J.D said:
... I have scanned a bag full of pull tabs on the etrac and have those sections blocked out
There is a big difference between what you are doing, and what Jack is doing. You CAN INDEED do exactly what Jack is doing, with your E-trac. The problem is with each-of-your's respective interpretations/definitions of the above quote.
^ ^
depending on HIS samplings of pulltabs and YOUR samplings of pulltabs, is going to make all the difference in the world. If he were to take YOUR "bag-full of pulltabs", and adjust his detector so that they got rejected (crackled, nulled, or whatever), then TRUST ME : He would be missing most of those gold rings he's now getting.
I have a suspicion that his tab collection he's assembled is very concise. You can do that too: Take 100 tabs off 100 soda cans, and sample them for reject-setting purposes. But the trick is, that just like sampling 100 nickels, surprise surprise, they're all going to read the same. And sure: You can factor in various types of tabs (the 1970s type, the various competing brands of soda types, the modern types, etc...). But you are STILL left with perhaps 6 or 7 recurring types. And yes, after editing out ("notching") just those types, there are still various size gold rings that do NOT fall into those ranges.
Versus if you took a sampling of typical junky park aluminum junk (chopped up and/or bent up tabs, foil wads, molten can nuggets, can slaw, etc...) and edited out all THAT too, then you would eventually get to where you've edited out all your low conductors, including all gold rings . Unless the gold ring were big enough that read up into the zinc range, that is.
I also have a suspicion that Jack is hunting particular type areas that don't abound in the mountains of other aluminum junk (that don't happen to read in his concise tab coordinate zones). There are admittedly some types turf and sandboxes that will be prone to less junk. Upscale soccer fields where they're not eating and picnicking for instance. Or baseball fields that aren't used as cross-over picnic fields, etc... In that case, it's boiling down to a factor of "location location location", and not some trick of machines, disc. patterns, audio, etc....