How Metal Detectors Work

Many people often ask me the question of how exactly do metal detectors work…  So, how does a metal detector work? The basic treasure hunting metal detector has four main components: The stabilizer, control box, shaft, and search coil. The stabilizer is simply a cradle for your arm to help stabilize the metal detector motion while searching. The control box contains the microprocessor, PCB, batteries, speaker, headphone plug. Some metal detectors have a really complex and powerful microprocessor with complicated digital displays, and some others do not have display features at all. The more sophisticated metal detector s on the market are able to discriminate the type of metal that is being detected, the depth of the object, and some can even tell  you the size and type of object (relic or coin).  Most metal detectors are mono-frequency detectors and are going to use VLF technology (very low frequency). This frequency is usually in the 6.4kHz range and is good for detecting non-ferrous metals deep in the ground. The metal detector coil is made up of an outer coil and an inner coil. The outer coil transmits a magnetic field and the inner coil receives or detects resulting magnetic fields coming from objects in the ground. The outer coil transmits a magnetic field into the ground and if there is any conductive (metal) objecst then it amplifies the respective magnetic field of these objects. This causes the object to transmit a magnetic field upward toward the surface of the ground that the inner coil (receiver coil) will detect and transmit the information (intensity, frequency) back to the control box.  Based on how strong the signal is, the detector can approximate the size and the depth of the object being detected.

VLF metal detectors can also distinguish between different metals by analyzing the magnetic field frequency that is received from the buried object. This is done by looking at a phenomenon called Phase Shift. Each type of metal, depending on its inductance and resistance, will give a different frequency back then is received from the transmitter coil. By analyzing the phase shift (difference in frequency) that is received from the object, the advanced metal detectors can approximate what type of metal is being detected below the ground. This can become complicated when you consider that many target objects will be alloys of different metals and that some precious metals have the same phase response as junk metals. Most VLF detectors have the capability to adjust the discrimination to allow for canceling out the signals of ferrous metals like iron (low phase shift) that would account for most of the trash that would be found.

This is the basic operation of the common treasure hunting VLF metal detector. With modern VLF detectors, you can discriminate most trash and find precious metals at significant depths and have a good idea of what it is you have found before digging anything out of the ground.

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1 comment

  1. I’ve always been curious about metal detectors. Thanks for the detailed explanation!

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