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Steve Herschbach

Fisher Impulse AQ Discrimination Explanation

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Steve,

Absolutely brilliant post.  I think most of us beach hunters would agree with you that what we are hoping for is that the AQ hits that "sweet spot" like no other machine.  How do you think it will handle platinum?

Bill

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Good question. Platinum rings can read real low, and smaller ones down in the foil range. I would not be shocked if some platinum gets lost with the AQ discrimination setting. The talk focuses on gold for a reason. Like I have mentioned in another post getting our hands on this machine and doing a lot of target testing will be critical to determine where the lines are drawn and if they can be moved at all.

Let's face it, marketing will talk about what the machine does well and will let other stuff go unmentioned. We will have to figure out the details ourselves. In a nutshell, where are the mid-range cutoffs both high and low? And what trash is still going to show up in the mid-range? I have an idea what to expect but only actual use will prove the reality of it all.

There are bound to be people who just freak out over the idea of missing anything. The fact is unless you are using a high power PI and dig every target we all miss stuff. The more discrimination you employ, the more inevitable it becomes. For me it is purely a practical matter involving how much time I have and in some cases how many targets I can physically dig. Moving the odds in my favor by eliminating worthless digs at the cost of some possible good targets missed is one of the most important strategies we employ in metal detecting, and getting it wrong too far in either direction can be costly.

I have often used VLF discrimination to eliminate zinc penny and higher targets while park detecting for rings. People will say "yeah, but some rings read like zinc pennies". Yup, that's true. But rather than fill my pockets with coins I prefer to concentrate on the mid to low range where the odds are better for rings and save a lot of time digging coins I am not looking for. On the very low end I may also pass on the real small foil and can slaw bits. This is what the AQ is also doing.

It sounds like lots of time has been spent optimizing where to put the cutoff points and I am going to trust the testers have chosen wisely. Nothing is perfect. I am all in for a PI that concentrates on the main sweet range while eliminating a lot of deep trash digging, the bane of any PI detector. And if the trash level is low anyway, just switch to all metal and dig it all. Best of all it's not an insane 7 lbs like my old ATX!

I expect the AQ discrimination system will have limits but none that will surprise me and I am willing to accept the limits to get some form of decent discrimination while retaining PI depth in black sand laden beaches. If the machine does as expected I don't see huge benefits for low mineral white sand beach hunters. The main benefit will be west coast and volcanic islands where a PI will shine. And in those areas heading for the trashy places that normally drive PI hunters away will be a good idea.

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Welcome to the forum Alexandre! We are all excited about your new detector and speculating about how the discrimination system works. The goal on this forum is only to help people, so if anything does not seem helpful please let us know.

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The following is a discussion of existing PI detector features that I find to be useful for relic hunting that goes beyond the target discrimination break-point discussion that Steve articulated so well above.  Beside the high mass-high conductor/low mass-low conductor discrimination breakpoint setting - another useful feature implemented in the ML GPX and to a lesser extent (or less successfully, based on my experience) on the Garrett ATX is iron identification, rejection, or blanking.  The GPX implements a blanking function on probable "high" (low tone) corroded iron signals with the peak of the signal tone blanking out.  Not 100% effective as it can also provide "false positives" on non-ferrous targets but it is nevertheless a pretty useful feature.  Similarly, the ATX has an iron grunt check, which is both ergonomically inferior (requires you to engage a separate button during your swing) and I have found it to be less reliable (almost to the point that it is a 50/50 proposition if you do get an iron grunt).  Blanking has it's drawbacks of course.  Besides reliability, iron reject/blanking on the ML can only be utilized with DD coils and can become overwhelmed in thick iron situations. Anyway, Alexandre or Rick (once you get your machine), if either of you can shed light on whether such a feature will exist on any or all iterations of the Fisher Impulse, that would be enlightening. 

Finally, compared to the Garret ATX, I find that the as you gain experience digging innumerable nails, the tone "shape" of the GPX provides a wealth of information.  For example, the classic "W" double peak high tone (low conductor/small mass target) is often a clue that you are swinging along the long axis of a nail.  Turn 90 degrees and that nail sounds like a sweet high tone indicative of round brass or small lead.  Bent nails, however, are very difficult to ID, they sound great.  Higher mass minie balls give a nice smooth low tone that will not break unlike a large rusted nail or other piece of iron.  The ATX uses more of a processed doorbell high-low/low-high tone, so it cannot convey much additional target information other than the high conductor/high mass or opposite characterization.  So I am interested whether the tone implementation on the Fisher Impulse target audio will convey analog-signal-like target characterization (e.g., irregular or regular shape) information.

