GPX4500/4000 Using Double-D coils

Well, we all know that the strong point of the GPX detectors (such as the GPX-4000 and the GPX-4500) is that they can use monoloop coils on more mineralized ground than other models. Sure, there's hot soils that can only be handled with a Double-D but everyone hunting with a Goldstalker or a Nuggerfinder coil is using a Mono these days. Most know that the Double-D coils are capable of discrimination (near the surface) yet the Monoloops are not. Not so long ago, between 2003 & 2004, every large nugget recovered from the Golden Triangle during that year was (according to Jim Foster) found with a Double-D and not a monoloop. Not a single one! Part of this reason was no doubt because the GP3000 and similar models popular at the time etc needed a Double-D to handle the soils there. The Double-D is shallower than the Mono but the cone shape of the monoloop's EMF means you really need to overlap your coil sweeps and work more slowly. And just how much more shallow is that Double-D coil by comparison? I know the 11" DD stock coil has found me good and bad targets exceeding several feet in real bad iron-clay soil littered with iron ore stones. But I find a larger Monoloop to be shallower in some instances. Can't quite explain why although it could be related to the following:

Something often overlooked is that there's usually two mineral barriers: one on the surface (which needs to be scraped away prior to rescanning the target) and another, down deeper. Often, good nuggets lie below this second barrier, usually below the layer of ancient gravel wash which has trapped nuggets ... and this barrier can deflect or even absorb the signal from a monoloop. Large nuggets and numerous gold finds have been ignored in such conditions by other coils and detectors. Those using Double-D coils with the earlier models like the GP series have often noted that they recovered good nuggets that otherwise would have been ignored by the deeper seeking monoloops. Some spectacular find were made with Double-D's.

Who uses a Double-D coil with their GPX and why is that preferable for your location? Or is everyone stuck on their Monoloops nowdays?

Yes your right, I think mono`s are the flavour of the month at the moment, and your quite correct about the mineralised layers that the mono`s won`t penetrate I remember the boys at coiltek explaining this to us a couple of years ago, so don`t throw your DD`s out just yet LOL, still could be a handy tool.
cj

Nope, not all true mate. No large nuggets were found with monos in the early days? Garbage, I know of numerous large nuggets found with monos on sd2000S 2100s. 2100S worked ok on large monos its just that those without proper experience and no patience couldnt handle them and used DDs. Meanwhile others who knew they were punching down deep used them and found heaps. If you were around in those days and not just reading about it then you would have seen how much really came out with monos, thousands upon thousands of oz.
Ps: Best large nugget with a mono I saw..200oz.

I think it depends which state he`s talking about.
cj

I said that everyone looking for Large Nuggets is not necessarily finding them with the GPX detectors....and that there's a much higher strike rate with older detectors... Which use Double-D's. Logically, this shouldn't be the case. I personally feel the GPXs are ahead of the game so why is there an inconsistency in the success of certain unit/coil combinations? The GPXs certainly find smaller gold than earlier models so it's logical to assume they're going to find larger nuggets as well but if the wrong coils are employed for this task, the results may be unsatisfactory for hardcore nugget hunters seeking the larger-than-life monster nuggets.

I suspect this may also be because the GPXs are more sensitive and react to more targets and more junk... at depth too. So the SDs and GP3Ks (being less sensitive to smaller targets) are going to sound off less often and the successful strike rate would be statistically higher. Doesn't mean any of the detectors are better or worse but it sure looks to me as though people are only using Monoloops and are picking the wrong coils for the wrong soils. Spoke to a guy today with an SD2000 and a 4500 and even he agreed that he found all the larger nuggets with his SD.

Today I was speaking with an associate who comes from two generations of Prospectors. He's spent a lot of his life with metal detectors and watched the evolution from earlier days to the current generation. I explained my theory to him because of something he once said to me when we were talking about coils late last year. He and I have a bet running because he's never seen a GPX unearth a large nugget locally. He said people talk about it but that he's not see it happen in NSW. I know of cases in other states where multi-ounce specimens and nuggets are found with the GPX. But the sheer number of people finding large nuggets with older detectors using Double-D's has me p-perplexed. Remember that prior to the GP3500 and possibly the GP3000, Prospectors almost refused to even consider Monoloops. Even when a Minelab rep was attending meetings to tell them they'd find deeper targets with monoloops.

After talking to my associate today, he agreed that the issue of dual layers of mineralization is a plausible consideration. There will be smaller nuggets above mineralized wash gravel and even larger nuggets out in the open.. all of which can be found with Monoloops and other detector-coil combinations. But people hunting larger nuggets will tend to find them deeper. Especially these days since the surface layers have been picked over pretty good. But if the Secondary layer occurs (which it often does) and since large nuggets can be found inside and below this secondary belt of mineralization, and since this secondary level can (and will) block, absorb and reflect the signal from a Monoloop whilst allowing the signal of a strong Double-D coil.... well, you guys do the math.

I'm starting to think a few others have considered this too. As CJ mentioned, the Monoloop is "flavor of the month"... and a very practical coil: I use one myself... but perhaps larger nuggets are being missed with the GPXs because people are stubbornly refusing to use a Double-D coil. Everyone I've met with a GPX has a LARGE Monoloop and the stock 11" DD coil... but not a large DD coil. Curious.

Please note the image I've made tonight to illustrate what I'm referring to. It's the more complex windings in the Double-D that allow the field to better penetrate this secondary level of mineralized material.

Cheers.

Gday

Since the gpx detectors hit the market everyone from minelab to the dealers have touted the use of mono coils over dd coils, and have actively advised users that monos are the way to go, so its no surprise to me that people are using big monos, one of the other factors would obviously be the weight.

Big dd coils are usually heavier, and not the sort of thing you would use for general detecting, but when you can swing a 16" mono that is lighter than the original 11" dd why wouldnt you?, as far a mineralisation layers go well there is probably a lot more we dont know about the mineral make up of some goldfield areas than we do know, its a big statement to say that a dd coil will definately out perform a mono coil or vice versa.

