I slowed down my swing, this time passing the coil carefully along the exposed bank of the gully. There it was again. This wasnt a strong response, like crossing a bit of iron or lead, but more subdued, almost as if it were a patch of noisy ground. I was intrigued by this signal, mostly because it was repeatable. No matter which way I approached it with the coil, I always got a disturbance in the threshold. The detectors automatic tracking feature refused to balance it out, so I knew this target was worth investigating.
Using the side of my boot, I cleared away a thick layer of pine needles and black humus, taking it down to bare earth. A quick sweep of the coil let me know the target was still buried. The clay-like material peeled up easily with my pick and, within a matter of moments, I had excavated a hole approximately 8 deep. Rechecking with the detector, I found the target had been moved into the freshly dug pile. I began running handfuls of dirt across the coil until a sharp ZIP rang through the headphones. I spotted a small quartz stone about the size of my thumbnail that looked incredibly ordinary, until I rolled it over and discovered the entire backside was coated with gold! This was by no means the biggest nugget that had come off this patch, but it was my first piece with the new Minelab GPX-4000.
During the past 10 years, I have been asked by a number of companies to field test their equipment and publish the results. Most of these have been metal detector manufacturers and I jump at the opportunity to hit the goldfields with the latest, greatest equipment. Over the years some of these new metal detectors had me saying WOW, while others had me scratching my head wondering what on earth the engineers were thinking. I am currently several weeks into my testing here in Arizona and feel confident enough to say that these engineers have me smiling.
Released in October 2006, the new GPX is the most technologically advanced gold machine the Australia-based company has ever produced. Veteran Minelab users can rest assured this is not an upgraded GP 3500; it is an entirely new machine with features that are, in my opinion, unmatched by any other gold machine currently on the market. Before getting to my personal findings, lets first cover some of the changes Minelab has made.
One of the most obvious changes I noticed was the addition of an LCD Display. The LCD was necessary since Minelab took a step toward the digital world leaving many of the analog components found on earlier models behind. Located on the rear of the control box, it is here the user interfaces with the machines advanced digital control system (DCS). The DCS offers the operator an increased level of control. Not only are there pre-programmed search modes to choose from, now nugget hunters can select their own combinations of settings to optimize performance for different areas. Learning to navigate the Menu took me a few hours, but I came to appreciate the accuracy of the numeric settings as opposed to relying on the position of a knob, which can accidentally be changed while detecting or during transit. Another handy feature is that all changes are saved automatically on shut down and reloaded when the detector is turned back on. The user can also restore the GPX back to factory pre-sets at any time.
Another monumental change is the GPXs power supply. Minelab ran with the high-tech theme by supplying their newest creation with a Lithium Ion battery capable of providing over 12 hours of continuous use! The intelligent charging system is self-contained in the aluminum housing, offering rapid recharge times of 3 to 4 hours from either a 12V or 110V source. The batterys power can be tested at any time through the LCD display. I have nothing but praise for this system; its easy to use and recharge. This sleek, lightweight lithium is a far cry from the 5-pound 6V brick previously supplied with the GP Series. I am not alone when I say, Thank you Minelab, thank you, thank you!
Other noteworthy improvements include: Motion, Gain, Smooth, and Manual Tune. The Motion setting ties in with sweep speed (the speed at which a person swings the coil from side-to-side), and is another first for a Minelab PI detector. The Motion function allows the operator to indicate their intended sweep speed, allowing the detector to provide the best threshold stability and target response. Gainit is a word we dont normally associate with a pulse induction detector. On the GPX, Gain is a numerical function that affects the overall sensitivity of the detector. Now the user has the ability to increase or decrease sensitivity; a powerful tool that can amplify faint signals or help quiet down noisy ground. Adding an adjustable Gain to this detector was a brilliant move.
The GPX has another sensitivity timing option called Smooth which considerably reduces signals from hot rocks and ground noise, yet remains responsive to small targets. The Manual Tune helps reduce electrical interference by allowing the user to fine tune the detector after performing an auto-tune. The manual tune has a range from 0 to 225 and, as I discovered, can be very useful for eliminating cross talk from a nearby detectorist or distant power lines. One of the first places I decided to run the GPX was on a nugget patch in the northern Bradshaw Mountains of Arizona. This area had given up some beautiful gold over the years, but, like most of the easy-to-get-to spots, had seen its fair share of coils. Three separate components came together to make this one of the more challenging patches Ive ever encountered. Component #1: the abundant hot rocks. Lying atop the surface was a mixture of basalt and greenstone; all of which was incredibly hot. As I had learned from previous trips, many of these stones rang out just like a piece of metal. Their unusually high conductivity prevented them from being ground balanced out and drove many a prospector crazy. Component #2: the soil. Hiding beneath the blanket of hot rock was the ground itself which was just as rich in iron oxides and just as noisy. Component #3: the interference.
