VLF stands for Extremely Reduced Frequency and is identical engineering used in detectors constructed for coins, jewelry, and relics. VLF detectors broadcast at frequencies measured in kilohertz starting from three kHz to 70 kHz. Listed below are 5 explanation why you could choose a VLF in excess of a PI https://archives.profsurv.com/forum/Professional-Surveyor-Magazine-Discussion/Education/-151484.aspx