My guess on that subject is that there are probably many layers of mineralisation and that would come down to the geology of the particular area, and the ground depth would be an obvious factor, and the type of gold whether its more specimen gold or solid nuggets, whether they are round or flattish, too many variables to be positive about the strength of one type of coil over another.

I have worked some areas in the past that make it impossible to use a mono coil, the mineralisation seems to flood the coil with false signals, and the constant moaning and groaning is worse than being on a salt lake, I have already briefly used the gpx4500 on one spot with some surprising results, not gold wise but the machine running smoothly on some ground that sent me packing on other occasions with the gp extreme and then the gp3500 and I was using monos and dd coils on them.

There are definately going to be areas where a larger dd coil will be the go, and I personally would keep a coiltek 14" dd pro coil in my kit just for that reason, one of the other areas that I spoke of produced some good gold to the dd pro and commander coils as it was just too hot to use anything else, I dont know about the nuggetfinders as they were new to the market and little was known about them.

Anyway this season I intend to target those areas and we will see, but it makes sense to carry a medium sized dd coil with you just in case, its all a case of suck and see I suppose.

cheers

stayyerAU

...very interesting post Marco...makes alot of sense and may answer why I have gone over areas and later found gold..just have to remember what coil I was using...

Ray

G’day,

Maybe the lack of large nugget finds by the GPXs is due to the almost exclusive use of Sens/Smooth and Enhance as opposed to Normal or Sharp. JP has confirmed that the former timings while reducing ground noise beautifully charge a price by way of attenuating the signals from deeper gold, so that means that any large deep nuggets which will already have a broad weak response on the later timings may become inaudible in Smooth and Enhance.

Secondly, I find it difficult to understand why a DD would penetrate mineralised ground better than a mono. The only reasons I can see why it may appear too, could be due to the spatial displacement of the transmitter and receiver coils in a DD where the receiver coil is listening at a slight offset angle to the path that the pulse field took. However, that would only be plausible if the transmission field artefacts somehow inhibited the targets response following the same path back to the coil?

It doesn’t seem likely that the few inches displacement of the Tx Rx coils would overcome this if it were the case. So it’s more likely the former reason stated or, the reduced ground surface response the detector would have to compensate for with the DD. The spatial displacement of the coils causes a ground nulling effect within a few inches beneath the coil, disposing of your primary layer Nero. Ground exclusion circuits tend to reduce target response amplitudes, so by getting rid of the near surface component, the electronics only need to subtract the response from the deeper mineralised layer, thereby leaving much more of the target response intact to create an audible indication.

My gut feeling is that it’s a mixture of all the above. We’ve seen JP dig some deep holes for some nice lumps on his vids, so it may take some operator technique and skill to recognise the much more subtle way the GPX indicates a deep biggin.

I reckon it’s not just as simple as DD verse Mono, it’s also the timings used.
The ultimate big nugget combo on the GPX maybe a big DD running Sharp.

Cheers
Grey.

PS let me qualify the above statement...it would all be ground response dependant...how long is a piece of string?

Double-D coils on a GPX
by nero_design on Sun Feb 08, 2009 3:04 pm

Well, we all know that the strong point of the GPX detectors (such as the GPX-4000 and the GPX-4500) is that they can use monoloop coils on more mineralized ground than other models. Sure, there's hot soils that can only be handled with a Double-D but everyone hunting with a Goldstalker or a Nuggerfinder coil is using a Mono these days. Most know that the Double-D coils are capable of discrimination (near the surface) yet the Monoloops are not. Not so long ago, between 2003 & 2004, every large nugget recovered from the Golden Triangle during that year was (according to Jim Foster) found with a Double-D and not a monoloop. Not a single one! Part of this reason was no doubt because the GP3000 and similar models popular at the time etc needed a Double-D to handle the soils there. The Double-D is shallower than the Mono but the cone shape of the monoloop's EMF means you really need to overlap your coil sweeps and work more slowly. And just how much more shallow is that Double-D coil by comparison? I know the 11" DD stock coil has found me good and bad targets exceeding several feet in real bad iron-clay soil littered with iron ore stones. But I find a larger Monoloop to be shallower in some instances. Can't quite explain why although it could be related to the following:

Something often overlooked is that there's usually two mineral barriers: one on the surface (which needs to be scraped away prior to rescanning the target) and another, down deeper. Often, good nuggets lie below this second barrier, usually below the layer of ancient gravel wash which has trapped nuggets ... and this barrier can deflect or even absorb the signal from a monoloop. Large nuggets and numerous gold finds have been ignored in such conditions by other coils and detectors. Those using Double-D coils with the earlier models like the GP series have often noted that they recovered good nuggets that otherwise would have been ignored by the deeper seeking monoloops. Some spectacular find were made with Double-D's.

Who uses a Double-D coil with their GPX and why is that preferable for your location? Or is everyone stuck on their Monoloops nowdays?

This post reminds me of an article that Jim Foster had published in the GG&T magazine a few years back when he was working for Coiltek Maryborough. Straight up I will tell you I disagree with Jim on a lot of points especially when it comes to the DD coils seeing through a layer of mineralisation that the Mono coils are being deflected by. The reason is pretty simple, basically speaking DD and Monoloop coils are exactly the same from a transmit point of view, things only change when they receive. So then why is it that a DD seems to punch in deeper than a Mono in some ground types? It is due to the second winding of the DD coil and also the size of the windings which gives them an advantage, if for example we take two identical sized Mono and DD coils then take a peek inside you will soon see they are not created equal, the Monoloop has a winding that pretty closely mimics the outside edge of the coil whereas the DD windings cut the coil in half. Obviously this means the DD coil is only transmitting from half the sized winding as the monoloop, but also at the same time it is only receiving from only half the same sized winding as the Monoloop (keep in mind due to the smaller windings there is also an inherent gain in sensitivity).

