a nail or piece of wire, the agglomeration of rust is linear, which is to say highly anisotropic. It's gonna sound off.
Now, you dig, and break it up. The particles get distributed into the soil and reoriented at random. No more "target".
--------------------------
--Dave J.
A
Anonymous
Guest
Hi Eric
As Arthur Daly would say "some has been telling you a porky" here in Vic if go to some of the ground in the golden triangle,the ground can sound like there is 20 oz nugget every foot or so and that is with a DD.I think that Chris idea to get hold of few bricks could be your best answer to the problem.
Regards Frank Wallis
As Arthur Daly would say "some has been telling you a porky" here in Vic if go to some of the ground in the golden triangle,the ground can sound like there is 20 oz nugget every foot or so and that is with a DD.I think that Chris idea to get hold of few bricks could be your best answer to the problem.
Regards Frank Wallis
A
Anonymous
Guest
Very interesting info Dave, but in the case of my pictured patch the question is what does the detectors EM field do when it hits the magnetic red clay layer at about 8" deep ?. The current thinking is that it distorts horizontally across the top of the clay layer, taking the path of least resistance rather than penetrating deep into the ground. Most of the nuggets I found after scraping the patch should have been detectable from the surface and in "normal" ground I'm sure they would have been detected by the SD's and Coiltek coils. Regards, Chris Hake.