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Date Posted: 16:49:27 03/26/02 Tue
Author: Ralph
Subject: Understanding VLF - Ground Balancing

Perhaps one of the most difficult and least understood aspects of gold detecting for the beginner is that of ground balancing. What is it, how is it done, and why are the most often heard questions from those new to gold detecting, or even detecting in general.

First of all, mineral conditions within the soils where we detect for metal objects often contain certain minerals.... oxides and sulfides of certain types that have some degree of conductivity that creates a "target signal" for the metal detector. While these minerals often produce a very low level of conductivity, when we are dealing with an "all-metal" metal detector especially, even these slight levels of conductivity can create problems, since the detector is seeing the entire ground under the coil as one big target.

Now, there are basically two ways that a metal detector can be designed to deal with such mineralization problems.

First, there is the "discriminating" metal detector, those most often used to hunt for coins and other items which give the operator the ability to discriminate out certain levels of metals that fall within a certain area of the conductivity scale. This is a "relative scale" based on an arbitraty valuation of "100" for annealed pure copper at a specific temperature. This is the "base" that is used to compare all other metals on the relative coductivity scale, with most metals falling below this valuation, and silver falling above this level at approximately 106.

So, when using a discriminating metal detector, it can clearly be seen that the best way to deal with low levels of ground mineralization is to simply "chop off" that lower area of the conductivity scale in which this mineralization exists. There is a little more to it than that, and this is a simplified example, but this is basically how certain discriminating metal detectors handle ground mineralization by design. And it's also important to remember that the more ground mineralization that exists, the more difficulty ANY detector will have in peering through such ground to find a metallic target. Mineralization simply reduces the effectiveness or depth ability of the detector when dealing with induction balance, because the transmitted and received signals are a constant, keeping that ground mineralization saturated with an artificial electromagnetic field that creates what might be compared to a visual "fog" between the target and the detector itself.

Now, in dealing with "all-metal" type gold nugget detectors, obviously there is no "discrimination" circuit to help deal with the ground minerals, so a different approach must be taken. And while some discriminating detectors use some level of this approach also, most simply "cut off" lower level conductivity signals as explained earlier.

With "all-metal" type detectors, the ground mineralization problem is approached as a true "balance" problem. If you will remember, we talked about the "induction balance" design in the first part of this series, and how the transmitted and received signals were simply "balanced" to each other so that the TX or transmitted signal was seen the same when it comes back into the detector by way of the RX or receiver circuits.

In ground balancing an all-metal type machine, we are simply expanding on this "balance" approach by "re-balancing" the TX and RX signals to "ignore" the signal from the ground mineralization as if there were no signal at all coming back into the machine. This is a sort of "clipping" process if you will, whereby the circuits of the detector send out a "known" signal that is compared with the "unkonwn" signal that the RX circuits receive back from the ground. The difference in the two is then effectively subtracted from the received signal so that the detector "thinks" it is again sending and receiving the same signal..... i.e. NO TARGET RESPONSE. On manual ground balance machines, this is done by the operator to his desired operational level, while automatic or "auto-tracking" detectors are designed to do this automatically within it's circuitry. They are effectively doing exactly the same thing.

Now, from a practical standpoint, it is "usually" to your advantage to tune your detector to a slight degree of "positive" ground balance. That is, to the point where you get a slight increase in target response tone as you lower your coil toward the ground. This should ONLY be a slight response. The reason for this slight positive balancing is to prevent the detector from missing or giving too slight a response to already faint smaller and deeper targets. Anytime you have a negative ground balance, the audio circuits of the detector actually have to make up for the "dead zone" in the detection level before beginning to give a good target signal. This can be looked at in much the same way you might look at the audio threshold of a machine, something we will discuss in a later section. But keep in mind that under "most" conditions, it will be to your advantage to use both a slightly positive ground balance AND a slightly positive audio threshold. It simply gives the detector a "head start" in producing the signal that tells you there is a target under the coil.

Where practical manual ground balancing is concerned, it is a very simple process that for some reason beginners have the most trouble with when using a manual GB detector.

The easiest way to manually ground balance a gold machine is to simply pump the coil toward and away from the ground while listening for the response of the audio. If the audio signal increases as the coil approaches the ground, you are balanced too POSITIVE, and need to TURN DOWN the ground balance level. If the audio signal decreases as the coil approaches the ground, but increases as it moves upward and away from the ground, then you are balanced too NEGATIVE and you will need to TURN UP your ground balance level. Once you find the point where there is no response in the audio signal when either lowering or raising the coil, then you are technically at the "balance" point. It is at this point that your machine no longer "sees" the mineralization in the ground as a target. Again, you will normally be better off adjusting to a very slight positive balance at this point so that you can HEAR what the ground beneath your coil is doing..... whether it is changing, staying the same, or becoming very inconsistent as you move from one area to another.

Another quick tip on ground balancing a manual induction balance machine.....

While ground balancing your detector, try to do so as you pump the coil up and down and while moving the coil in a wide arc across the ground from left to right and visa versa. In this manner you will get more of an "average" for that particular spot rather than just at one point under the coil. And remember to be absolutely SURE that you are not trying to balance your machine over a metal target in the ground..... one of those simple "DUH" concepts that is often overlooked.

While it may appear complicated to the uninitiated, with time, practice, and UNDERSTANDING, ground balancing will become almost second-nature, and you will eventually find yourself doing it "on the fly" without ever missing a beat, and never giving it a second thought. Practice, patience, and persistence make it all come together.

Where so-called "auto-tracking" machines are concerned, there are advantages and disadvantages to having the machine do your ground balancing for you. As a quick and easy approach for the seasoned prospecting vet, or a simple method for the beginner at gold detecting, they can be a time saving feature that will work very well in "most" conditions and under most situations. In some areas of extreme mineralization or certain types of hot rock infestation, auto-tracking circuits CAN do more harm than good, and contrary to some popular opinion, these types of machines can and will "track into" certain good targets, either causing the machine to ignore good targets altogether, or simply miss them due to surrounding or close proximity mineralization or hot rocks. Suffice it to say, they are not the "fix-all" of the all-metal detecting world, although improvements seem to be made with each new circuit design that is introduced into the market. As those designs improve, so obviously will their performance in the more difficult environments.

I hope some of this will be of some help......

Ralph

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