Here is a good Sunday read for you. Reg wrote what is still the best introductory text on PI detectors. Recently he added extra chapters at the Findmall forum. Even if you read the original before it is worth reading again.
Understanding The PI Metal Detector by Reg Sniff
http://chemelec.com/Projects/Metal-1a/Understanding-the-PI-Detector.htm
Deepest PI Detector by Reg Sniff
Part One Link deleted since Findmall update broke old links
Deepest PI Detector by Reg Sniff