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Maintaining Threshold Stability

Maintaining Threshold Stability
Tips for using the Minelab GPX Detectors

With the sharp rise in the spot price of gold, there has been a marked increase in the number of people getting involved with recreational gold prospecting. Gold Club memberships are up across the country and so are sales of mining equipment. Hopeful prospectors are still buying plenty of gold pans, but in the past few years I have noticed a general shift in their interest towards something a little more "high-tech". Many are now investing in a relatively new gold finding tool that didn't gain much attention until recently. That tool is the metal detector. So what is it that makes the metal detector so appealing? I personally believe it is because this piece of equipment can be used virtually anywhere. Unlike a dredge or sluice box, the metal detector does not require water, so it is ideal for scanning desert regions. There are no motors or hoses, so there is essentially no maintenance, plus it can locate all types of valuable metal objects, including nuggets, coins, ancient artifacts and meteorites. While there are many different brands available, and all are selling well, one brand in particular that seems to be gaining momentum is Minelab; especially their GPX Series machines.

The Australian company has been manufacturing high performance detectors since the early 1990's, but since most carried a price tag of several thousand dollars, sales have historically been limited to a niche group. Now with gold peaking at over $1300, many folks are willing to take a gamble on a high priced machine. Speaking from over fifteen years hands-on experience with the Minelab brand, I can say that they do offer incredibly impressive results. They can handle severe ground mineralization and penetrate deeper than virtually any other hand held detector on the market. However, even someone armed with the latest and greatest in technology can easily walk over the gold if their machine is not tuned properly. In order for the Minelab GPX detectors to work at their optimum, the user must maintain a clear and steady Threshold.

The Threshold is the constant background "hum" emitted by the detector. This sound is extremely important because it is a reference point that lets you know what the coil is sensing, whether it is a metallic target, ground noise or interference. Ideally it should sound smooth and stable, almost like a small mosquito buzzing in your ear. If the detector is tuned properly and the Threshold is running smoothly, you will find it very easy to hear the faint signals produced by small or deeply buried nuggets. However, if the detector is not tuned properly the Threshold may become highly erratic, making it difficult to hear these subtle signals. Think of it like trying to hear a whisper in a crowded noisy room. An unstable Threshold is always a bad thing and will cost you gold if left unchecked. Fortunately there are a few simple steps that can be taken to help ensure your GPX detector is running as smoothly as possible.

Ground Balancing

One of the most important aspects of hunting with the Minelab GPX detectors is learning to properly ground balance. Even though these detectors will handle higher levels of mineralization, they must still be balanced to maintain a quiet Threshold and achieve maximum performance. Ground balancing is a process through which the metal detector is able to cancel out the negative effects caused by ground mineralization. It is a very important part of detecting as it will allow you to distinguish a true target from the false signals produced from the ground.

The Minelab GPX detectors offer both automatic (Tracking) and manual (Fixed) balancing capabilities. Tracking will automatically monitor ground mineralization levels and adjust the balance setting while the operator is sweeping the ground. Fixed, on the other hand, does not monitor the ground, but rather holds the last balance setting. Each has their pros and cons. For myself, I tend to run the Ground Balance switch in the Fixed position, as I feel it provides great depth, sensitivity and sharper target signals. However, if the ground conditions are severe, or there are lots of hot rocks, I will often flip into the Tracking mode. Sometimes a slight loss of sensitivity and depth is more desirable than constantly having to re-balance the machine every few steps. In either mode, the process of balancing is essentially the same and I will describe my method to you. First, I look for a piece of ground that is clean of obvious surface trash. Then, I turn on the detector and wave it over a stretch about three feet wide, listening for any signals which signify the presence of buried targets. If I hear targets I will move to another spot and try again. If I don't hear anything, I will start the procedure. To achieve maximum performance from the GPX it is very important to never ground balance directly over, or in close proximity, to a metal target. Be sure your pick or digging tool is well out of range.

First, I make sure the Ground Balance switch is in the Fixed position. Then, I hold down the green Quick-Trak button on the handle. Next, I begin by pumping the coil over the ground and listening to the Threshold. I keep the coil horizontal to the ground and use short, steady strokes. The speed at which I pump the coil is neither fast or slow, but closer to a medium pace. I will usually bring the coil within an inch of the ground on the down stroke. If I hear a rise in pitch as the coil is lowered towards the ground, or as the coil is lifted away, I know immediately that the detector is not balanced correctly. If this is the case, I continue pumping the coil over the ground while holding in the Quick-Trak button. Once I can pump the coil towards, or away from the ground, without a noticeable change in the Threshold, I know the detector is balanced and I can release the button. If you prefer to hunt in Tracking, make sure the Ground Balance switch is in the Tracking position. Then, press and release the Quick-Trak button while pumping the coil towards the ground. You will know the detector is balanced when there is no noticeable change in Threshold when moving the coil towards or away from the ground.

