Whites TDI Classroom

I have VERY good ground in my yard. I've used the ground probe feature on my V3i and seen that my dirt is -93 vdi and has mineralization of 2.2%, arguably the best ground I've ever hunted. In this yard I have a penny and a 58 caliber minie ball buried at 10" each for testing. At first I was unable to pick them up with the TDI having ground balance turned ON. Off was no problem. Then in my box I found the quick reference card that explains how to set up the machine for two tone operation with ground that does not require balancing. Having tried each of the suggested settings I was still unable to pick up the targets. Then I just started playing around with the detector. With gain at max, pulse delay at max and ground balance at max I was able to pick up each target loud and clear with low tone ID. My question is this...the quick reference card suggest setting a lower ground balance number for higher pulse delay settings, the exact opposite of what I needed to do to get the detector to do what I wanted. What gives??

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Diggin with a Minelab CTX3030 and a Whites TDI

I dont go by the recommended settings, it all depends on the ground your hunting in. They give you a general ref of where to start but the final settings are totally dependent on the area your in.

That makes sense, I was just surprised that the settings I needed were completely opposite. Once I got it set right, it worked great!

Hello Downdeep, your experience tracks directly with the data provided by Reg, the local expert.

As already noted above, the guidelines in the manual for setting the GB (based upon your Pulse Delay setting) is designed to cancel significant ground mineralization.

Since your ground didn't need balancing, your biggest detection depth is with GB in the Off position. However, if you want to be able to discriminate targets based on conductivity, you are lucky that the ground mineralization was weak since it allows you to move the GB control just about anywhere you like without getting false signals from the ground.

In that case, here are Reg's guidelines:

if the object has a low tone (high conductivity) and the GB is reduced, then the signal strength will reduce (your first experience)

if the object has a low tone (high conductivity) then increasing the GB will cause the object signal to become greater (your second experience)

Of course, for low conductivity targets, the reverse directions hold true.

Thanks for the info. I was pointed to the TDI field Manual as well and printed it off. I think I'm going to have fun with this detector!

Its a great detector, I go to it more than my GPX5000. I also have the Jimmy Sierra coil for it, I havent had it out with that coil yet but it sure will be nice in the tight areas I like hunting.

Hi Downdeep,

You did something everyone should do and that is experiment with your detector. The reason is you will find out more about it if you do. Now, with that said, also keep in mind that what you think you learn may not be the best solution even though it might work at the time. So, the point is, don't stop just because you find a working combination.

Now, with that said, there is a lot of info out there about the TDI and some of it will not be as accurate as it could be. This happens when one first begins and then posts what they find. The more people who do this, the more confusing it can become.

So, the best solution is to learn the fundamentals of the detector and know how to apply those fundamentals. One such fundamental is the one about the GB control and the tones mentioned earlier. That is, if you get a low tone from a target, the increasing the GB will give you a stronger signal and normally will allow you to detect that target deeper. This holds true for larger targets where noise isn't a problem.

The fundamental tone is a high tone. This is what you get when the GB is turned off. Turn the GB on and some targets may become low tones and remain that way throughout the GB control range. Other objects may change tones at some particular setting. In the case of targets that are identical or nearly so, the setting at which the tone change occurs will generally remain the same.

Other targets that can change for various reasons, may not display the same results. In the case of repeatable targets, you can take something like an Indian Head penny and you will find it will change from a low tone at a very low GB setting. This setting will be reasonably consistent when you change coins. The reason is, the coin size and composition remain the same. Thus the results generally will remain the same.

Now, take a nail and you may find the tone changes over a wider range. This can happen because of several factors including the change in the size of the nail or the deterioration of the nail as it rusts. Even the chemistry of the nail can change and this can have an effect on the final results. Years ago, steel that didn't meet certain specs would or could be made into nails. As a result, one might find older nails to vary in hardness among other things. BTW, I used to work in a steel mill that made nails so this isn't hearsay.

Getting back to the controls on your TDI, keep in mind there is an interaction when different controls are changed. Change the delay and the settings where a target changes tones can change. In other words, lets say a target changes tones at a GB setting of 7 with a delay of 10 usec. If the delay is changed, the GB setting at which the tone change occurs will also change.

Now, increasing the delay will result in a reduction of depth providing the GB is off. In other words, the signal strength of the object decreases as the delay is increased. However, because of the interaction of the GB control when the GB is turned on and things change. High tone responses can change to a low tone response depending upon the target and the GB setting. If this can happen, there will be some point where the tone changes from a high tone to a low tone. At that point, that same target may display very little depth of detection. The farther the GB setting is away from that tone change position, the stronger the response.

Now, the next thing to remember is targets vary in how they are detected for many reasons. Small targets are almost always high tones. Small targets also generally have a very short time constant. What this means is if you turn the delay to a longer time, then you will get a much weaker signal from such objects.

