GPX 17 Coil Commentary

[Steve Herschbach]

20 hours ago, VicR said:

When i used to work in Papua New Guinea the local saying was "expect the unexpected" which i think applies to metal detecting - you continue to learn and be surprised.

Yesterday three of us decided to test the GPX6000 with 11 inch mono, 14 inch DD and 17 inch mono and GPZ7000 with 14 inch stock coil at a local test site that has targets in situ for a number of years.

We were particularly interested in the deepest target that was buried at 400mm (about 16 inches) representing a large deep nugget.

The 7000 could not detect it in difficult setting only in normal setting -  i was told by the owner of the 7000 that normal is not his preferred setting.

The 6000 could not detect this deep target  with the 11 inch and even the 17 inch (we tried all settings)   - but get this - the 14 inch DD with EMI cancel on could hear it as good as (if not better) than the 7000 in normal mode . We were not expecting this result.

We though about this for a moment and the only conclusion we could come to was that the high environmental EMI was affecting the performance of the other coils & favoured the DD coil.

Any other thoughts as to why we had this result?

 

DD coils back in the day were the go to coils on early Minelab SD and GP PI detectors, and it was not until the GPX series that a switch was made more towards mono coils, as the GPX design tended to favor those coils. However, DD coils do have inherent ground canceling abilities that mono coils lack, and in the case of the GPX 6000 extra EMI canceling properties (or salt, depending on the setting). Your guess as to what produced the result is as good as any, and shows how preconceptions based on simple tests can run afoul of reality. People have generally been writing the DD coil off as a no go due to simple depth comparisons in ground where the DD coil has no advantage. But in this case it shows that the GPX 6000 DD coil does have a place, and can produce good results. Areas with high mineralization, salt, and EMI, alone or in combination, may benefit from use of the DD coil. It's not that the DD coil goes deeper, it is that it loses less depth than other coils in certain adverse conditions. It's being included as the option in the U.S. and Oz for that very reason.

[Jonathan Porter]

EMI/Sferics are depth killers, this is why the GPX5000 came with the Stabilizer setting set on half way, however the GPZ7000 is far better for Sferic noise than the previous GPX machines but can suffer from it nevertheless. Salt is also a depth killer, in a lot of ways it is far worse than EMI due to swing speed having such a variable interaction with the noise generated (from micro variation to excessively loud depending on the location and speed of the coil).

400 mm is not that deep relative to what both these machines can achieve, I’ve dug stuff not much bigger than a gram at similar depths in the right locations, so I would say there was a combination of Salt and EMI present at the test site.

IMHO the Salt mode has better outright depth compared to Cancel assuming EMI is minimal (Cancel is the default setting when a DD coil is attached to the GPX6000) , Salt mode does have some noise cancelling characteristics so if the EMI/Sferics is not too bad then I will use that first before going to Cancel mode. There is a good reason for having a 14” coil size as the default DD coil with the GPX6000, essentially the Rx of GPX14DD it is a 7” Monoloop so the sensitivity to surface targets is off the charts but also the 14” overall size brings up the depth which is compromised when using a Cancel or Salt Tx/Rx format compared to a traditional DD receive or compared to to using a dedicated monoloop coil.

When using the DD, small targets are most sensitive on the left hand edge of the coil with some sensitivity in the centre like a traditional DD, for deep targets the whole left winding is alive with the bulk of the deep signal coming from the centre. It helps with visualisation to think of slower broad deep target noises coming from the centre of the coil. 

