The original Metallica-Scope was invented after Mel Fisher's father, tested a loop for the Navy. Radio stations were used to navigate ships, by turning a loop from a ship. This is much the same priciple the early 2 box detectors used. The Navy discovered in areas with mountains, the signal became redirected in certain geographical locations. RF (radio frequency) detectors soon were invented, as a way to detect metals. Ore veins can produce feedback signals from distant radio stations.
Your compass reading could be thrown off from an abundance of iron minerals.