little video to show why you need to scan all permissions in grid patterns if you use DD .dont declare fields barren unless you have .even a kids toy can find coins that some expensive machines miss.
The old TR machines back in the 70's would see coins directly beneath iron, with the Compass 77b being legendary in that respect, something a VLF will never do, no matter what coil or how much the machine cost. I used to hunt behind VLF's at old house sites with a TR, lots of nails, and lots of coins the VLF's missed, and whose owners declared the site "hunted out".