OK so we've established the two coil types are not the same but have similarities, now we need to take a peek at why a DD coil runs quite. This happens because the windings of a DD coil pass over each other which acts as a damper if you like to the receive circuit of the detector, to prevent the coil from seeing itself the coil has to be nulled (similar to the detectors method of nulling out the ground so you can detect). The problem with nulling is a certain amount of performance is lost but in the case of a DD coil this can be advantageous, because sometimes less really does equal more . If we did not have to deal with ground signal then our PI machines would have phenomenal depth (this has been demonstrated by competitors to Minelab in the past with air tests (especially with military machines for de-mining) where they demonstrated incredible depth yet had poor performance in even mild mineralisation).

DD coils have been around for donkeys ages, the detector sends a signal down one winding then receives back through the other, because of the two windings measurements can be taken between the transmit and receive which allows us to make informed decisions about the conductivity of a target (discrimination), and at the same times removes a large amount of ground signal due to the damping effect of the two windings close proximity to each other (in other words DDs ignore ground noise), they also generate their signal response in a blade type pattern due to the two windings crossing each other which allows a better coverage of ground from a receive point of view. Monoloops are a pure power performance option, in other words they pump out the maximum energy into the ground the detector is capable of over the full circumference of the coil and then after internal switching (they transmit and receive off the same winding and through the same cabling) also receive over the full circumference of the coil. Because of these characteristics they have incredible sensitivity and depth but by their very nature they also become their own Achilles heel because in noisy ground the electronics can become swamped by the reflected energy from the immediate conductive surface of the soil (swamping effect where the immediate mineralisation can overload the electronics).

All this raw power can come at a price because the more subtle signals that are deep and faint can easily be masked by the effect of the mineralisation on the surface, so to some extent they will lose depth or more accurately they can mask deep targets due to the noise generated by the surface mineralisation (hence less depth). DD coils on the other hand are not affected so much by the surface mineralisation so generate less ground noise than a Mono in the same ground type so faint deep targets will stand out more from the feedback of the detector. Enhance and Smooth mode of the GPX range basically do the same thing but with the full transmit and receive of a Monoloop coil, this explains why so much mid ranged sized gold is being found in noisy areas that have been well done over with DD coils in the past, the gold being found by the new timings represents the gap or blind spot if you like of the previous coil options.

I would say the reason there are not so many large nuggets being found with Smooth and Enhance (to my mind it is still early days yet though) is because a large chunk of gold at depth in mineralised ground would have stood out much more with a DD coil once the ground noise was filtered out, hence those targets have already been removed.

So to re-cap, DD and Monoloop coils transmit and receive exactly the same, so if a ground type is going to deflect one then it will deflect both of them, the picture above does not explain what it is trying to demonstrate clearly enough, is the author referring to the receive field generated by a conductive target or the transmit field of the coil? (to my eye it looks like the transmit field is being demonstrated and as such is incorrect). The differences between the two coil types is in the size and shape of the transmit (DD coils are more elliptical in shape) and the method used to receive the resultant field back from the ground (either through switching or a second cable) otherwise they are exactly the same. Because of the smaller transmit and receive sizes of a DD coil and the way they null it is best to use a slow methodical swing speed whereas a Monoloop benefits from a brisker pace to alleviate surface ground response (swamping effect). A DD coil benefits from a very close to ground swing height whereas a Monoloop prefers a gap of at least 25 mm to alleviate swamping (remember the Monoloop has a much larger transmit/receive so has plenty of performance to spare).

Hope this helps explain a few things,

JP

PS In some circumstances a DD coil used in Normal timings will outperform a Monoloop in Enhance, but as has been largely demonstrated Mono coils in Enhance timings are finding plenty of gold missed by the DD/Normal timing option.

hi,
from memory, i think jims reply when someone asked how he could prove
this theory was= it's not for me to prove i'm right, it's up to you to prove me wrong.
i have no idea if he's right or wrong, but it would be good to know for sure either way
cheers fencejumper

A very great post Jonathan, I am always learning. Marco, I agree with your theory on DD's being deeper than Mono's in certain circumstances like layered and very mineralized tough ground. I always loved DD coils and agree they have terrific depth and underrated in this Monomania. You can also listen for small signals as DD's run quiet as well.

IMHO, I believe that Mono coils are only marginally better(in depth and sensitivity) over DD's of the same diameter(size) for people who want every advantage they can get and also have to be overlapped more. But Monos can be swung faster than DD's as Jonathan said. JP also said Monos at about 2.5cm and DD's the closer the better to the ground. In a purely factual way the mono coil is deeper, however (and I do mean however) the deepest penetration point of that Mono coil is so small its real easy to miss targets at that ultimate depth. What it does allow is better separation of targets at that depth than you get with a large DD coil. The DD coil doesn't miss much at its ultimate depth though, because of the wiper blade like pattern it uses, instead of the upside down cone you get with the Mono.

A new twist on the situation below. Please take a look at this below as it took me alot of work to get this together:

DD vs Mono in shallow depth
http://arizonaoutback.ipbhost.com/index.php?showtopic=6735
>>>Jun 11 2008, 06:37 PM Post #14
"...You will notice that the DD coil has a stronger field than the Mono up to about 5.2 inches from the target and begins to fall below the Mono beyond this point. As you move further away from the target, the field strength of the DD falls rapidly compared to the Mono field strength. Note how the DD is appx. 55 compared to a field strength of appx. 75 for the Mono at 10.5 inches. Remember that you want the field strength to be the largest number possible for maximum depth penetration. A stronger field will produce larger eddy currents in the gold which gives you the best possible chance to hear the target. The absolute number of the magnetic field strength gives you a way to compare different coils at a particular height (depth).

After a quick think, you can see that the DD coil will have a stronger field than the Mono from about 0” to about 5.2”. As with all coils, the smaller the diameter (all things being equal) the stronger or more sensitive the coil will be in the close proximity of the coil. Even though the coils are of the same diameter, the transmit portion of the DD coil is much smaller than the diameter of the Mono coil which is why the DD coil has a much stronger field up close to the target than that of the Mono coil!

