VLF stands for Extremely Reduced Frequency and is the same technologies Utilized in detectors developed for coins, jewellery, and relics. VLF detectors broadcast at frequencies measured in kilohertz starting from three kHz to 70 kHz. Here are five reasons why you would possibly go with a VLF around a PI http://leydis16.phorum.pl/viewtopic.php?p=1034261#1034261