My first post pointed out some of the features as to what can cause one PI detector to detect deeper than another. Issues such as current into the coil and unique design that generates very low noise are two key issues. However, there are more things and issues that are sort of intertwined that also need to be factored in.
Talk to a Minelab PI owner and sooner or later the discussion will get around to noise. The discussion may include everything from solar flares to lightening, and of course power line noise. On some of the OZ forums, they even discuss how power line noise extends out because of air currents.
Now, in all fairness I really need to explain that Minelab has pushed the limits on sensitivity and taking the sensitivity to such limits causes their detector to be much more sensitive to some of the noise problems. This increased sensitivity is part of the key as to why the ML normally has greater depth than other PI's.
Other PI's are also affected by some of this external noise but usually the effects are not as dramatic or obvious. Try to use a ML PI in town and usually any depth comparisons will be dismal at best. Change the location to a more remote area and the ML really shines and displays exceptional depth, usually far superior to most competition.
The point is, all PI's are affected by noise and this effect can be quite dramatic. In my previous post I mentioned that with my low powered PI, I could get a wide depth range on a nickel and the reason for that depth range was simply noise. One thing that many PI owners don't notice or realize is if they raise the coil above the ground, higher and higher the noise level most likely will increase. Depending upon how close one is to the noise sources, the amount of increase in noise as one raises the coil will usually determine just how much influence the noise has and this can vary significantly just because of the noise source itself. In other words, the issue is far more complex than many people realize.
One other effect many PI owners don't notice or realize is the coil edge effect or stated differently, the coil edge sensitivity. Take a very small gold nugget or any other non ferrous object and pass it under the center of a large coil and chances are you might not get a noticeable signal. However, take that same object and pass it right under the coil winding on the edge of the coil housing and bingo, it is very possible to get a decent signal. The reason for this occurs because of the coil field effect. In simple terms the field strength is quite intense right under the winding itself and this intensity extends out a few inches depending upon the coil size and the object itself.
Now, take this edge effect and a unsuspecting operator and it is quite possible for someone to believe the object they are detecting is under the center of the coil when in fact it is under the edge of the coil. Because the signals sound the same, it is extremely easy to have this happen. Alter the angle of an object such as a flat coin in the ground and the maximum signal doesn't necessarily happen under the center of the coil. Since in most cases the object is centered at the largest response, the natural thought process says that is where the object is when one begins to dig. Usually, the hole gets quite deep before the object drops down into the hole and disappears. When this happens we don't give up but dig deeper, which then recovers the object. The next logical condition is by chance the object is scooped out just prior to it falls but the hole is still much wider and deeper. In any case, the first thought is the buried object had to be at the deepest depth. Unfortunately, if one tries to repeat the process, this same object won't be detected even close to the depth one thinks it was found. Unfortunately, this causes people to believe in the halo effect is what caused the object to be detected deeper and now can't be detected since the "halo" was disturbed.
Keep in mind that the halo oxides of any metal are such that they do not produce a signal that is detectible. The exception really isn't an exception is rusty ferrous metal. Normally as the object deteriorates, the signal strength gets weaker. However if the rusty object deteriorates to an iron oxide and becomes part of the ground minerals and than that oxide is transformed by intense heat to maghemite, then the ground signal will change. Something as basic as a fire or a lightening strike can cause the ground to suddenly change which can cause a signal quite often sounding like a deep target.
Then there are all the strange anomalies that occur that seem to enhance a signal. Normally, these seldom happen but they can and do. Something as strange as iron oxide such as black sand can concentrate or distort the signal such that in certain cases objects can be detected deeper than they would be under more normal conditions.
One other unique issue is many people firmly believe that a PI will or can detect burnt wood, so when they come across an old campfire they will get signals from the burnt wood. Unfortunately, this is not the case. What has happened is the intense heat from the campfire causes a transformation of the ground mineralization to maghemite which is quite detectable. Old red bricks were made using different levels of iron oxide to get the brick the red color. Test an unfired red brick and nothing, nil, nada in terms of any distinct change. However, subject that same brick to the heat used in firing that brick and suddenly that same red brick becomes a strong ground signal change. In fact, one can use an old brick to simulate some of the old positive indicating hotrocks when testing a VLF. On a PI, the signal comes through as a sudden change in the ground signal but may sound like a very large deep target response.
Now, do any of these things I mentioned occur even to those really experienced operators? Of course they do. In fact, I have seen them directly, and in the case of one small nugget had very well known AZ nugget hunter stumble when he tried to show how his detector was so effective he detected that small nugget at a depth of several inches. Unfortunately, when he tried to show me how well his detector did on detecting the small gold by passing the same coil over the small nugget he couldn't get the slightest of signals. Because of my knowledge of this nugget hunter, I knew he found the nugget with his detector. So, what probably happened was something enhanced the nugget signal allowing the guy to get a decent signal, which finally led to the recovery of the small gold piece. As I think back, I realize now we didn't check the coil edge to see if that would generate a response.
So, in my 40 plus years of metal detecting of which 25 years was dedicated to serious nugget hunting and the last 12 years with a PI, I have pretty well seen it all. Compound this with the fact, I needed a light weight ground balancing PI when there wasn't one available, led me to learn what I could about PI's. As a result, I ended up modifying different PI's until I was able to buildup a low powered PI for my dad to use because his bad back wouldn't allow him to hunt with any of the heavier PI's any more.
The learning process, along with the experimenting needed to design and build the changes forced me to learn an awful lot about the PI and how it works. My BS degree in Electronic Engineering Technology gave me the technical background from which to build.
What I can greatly attribute my PI knowledge and experience to is to Eric Foster and his willingness to share his knowledge and wisdom over the years I have known him. Eric was and still is a wealth of knowledge when it comes to PI's and what makes them work the way they do. BTW, I still search a lot of his earlier posts on this forum when looking for answers, even today.
As a final note, some of you won't believe much of what I have written and that is fine. Feel free to experiment and prove me wrong. That is easy to do on a one time basis, because of all the potential factors that have an influence. However, feel free to do it on a consistent basis. The key to really learning the PI is something that takes years of experience and knowledge to fully understand just what is going on. I strongly recommend people take the approach to prove or disprove an issue and do so on a regular basis. What you will find is what seems logical when you first start out, may change dramatically as time goes by.
I mention this because for many years people believed that gold would be detected using somewhat of a constant thought on gold itself. By this I mean a 1/4 oz nugget would act much like any other similar size nugget. Unfortunately, nothing could be further from the truth, but that is another issue that has been discussed and can be discussed again.
So, getting back to which is the best PI with the greatest depth, the answer can be quite complex since what works in one location may not work that well in another. Noise, ground conditions, hot rocks, and other factors all can influence how well a detector works.
Now factor in a person's experience with only one detector and it is easy to understand why questions get so many different answers as to which is the best today.
Reg
Edited 1 time(s). Last edit at 09/14/2012 01:49AM by Reg.