An interesting observation that was made while doing the test on most DD coils compared to the Mono coils (with equal diameters) was the fact that the crossover point (the point where the field strengths are about equal) seems to always be appx. 4.5 to 5.5 inches no matter what the diameters are as long as they are equal. It is the particular geometry of the coils (ratio) that explains this crossover point.

The DD coil in this case would be ideal to hunt with if you are in shallow ground or on bedrock compared to the Mono of the same size.

Hope this helps a little bit and gives you a clearer picture of what is happening with the coils. Stay tuned for more comparisons and analysis. Please excuse me in advance if I’m delinquent in answering any questions that might be posted, as my days seem to be filled with things to do!

Enjoy, Steve Gholson"


>>>Jun 11 2008, 06:54 PM Post #15
"...that coil performance generally cannot be accurately judged by air tests,...
...Thank you for this info...Cheers, Ron"


>>>Jun 11 2008, 07:38 PM Post #16
"Ron,
I don't feel that way at all. The magnetic field that you see in the plots represents a relative number.
The greater the magnetic field strength the larger the eddy currents will be.
It does not matter if we are looking in air or in the ground,the coil that has a greater field strength
will have more "brute" force to make things happen.
These numbers were generated with air test and even though they would be different for in ground
measurements, the ratio or difference between the two would be close regardless of where the test
were conducted. The test were conducted in air only for the fact that it is much simpler to work with.

The soil absortion of the transmit field and any nasty things that happen in the ground, will effect
both fields equally. If the ground is nasty, a DD would preform better only because you are able
to hear (receive) the signal better because of the ground noise that is eliminated with the DD.
We are not talking about ground coverage, etc. only the brute force transmit signal.

Just my thoughts! Regards, Steve G"


>>>Jun 12 2008, 03:40 PM Post #18
"...Notice that the crossover point on these coils is about 5.4 inches.
Once again, the DD coil has more sensitivity up close and begins to fall off (perform worse) once you strike the 5.4” level. Beyond this 5.4” level, the Mono starts to pick up steam and out performs the DD coil. Please remember that we are talking about brute strength here regarding the electromagnetic field.
If you refer back to the prior graph ( 16” DD / Mono) you will notice that the 11” coils have less magnetic flux density at depth than the 16” coils.
The 16” Mono has a level of appx. 75 at 10.5” while the 11” Mono has a level of appx. 60 at 10.5”.
This plot again shows that the DD coil has more sensitivity up close to the target and starts to fall off at depth vs. the Mono of the same diameter...."

...I will continue to show more graphs, power loss of coils, formulas, etc. in future post.

Enjoy, Steve Gholson"


http://arizonaoutback.ipbhost.com/index.ph...=6735&st=20
>>>Jun 16 2008, 10:33 AM Post #27
"...The data that is used to construct the plots is taken from the point where the test system shows the largest magnetic flux density. It just so happens that this point is dead center of the coil...

...I’m not sure this data would be beneficial (strength of field on edge) but if so and time permits, we could plot it.

Hope this clears up the picture a little bit!

Regards, Steve Gholson"


>>>Jun 17 2008, 09:52 AM Post #34
Hey Steve,
Thanks for all the great information, I am strongly considering the purchase of a 16"RDD now that I have seen the advantages on smaller gold at lesser depths between 16"RDD, and 16"RM.
I have an area where the gold is rather shallow. Plus from what I hear DD is smoother in wet or highly mineralized ground. All I have ever used are mono coils...

...Thanks again for the education. cool.gif
Gus-

Seems Fencejumper has gone and edited his post while I was posting this, sorry for the confusion.

fencejumper wrote: Hi all,
JP just wondering,if a mono can become swamped by REFLECTED energy in some ground but a dd will not,
does that mean you will get targets deeper with a dd than a mono in that ground?
cheers fencejumper

All the coils are susceptible to swamping, just the DDs are less prone due to the NULL of the coil (basically speaking a DD coil is desensitised due to the proximity of the second winding). A Mono coil in the right hands will still provide the best overall depth (it has to because with similar sized DD coils the Mono has a much larger transmit and receive), but the resultant ground noise or ground signal if you like can mask a good target in the clutter of the threshold.

As an example a 24" mono is a pure 24" of transmit and receive whereas as a 24" DD is really only a 12" transmit and a 12" receive and to add more dimension to the equation the windings are out of round which also impacts on depth etc, so for maximum depth the bigger the DD coil the better. The thing is we have another limitation to take into consideration (this limitation helps with the DDs BTW), realistically speaking the most we can expect depth wise from our PI machines is 3 feet max (yes large objects have been dug deeper but very rarely), but the thing is a 12" coil is also capable of detecting a LARGE nugget at those depths too if the conditions allow, so this means that a DD coil has an advantage because the reduction in ground signal far outweighs its depth abilities, this means a good operator who is prepared to keep the coil swing speed down, and coil height as close to the ground as possible has a very good chance of finding large nuggets at depth in high mineralisation using the optimised timings for full depth (Normal timings).

Hope this explains things a little more,

JP

G’day,
Yesterday I happened upon the Minelab Times of September last year which provides a graphic comparison of the 4500’s timings. For those who’ve not seen it, download the link <pdf> and look at the bottom of page 3.

It illustrated to me the multitude of variables at work here. There are very few consistencies, take mineralisation for instance it can alter minimally from day to day with fluctuations in heat, moisture, and the earth’s magnetic field. Interference is also far from static with intensities varying with the weather, man-made emissions, and ionospheric disturbances.

The basic performance of one particular coil in each of timings is fairly consistent, but the operator needs to be a little open minded to experiment from day to day, and season to season, to ensure that the mix of variables, EMI, target depth, size & composition, mineralisation, coil, timing, weather conditions etc., are optimal.

This doesn’t begin to consider detector variables such as Gain, Stabilizer, Audio mode, Motion, etc.,

The power is in our hands it’s a matter of using our grey matter and spending a little time experimenting so we learn to use it to its full potential in any given setting.

