Double-D coils on the GPXs

[nero_design]

I have a question that my fellow Australian Prospectors repeatedly seem to dodge when asked. I wonder if our friends in the US or elsewhere could offer a comment or even an opinion based on their own experiences.

Double-D coils were always the popular choice with non-GPX detectors (SDs & GPs) for a number of reasons. The earlier models were unstable with Monoloops and so Double-D's were always the coil of choice. Before the GPX's came along, the vast majority of larger nuggets were being retrieved via Double-D's. Between 2003 & 2004, not a single large nugget was reported to have been found with a Monoloop in Australia's "Golden Triangle" (according to our Gold/Treasure Magazine here in 2004). One of the best reasons to use a Double-D was/is when dealing with a mineralization layer. Often there are two, one on the surface and one over the subsurface gravel layers deeper down. Whilst the GPX's excel with the use of a Mono to give them more depth than a Double-D, the signal from a Monoloop can be reflected or even absorbed by the secondary mineralization layers, effectively concealing nuggets... especially LARGER nuggets.

Perhaps this is why the older machines (forced to run with a Double-D) are finding larger nuggets at depth compared to the GPX detectors? I am finding the subject intriguing and would like to hear from any Nugget Hunters who have an opinion either way over the use of Monoloops & Double-D coils. I was always of the opinion that the GPXs would shine with a Monoloop (which they do) but recent research is indicating that it's the older detectors using Double-D's that seem to be finding the larger nuggets. Since nearly everyone is running their 4000 or 4500 with a Mono these days (because they can), it might account for the strange inconsistency if they are walking over nuggets that hide below the secondary mineralization layer (usually larger, heavier nuggets too).

Who has used a Double-D with a GPX and what were your thoughts or experiences?

Those of you with earlier models like the SD often report larger nuggets... Are you using a DD coil on that detector?

Thanks in advance,

Marco

[nuggethunting]

Hello Marco,

Good question. I can tell you many professional nugget hunters were finding gold nuggets with Monoloops on those ground conditions. Many of them prefered Monoloops even though the ground was noisier and they kept their searchcoils off the ground a bit to ignore the mineralization noise.

I agree with you that the majority of the nuggetshooters were using DD's on that mineralized ground, but not all of them.

I'm able to hunt locations with a Monoloop on the GPX series where I couldn't before with a GP series. I would have to use a DD or DD Pro coil on the GP series to hunt many of these locations, but my GPX with a Mono just purrs through the spots with no ground noise.

Take care,

Rob Allison

[Bob777]

Hello nero_design,,,

This is bob777. I have only been using my machine for a year and ahalf. I have a SD2200 2v. For deep and shallow nuggets which coils do you think would work great with my SD2200 2v? Your advice is greatly appreciated. Thank-You, for your reply...bob...

[AZNuggetBob]

Hi Marco Im a big mono coil user and always have been. one of the reasons the dd coil does so well in the golden triangle is they have some of the hottest ground in the world there. even here in the U.S. there are hot ground areas were the dd coil will out shoot the mono. the mono also picks up iron hot rocks more than the dd so your going to be digging more false signals and iron hot rocks but in medium to mild ground conditions the mono will out shoot the dd especially on larger deeper nuggets. Ive found more larger deeper nuggets with my mono's than the dd"s. you just have to decide how hot is too hot for the mono coil . AzNuggetBob

[nuggethunting]

Hey NuggetBob,

Very good advice about the Mono vs. DD coils. I agree, on the older SD's and GP series a Mono coil can pick up a bunch of ironstones and volcanics, which can make detecting hell if there are tons of them. A few here and there you can just kick them out of the way for the most part.

On the newer GPX series, they have several "timing features" such as Sensitive/Smooth the GPX-4000, and "Special/Smooth & "Enhance" on the GPX-4500 to knock out a good portion of the ironstones and volcanics even if you're using a Mono coil.

On mild to medium mineralization a Mono coil will normally get a bit more depth if you're playing close attention to signal responses. It will also be a bit more sensitivity to small gold and you can use the outer edge to pinpoint targets. When you start getting into tons of ironstones, volcanics or very mineralized ground conditions, a DD might beat out a mono due to the DD havingbetter mineral immunity on hot ground (less sensitivity vs. mono).

Like Nuggetbob, I've always been a bit fan of Mono's when you can use them!

Hope this helps,

Rob Allison

[nero_design]

Thanks for taking the time to reply everyone. Someone recently suggested to me that the reason the GPX isn't hitting nearly as many large nuggets Vs older detectors may be related to the use of the new Smooth Timings. Whatever the reason, it appears that others are starting to notice that the success rate with older model detectors using Double-D coils is indeed proportionately higher.

As for mineral barriers, the concern that GPXs might be unable to read a target below the second barrier only applies to areas where twin layers occur and a Mono-loop is employed for the task. For single layers where a deeper layer of mineralization doesn't exist, I'm sure the Monoloop will have the advantage of depth over a Double-D. Back as early as 2003, a few people began to question the difficulties of Mono-loops with deep targets when gravel wash was encountered. It was explained then that the secondary layer (where nuggets often lay in or below) contained a high degree of mineralization that seemed to mask the larger nuggets from the Monos but not the Double-Ds. A few Australian prospectors let me know this week that this might explain why they missed nuggets with their Ellipticals and it occurred to them only when they re-visited areas again and unearthed nuggets with their Double-Ds at a later date.