For VLF type detectors, interference really isnt an issue at all; for PI machines like the Minelab SD & GP Series it is a different story all together. While pulse induction machines are better than VLFs at coping with severe ground mineralization and hot rocks, and offer unmatched depth, they are susceptible to external interference. Power lines, radio waves, aircraft radar, underground cables, electric fences, and even climatic conditions can all adversely affect the performance of a PI detector. The proximity of this patch to a nearby town meant it was plagued by a steady stream of this invisible garbage. I hoped the GPX-4000 would fair better than its predecessor. After a short hike I arrived at the patch and was pleased to find I was alone. I switched on the machine and started the ground balance procedure. The GPX balances in much the same way as the GP3500. Simply push the green button, pump the coil above the soil for a few seconds, and go. As far as ground balancing goes, it doesnt get any easier. There are two settings that affect the balance: Fixed and Tracking. If the soil is fairly quiet, or only lightly mineralized, the Fixed position will offer best results. However, if the soil is noisy and highly mineralized, I strongly recommend Tracking in Medium Speed.
Once balanced, I listened carefully to the detectors Threshold; something was wrong. It was way too smooth and steady. Where were the fluctuations? Where were the familiar warbles that I expected to flood my ears? Thats when I realized nothing was wrong; the GPX was just this quiet! I lifted the coil waist high and listened; nothing happened. I lifted the coil above my head and listened; still nothing. The ability of the GPX to cut down electrical interference is definitely impressive. As I swung through the patch, I discovered many of the hot rocks were still there, which I expected, but the response from the smaller fragments of rock and soil were essentially gone. By running the Audio in Quiet, Sensitive in Smooth, and backing off the Gain slightly, I was able to eliminate a vast majority of the false signals that had annoyed me in the past. These settings gave a clean Threshold, but this stability came at a price. I found this out while passing over a signal I suspected was gold. The signal was enough to stop me, but had I been swinging any faster I would have missed it. Out of curiosity, I switched from Smooth to Extra and passed atop the target again. What a difference - the signal really boomed! I couldnt stand the suspense so I scratched away about 6 of the red-stained dirt and was rewarded with a coarse nugget about 3 grams. I wandered the hillside for the better part of three hours, probing my coil into the thick brush, until I finally got another hit. I expected to see a bullet roll out from the hole; instead it was another bit of yellow metal weighing nearly a gram. It seemed like the ole patch still had a few treasures to give; unfortunately it was late in the day and time to call it quits.
At the time of this writing I am nearly a month into my testing. I still have a lot of learning to do, but I feel confident in telling new owners the following. The Quiet and Smooth functions are powerful tools, as long as the operator remembers that some nuggets at depth may be passed up; especially when used in conjunction. Quiet reduces signal strength, and Smooth sacrifices some depth. They are both incredibly useful features; however if conditions permit (i.e., low interference and low ground mineralization), I suggest running the Audio in Normal and Sensitive in Extra to ensure maximum performance.
The detector is supplied with an 11 round Double D coil that will give best results in noisy ground, but will not penetrate as deeply as a Monoloop of the same size. The improved circuitry of the GPX allows the use of mono coils in areas where DDs were traditionally a must. Because of this, I suggest adding a monoloop coil to your arsenal. Coils that work with the GP and earlier SD Series detectors are compatible with the GPX. Some of my best results have come using the aftermarket Nugget Finder 10, 14 and 16 mono coils.
During first few hours on the GPX, I was tempted to head back to the truck and swap it out for my trustee GP3500. The Menu took some getting used to and the audio was completely foreign, but eventually I learned to understand its language. I forced myself to stick with it and, after a few successful visits to my old patches, Im thankful I did.
The GPX excels in its ability to handle the highly mineralized soils of the goldfields, its immunity to electrical interference, excellent depth penetration, and a broad range of sensitivity. Some of its weaknesses are an iron discriminator which only works on large ferrous targets. As a result, 99% of the time I operate in the All Metal Mode and rely instead on my mental discriminator. Incredible detector, but, like the GP3500, a quality discriminator is still lacking. The position of the Ground Balance switch has been moved on the front panel. Minelab should have left it above the Coil/Rx switch where it was easier to get to. Other considerations are incompatibility with some aftermarket booster/audio amplifiers, batteries, and power cables and, of course, the price. The newest Minelab will cost you a buck or two shy of $4ꯠ(about $500 more than the GP3500).
True, its the most expensive hand-held gold detector on the market, but how many other machines can cruise right through some of the most highly mineralized ground in the country with only an occasional adjustment of the ground balance? Better yet, how many can detect a multi-ounce piece at over 2 foot, but still have the sensitivity to find a nugget weighing a mere 0.1-gram? Serious nugget hunters wishing to arm themselves with every possible advantage will definitely want to look into the Minelab GPX-4000. This is the most versatile, technologically advanced, yet easy to use detector Ive had the pleasure of testing in a long time two thumbs up from this prospector! For more information on this and other Minelab products, please visit them at www.minelab.com, or phone their U.S. office at 1-702-891-8809 to locate a dealer near you. Dont forget to mention you read about it in Lost Treasure Magazine!