In the end though, no matter how the detector is setup, to achieve outright depth on big nuggets may require the operator leaving the comfort zone of a silky smooth threshold and enduring the wavering jerky one of a detector at its limits, because as JP has said many of the big nuggets under well trodden paths are gone already. Whether that means a BIG DD running Normal or Sharp if the situation will allow, or a BIG mono running Enhance, you be the judge, it all depends......

Cheers
Grey.

yeah sorry about that jp i reread your reply and already knew the answer,tried to delete but did not
know how so i just edited
sorry cheers fencejumper

I just found the article written for the Jan/Feb 2004 edition of Gold Gem & Treasure by Jim Foster which was titled "Which Coil, DD or Mono?". I suspected he might have been working for Coiltek at the time because the article spoke very highly of the Coiltek DD coils... to the point of the article almost being a great advertisement. I'm curious to resolve this issue since I have a standing bet running with another Prospector concerning the 4500 as I believe it to be the most capable detector released so far and yet he thinks there's an unresolved issue because he's never seen a particularly large nugget retrieved by the 4500. We've heard of them... but just on the wind. When I saw this article from 2004, it struck me because it actually gives a valid explanation as to why the DD found such larger nuggets over the Monoloop.

The article was an interesting one and ran from pages 6, 7, 8 & 9 for those who have a copy. Jim said that on the GP3000 (which was the big detector at the time of publication) the DD Pro coil was as sensitive as most monoloops on small gold at depth. He followed up by saying that "recently it has become common knowledge that there is a limiting factor to the depth penetration of monoloop coils on highly mineralized ground, especially on larger nuggets. This is born out of the fact that in the last 12 months, not one large gold nugget has reportedly been found using a monoloop in the Golden Triangle. Every big nugget (and there have been quite a few) has been found using a DD, and most of them have been found using a Pro DD coil on a GP3000.". Jim goes on to say that there are usually two distinct layers of ground mineralization. He said that the second layer (not the surface layer) was the higher mineralization layer where most of the larger nuggets were being found.

Flash Forward to today: and we now have the GPX-4500 which is the 'hot, new detector' and NOW this leading edge detector can handle a monoloop much better than the old GP3000 could back when the article was written about 4 years ago. So with Monos being much quieter but offering slightly better depth than a DD coil on this detector, this has become the "accepted, must-have combination" for most nugget hunters. But we're not seeing nearly as many larger nuggets being found with the GPX-4500 as many would have expected. I keep meeting people who have retained their SD's because, even though they also own a 4500, they claim that they find all their big nuggets with their SD detector. Sure, there were some big ones found with the GPX-4000 when DD's were used. But lately, with the GPX-4500, everyone's using a Monoloop and not nearly as many big nuggets at depth are being reported as I would have expected. And I'm not the only person to notice this either. Dealers and pro-Nugget Hunters have all posed that question to me at one time or another. And after coming across this old article, it's got me thinking that perhaps the most obvious difference is in the fact that everyone's using Monos on the 4500. And those people finding larger nuggets with OLDER detectors are obviously going to be using DD coils (which they are). In my mind, the obvious difference is the use of new timings with a Monoloop coil. Perhaps the solution might be to reconfigure the 4500 to better resemble the previous models and then bang on a DD coil to see what the results are.

Now the earlier DD Pro coils from Coiltek were/are known to output a stronger, denser field (partially by drawing a slightly higher current from the battery to construct this field) and this was at time when the DD coils were known to run much quieter on the GP3000 than a monoloop could dream of doing.

Jim said that by utilizing the more defined and powerful field of a DD coil, and even more so with a Pro DD coil, this barrier can be overcome and the bigger, deeper, nuggets 'seen' by your detector.

On the subject of dual layers of mineralization, the author of the article said that it's what lies under the gravel wash that makes all the difference. Under the wash there is usually a bottom of clay, or in the case of WA, caprock. In this contact zone between wash and bottom lies another, heavier layer of mineralization that has leached down through the wash. When the wider, less dense field of a monoloop comes into contact with this heavy layer of minerals, it is often deflected to the sides and fails to penetrate further. "The user is under the false impression that there are no targets beneath the coil. But it is from beneath the second layer of high mineralization that most of the larger nuggets are now being found."

JP, I can see (and understand) why you find cause not to agree with Jim's comments on several issues of technical and design considerations. And what you've said makes a lot of sense. I'm not asserting that Jim is correct here either although the article he wrote has posed even more questions. I think there's a lot of interesting co-incidences here and I'm wondering if there's indeed a little modern relevance in such an 'old' article.

As noted above, the quieter the coil, the more likely the user is going to hear a target and so the DD coils are going to have a considerably advantage at the sacrifice of some depth. The question remaining is: (assuming the hypothesis of a DD penetrating the dual mineralization layers is factual or true) Would anyone abandon their Monoloops and then switch to a DD with their GPX-4500 in order to give themselves an edge with the deeper nuggets of more robust dimensions?

Cheers,

Marco
Note that quoted text from the article was restricted to "common knowledge" and was quoted where possible.

Good info Marco, I enjoy reading all your posts.

I was reminded of a post I made on another forum, and it maybe the reason for the supposed extra penetration of the DD over the mono in certain situations. According to Eric Foster, the best coil form factor for a PI, is a rectangle, 3 or 4 units long, by one unit wide. Now this is a mono coil, for instance 18” x 6” or 20” x 5” etc.

Eric hasn’t elaborated on why this is so, but it doesn’t take much to imagine how the magnetic field would be focused and concentrated into a wedge shape. The flux density increases as the sides of the coil are drawn together. It is quite possible that this concentrated field is less likely to be deflected while trying to penetrate the reactive ground components. We could illustrate it this way, if you were going to split some firewood, would you rather use an axe, or a sledge hammer?
The axe is like our rectangular coil with the field forced into a narrow blade, and the sledge resembles our round mono with the more diffuse field.

I suggested in the other post that a narrow elliptical mono coil (not a Commander) resembles Eric’s ideal more than a round mono and given relative sizes the elliptical in certain situations should penetrate deeper.