JP

[VicR]

7 hours ago, afreakofnature said:

Hey Vic (and G), I have a couple questions.  Vic, what was the target size?  Vic and G, do you think that because the DD should smooth out the threshold/EMI/ground that you were able to hear subtle repeatable targets or was the threshold still a little chattery but the target signals was obviou

Hi FON,

I was told the target was a largish piece of lead (maybe an oz) at 400mm, it & other targets was buried many years ago by a very experience local detectorist - now i have recovered a .5gram nugget at about this depth (same as JP) at Tibooburra NSW (desert country) with the 11 inch coil so i was very  surprised i could not pick it with the 11 and 17 inch mono coils on the 6000 but the DD 14 inch signal was clear as a bell. We had 2  6000 machines with us and both achieved the same results.  Also the 11 and 17 inch coils did not sound overly unstable - only when you put them up in the air could you tell you were in a high EMI area. After the testing we went detecting - I kept the DD coil on and could go right up to an electric fence without hearing the pulsing  - my mates with the 7000 and the 6000 with mono coil had to stay well clear of the fence as it made their machines very unstable.

JP and Steve - thank you for sharing your insights and contributions. Makes me realise i need to understand the environment and ground better to make the correct coil & setting choices. Nothing beats experience.

Also i have a unsubstantiated but credible rumour (apologies if this has already been report) that Minelab has already given a 3rd party permission to design and make coils for the GPX6000.

[dig4gold]

Are you not specifically referring to higher mineralised grounds. What about very mild ground & DD verse Mono. You still think the DD has it over a mono? Cheers ? 

 

[MSC]

JP, I have had tennis elbow a couple of times in the past, I purchased an elbow "sleeve" I guess you would call it. It took some time but eventually ( within months ) it healed and have not had problem's for a long time.  The tightness and heat to the elbow it provided is what helped me. Hope you get better. 

[PhaseTech]

On 8/18/2021 at 9:29 AM, VicR said:

Hi FON,

I was told the target was a largish piece of lead (maybe an oz) at 400mm, it & other targets was buried many years ago by a very experience local detectorist 

 

Hi Vic, myself and two other customers set up that test bed initially to test a range of coils. It was just supposed to be for a quick comparison, so we did some crude measurements and target size estimations, most of which have since been forgotten. But the targets have surprisingly remained buried for many years, and I have used the test bed so many times now that I know how different machines respond, so I have avoided the temptation to dig them up and record the weights and depths accurately. I still may do that some day. There is a hackly bit of lead, about 2.5g that is also buried quite deep, and this is a great "challenge" to newbies with any machine, as coil control and positioning is critical. 

From the best of my memory the largest target is a 7g lump of lead, and depth was initially at about 45 cm. The ground to the side of the hole is quite mineralised clay, so mono coils in Normal timings often perform poorly. 

The best combos I've found on this target are: 

Any GP series machine with Detech 15" DD coil

GPZ7000 in General/Difficult 

GPX4500 in Enhance with 12" Evo

GPX5000 with 17x13" Evo (Enhance or Fine Gold). The Commander 15x12" also responds well 

The 15" Evo & Detech 15" super deep also gets it easily, but the signal response is broader, not as crisp as the 17x13".

The 6000 with 11" mono really struggles. The 14" DD in Normal and Salt is the best combo. 

Ground moisture and EMI levels vary the results slightly, which can be quite interesting. 

[Northeast]

6 hours ago, PhaseTech said:

The 14" DD in Normal and Salt is the best combo. 

Very interesting.  Thanks Nenad  ?

[Steve Herschbach]

On 8/18/2021 at 5:06 PM, dig4gold said:

Are you not specifically referring to higher mineralised grounds. What about very mild ground & DD verse Mono. You still think the DD has it over a mono? Cheers ? 

 

I prefer the 11” mono over the 14” DD in low mineral ground, but advise doing your own comparisons. Ground varies and I’d never advise taking advice over actual tests you do yourself.  In high magnetite locations, the DD coil will easily detect targets that the 11” mono misses entirely, especially larger items at depth. So there is a point where the 14” DD does pull ahead of the 11” mono, but I do not know where that line occurs. The only way to know for sure is do some test target comparisons in the ground in question. However, for more depth over the 11” mono in mild ground, the 17” mono is probably your best solution.

