Principle of Metal Detector machine

A metal detector creates invisible electromagnetic field inside the detecting  head to detect a microscopic change in the signal of electromagnetic field  occurring when the metal passes through inside the electromagnetic field.  However, as a testing product containing water and salinity, can also change  the signal of electromagnetic field similarly to to a metal at the time of  metal detecting, the impact of changes in the signal given by the product  itself should be removed effectively in order to detect the finer metal  contaminants.