The interesting point in all this is, especially with an elliptical DD, the coils are closer to the 3:1 ratio and we would see the wedge penetration effect (Not to be confused with the blade like response, which is due to a type of differential cross-coupled cancellation effect, leaving just the component within the crossover area)

When we use the switch to select either Mono, DD, or Cancel on the GPX series, we are informing the microprocessor what type of coil we have attached, which in addition to Cancel may invoke quite different processing algorithms.

Whatever’s going on, there's a strong possibility of many buttoned lips not letting on until such time as the mop up has been completed.

Cheers
Grey

I’ve edited this to add a summation of what I feel could explain the proposition that a DD coil detects big nuggets deeper than a mono in certain situations. At least 2 things could be implicated.

1) The DD configuration that cancels near surface ground noise, thereby reducing the amount of target signal attenuation by ground exclusion circuitry.

2) The Tx and Rx windings within a DD coil (especially elliptical) more closely resembling the ideal 3-4:1 ratio, giving peak flux density with respect to energy input.

Marco, I do not doubt there are a number of varying layers in alluvial ground which present different GB profiles to the electronics of any Minelab PI machine (in fact I know there is). The reason the 4500's haven't produced any large gold yet is more likely due to a very simple cause, everyone is still going over OLD ground removing nuggets missed by previous techs so tend to spend more time in ground that is going to be productive to the Smooth/Enhance timings. If you went over the same ground with a DD combination no signals would be heard so more time would be spent investigating a larger range of ground type or in some cases the deeper areas, as time goes by the GPX users who have switched to Mono coil use will start to investigate those areas with less signals and I strongly feel the bigger lumps will start to come to light.

I would also like to point out your experiences seem to be mainly based in the lower eastern part of the country, why not do a ring around of the various dealers Australia wide and get a feel for what has been found since the release of the 4500. I would suggest you start with Finders Keepers in Kalgoorlie because when I visited with them late last year the lumps were coming in thick and fast (Kal is very similar to areas like Wedderburn in Vic which in the past were more traditional DD only areas).

Both Monoloop and DD coils transmit a field in the same way, the field then excites a metal target which then produces a field of its own which is picked up by the receive circuit of the 4500, because the DD coil has a second winding the effect of ground close to the coil is largely ignored which allows the coil to run much quieter than a Mono (compared to a Mono coil used in Normal timings), they also generate their response where the two windings cross (in a blade type pattern), but this does not mean the transmit field or the resultant field created by the target is blade shaped, just the receive point of the coil, it is important you understand this because a lot of misunderstanding can result if you don't.

I find it very hard to believe you did not read Jim Fosters article before you posted this thread , at the time of the article the GP series were in vogue and the only way to effectivley deal with some highly mineralised areas was to use a large DD coil. The popularity of the DDs came more into main stream when Coiltek Manufacturing released their 7.3V regulated power supply (SD2200 days) which had the effect of increasing the magnetic field out of the coil, this then made the use of a large Monoloop very difficult in areas of high mineralisation so operators were forced to use large DD coils. Just to clarify a 10% increase in magnetic field strength from the coil does not mean a 10% increase in depth, at best it might equal 1% but in some instances it can actually mean less depth due to the increased susceptibility to ground effect (DDs included). Jim's justification for his main argument was that the SD2200 was never a good detector with Monoloop coils, this I strongly disagree with as I literally found hundreds of ounces with all the SD series machines and every other Minelab PI with Monoloop coils, as I said above it is more to do with the higher voltage power supplies that caused this myth. When the 3500 came out the big advantage was the medium GB speed which allowed operators to deal with a lot of the clay domes prevalent in the goldfields.

Yes a lot of gold came out of Vic with large DD coils, guys there specialised in them and became very proficient. Tony Mills at Gold Search Australia was one of the better operators with a large DD coil (he had this incredible handle bar arrangement to swing them with), a couple of other guys who spring to mind are John Campbell and his partner from Bendigo plus I am sure there are lots of other good operators who got to understand their set ups intimately. I can also just about guarantee that those same people are now mopping up with their 4500's in Enhance mode using Mono coils.

As noted above, the quieter the coil, the more likely the user is going to hear a target and so the DD coils are going to have a considerably advantage at the sacrifice of some depth. The question remaining is: (assuming the hypothesis of a DD penetrating the dual mineralization layers is factual or true) Would anyone abandon their Monoloops and then switch to a DD with their GPX-4500 in order to give themselves an edge with the deeper nuggets of more robust dimensions?

I think your remarks are based on a basic lack of understanding of what the new timings of the GPX series really do, if maximum depth is needed with a GPX series machine then use Normal timings with whatever coil you desire, if a DD is chosen then you have the same performance of the GP series machine with the added benefit of Gain control, Motion control, SETA etc etc, if a Monoloop is chosen the same applies. However if you want the full depth on some targets in high mineralisation that a DD coil in Normal timings is missing gold in then Enhance is the best option because you have the near full power of a Mono coil with a far quieter detecting experience than a DD could ever hope the achieve.

Please keep in mind the options available on a GPX series machine are just tools to achieve a result, at the moment the Enhance timings with a Mono coil are in vogue because that is what is producing the results for operators in areas they know and have done well in in the past (myself included), I have been doing this professionally for over 15 years now and I can assure you the pieces over 10 Ounces are in the minority so that tends to mean we target areas that are going to be productive numbers wise (read: more number of nuggets at smaller sizes because they are more prevalent).

Marco (name edited after I realised I mistakenly referred to Marco by his surname (Nero) not his Christian name) I admire the rapidity with which you have come up to speed on the subject, your enthusiasm is to be commended and your willingness to help others less knowledgeable is appreciated by all who read these threads (not to mention the pictures you have put to the web are excellent). Jim Foster is a member of this forum (goes by the handle of Granite I think) so maybe he can come in here and clarify the ideas that you have touched on in your quotes. Just as an aside; from what I can gather the PRO coils made by Coiltek draw more current but that does not mean they produce a larger magnetic field out of the coil, maybe Trevor from Coiltek Manufacturing can come on here and clarify also?

