Metal detectors with vlf technology

Metal detectors were introduced in the early twentieth century with simple provisions to detect metals buried or shallowly placed in the soil, with limited capabilities and tubes and batteries which made the work with these gadgets very tough. The improvements in technology, the invention of micro chips and the advancement in battery technology allowed the improvement of metal detectors as well and the so called VLF technology was employed, in order to improve the metal detector's performance.
The VLF technology is based on a two coil principle, where the transmitter coil is being used to send an electrical impulse in a very low frequency through the coil, emitting a signal, which is being picked up by the second coil, the receiver coil, where the wire of the coil is being used as an antenna to pick up the reflected frequencies from objects which are located anywhere in the ground. The depth of functionality depends on the power employed and the strength of the antenna, but new models use amplifying to improve reception of the reflections and improve the protrusion into the soil.
The signal emitted by the transmitter is actually a magnetic field. The metallic objects are conductive and under the influence of the field, they generate a magnetic field back, weaker than the originally transmitted one. Since that the responding magnetic field is deemed to be opposite of the field emitted, the buried artifact seems to be answering to the emissions.
The coil which is designated as the receiving end of such reflecting magnetic fields is supposed to be entirely shielded from the emitting coil, but the receptive part is not shielded at all, capable to receive any and all signals incoming from the ground. The depth of buried objects is easily defined, the deeper a metal artifact, the weaker will be the response.
The distinction of different metals is easily done and explained. VLF metal detectors function, as previously said, with a very low frequency, hence the name, VLF. The mentioned reflection is supposed to be a mirror image of the frequency emitted, but because of the difference in inductance, which is the quality of conduction of electricity as well as the level of resistance; different level is applicable for all metals, the frequency changes the phase. This change is called VLF phase shifting.
Most metals have varied inductance and resistance, therefore a VLF metal detectors have the ability to distinct between different metals. An application of the notch filter additionally improves the distinction between metals, but also gives you an idea of what is above and below the artifact you are zeroing in.

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