This is not a technical paper by a technical genius. My smarts tend to be in the practical application of metal detector technology in the field. I am less concerned by the theory at work under the hood than the actual detecting characteristics revealed in actual use. I would very much welcome smarter people than I pointing out any obvious technical inaccuracies below!
It has been clearly stated that the GPZ 7000 is not a pulse induction metal detector. Yet it sounds and acts deceptively like previous Minelab pulse induction models, just enough to get people into trouble. I think it is people who are expert with Minelab pulse induction detectors that are having a harder time adjusting to the GPZ 7000.
My background comes far more from the VLF side of things. VLF is a misnomer, because it means Very Low Frequency, and was originally nothing more than a subset of induction balance or transmit/receive (TR) detectors. From
http://www.vlf.it/frequency/bands.html
ULF ultralow frequency 300Hz to 3000Hz
VLF very low frequency 3kHz to 30kHz
LF low frequency 30kHz to 300kHz
Thus it can be seen that the White's GMT at 48 kHz and Fisher Gold Bug 2 at 71 kHz are actually LF (Low Frequency ) detectors, not VLF detectors. And a White's V3i running in 2.5 kHz single frequency mode is running in the ULF range. But these days VLF has become a more generic term referring to induction balance detectors in general.
I used to always think in terms of Induction Balance and Pulse Induction as the two different methods used for inducing currents into the ground by modern metal detectors. But things are getting more complicated. Most single frequency induction balance detectors use phase shift for ground balance and discrimination, whereas Minelab BBS and FBS multi frequency detectors rely on time constants, blurring the line between time domain pulse induction and frequency domain induction balance. Minelab is addressing that by now referring to all non-pulse induction detectors as Continuous Wave or CW detectors. Pulse induction detectors have a transmit period, and a non-transmit (zero transmit) period. From
http://www.minelab.c... Technology.pdf
"CW means Continuous Wave and includes all technologies that do not have zero transmit periods (almost all technologies other than PI)."
The GPZ 7000 is most definitely not a pulse induction detector in that the detector is always transmitting into the ground. But where a PI measures the time constant of items from the point where the transmit period ends the GPZ measures from the instant where the transmitted field polarity reverses. Again, like with the BBS and FBS detectors Minelab is mixing continuous wave transmission with time domain processing.
OK, blah, blah, blah in layman's terms and no doubt not entirely accurate from a technical standpoint. But close enough for me to have an epiphany recently while running the GPZ 7000. It acts like a super VLF detector more than a pulse induction detector. The ground responses, hot rock responses, salt responses, and gold responses are not that different than what I might expect from a mid-frequency VLF detector albeit one that is much, much more powerful.
Everything in metal detecting tends to be a trade off or come at a cost. The Minelab SD/GP/GPX series of pulse induction machines were a progression with later models designed to handle salt ground and hot rocks that earlier models could not handle. The problem is when you tune out a salt signal you tune out weak gold signals. When you tune out certain hot rocks, you tune out certain types of gold signals. Savvy operators always knew that by running in older "Normal" modes they would have to deal with more noise and possibly dig more hot rocks but they could also find gold that more aggressive timings missed.
When I run the GPZ 7000 from a purely operational standpoint it acts and feels like a super VLF and basically that is what it is, with a transmission mode that leaves the ground with less ground signal to deal with than that generated by a high power pulse, employing an advanced coil design, and advanced ground balancing algorithms to deal with ground mineralization while still deriving the extra power and sensitivity inherent in continuous wave detectors. I am curious where Bruce Candy's head was when he came up with the ZVT concept, but it appears to me to derive more from the BBS and FBS line of thinking than being something related to the PI machines.
The point of all this? Forget about how your PI acts and works and if anything treat the GPZ more like a Super VLF than a pulse induction detector. Certain old VLF tricks work with the GPZ. The main one being - get that coil off the ground!!
Saturable Soils
Again from
http://www.minelab.c... Technology.pdf
"The GPZ 7000 coil must be symmetrical about the leftright axis because ZVT transmitted signals cause a type of signal to be produced from the ground which is absent during PI receiving periods. This ZVT receive signal is from something called soil magnetic hysteresis. If an asymmetrical (unsymmetric) coil like a DoubleD is used by a ZVT detector (asymmetrical because the transmit coil is on one side and the receive coil on the other), a signal is produced which depends on the speed at which the coil is swept over the ground (and how close the coil is to the ground), and the audio tone will be higher going in one direction (for example left to right), and lower when sweeping in the other direction, which would clearly be unacceptable."
The text goes on to say the Super D coil design is employed to avoid this problem. A better word than avoid may have been reduce. The issue has been reduced but not totally avoided. Have you run into it yet? The GPZ producing a rising high tone as you swing one direction, and a descending low tone as you swing the other direction? Another way flattering way of thinking about it is the GPZ moans and groans as you swing. This is the common response over salt or alkali ground. The intensity depends on the amount of salt in the ground but also on the speed at which the coil is swept over the ground and how close the coil is to the ground.
The solution might include changing gold or ground modes of course. But irregardless of that one obvious solution of to simply slow down and keep a very constant swing speed. This will moderate the effect and still allow nugget signals to stand out.
The less obvious solution and one that many people will have a very hard time accepting is - raise the coil. Many VLF detectors will overload on bad ground and the simple answer is to raise the coil to eliminate the overload. This is counter-intuitive to people who always scrub the ground with the coil. How can raising the coil add depth? It can and it will if the electronics are being swamped with excessive receive signal.
Once again from
http://www.minelab.c... Technology.pdf
"In any of the above settings, it pays to swing the coil an inch or so above the soil surface if the soils are considered saturable (VRM). Saturable means that a detector ground balances well if the coil is raised and lowered down to about a few centimetres above the soil surface, and for the worst saturation, down to several centimetres, but not if the coil is swung up and down to a height lower than these saturation height thresholds (e.g. down to the soil surface.) In addition, the degree of (VRM) soil saturation is considerably less for Difficult or Severe than Normal. As the metal detector coil is moved towards a soil, the transmitted magnetic field in the soil gets stronger. This causes a (very) small degree of VRM signal saturation that happens to cause the resistive signal relative slope of the tilt to change. This is why the amount of VRM soil saturation is far less for Difficult and Severe than Normal. Soil saturation often requires the user to operate the coil several centimetres above the soil surface for best results.
However, whilst soils do have resistive signal that are very accurately loglinear, unfortunately this is not perfectly accurate for some soils, and, because the GPZ 7000 has such very high sensitivity, even miniscule deviations in the straightness of the line of the loglinear resistive signal will cause ground noise signals. Severe is less sensitive to these miniscule deviations than Difficult. Whilst the GPZ 7000 does not have a dedicated salt detection setting (saline soils), the best Gold Mode setting for salt soils is Extra Deep."
Extra Deep is an extra long time constant mode and so is less sensitive to weak signals from both salt ground and small gold. It is the classic metal detecting trade - you cannot get super sensitive to gold without getting sensitive to salt. Machines can totally eliminate salt effects but never think it does not come at a cost in lost gold. The trick is to get the larger stuff, and the tips above are the same you would employ with any hot VLF in an attempt to deal with saturable ground. The simplest one was the one I wanted to highlight because it just goes so much against the grain.
Raising your coil above the ground can actually add more depth!
None of these solutions is a magic bullet and will totally eliminate the issue on the GPZ 7000. It will succeed to a greater or lesser degree in different locations with different soil conditions.