I made an image (below) to try to explain what I mean by dual layers of mineralization. Again, this subject relates only to GPXs used Monoloops on soil which had dual layers of mineralization.

Hello nero_design,,,

This is bob777. I have only been using my machine for a year and a half. I have a SD2200 2v. For deep and shallow nuggets which coils do you think would work great with my SD2200 2v? Your advice is greatly appreciated. Thank-You, for your reply...bob...

Hi Bob777,

I think detector coils are a little like camera lenses in that there's never a single coil that does it all. For shallow ground I try to use a more sensitive coil although I prefer to just use the one coil. Since smaller nuggets are far more common than large ones, I try to research the area I wish to detect first to determine if larger nuggets have ever been found there before. A smaller coil on your SD2200v2 is going to find more smaller nuggets. The larger coils will find medium to large nuggets at depth but will usually ignore the sub gram nuggets altogether - unless they're sitting near the surface.

One of the best coils for smaller nuggets and for searching areas like creeks (where the bedrock is closer to the surface) is the

Coiltek DD 10" Elliptical Coil. This is almost identical to the Commander DD 10" Elliptical coil and I would be more inclined to lean towards the commander for slightly more output (marginal) and perhaps a little less power consumption - if that's important to you. If you just wanted to get a single coil, I believe the 14" DD Coiltek Super Detector Coil is really impressive and hard to beat. It will find deeper nuggets and larger ones as well. Should be good on smaller nuggets in the subgram category although not too small. Anything close to .50 of a gram might be too small for this coil. Most of the people I've spoken to suggest that anything like a gram at depth might be too small with that coil. I asked a couple of experienced detectorists what their suggestion would be for a deep seeking coil for larger nuggets and they said Coiltek' Mini-UFO (17" or 24") would be ideal. Might be worth asking around on your end there. I believe the Goldstalkers and Nuggetfinders are designed for the GPX series and may not be as suitable on the SD2200.

Hope this helps a little. I'm sure someone else can contribute here.

[AZNuggetBob]

Hi nero_design I assumed maybe what you may be referring to when you mentioned a double layer is what I refer to as a cap layer caused by surface erosion concentrating heavier hot rocks on the surface and creating a surface wall. sometimes you will encounter other types of surface caps such as a later volcanic overlay. both of these type deposits are not the best place to use a mono coil. If the cap rocks are scattered you can shoot in between them or kick them aside. also the primary layer is on the bottom, the secondary is on top. I think its important to look at it from that point of view.

AznuggetBob

[dave wiseman]

Aren't you the same Nero from Jack Lang's old forum?

[nero_design]

Aren't you the same Nero from Jack Lang's old forum?

Yes I am. I heard jack retired and his forums were discontinued by the new owners of his business.

But yes, I'm the same Nero.

[Jonathan Porter]

I have a question that my fellow Australian Prospectors repeatedly seem to dodge when asked. I wonder if our friends in the US or elsewhere could offer a comment or even an opinion based on their own experiences.

Double-D coils were always the popular choice with non-GPX detectors (SDs & GPs) for a number of reasons. The earlier models were unstable with Monoloops and so Double-D's were always the coil of choice. Before the GPX's came along, the vast majority of larger nuggets were being retrieved via Double-D's. Between 2003 & 2004, not a single large nugget was reported to have been found with a Monoloop in Australia's "Golden Triangle" (according to our Gold/Treasure Magazine here in 2004). One of the best reasons to use a Double-D was/is when dealing with a mineralization layer. Often there are two, one on the surface and one over the subsurface gravel layers deeper down. Whilst the GPX's excel with the use of a Mono to give them more depth than a Double-D, the signal from a Monoloop can be reflected or even absorbed by the secondary mineralization layers, effectively concealing nuggets... especially LARGER nuggets.

Perhaps this is why the older machines (forced to run with a Double-D) are finding larger nuggets at depth compared to the GPX detectors? I am finding the subject intriguing and would like to hear from any Nugget Hunters who have an opinion either way over the use of Monoloops & Double-D coils. I was always of the opinion that the GPXs would shine with a Monoloop (which they do) but recent research is indicating that it's the older detectors using Double-D's that seem to be finding the larger nuggets. Since nearly everyone is running their 4000 or 4500 with a Mono these days (because they can), it might account for the strange inconsistency if they are walking over nuggets that hide below the secondary mineralization layer (usually larger, heavier nuggets too).

Who has used a Double-D with a GPX and what were your thoughts or experiences?

Those of you with earlier models like the SD often report larger nuggets... Are you using a DD coil on that detector?

Thanks in advance,

Marco

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).

I should also point out that I have used Monoloop coils almost exclusively for the past 14 years with nuggets up to 83 ounces in weight in recent times, just because GG&T doesn't publish an article showing a large nugget does not mean they are not being found.

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. Please remember it is all relative, the noisier the ground the greater the need for a DD coil or a set of timings with a Monoloop coil to deal with the noise, if no noise is present or if it is minimal then I suggest you stick with the Normal timings or even the Special Xtra (old sensitive) timings.

[AZNuggetBob]

Hi JP, Great post and very well explained. Personally I find mono's a great advantage in the right situations Ive seen them produce more nuggets from hard hit patches including patches that have been pounded with DD's. Monos's in hot ground situations require more user skill, experience and patience than the DD'S and that is why Ive always referred to the DD's as comfort coils, easier to use and more stable but the pay off with the mono's can be well worth the trouble. BTW JP Great DVD's Take Care AzNuggetBob