Geez if minelab ever build something like that you'll be able to just kick back on the camp chair with a beer and watch the robot go and find nuggets for you, bloody awesome.
I wonder how the electronics in the machine affect the detectivity of the coil.
Maybe get one of those electronic vacuum cleaners and let it loose on the packed sand at the beach with some form of shielding between the motor and the coil?
My son is studying Robotics and Mechatronics (1st year). Don't worry I've already discussed the robotic gold detector. Can't give all my secrets away. But it's a ripper. Just hope my son sticks with it and we actually develop the idea. Don't tell anyone.