Hi Bill,
Eric is absolutely right, the website you posted has a very good general explanation. In fact, it is one of the best condensed articles I have read.
I struggled in trying to answer your question before you posted the site. The reason is, nothing is perfect, and there are exceptions to every rule. One should read the article for the foundation and remember that there will be a whole lot of buried objects that will defy what was written in the article.
The truth is, it would take a book to fully explain just what to expect or what might be the response from a buried object.
You asked if iron and steel are the easiest to eliminate. The answer is yes and no. Given a new large piece of iron or steel on the surface or freshly buried and the answer is more yes than no.
Have a piece of old rusty metal in the ground, especially an old nail buried at an angle, and the answer is no. Toss in small pieces of an old rusty can and the answer is maybe.
When I have been asked how iron objects or even gold objects respond on a metal detector, my best answer is "anyway they want". Simply stated, there is no solid or reliable answer.
The article sort of implies iron signals can change and this is noticeable on a TID detector when it is stated the indication may change dramatically and the signal is orientation sensitive. This is absolutely true and should be emphasized. People should be made aware that detectors are not perfect in their analysis. More importantly, it is not the fault or limitations of the detector, but the limitations of the technology. Mother nature has done a wonderful job of making it impossible to perfect a detector that will be correct in its analysis.
Gold objects produce their own problems. As an example, a friend of mine tested his new VLF on a very small nugget I had found with my PI. The nugget was just a little over a grain in size. With the nugget sitting on the ground in plain view, he made several passes over the small gold object and, depending upon the height of the coil, got an indication the object had a probability of 30% to 60% gold.
If we would have moved this small piece of gold to a different spot where the ground had a different mineralization level, the numbers would have changed.
Was there something wrong with his detector? Absolutely not. In fact, it is one of the more accurate TID (target ID) detectors I have seen. The problem is the fact there is no way to completely analyize a target and obtain perfect results. One big factor is the ground mineralization. It can distort the signal significantly.
Since this is a PI forum, I will get back to discussing PI's and the limitations they encounter. Like the article stated, some iron objects have a very long decay time. However, some do not. Many small pieces of something like an old rusty can, will have a decay time very similar to a gold nugget.
Adjust the delay on a PI up to 50 to 70 usec or so, and most nuggets less than 1/4 oz in size or most gold rings will no longer be detected. Unfortunately, small pieces of an old tin can can also display the same thing and disappear at the longer delay. To make things worse for gold hunters, basalt which can give a strong signal at a short delay will also disappear at the longer delay. On the upside, a solid steel bolt or washer will generally produce a very strong response at the later delay. On the downside, larger pieces of gold may still produce a reasonably strong signal.
The article mentions PI's are not that ground sensitive. This is true when the delay is over 25 usec or so. Adjust the delay down to 10 to 12 usec and the ground may produce a very strong signal, depending upon the ground itself.
VLF's "scream" when the coil passes over magnetite black sand. PI's generally produce nothing but an ocassional whimper over the same area.
VLF's can be adjustd so red clay ground conditions barely cause a whimper. PI's without ground balance can produce a very strong response over the same area. Even with ground balance, a sudden bob of a PI coil will generate a strong signal.
The bottom line is, the website you posted provides an excellent overview of VLF and PI instruments and how they work in general terms. However, specialized detectors and some "trash" objects will produce signals that are quite contrary to some of the information mentioned.
One of the most common mistakes is thinking there is something wrong with the detector when a conditon doesn't fit the norm. What I have found is detectors seldom fail and when an abnormal signal is encountered, it is because of an abnormal condition and not a detector failure.
Reg