The time constant is the time it takes for an object signal to dissipate. Short time constant signals decay or dissipate quickly. Long time constant objects take a very long time for the signal to go away. The higher the conductivity and the longer the time constant. As a general rule, the larger an object the longer the time constant. So, combine the two, and something like a silver dollar has a much longer time constant than that of a dime. A much smaller silver coin such as some of the early Roman coins may have a shorter time constant even though it is made of silver, simply because of its size.

So, both size and type of metal involved are determining factors on how an object may respond. Add in the fact that surface characteristics and thickness are also factors, and one can begin to understand the complexity of trying to figure out just what an object will do signal wise.

The point to all of this is to inform you that the factors that cause a tone change or a target response are more complex than what one might think. So, don't be surprised if something doesn't seem to fit or react like you think it should. The more you use the TDI the more this will make sense.

Now, you found out you can adjust the TDI and detect deeper targets such as something such as a bullet. You also found out the depth of detection increases if the GB is turned off. Also, when the GB is on, you also noticed that some targets change tones. So, if you combine the information, then the GB must do something that will cause a tone change and a depth loss.

Now, if you think about the ground signal and the fact the GB control is there to minimize that ground signal, then adjusting the GB control is designed to minimize that ground signal. To do this, the GB control adjusts another signal that is subtracted from the main target signal and if the subtraction is just right, the ground signal will be subtracted out. Unfortunately, in this process, other signals are also reduced by this subtraction process. Alter the GB setting and the ground signal will change. So, by reducing the GB to something like 4 or 5 and the ground signal will be a positive response, but a signal from many nails will be minimized. Unfortunately, the signal from something like a 3 ringer bullet will also be reduced dramatically. So, in the process of trying to eliminate nails, you also reduce the depth of detection of certain bullets.

Now, with the settings you mentioned, you found you could detect the minnie ball deeper with certain settings. Turn the GB off and the depth of detection is potentially maximum. Turn the GB to max with the GB on and as a general rule, the depth of detection of a minnie ball will increase. The difference between the GB off and the GB on will also result in a the tone signal change of the minnie ball. Since the tone changes, then so will the depth of detection. Find the exact spot on the GB control that the tone changes and you will find the worst setting for detecting that object. Keep in mind that this is the concept of ground balancing, which is why the GB control is there to begin with. It just happens on this detector, this same feature also results in the ability to separate targets as well as detect some objects while ignoring others.

So, as you can see, this gets complicated, especially to one still learning the detector.

Finally, keep in mind the primary reason for the GB control is to minimize the ground signal. That was and is the reason for the GB control. It was later found out that this same adjustment could also help in separating and detecting some targets while ignoring others. So, think of this disc feature as a bonus and not the primary reason for the design.

Well, this has been long winded, but I hope it helps a little.

Reg



Edited 1 time(s). Last edit at 05/07/2011 02:51AM by Reg.

Wow Reg, thanks for all the info! I'll have to read it a few times to digest it I'm sure! lol While I'm learning the machine I'll do my best to keep this forum active and get you guys that know a good bit to keep sharing. Thanks again.

Hi Reg, I was just playing around with my detectors (MXT & TDI) in my backyard and noticed something that disturbed me a little. I found an object buried with my MXT that was showing up in the "foil" range. I then checked it with my TDI with my 10x14 mono coil and could barely hear it. In fact if I didn't already know it was there I would have missed it. I was disturbed because my MXT with a 6x10DD coil was reading way stronger than my super-powerful TDI, which is my go-to gold prospecting detector. The TDI was set on Pulse Delay of 10, Gain 6, GB on 8, Threshold and Frequency and Volume all around 12 o'clock (straight up). My battery was fully charged right off the charger from an overnight charge. My back yard has only medium mineralization (GND = 55 on my MXT, and 6-7 on my TDI). So knowing how the GB setting can be moved around to null out different conductive metals, I started moving it around. At a setting of 1, I could hear the target very well - just as expected = most low conductive targets signal louder on lower GB settings, as documented by Reg. At a GB of 6 it was very low, and when I moved it up to 8 & 9 (where it would be set when prospecting in the California Gold Country), I couldn't hardly hear it at all. This told me that small gold nuggets which would fall into the same conductivity range as this piece of foil might be passed over. In the Gold Country the mineralization is so high that I could not possibly set it at the low end of the scale - it would have to be in the 8-9 range in order to quiet the ground noise.
There was a chart I dug up from a while back titled, "TDI test by Geo." I pulled it out today and it confirmed this same concept - a gold coin showed it's lowest response when the Pulse Delay was set at 10 and the GB was 7.
So this being the case, I'm a little concerned, now, with regards to the TDI hunting gold nuggets. Your thoughts on this?

Hi Goldchaser,

First, you have to realize that PI's don't do as well as a VLF on very small targets. This will also include very thin items such as foil.

Next, the coil size you mentioned certainly isn't the best for finding small gold. If you were using a 6" by 10" coil on the TDI, I am sure the response you would have gotten would have been better. In fact, it reminds me of a situation where I was hunting with a dealer of another PI who happened to find a very small nugget and when he tried to show me how well his detector did on that small nugget, he couldn't get a beep no matter what he tried.

