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
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
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
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!