Hi Bill,
First, I need to thank Buzz for sending me three hotrocks commonly found in the area where he hunts for gold. These are some of the strangest hotrocks I have encountered to date. I will explain why in a minute. I should also mention that I just ran a few quick tests using a 11" DD coil and a similar size 11" mono coil when testing these rocks. So, other size coils may produce slightly different results.
Now, I would like to also explain that there are several types of hotrocks that will generate a response on a PI. I should also mention that the tests were done using an Eric Foster designed PI so there could be slight differences in response when using a different type of PI.
Also, there have been discussions on this forum indicating PI's have a tendency to ignore hotrocks. This is true of some of the more commonly found hotrocks in gold producing areas but is not true for all rocks. In other words, a person should not expect a PI to simply ignore hotrocks in general. More importantly, the different types of hotrocks do not actually respond the same although some may produce a similar audio response on a PI equipped with a mono coil.
I suspect the above statement doesn't make much sense so I will dwell on it a little more.
The more typical rocks found in gold producing areas that produce either a very strong positive or negative response on a VLF depending upon the ground balance setting generally produce little or no response on a PI. These are usually of the magnetite family of rocks. They are generally black or possibly almost a blue-black in color and are very similar in nature to the black sand commonly found in gold producing areas.
When the delay is very short (10 usec or less) one may get a response when the coil is extremely close to this type of rock (within an inch or two) but normally this signal subsides by elevating the coil and the rock is simply ignored.
Rocks such as basalt commonly found in areas where there was a lot of volcanic activity will usually produce a relatively strong positive response as will various of other rocks containing a slightly different form of iron oxide.
Of course, there will be other rocks that may produce a positive response due to a different composition including those containing small amounts of ores. So, one shouldn't try to catagorize or assume all hotrocks to be the same.
Now, the rocks that Buzz sent me produce a very strong positive response when using a mono coil on my PI, much like a metallic object. However, when the PI is equipped with a DD coil, these same rocks produce a negative response quite similar to what magnetite does on a VLF. Strange!!
One thing that hasn't been discussed at length is a DD coil will produce a negative response on iron objects if they are close to the center of the coil. Pass a small nugget over the center area of a DD coil and you get a nice positive indication. Actually, when a nugget is passed close to the coil, the nugget may produce a positive response when it is close to the outer windings, then produce a negative response between the outer windings and the center overlap zone, and finally produce a postive signal in the overlap zone.
Pass a piece of iron over the same area of the coil and you get a negative or opposite response. This is one nice feature of a DD coil that is nulled.
Iron objects even as small as a thumbnail piece of an old rusty can produce a negative response when directly under the center region of a DD coil. This same metal will produce a positive response on either side of the center zone. So, iron objects close to a DD coil will usually produce a double blip type signal much like a nail does on a mono coil. When the iron object is farther from the coil, the center zone negative signal disappears and you are left with a wide positive indication.
The rocks that Buzz sent me respond much like a weak piece of iron. When the coil is close to the rock, I get a negative response when the rock is under the center of the coil. Fortunately there isn't the typical iron type positive response on either side, and raising the coil a couple of inches and passing over the same rock will cause the rock to produce little if any signal. So, it is easy to make a quick check for them.
Reg
