GPZ High Yield, General, or Extra Deep?

[Steve Herschbach]

I have commented in the past that as far as how it actually acts in practice the Minelab GPZ 7000 reminds me more of a "super VLF" than a PI detector. I want to carry that analogy a bit father in an attempt to help clear up a common question about the detector. What Gold Mode should you use?

Minelab makes a unique prospecting VLF called the Eureka Gold. What makes it unique is that is can be switched between three frequencies - 60 kHz, 20 kHz, and 6.4 kHz.

The high frequency 60 kHz mode is the hotter on small gold and in mild soils also gives excellent depth on all other sizes of gold. If you were to air test the detector this is the mode that would air test the best on a wide range of targets. It is the go to mode for mild ground and at high gain levels can be very impressive.

The problem is a hot high frequency setting that really lights up the gold can also light up bad ground and hot rocks. This in turn can obscure desired targets. If the ground mineralization is bad enough penetration is severely impacted because sensitivity/gain levels must be drastically reduced in an attempt to reduce the response from the ground and/or hot rocks.

If this is the case going to the 20 kHz mode will make the machine less sensitive to ground feedback and hot rocks. Some sensitivity to the smallest gold is lost, but by reducing interfering ground responses depth on larger gold is retained.

In the very worst ground and hot rocks however even 20 kHz mode can be too hot. In a worst case scenario going to the low frequency 6.4 kHz mode can eliminate all but the absolutely worst ground feedback. Sensitivity to smaller gold again takes a hit but depth on the larger gold again is to a large degree retained under difficult conditions.

The coil employed plays into this. Larger coils "see" more ground and are more susceptible to the ill effects of bad ground. This can be offset by using lower frequencies. In fact, the 15" WOT coil for the Eureka Gold comes with a note saying that best performance can be expected in the low frequency 6.4 kHz mode.

The problem is that people are usually thinking about what mode gold responds best at. It is often more useful to think in terms of in what mode ground and hot rocks will be less inclined to create issues. If you can't get the machine to play well with the ground then finding gold becomes a bit of a moot point.

Think of the GPZ High Yield Mode as being the equivalent of the 60 kHz mode on the Eureka Gold. General would be the equivalent of the 20 kHz mode, and Extra Deep a match for the 6.4 kHz mode.

The GPZ Gold Mode names confuse people. Extra Deep should be the deepest, right? Well, yes and no. In mild ground General will outperform Extra Deep. However, in extremely bad ground or when employing yet to come larger coils Extra Deep will reduce or eliminate ground noise that keeps General from attaining the depths possible in milder ground. It is not that Extra Deep goes deeper. It is that Extra Deep is quieter allowing nugget signals to not be masked by ground noise. Which in turn delivers more effective depth.

People are constantly confused I think by the fact that performance of metal detectors is not measured in absolute terms except in air tests. Which is why I consider air tests to be generally worthless when evaluating prospecting detectors. The most important thing by far is relative performance, that is, performance relative to the ground and hot rock situation. Machines can deliver spectacular air tests and fail miserably in the ground. In fact, the best air tests are most likely to fail in the worst ground.

So for me High Yield is the go to mode because I frequent milder ground locations and unfortunately smaller gold is my bread and butter. In hotter ground where average nugget sizes are larger General might be the go to mode instead. It just depends.

Extra Deep is a special mode that is very, very much equivalent to that 6.4 kHz mode on the Eureka Gold. It is going to actually be the less sensitive mode in mild ground. It only shines in the worst of ground and even more so when using a larger coil. Frankly, in 99% of the situations one might encounter with the stock coil High Yield and General are going to do the trick. Extra Deep is only really going to come into its own when a large coil is introduced for the GPZ 7000. If a person is running a large coil in really bad ground searching for multi ounce nuggets, Extra Deep is likely to be the go to mode.

Hope that helps.

[fredmason]

An excellent analogy, Steve. Equating the three selectable khz modes on the Eureka to the three selectable modes on the GPZ should make the matter clear to experiences detectorists, it certainly is good reminder to me! 

I will mention to those not as experienced that the selectable modes are not the same as a multi-frquency vlf that runs through multible frequencies all the time.

fred

[Sourdough Scott]

Thanks Steve for putting it in layman's terms. That gives me way more insight on the different modes. I can totally process this now. Even me having 2 PHD's. Things get convoluted in my enormous noggin. BTW my 2 PHD's are a (Portola High Diploma) and a (Post Hole Digger) lol

[Lunk]

Great post Steve, that really makes the Gold Modes undersandable and explains the different types of ground that each mode should be used in. However, some may be confused wondering what, then, do the Ground Types of the GPZ 7000 accomplish? I've found that the Gold Modes act as a sort of "fine tune " for each Ground Type; for example, when I'm hunting in the Normal Ground Type and High Yield Gold Mode and the ground is too hot for this combination, instead of simply going into the Difficult Ground Type I will first try General Gold Mode and if the ground is still too hot, Extra Deep. Extensive testing on undisturbed targets in a variety of ground types has revealed that more sensitivity is retained by this fine tuning method than by just choosing the Difficult Ground Type right off the bat.

[Tortuga]

Great post Steve, that really makes the Gold Modes undersandable and explains the different types of ground that each mode should be used in. However, some may be confused wondering what, then, do the Ground Types of the GPZ 7000 accomplish? I've found that the Gold Modes act as a sort of "fine tune " for each Ground Type; for example, when I'm hunting in the Normal Ground Type and High Yield Gold Mode and the ground is too hot for this combination, instead of simply going into the Difficult Ground Type I will first try General Gold Mode and if the ground is still too hot, Extra Deep. Extensive testing on undisturbed targets in a variety of ground types has revealed that more sensitivity is retained by this fine tuning method than by just choosing the Difficult Ground Type right off the bat.

This is how I use my GPZ as well. Each setting for me is just another way to "dumb down" the detector in tough soil.

In mild soil I use Audio Smoothing: Off, Normal Soil and High Yield.

If the ground gets hotter I turn Audio Smoothing: Low.

If it gets worse I switch to Difficult Soil and continue down the chain until things get quiet. Fortunately I don't have to switch settings much where I detect but maybe somewhere like Rich Hill with a little tougher ground mineralization might require me to "dumb down" my GPZ.

I haven't even touched the Sensitivity or Threshold settings on my GPZ yet. The stock settings seem to work fine for me.

[beatup]

Steve a great way to get one's self to really think about the 3 gold modes on the GPZ and how they work. Lunk,your bit of info is also spot on as it's also what i try do out in my practice patch .

[Steve Herschbach]

I probably have not experimented with it as much as you Lunk but I do agree. Like all analogies mine is an oversimplification, and once you toss ground modes, gain, and audio settings into the mix it can get pretty complex. Obviously the ground conditions where a person operates most determines in the end what settings work best. These days I run the machine really hot and tend to adjust for extra noise more by modifying my audio settings than by "downshifting" the underlying horsepower.

My post more than anything was a response to Scott querying on another thread what that Extra Deep thing was all about.

[Lunk]

Steve, it's an informative thread and so far it seems that all of the methods of dealing with hot ground that have been mentioned are fairly similar.