So lately I've been noticing when out detecting with Kiwijw his GPZ will find tiny bits of gold quite deep, that weigh less than a shotgun pellet and after often smaller in size, but appears to miss a majority of the pellets very close to the surface, where most pellets call home but not much of the gold calls home.
At this one particular spot we went to the other day I decided I'd just use my Equinox and 6" coil, and he used his GPZ with it's 14x13" coil.
I had at least 60 pellets by the end of the day, he had about 15 or so, there was no exact counting but the difference was obvious. We were both on the exact same ground, a patch about 20 meters by 20 meters at most as a guess. We both covered a majority of the same ground, starting at one side each and worked our way over to the other person's side. When I got to his side I was digging pellets like mad, even in his dig holes where he managed to get bits of gold out of like the GPZ hardly noticed the pellets. I was puzzled by this as I was wondering how he found the bit of gold no bigger than a pellet in amongst the 6 pellets I got out of his dig hole. John did mention he often scapes away the surface soil and if the signal continues down he'll dig it thinking it's possibly gold, is this the reason he doesn't get as many pellets or does the quirk go deeper?
This has had me stumped for some time and John wasn't sure why he digs less pellets as he is getting gold smaller than a pellet very often with the GPZ but we both put it down to the HF VLF sensitivity. It was seeming like the GPZ just loves gold.
I've come to the conclusion from my research this afternoon it's due to coil size more than anything. I found a Treasure Talk on the Minelab site that had the information I had been searching for.
Here are the key statements from it that make sense to my dilemma and in red are the key points,
We all basically know that large coils detect deeper than small coils. Why is that? The relationship between the size of a coil and the detection depth depends on a number of factors related to both the construction of the coil and the particular target itself. Starting with the coil itself, to compare the relative sensitivity of a large coil to that of a smaller coil, we need to determine the strength of their respective transmitted magnetic fields and the receiver sensitivities. These can be calculated by applying the Biot-Savart law, which describes the magnetic field produced by an electric current.
For this, we need the exact geometry of each winding, the number of turns and the size of the wire. Once we chose a shape, the number of turns and the wire size depend upon the constraints we apply to the design. For example, we usually require the inductance and the resistance of the transmit winding to be the same regardless of the size of the coil, such that the transmitter electronics operates optimally with any size coil. This leads to a reduction in the number of turns as a coil gets larger or conversely an increase in turns as a coil gets smaller. Based on this, we can understand that a small coil with more turns creates stronger localised fields, while a large coil creates a field that is weaker in the immediate vicinity of the coil, but decays more slowly with the distance from the coil. Thus, further away from the coils, a larger coil has a stronger field.
We now need to consider the target, for example a particular nugget: if we use one coil as a reference, at what depth can we just detect the nugget with it? Once we know this, we can utilise the previously calculated relative sensitivity to determine at what depth we can detect the same nugget with the other coil. Doing this for a range of nuggets, we find that small coils are better suited for very small nuggets near the surface, while the larger coils are better for intermediate and deep nuggets.
Is this the reason the GPZ is missing the tiny pellets near the surface but getting bits of gold amazingly deep that are smaller and lighter than a pellet? I have noticed a similar quirk on my GPX 4500 with it's 14x9 EVO coil where it will miss a lot of shotgun pellets but find tiny gold deeper but it doesn't seem as pronounced as the GPZ but again that's an amateur operator vs a professional. I will try emulate this on an air test when I get the chance but I can't run my GPX around my property as there are power lines nearby that it really doesn't like. Are the pellets KiwiJW is getting the ones that have somehow made their way deeper and he's missing the more shallow pellets? I understand John's a very skilled GPZ operator and it could just come down to operator skill levels but if I am getting multiple signals with 4 shotgun pellets and 1 bit of gold all under my coil at once I'd just be confused and dig all 5 things, meaning I'd end up with all the pellets in my pocket also. John seems to manage High Frequency VLF performance out of his GPZ but misses a lot of the downsides of using a HF VLF, the tiny nuggets he can find on that thing and the depth they come from are just mind boggling. It has to be seen to be believed.
Any help on clearing up this mind boggling quirk would be appreciated.
A big thanks to Phil Beck for the info I've quoted from his Treasure Talk for this post. Here is a link to the entire Treasuretalk https://www.minelab.com/anz/accessories-1/gpz-19-1
By Steve Herschbach
Published on Apr 12, 2018 by steelPHASE
In this video I talk about the settings on detectors, aiming mainly at the Minelab GPX, SDC and GPZ series. My aim is not to tell you what settings to run, but rather give you the knowledge to make informed decisions on settings out in the goldfields. Now I am no Einstein so I do stumble and fluster in a few spots but I have edited in some text to make things clearer (hopefully).
By Stephen newell
You have a new brother in the family.
Special thanks to each of you that gave me advise on this decision. Extra call out to Brian for checking the Zed out for me in person. Lot of respect for the tight group we have here.
Feel free to drop tips and hints in here or pm. I have read so many threads my eyes are bleeding lol. Come on 1 oz Nug.
JW looking forward to nugget find sharing with you.
Fred low and slow, Check!
Nenad hit me up with more of those tips, so I can pay for this thing lol lol.
By Sourdough Scott
Came across a post on Facebook, about this gentleman testing a few aftermarket coils. This really opened my eyes this morning. I was under the impression that the Minelab coils were chipped and no aftermarket coils could be used. If this was the case? Why did they wait so long? The smaller coil would suit my needs just perfectly. http://golddetecting.forumotion.net/t26022-gpz-18-coil-test-report
Ok I am just passing this link on, as I found it of interest I feel others on DP perhaps may be interested. This is the first I`ve heard of such thus have no idea if it is for real but I hope it is so, we want lighter, smaller, even bigger coils for our Zs. and as we`ve experienced, with the aftermarket coils for SD-GPX series, increased finds, why not for the Z?