It is important to check your ground balance often, even if the machine seems to be running fine. This is especially true if you move to a new location where ground conditions are likely to be different. This can easily be done by periodically pumping the coil over the ground and listening to the Threshold. Never settle for a mediocre balance. If you get a disturbance on the down or up stroke, be sure to make the necessary adjustments. Remember, you want the detector to run as quietly as possible. Less background noise equates into a better likelihood of hearing a tiny or deeply buried nugget.


Tuning is another key element in helping to keep the GPX machines running smooth and stable. Many prospectors confuse this function with ground balancing. Both are designed to reduce noise, but they are two distinct functions. While ground balancing deals with the noise generated by the soil and rocks beneath the coil, tuning actually deals with noise that is generated by electricity or radio waves. This type of noise is commonly referred to as interference. It is most often caused by power lines, radio towers, electric fences, climatic conditions or other metal detectors. If you are not familiar with the sound it causes, turn on the GPX inside your home with the coil raised in the air. That constant oscillation or wavering of the Threshold you will hear is a result of interference. This type of noise cannot be ground balanced out, but it can be Tuned out.

The engineers at Minelab have provided us with a very handy tool for combating interference. This feature is known as the Auto Tune (AT) button. It is a small, black button that can be found on the front control panel beside the Threshold knob. When activated, the AT instructs the detector to automatically scan each available channel and select the one which provides the quietest operation. The range is from 0-255. Using the AT feature before detecting can greatly improve overall stability. The tuning procedure is easy to perform and should take less than a minute. Simply turn on the detector with the entire machine held at waist height. Slowly turn in a circle, listening for the strongest source of interference. Once it is found, keep the coil pointed in that direction and reach down and press the AT button. Do not hold the button in. You may want to remove your headphones during this process as it can get fairly loud. Keep the entire detector as motionless as possible and do not pass any metal in front of the coil. The detector will cycle through all available channels and give three "beeps" when finished. This process usually takes about forty-five seconds. If you would like to know what channel has been chosen by the AT you can access the Manual Tune screen and check the number displayed.

Once the AT has been performed you will need to ground balance the detector using the steps described above. The number of times you will need to use the AT will be dependent upon the area you are detecting. If you are in a remote location, it is unlikely you will need to tune more than once. However, if you are searching close to civilization where there is a constant stream of interference, you may find it necessary to tune several times a day. I have also found that interference levels differ throughout the day. In my experience, I have noticed that early morning and late afternoon are the quietest, whereas as mid day is typically the most noisy. One final note regarding tuning with the GPX detectors. It is possible to tune them by hand using the Manual Tune feature. This option does offer the user the most control, however I highly recommend the AT for new owners. It is faster, easier to perform, and based on my testing, has an accuracy of 95% or better.


The final feature we will discuss is the Motion setting. This setting can have a profound effect on Threshold stability and can be accessed via the LCD. The speed at which you sweep the coil will influence target response and ground balance adjustment. It also effects how susceptible the detector is to interference. Therefore, matching your coil swing speed with the corresponding Motion setting can greatly reduce noise and improve target signal responses. There are different Motion settings available, depending upon which model of GPX you are using. For example, on the new GPX 5000 there are four possible settings: Very Slow, Slow, Medium and Fast.

Slower Motion settings offer the best stability and less interference will be heard, but the coil must be moved slower. Faster settings allow more interference to be heard, but the coil can be swung very rapidly. The speed you pick will ultimately be determined by the type of detecting you are doing and how much interference is present. For instance, when prospecting new country or patch hunting, you may find the faster Motion settings to be the best option, as they will allow you to cover more ground quickly. However, when re-working an old patch it is likely that you will be moving the coil very slowly trying to listen for deep nuggets that may have been overlooked. In these cases, the slower Motion settings will definitely be the way to go.

You can hear the differences for yourself by toggling between the slowest and fastest settings. First, select Very Slow and listen to the Threshold for a few seconds. You will notice that the Threshold sounds smooth and there is very little fluctuation. Next, select Fast and once again listen to the Threshold. Chances are you will notice that the Threshold now sounds very "jittery" or erratic. The faster Motion settings do have their place, but the added noise can be a distraction and actually make it difficult to hear faint targets which are often deeply buried nuggets. I never recommend the faster settings in areas of high interference. For myself, I almost always run my GPX in either Very Slow or Slow. This means I cannot cover as much ground in a day, but with the added Threshold stability I can be confident that if I do walk over a nugget I will be sure to hear it.

Learning to properly adjust your GPX detector is one of those things that will take time and practice. Even though I have been using my GPX 5000 for several months now, I am still discovering new ways in which I can improve the performance. I am constantly experimenting with the controls to see how they effect the machine's stability, and you should too. I know it can be difficult to use up precious daylight trying out different combinations, but if you take the time to really learn the machine it will only make you a stronger more successful hunter. I wish all of you the very best of luck!

* All rights reserved. Not part of this article shall be reproduced or transmitted in any form or by any means, electronic or mechanical, without written permission from the author.

Minelab, Nuggetfinder coils, gold nugget prospecting with metal detectors.Where to find placer gold.
Maintaining Threshold Stability


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