VLF stands for Incredibly Very low Frequency and is similar technologies used in detectors built for cash, jewellery, and relics. VLF detectors broadcast at frequencies measured in kilohertz ranging from 3 kHz to 70 kHz. Here's 5 reasons why you may decide on a VLF over a PI The depth http://opensource.platon.sk/forum/projects/viewtopic.php?p=12840358#12840358