The 17” mono versus 14” DD in extreme ground is interesting. The DD is easier to handle as far as ground response, and my general preference at this point in extreme ground, but I can get the 17” to work. It benefits from being lifted off the ground a bit, and the tones may reverse compared to the DD. In either case sensitivity may need to go to manual 5 or lower, or Auto. When I say extreme ground, I mean it, like 50% magnetite content or higher. Think decomposed serpentine ground, or beaches. The kind of stuff you drop a magnet, and when you pick it up you have a golf ball of magnetite. Not sure how this translates as far as maghemite in Oz. In the U.S. our culprit is usually magnetite, in Oz it’s more often maghemite causing issues. As usual any commentary I make is more U.S. specific, though may offer clues for the Oz users. The following helps shed light on this basic difference between the U.S. and Australia.

From Bruce Candy at https://www.detectorprospector.com/files/file/52-metal-detector-basics-and-theory/:

"In geologically new soils, the degree of mineralisation is usually weak, except for some volcanic soils. These relatively new soils are commonly found in North America and Europe (from glacier scrapings during the last ice age and mountain erosion etc). In contrast, surface soils which have remained surface soils for a long time often have high mineralisation, because the action of water, over a long period, causes iron compounds to migrate to the surface. For example, Australia has old soils, having had no glaciers recently or significant mountains to be eroded. Some volcanic rocks or sands, known as black sands, may be highly mineralised and are found, for example, in a few USA mainland and Hawaii areas. These black sands (or rocks) are made of mostly magnetite, an iron oxide called ferrite. These typically produce almost entirely X signals, and almost no R. They are heavy, that is they have a high density, and can be identified because they are strongly attracted to a magnet. Small roundish magnetite/maghemite pebbles (a few mm in diameter) are also attracted to a magnet. These, for example, may be found in many Australian goldfields, but do produce significant R signals. Thus, USA goldfields are typically different from Australian goldfields:

• The USA soils are mostly mildly mineralised but in some areas may contain either nearly pure magnetite black sands or rocks, which  are problematic for metal detectors as they have very high X components (strongly attracted to magnets).
• Australian goldfields have highly mineralised soils, but very few black sands or rocks that contain nearly pure X magnetite. The magnetic materials are in the forms of magnetite-rich small pebbles and rock coatings, clays and general “sandy” soils. These all contain magnetic materials that produce high levels of X signals as well as R. The ratio of X and R is random, and the R component arises from extremely small magnetic particles called superparamagnetic materials, which are discussed below."

More details on mineralization:

https://www.detectorprospector.com/forums/topic/1599-gb-numbers-mineralization/?do=findComment&comment=19002

[Jonathan Porter]

Steve as always a very informative post from you and interesting insights into the GPX6000 in your neck of the woods. The 6000 behaviourally is quite different when operated in Normal compared to Difficult so different in fact you could consider it to be almost 2 detectors in one. 

As an example I was working some benign ground recently but had just come from some adjacent variable soils so was still in Difficult mode (using the 17” mono). The overall soils in the area where derivative of granite contacts so I was using the Auto+ mode which really ramps up the sensitivity if the soils will allow (much higher than full manual mode). I was noticing the detector was becoming increasingly jittery the further I moved away from the red stained ground into the paler soils which got me thinking as I was expecting the reverse considering Difficult is so good at ironing out mineralisation, so out of curiosity I switched to Normal, when I did that the whole detector just went silent and smooth. 

The only thing I can put it down to is the inbuilt gain of the difficult timings is much more aggressive than Normal to lift the left over target signals after the ground has been cancelled out, as such combined with the increase in sensitivity in the quieter soils from using the Auto+ the sensitivity of the detector had elevated to the point where the noise floor of the machine was down to the electronics level so Sferics, EMI and some electronics noise was getting into the signal train (all supposition on my part). 

The other thing I have noticed is the channel flipping is much more evident in Normal mode, so you have to be careful on those dipping low toners as they can herald a really nice piece at depth. I have not focused much on chasing deep signals with the 6000 as I have the GPZ7000 for that, but with care and attention there is a huge amount of subtle information with this detector which will improve with time spent in the field.

JP