Lastly, a GPX 4500 is a much more refined version of the GP3500, that means the energy directed into the ground is the same and the receive is the same, just the 4500 has more control for the end user plus the added trickery of software enhancement in the form of Smooth and Enhance which then gives it that extra edge. If operators want the same performance perceived by the previous GP series all they have to do is forsake the Enhance/Smooth timings and use the Normal timings with the coil of their choice but with the added advantage of the digital platform, seems like a win win to me?

JP

Nero I find it strange you: had not at some stage seen Jims work prior to starting this thread. Jim Fosters article " witch coil" was the most discussed topic in my group of prospecting friends and acquaintances for quite some time .
I would have thought one of the pro nugget hunters or dealers you have been talking to
might have steered you in the right direction earlier.

Hi Adrian,
Nobody said anything. Even here. I'm sure the article was a popular conversation theme when it was published but people seem to have forgotten about it. And it's not so much the article that has me thinking but the mineralization layer issues which Jim raised. I was looking for a reason why the 4500 wasn't landing all the big nuggets all by itself. And why so many seasoned users were puzzled and still used an older SD for their bigger nuggets. Nobody else I spoke to seemed to make a connection either until I brought it up in recent weeks. The subject of DD Vs Mono is/was always a main theme for detectorists... for all sorts of reasons... but it appears that Jim's article might (possibly) shed some light on the reasons why the 4500 isn't the only detector preferred for the job.

JP's compared many similar coils in both Double-D and Monoloop side-by-side. He's very interested in the technical performance of coils and equipment and his considerations have a lot of relevance when it comes to the 4500. If i understood his reply above, he seems to feel that the dual mineralization barriers raised by Jim years ago should not be causing performance issues with the Monoloop on a 4500. And I'm inclined to take anything JP says about the GPX seriously because he's had a lot of experience with this detector in a lot of different geological locations. So perhaps Jim was wrong. If he was right, I'll have to go dig up a large DD coil and spend a lot of time running over my older haunts. Plenty of food for though though.

Cheers,

Marco

Jonathan Porter wrote:
....Just as an aside; from what I can gather the PRO coils made by Coiltek draw more current but that does not mean they produce a larger magnetic field out of the coil, maybe Trevor from Coiltek Manufacturing can come on here and clarify also?
JP

Hi JP
I missed this bit when I read your post earlier (sorry). Thanks for your kind words. I'm going to get it wrong plenty of times.... and sometimes I do. But I do find myself having to defend observations or even completely unrelated activities from some of the more aggressive board members around the globe. As you've no doubt noticed, some people are not open to discussion or the exchange of information. On this subject (Double-D's on Double Mineralization), I think there's something interesting here that was probably worth bringing up for discussion.

Now I've already phoned and spoken to Trevor earlier this week AFTER purchasing some Coiltlek coils for my own experiments. Whilst that conversation was private (sorry Goldpig) - he's otherwise a very polite and helpful person. The article written by Jim Foster in the 2004 GG&T magazine mentioned specific DD Coiltek coils and that they construct a "bigger and denser electromagnetic field" ensuring "better depth and sensitivity capabilities than a standard Double-D coil" based (in part) on a more complex winding - at a "20% more energy" sacrifice on battery power. Again, this information was in relation to a specific DD coil type... not all DD coils. And it came from Jim Foster at a time when he was probably working over at Coiltek.

FOOTNOTE: I hear JP and Trevor have been invited to Arizona for a get together with prospectors out that way in the USA. I read elsewhere that Trevor said that he's going to bring a few new toys over to show the Yanks. Sounds like there might possibly be some new coils in the works. (Big Goldstalker DDs??) Wish I knew. (seriously, I don't).

Cheers,

Marco

Hi All
A sd2200 with a 18 coiltek DD pro pulls a 35 ouncer at 32 inches.So if what I read above is correct A 9 inch Mono would do the same.Seriously ?
Cheers Dig

Dig24crt wrote:Hi All
A sd2200 with a 18 coiltek DD pro pulls a 35 ouncer at 32 inches.So if what I read above is correct A 9 inch Mono would do the same.Seriously ?
Cheers Dig

My younger brother found a 30 odd ounce piece at 30 plus inches with the Minelab 8" mono on an SD2100 back in 1997.

JP

Jp
Am always open to conversion.Will do some serious inground testing tommorow.Will run my Gp3000 and Gpx4000 with coiltek 14 mono against Sd2200 with Coiltek 24 inch DD.Will post results tomorrow night
Cheers Dig

If I may be so bold Dig ... could you note and post the settings you use, very interested to know where you achieve best depth, in

particular with the 3000. Have done some comparative tests myself with interesting but not conclusive results. cheers aa

Jp
Am always open to conversion.Will do some serious inground testing tommorow.Will run my Gp3000 and Gpx4000 with coiltek 14 mono against Sd2200 with Coiltek 24 inch DD.Will post results tomorrow night
Cheers Dig

The response on a DD might sound more obvious over a small mono as the receive is different, in the case of large nuggets with small monos I am pretty sure most are ignored as ground noise (very broad response) not because the coil is incapable of responding to it whereas a DD generates its response through the interaction of the two windings so can sound more defined (this is all relative to ground signal reduction of a DD coil as well).

JP

nero_design wrote:
Hi JP
I missed this bit when I read your post earlier (sorry). Thanks for your kind words. I'm going to get it wrong plenty of times.... and sometimes I do. But I do find myself having to defend observations or even completely unrelated activities from some of the more aggressive board members around the globe. As you've no doubt noticed, some people are not open to discussion or the exchange of information. On this subject (Double-D's on Double Mineralization), I think there's something interesting here that was probably worth bringing up for discussion.

Now I've already phoned and spoken to Trevor earlier this week AFTER purchasing some Coiltlek coils for my own experiments. Whilst that conversation was private (sorry Goldpig) - he's otherwise a very polite and helpful person. The article written by Jim Foster in the 2004 GG&T magazine mentioned specific DD Coiltek coils and that they construct a "bigger and denser electromagnetic field" ensuring "better depth and sensitivity capabilities than a standard Double-D coil" based (in part) on a more complex winding - at a "20% more energy" sacrifice on battery power. Again, this information was in relation to a specific DD coil type... not all DD coils. And it came from Jim Foster at a time when he was probably working over at Coiltek.

