Garrett, BAH!
Minelab - throw them in the trash!
Get yourself a Ground PENETRATING Radar metal detector.
Dunno why I wasted thousands on my GPX, or why idiots blow 10 grand on a GPZ.
This can detect 15 metres below, and give you 3D images!!
For the reasonable price of $35,000
http://accuratelocators.com/usa-xl16-imager-plus.html
Good luck digging through 15m of granite