This Article Appears in the July 2004 Gold Gem & Treasure Magazine.
© Copyright to Gold Mining Centre® All Right Reserved - 2004

Garrett Infinium LS Discrimination.

By Phil Stearnes, Gold Mining Centre

Since the Garrett Infinium LS was released, I have been watching reports on the web and it has become very apparent that the majority of these people who are posting on the many forums, have no real knowledge about using the two types of discrimination on this unit. Certain people have been making statements that the discrimination doesn’t work, and this is only after several hours of use, or even less. Because of this problem and misinformation being given by some of these so called experts, I believe it is time for me to write this article to get the true facts out to new and existing customers.

It is a well known fact, that good discrimination with pulse induction is very hard to achieve with any real success. Pulse induction loves steel and iron targets, which is basically hard to eliminate. Others have tried, but generally it was only surface elimination which really only worked on larger targets of iron and steel and also, larger nuggets of gold were known to have been eliminated by this system. There was no system of discrimination with pulse induction to give indication that smaller nuggets were good targets and wire and other items of steel were possibly bad targets, and definitely not at depth.

During the three years that I was involved in the testing and design of the Garrett Infinium LS with the Garrett engineer, many ideas were looked at, including discrimination. When he came over to Australia for testing purposes, I told him that I had an idea for discrimination and would he have at look at it and see if it was viable. He took one look at it, looked up and said, you’ve taken my idea. At this point, I knew that we were thinking along the same lines of design, and this had to be good, and also that we were onto something that had not been done before.

Looking at the combination of the Dual Tone Discrimination that the engineer had implemented in the Infinium LS and the Reverse Discrimination that both of us had agreed upon, I knew that we had something more advanced than had ever been achieved before. And not only that, but a system that basically works to the depth of the target, as long as there was a readable signal to work with.

The first thing to understand, is the way that the dual tone discrimination works and this may take a bit of understanding for some, as it is different to the standard VLF detectors. With VLF metal detectors, steel is normally lower conductivity than gold, with copper and brass being higher conductivity than gold. Some of this is still true with a pulse induction detector, but steel and iron differs greatly. Steel and iron is highly conductive to pulse induction detectors and this why it is so hard to eliminate. This is due to the eddy currents produced in different material by the pulse from a pulse induction detector, and the decay time properties, but I do not intend to go into this technically, as the most important thing is to recognize this factor and how to use it in the field.

To explain where gold lies in the conductivity range with pulse induction against steel, I will use the numbering scale from “1” to “12”, that Garrett uses on their coin detectors. This will help the people who are used to the discrimination on VLF detectors to understand. Most gold will sit somewhere between the range of “3” to “7” on this scale of “12” whereas steel will sit generally in the top scale around “11” to “12”. Copper and brass will normally come in somewhere between the gold and steel readings.

The dual tone discrimination takes this conductivity range into account and the two different tones have been designed around this fact. Any low conductive material like gold, will normally give a high tone first and then followed by a low tone, whereas a high conductive material such as steel, will normally give a low tone first, followed by a high tone. You will notice that I said normally, but there are other factors that can occasionally cause the opposite to happen and this is where operators can get fooled and declare that the discrimination doesn’t work. The problem is that they haven’t learnt how to extend the use of the advanced discrimination capabilities of the Infinium LS detector.

Once you have located a target, first of all, listen to the tone you are getting. If the target signal is quite strong, raise the coil so as to reduce the signal strength and then check the tone given. The reason for this is that a large signal can overload any electronics and give an incorrect reading. If you are getting quite a good signal and it is a high tone first, then this is indicating a low conductive target, which is where gold normally indicates. If a low tone is obtained first, this is indicating that it a high conductive material. This does not necessarily mean that it is steel, but generally higher conductivity than most gold.

Now comes one of the important tests that you should carry out, and this is where the reverse discrimination comes into affect. Turn the discrimination right around to iron check. If the target completely disappears or you are just getting it, then it should be checked, as this is indicating possibly a low conductive target.

The next part is where many people are misunderstanding the discrimination. If the target remains quite strong in the iron check mode, then most people would assume that it is steel, but this may not be the case. You must also take into consideration the tone you are receiving. If the target is giving a low tone first and the signal strength is still quite strong, then I would suggest that it is most likely steel or similar material, but if the tone is high first, then it is probably not steel, but something that has a higher conductivity than most gold or lower conductivity than steel. This may be targets such as silver, brass or copper. It may even be gold, as some gold that has been found in Queensland, has quite a high concentration of silver in it, and it won’t disappear when the discrimination is turned to iron check, but it will still give a high tone first, indicating a lower conductive material than steel.

Other procedures to fully utilize the Infinium LS discrimination are as follows. When a target is detected, take note of the tone you are getting first. Then move only the front toe of the coil over the target and check the target response tone. You will quite often find that a target may be giving a high or low tone first, but by using the front toe of the coil, this tone will change. I have found this discrimination test a more accurate way of determining a targets conductivity. Sometimes a plated bit of steel will give a high tone, but by using the toe of the coil, it will change to a low tone, indicating that it is more likely to be steel. Then use the reverse discrimination to confirm the targets possible identity.

Another check that can be used on a target that is giving a low tone, which is indicating possibly steel, is to turn the discrimination up to about 3 to 4 on the dial. If the tone suddenly changes to a high tone, then I know that it is most likely not steel. To try this test, use an Australian $1.00 coin as the conductivity falls into the area just above gold and will generally give a low tone with the coil passing directly over it. Using the toe of the coil, or setting the discrimination to approximately 3, the tone will change to a high tone. Only use this test to check the discrimination capabilities and nothing else. Do most of your tests using gold nuggets, as this is what you are generally looking for.

Also listen for the length of the signal that you get. You will find that steel will quite often give a drawn out signal with multiple pulses following. Numerous operators have been using some of these techniques already with the Garrett Infinium LS with great success. This can be seen from some of the photos in this article.

The thing that must be remembered, is that if a target is indicating a high tone first, never walk away from it. If the target completely disappears or is greatly reduced when the discrimination is turned to iron check, never walk away from it. Use the tests that have been explained in this article to determine if it is possibly a good target or a bad target. The thing that must be accepted is that nothing is perfect, and some targets may give the incorrect response, but by using the techniques described, most targets will be able to be categorised correctly.

The good thing is that the discrimination on the Infinium LS, works at depth and not just surface. If the target is too weak to get a correct reading, then remove some soil and then test the target again. You will also find that the discrimination works just as well with the mono coils.

One other hint that has nothing to do with the discrimination, but with the signal strength, is to listen for the smaller variations in the threshold, As with other pulse induction detectors, a slight variation in the threshold can indicate a deep large target or a very small target close to the surface. You may even see a slight reduction in threshold (going negative) when going over these small targets. If you are getting a small variation and it is in both directions, check closer for a possible target.

Hopefully this will help understand the true capabilities of the Garrett Infinium LS. There isn’t a pulse induction detector on the market that can match the discrimination of this unit and the mineralisation doesn’t appear to have any real affect on the discrimination capabilities. Remember to use both discriminations in conjunction to obtain the best accuracy possible.