Thanks for reading.  Excited to see the Fisher Impulse in action.

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In all this discussion, it ought to be remembered that the AQ is very tightly focused on recovering gold jewelry at salt beaches. That doesn’t mean that it will not be useful for other applications, but to the extent it may prove not ideal for those, the answer will be that tight focus and optimization for a purpose means that other purposes may not be as well served.

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Thanks Rick, your opinions on gold coins on the beach? Will it depend on the size of the coin?. It's the excitement before Christmas!

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2 hours ago, Chase Goldman said:

another useful feature implemented in the ML GPX and to a lesser extent (or less successfully, based on my experience) on the Garrett ATX is iron identification, rejection, or blanking.  The GPX implements a blanking function on probable "high" (low tone) corroded iron signals with the peak of the signal tone blanking out.  Not 100% effective as it can also provide "false positives" on non-ferrous targets but it is nevertheless a pretty useful feature.

Long story short this has never been mentioned at any as a potential feature on the Impulse AQ. Perhaps it may be seen on the land prospecting version since the AQ methodology is not very application to ground balanced nugget hunting.

The main problem with the ferrous blanking as implemented in both the Minelab PI detectors and the ATX is it only works on shallow targets, and not on the targets where you need discrimination the most - the really deep ones. I don't mind digging 6" nails, it is the two foot nails that I find annoying. This inability to function at depth made the GPX iron blanking pretty useless for most of my applications where deep ferrous is a common occurrence, whether on a beach or in tailing piles. That, plus numerous instances of my seeing quite large and valuable gold specimens rejected as ferrous by the GPX meant I never used it. It might be acceptable to miss bullets due to improper identification but not $500 gold specimens.

Now don't get me wrong, the surface ferrous blanking function can be very useful for some people and some applications. I can see where it would be a great benefit for relic hunting in particular. I'm just pointing out it has some serious limitations such that most gold prospectors at least never use it except in rare circumstances.

This is typical. This location produced some real nice gold nuggets. I got a bare whisper signal with the GPX 5000. I had a Gold Bug Pro on hand to act as a pinpointer and discrimination tool, but no signal from it until well over a foot. It started reading ferrous but by that time I was close anyway, and wanted the potentially target masking nail to be gone. So after quite a bit of effort in packed rocky soil out comes a large nail at about two feet. That is what I hope the Impulse AQ will help with. On the beach in particular the effort expended goes up exponentially with depth as the hole is likely caving and filling almost as fast as it can be excavated. I have had to dig some huge pits employing a PI in the surf with mask and snorkel only to retrieve a large ferrous item.

steve-herschbach-digging-deep-nails-minelab-gpx.jpg
Heading for China with a shovel

steve-herschbach-digging-deep-nails-minelab-gpx-5000.jpg
And my reward - did not help it was wedged under that large rock in the hole

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      Da Costa, A.C.S, Bigham, JM, Rhoton, FE, and SJ Traina. 1999. Quantification and Characterization of Maghemite in Soils Derived from Volcanic Rocks in Southern Brazil. Clays and Clay Minerals, v. 47, no. 4, p. 466-73.
      Hunt, CP, Moskowitz, BM, and SK Banerjee. 1995. Magnetic Properties of Rocks and Minerals. In Rock Physics & Phase Relations: A Handbook of Physical Constants, Volume 3.
      Koch, C.B, Borggaard, OK, and A. Gafur. 2005. Formation of iron oxides in soils developed under natural fires and slash-and-burn based agriculture in a monsoonal climate (Chittagong Hill Tracts, Bangladesh). Hyperfine Interact 166, 579–584.
      Rivers, JM, Nyquist, JE, Terry, D.O., and W. E. Doll. 2004. Investigation into the Origin of Magnetic Soils on the Oak Ridge Reservation, Tennessee. Soil Science Society of America Journal, Vol. 68 No. 5 p. 1772-1779.
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