By Steve Herschbach
For those of you more inclined to viewing instead of reading, Chris Ralph has put together an impressive collection of videos here. He is working hard at it adding a new video every week. Chris is very knowledgeable on various aspects of prospecting and mining with in-depth knowledge on geology. Take a few minutes and check it out.
Also check out Chris’ book, Fists Full of Gold
By Steve Herschbach
I consider myself fortunate to have met Garrett's Senior Design Engineer, Brent Weaver, many years ago during prototype testing of the Garrett Infinium. I think Brent deserves a great deal of credit in making Garrett the leader it is today with a series of incredible innovative bang-for-the-buck metal detectors, first and foremost being the AT series. Brent is just a real nice guy - comes across as a regular Joe not a head in the clouds engineer. I think this video shows that. It also has an extremely interesting discussion of pulse inductions detectors, ground balancing methods, and the dreaded "hole" that occurs when ground balancing. There is explanation of why the holes occur and how some designs try to alleviate but not eliminate the problem. Good stuff, starts around the 3:40 minute mark.
The audio quality in this video truly sucks. However, this is important information, and so repeated listens, typing, and listening again on my part produced the following transcript. Again, this is from Brent Weaver, Senior Design Engineer, Garrett Metal Detectors
Transcript starting at 3:40 mark:
"If you look at the various pulse detectors that exist on the market, you have your simplest pulse machines, which are single pulse machines that are non-ground balancing. Those are detectors that are typically exclusively used for the beach, for example, the Sea Hunter or something like this.
The next step in advancement there is to have a ground balancing pulse machine, those can still be a single pulse detector. This pulse channel is able to ground balance, which means it can neutralize the ground. The problem with that sort of technology is when you ground balance out the ground, you can also balance out any target that has a decay characteristic that is similar to the ground. We call this the "detection hole". Essentially, it is a hole in the conductivity... when you look at the conductivity of gold or any material, you look at a conductivity spectrum. The decay rate of that... pulse decay is a function of the conductivity of the material and also the thickness and shape of the material. It all factors into effective conductivity.
For a ground balancing pulse detector, again, if the conductivity of the ground is similar to the conductivity of the target, when you ground balance out the ground you also ground balance out the target. To eliminate that problem, you create a second pulse which is substantially different than the first pulse, such that when you ground balance out the ground on the second pulse, its conductivity decay as far as the target is concerned, the conductivity decay on the first pulse and that on the on the second pulse do not match each other. As a result, as you ground balance out the ground on the first pulse, you create a hole, and you ground balance out the ground on the second pulse, it also creates a hole, but it is in a different location.
And so, those two pulses working together in unison, working simultaneously, one will always fill in the hole of the other, they overlap such that you never have a detection hole. If you only have a single pulse detector, and it ground balances, it will have a detection hole. There are various products on the market, some are less expensive than the ATX, some are more expensive than the ATX, but if they are a single pulse detector, they are going to have a hole in their detection, period. You are going to miss gold. You are absolutely going to miss gold.
Now where that hole falls depends on the mineralization conditions, and where the ground balance is set for the detector. As the ground balance shifts, the hole shifts with it. If you have a detector that has continuous ground tracking, such that you cannot switch it off, as that ground track moves around for the various ground conditions, the hole moves around with it and the targets are disappearing into that hole. You never know where the hole is at any time.
Again, to eliminate that problem, the more advanced, the most advanced detectors, like the ATX, use multiple pulse technology. They don't just have one single pulse that repeats, they have different kinds of pulses, and those pairs repeat. That is one of the differences in a true high end performance product like the ATX, versus some of the other products on the market. They are good products, and they are ground balancing pulse detectors, but if they are a single pulse technology, they have a hole in their detection that will miss targets."
Now, for me listening to that it is obvious that Garrett was clearly gunning for the White's TDI. How do I know that? Because it was Minelab that originally put multi period pulse detection into the consumers hands, via their MPS (multi period sensing) technology beginning with the ground breaking SD 2000. The White's TDI on the other hand is an older design, and in fact is basically just an Eric Foster Goldscan stuck in a White's labeled box. The Goldscan and the TDI are a single channel ground balancing pulse detector with the problem that Brent outlines in the video. Bruce Candy of Minelab saw the same issue and the SD 2000 was specifically developed as a multi channel or multi period device for this very reason.
MPS was patented, and so I am not sure if it was the patent expiring or Garrett simply using a method that got around the patent, but the ATX is using a similar multi period design as the SD Minelabs. Having used the Minelabs and the TDI plus the ATX I can vouch for the electronics in the ATX as being very capable and I do believe superior to that in the TDI. My ATX in fact tested favorably compared to my GPX 5000 with the 5000 having the edge, but not as much as I expected. I think in large part that is simply due to the Minelab using a much more powerful battery pumping far more power into the ground than the ATX with its eight AA batteries.
Unfortunately in my opinion Garrett made a huge mistake in taking these excellent electronics and hobbling them with a housing that did not take advantage of one of Minelabs biggest weaknesses - ergonomics. The TDI had and still has a distinct edge in that regard, and at a lower price than the ATX. Ironically, it is also that detection hole and the ability to manipulate it via a manual ground balance that has become, not the big problem as laid out in this video, but a feature of sorts in favor of the TDI. People have learned how to manipulate the hole to help identify targets by using the ground balance control as a sort of reverse discrimination control. Combined with the unique conductivity switch on the TDI knowledgeable users can become very proficient at identifying various target classes, and this has made it very popular with relic hunters in particular.
What history in hindsight reveals, in my opinion, is that Garrett missed the boat with the ATX as regards Minelab. The ATX hit the market before the SDC 2300 and had a window of opportunity to really make inroads if it had been in a lighter weight dry land package with a light dry land coil set designed specifically for desert prospecting. The electronics are there; it is the heavy housing, and heavy knock sensitive coils overpriced by an attached telescoping rod assembly that really hurt the machine. On the flip side, I don't think they did too much damage to the TDI also with the TDI being a less expensive more ergonomically friendly unit. Like the Garrett Infinium before it, the ATX has settled into being more a beach detector than a prospecting detector.
I have of course been making hay over this since day one and continue to make an issue of it at every opportunity in hopes that we may see a Garrett LTX some day. Dry land design only for the absolute lightest weight possible machine with light weight knock resistant coils to match, it would be a winner. There is a market still I believe for a $1500 -$2000 ground balancing PI detector that clearly has more power than a TDI in an all in one package lighter than anything Minelab currently markets. The only question in my mind is whether Garrett will finally get it right or will it finally be a moot point when new Fisher, White's, and Nokta/Makro machines waiting in the wings finally arrive.
Some folks may be interested in Xp Deus for coin hunting. This video shows airtest ID distance potential of detector. The ground as mineralization rises will reduce the accurate ID at distance of buried targets.