Hi Billm,
Ground mineralisation is a bit of a misnomer, as all soils and rocks and ground contain minerals of one sort or another in their makeup. In relation to metal detectors, it is simply the minerals that cause an undesirable signal that are a problem.
These minerals can be divided into two classes; one that give rise to a conductive response - similar to metals. And the other, that gives rise to a magnetic response.
The conductive response is particularly noticeable on a wet ocean beach. This is because the sand is saturated with sea water and its dissolved salts, which constitute a weak electrical conductor. If you take a one gallon plastic can of sea water, you will get no signal from it with a PI detector. However, since the volume of sea water within a beach is so huge in comparison, it gives a signal simply because of the large volume that the coil is seeing. By comparison, fresh water beaches give no response, because the dissolved minerals are much less.
The magnetic response is caused by iron minerals, and is a result of the magnetic field from the search coil, magnetising the iron particles, which don't return to their unmagnetised state very quickly, when the field switches off at the end of each pulse. This is called magnetic lag, or magnetic viscosity. Many rocks, particularly volcanic lavas, give strong signals for this reason. Also some beaches on volcanic islands have very strongly iron mineralised sand, in addition to the sea water minerals.
However, some iron minerals do not give this magnetic lag signal. Many beaches where I live in South Devon, have brick red sands, and cliffs and rocks of the same material. These however, although rich in iron, do not affect a PI detector.
Australia has many areas which are very rich in the iron minerals (maghemite) that give a strong response on a detector, and makes working these areas almost impossible, unless an electronic method of cancelling the ground signal is used.
Eric.
