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Minelab GPX-4000 Field Test - Arizona Outback

Field Testing Minelab’s GPX-4000



I slowed down my swing; this time passing the coil carefully along the exposed bank of the gully. There it was again. This wasn’t 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 detector’s 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 & 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 let me know the target had been moved into the freshly dug pile. I set the detector down beside me and began running handfuls of dirt across the coil until a sharp ZIP rang through the headphones. I scratched through the material in my palm until I spotted a small quartz stone about the size of my thumbnail. The quartz 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 beautiful, and better yet, it was my very first piece with the new Minelab GPX-4000.

During the past ten years I have been asked by a number of different companies to field test their equipment and publish the results. Most of these companies have been metal detector manufacturers and each time 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!” 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 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 let’s first cover some of the changes Minelab has made.

As I unwrapped the control box, 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 that the user interfaces with the machine’s 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, but 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 GPX’s 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 battery’s power can be tested at any time through the LCD display. I have nothing but praise for this system; 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, and is another first for a Minelab PI detector. Sweep speed is essentially the speed at which a person swings the coil from side-to-side. The Motion function allows the operator to indicate their intended sweep speed, allowing the detector to provide the best threshold stability and target response for that particular sweep speed.

Gain…it is a word we don’t normally associate with a pulse induction detector. What is Gain? 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 on Minelab’s part.

In addition to the Gain, the GPX has another sensitivity timing option called Smooth. The all-new Smooth 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, AZ. This area had given up some beautiful gold over the years, but like most of the easy-to-get-to spots, it had seen its fair share of coils. My best take was in 1998 when my wife and I had a 3.5-ounce day with the Minelab SD2100’s. I’ve found nuggets since then, but they have become scarcer. Actually, I struck out there my last trip in with the GP3500. I knew in my gut that the pickings would be slim, but I also knew it would be a brilliant test area because of the horrible conditions.

Three separate components came together to make this one of the more challenging patches I’ve 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 isn’t 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 VLF’s at coping with severe ground mineralization, 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 that 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 all alone. “Good,” I thought, at least no one would be offended when I started cussing the hot rocks! 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 doesn’t get any easier. There are two settings that affect the balance: Fixed & 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 detector’s 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? That’s 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. I felt a smile spreading across my face and I did cuss, but it wasn’t out of frustration! Awesome is definitely the word I would use to describe the ability of the GPX to cut down electrical interference.

As I swung through the patch I discovered many of the hot rocks were still there, which truthfully 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 know I would have missed it. Out of curiosity I switched from Smooth to Extra and passed atop the target once again. What a difference - the signal really boomed! I couldn’t 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 a better part of three hours, probing my coil into the thick brush until I finally got another hit. I had wholeheartedly 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 ‘ol patch still had a few treasures to give; unfortunately it was late in the day and time to call it quits.

I have since been fortunate enough to add several more nuggets to the collection. 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, so 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 & low ground mineralization), I suggest Normal and Extra to ensure maximum performance.

The detector is supplied with an 11” round Double D coil. This coil will give best results in noisy ground, but will not penetrate as deeply as a Monoloop of exactly the same size. The improved circuitry of the GPX allows the use of mono coils in areas where DD’s 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” & 16” mono coils.

For me, the first few hours on the GPX were miserable. 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. Discouraged or not, I forced myself to stick with it. After a few successful visits to my old patches I’m so 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: a sorry excuse for 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. Other considerations are incompatibility with some aftermarket booster/audio amplifiers, batteries, & power cables, and of course the price. The newest Minelab will cost you a buck or two shy of $4,000…(about $500 more than the GP3500).

True it’s 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 I’ve had the pleasure of testing in a long time – two thumbs up from this prospector!

Minelab GPX4000 gold metal detector field test.
Minelab GPX-4000 Field Test - Arizona Outback


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