All three detectors we tested are of the VLF (very low frequency) type. It is the most common, works well in most ground conditions, and is used on both the cheapest and some of the most expensive machines. (The other main type, pulse induction, is strictly high-end.) A VLF detector works by generating oscillating electromagnetic fields via the coil. The field penetrates the ground and then reverses polarity, and, in a sense, echoes back to the coil. If the field encounters a metal object in the ground, it induces an electromagnetic field in the object. That, in turn, alters the nature of the echo, and the electronics in the control box can interpret the change and indicate the likely identity of the metal—they all respond differently to the electromagnetic field, yielding different echoes. (That said, the interpretation is far from exact: The depth, size, and orientation of an object can all change how the machine “reads” it. In my experience, aluminum pull tabs often registered at frequencies that could have indicated silver or copper. Alan, with 30 years of experience under his hat, said, “I just dig everything” the machine picks up.)