FOOTNOTE: I hear JP and Trevor have been invited to Arizona for a get together with prospectors out that way in the USA. I read elsewhere that Trevor said that he's going to bring a few new toys over to show the Yanks. Sounds like there might possibly be some new coils in the works. (Big Goldstalker DDs??) Wish I knew. (seriously, I don't).

Cheers,

Marco

I have found the whole subject very interesting and I am sure others have too, I have to admit though the original article kind of ruffled my feathers the way it was written (and it seems others too) because a lot of what was said I disagreed with. I haven't heard about an invite to Arizona with Trevor, but I am sure he will have a wonderful time of it once he gets his head around pronouncing Fajitas and Saguaro correctly. I loved Arizona while I was there and saw a lot of potential of good nugget finds in some of the areas I was shown.

The beautiful thing about human beings is we all have an opinion, so long as we can continue respecting each others point of view without sliding into back biting then forums like this will continue on providing a firm base for those who are willing to learn. I also suffer from the odd DIG on various forums and sorry to say I tend to go into discussions blinking wildly these days which tends to impact on some of my wording when I start to jump at shadows, if I have come across as a little grumpy at times please forgive as being a bit of a veteran of gold prospecting forums has left me with a permanent squint.

Regards

JP

Hi All
After four hours of inground testing will post info under new topic Small Powerfull Monos.Ps My Mate forgot to bring his 24dd which I was going to match against my 14 mono.But in a way that was good as we could try and recreate his find with the 18dd and so I called into coiltek and Andew and Dougy lent me a commander 11mono.Go to new topic Small powerfull Monos.Its Data that has changed my mind.I will post an intro then the data as I still havent managed to get long posts in one hit

G'day,

Jonathan Porter wrote:
Both Monoloop and DD coils transmit a field in the same way, the field then excites a metal target which then produces a field of its own which is picked up by the receive circuit of the 4500, because the DD coil has a second winding the effect of ground close to the coil is largely ignored which allows the coil to run much quieter than a Mono (compared to a Mono coil used in Normal timings), they also generate their response where the two windings cross (in a blade type pattern), but this does not mean the transmit field or the resultant field created by the target is blade shaped, just the receive point of the coil, it is important you understand this because a lot of misunderstanding can result if you don't.JP

I've been doing a little research on the properties of coil geometry related to field density.
I came across some papers related to the application of Helmholtz coils to different applications which gave some interesting facts. It seems that a square coil configuration has a much more uniform field than a circular one. Of course you wouldn't use a Helmholtz setup on a simple metal detector but this unrelated application went onto show that windings shape is doing much more than most people appreciate.

Looking back on what Eric Foster and Alan Westersen have to say about rectangular coils dispels what I first believed, and proves what I said was wrong. Rectangular (and probably tight ellipticals too) do not produce a stronger field density at depth than an equivalent sized mono, but, they do have a relative higher density within a medium distance. This is in harmony to what Steve Gholson found in his coil test measurements, reproduced earlier in this thread.

However, according to the aforementioned designers rectangular monos do produce a blade or wedge like field, and this probably explains the higher density at medium distance. The blade effect on a DD is not only a product of the cross over of the windings but the bringing together of the windings, because the blade effect is also evident on rectangular monos, and not just DDs.

So the images of the DD as a blade and the Mono as a cone are correct, nothing new there, just watch that coil sweep overlap though, or the mono advantage will be readily lost because it may not be as much as we imagine.

The extra field uniformity found in a non-circular coil is very intriguing. In the application above it was used to detect eddy currents in scratches and cracks on the surface of metal parts, precursors to metal fatigue. We may see new things in coil design yet, Eric has always said there's still much to learn about the most simple part of the detector, the antenna. Without a proper antenna, the most expensive radio receiver is no better than a Dick smith special. If only we could see with our eyes what was really going on.

Jonathan Porter wrote: Just as an aside; from what I can gather the PRO coils made by Coiltek draw more current but that does not mean they produce a larger magnetic field out of the coil, maybe Trevor from Coiltek Manufacturing can come on here and clarify also?
JP

I'd like to know the answer to this one too, because the 'Q' of a Minelab PI coil is fairly constrained the Magnetic field is propotional to:
B = relative permeability x (number of turns / length) x the CURRENT.

Proprietary information?

Cheers
Grey

Hi All/ GP
Dit anybody ever used one off thoose coils on your gpx, they are from Detech and is a butterfly s.e.f.
SEF = "Symetrical Electromagnetic Field" coils. These coils have been 3 years in research and development. Extensive field tests on all types of ground have shown that over a standard D/D or concentric coil they have MORE depth, BETTER ground balance, BETTER stability, BETTER pinpointing compared with D/D, BETTER target I/D
S.E.F. is a patent protected system that combines the best features of D/D and Concentric coils in one coil. They open up a new world of metal detecting.
Personely I use them on my e-trek.
I now they make them in 3 sizes for the gpx range, and sinds I owe a 4000, I like to hear if some off you ever used one.
Meaby someone can tel me, technicaly, wath this, Symetrical Electromagnetic Field, means.
Kind regarts from Holland
Gerrit

G’day Gerrit,
I knew these coils were available for the VLF/IBs but didn’t realise you could get them for the PIs. I’m not sure what they’re all about, but for the IBs the coils could be tuned near to resonance much like the Nexus coils, which originally used a Double O coil.
I suspect the more circular coil design produces the “Symetrical Electromagnetic Field” as opposed to the DD, which may cause more field cancellation.

If the gains are due to resonance then there will not be as much performance increase on PIs as with the IBs, and I suspect that goldfield mineralisation will reduce this even further. They’ve certainly taken the coin and treasure fraternity by storm.

I too would like to hear if anyone downunder has tried them on a PI and how they performed?

Cheers
Grey