In this system, we do not need two different coils, transmitter and receiver; There is only one coil for both transmitter and receiver working for the same purpose. This technology sends short circuit pulses through a coil wire. Each explosion or impact generates or generates a short magnetic field. When the impact lands on a metal object, the impact creates an opposing magnetic field within the object. PI detectors cannot distinguish different types of metals as accurately as vlf. These devices can have problems with iron-mine floors. They are not as versatile as the VLF. If you search in wet sand or salt water, this type of device will be useful to you.