So, if he couldn't get a signal when it was out of the ground and sitting in a nice little plastic vial, why couldn't he find it?

Well, there are several reasons including the fact there may have been another object that was detected, but the logical explanation was the fact the large coil he was using wouldn't detect the nugget except right at the edge of the coil, which is one place he didn't try when trying to show me how well his detector functioned.

Most of the the stories of great depths on small nuggets found with a PI equipped with a large coil can be attributed to this quirk. What happens is the field of the coil is really quite strong right at the winding but that strong field only extends a few inches. So, if you happen to have the coil close enough to the ground and the nugget is in the range if that field, meaning it is right under the winding itself, then bingo, you get a signal. Move the coil so the nugget is centered and it is quite likely you will get no response on the same very small nugget. Since most people do not think about actually checking the target from a different direction the first thought is the object is under the center of the coil when it is detected. Wrong. Next comes the digging until the hole is large enough the nugget drops and disappears. Then it is a matter of digging it out of the hole thinking it was really deep to begin with. Next thing you know someone is claiming to find a grain or two nugget several inches deep, when in fact, it was most likely right on the surface to begin with.

Try a simple test of this odd field condition and try to detect a very small target with the edge of the coil and you will see what I mean. Generally, this edge field only extends 3" to 4" max.

Now, with that said, don't expect to find nuggets as small with your TDI as you can with your MXT because it won't happen. Also, don't use the same sweep techniques when moving from one detector to the other. The TDI requires a much slower sweep speed especially when trying to detect very small objects. This sweep speed isn't as critical on much larger items such as minnie balls or coins, but when it comes to nuggets in the few grain range, it is important.

Next, to have the greatest success, pick a coil that works best on the size of nugget you expect will be the most common size you think will be found at the location you will be hunting. If you are relatively new to the TDI, then practice using a very small nugget for a test target. Try to use the smallest nugget you can reliably detect and then try different techniques including sweep speeds with the nugget buried at different depths. I think you will find you will get the best response when the coil is swept slowly and evenly over the ground while keep that coil as close to the ground as possible.

There will always be quirks so a test of one has little to no significance. It is those repeatable signals you hear during testing that will become the ones to rely on.

As for the ground being bad where you find nuggets, well, I have seen extremes from almost no ground signal to very bad ground response. Generally, the more clay the worse the response. On the upside, generally, such areas are usually quite smooth. Areas that are mostly gravel may display very little ground signal. In fact, I have hunted such areas with the GB off but used a DD coil in the process. Actually, DD coils work a little better when trying to find very small gold but do require a very slow sweep speed for the best results.

Now, as for the test by GEO, it is good that you looked at it because it shows what happens on larger gold objects. As a nugget with high purity approaches 3/8 oz to 1/2 oz, the depth of detection will diminish and can do so by quite a bit. Fortunately, or unfortunately, such size nuggets are really quite rare. So, searching for that particular size nugget should be done using different techniques. In other words, search an area at least twice but preferably more times if the area is known to produce gold. Change settings to aim at different size gold. In other words, if you look at the test by GEO you will note different settings that will increase the depth of detection of that gold coin significantly. So, it is only logical that one should try the technique that works the best for the gold one hopes to find. This is one of the tradeoffs of hunting with a detector that costs 1/4 as much as one of the newer high priced units. Personally, I will hunt an area several times using different techniques to maximize my changes of finding gold.

Several years ago I found a half gram nugget with my low powered PI. Now this nugget was missed by all the more powerful PI's who had covered the same area over and over. The club I belonged to had a push several days before and the area had been beat to death by PI owners trying to find that last pieces of gold that could still be found. Well, I found one of those nuggets that had been missed.

Once that nugget was out of the ground, one new PI owner wanted to test his detector to see how well it would do on that nugget to see if he could figure out how he could have missed it. So, we compared our ability to detect that same nugget and guess what, my poor low powered PI matched his high powered PI for depth. This simple test in front of a bunch of people made them think twice. My success was attributed to the coil I was using and not the ability to detect the nugget deeper. I didn't explain that to the onlookers but that is ok.

The best thing you can do is to practice and experiment, rather than try to find a quick answer as to what should be the best technique to use. Many articles I have read make me cringe because they provide information that will help a person miss good objects, rather than find them. So, I always recommend a person take the time to experiment, rather than try to take the shortcuts or the easy way.

There is a simple fact when using a TDI and that is you need to know the detector well if you want to get the most out of it. Anyone can take it out of the box read an article or two and start hunting. With luck, you might even find a nugget or two. However, with areas being hunted out really bad, this simple luck is not as common any more. The more experienced and better trained will be the "lucky" ones today.

Reg

Reg, THANK YOU!!! so much - for the time and knowledge you have shared. I feel much better knowing it was the foil acting similar as a very small target.