A person has to be careful when it comes to PI detectors. Most are designed to be used in or around saltwater. Taking them inland can lead to a lot of problems 'cause, contrary to the claims made by the manufacturers, they WILL be affected by the ground. If you look at land hunting units you'll generally see either a manual GB or they'll have ground tracking. In regards to PI disc., at present it's rather rudimentary. The one advantage of the present PI disc. is (on the Infinium and Goldscan 5) that they're effective at full depth, which is NOT the case with VLF's. Another difference between PI and VLF disc. is that the PI version isn't affected by mineralization. ..Willy.