Metal Detection Technology

Metal detection
Basic principles of metal detection.

Metal detectors are the oldest and mostly widely used detection technology for detecting buried objects. A metal detector exploits Faraday’s Law of induction and uses low frequency electromagnetic induction to detect metal objects at or below the ground surface. The detector generates a primary magnetic field as shown in the figure.  A secondary magnetic field is created around nearby objects that are magnetic or have electrically conductive properties such as iron rich meteorites.

Metal detectors operate on either the continuous wave or pulse methods of transmitting and receiving.

Demining
Operation of a pulse metal detector.

 Continuous wave detectors induce and monitor magnetic fields continuously to sense any disruptions caused by a conductive object’s secondary field, whereas pulse detectors transmit an impulse-like magnetic field and use the receiver to monitor the decay of the secondary magnetic fields.  Both methods have advantages and disadvantages.  Continuous wave detectors offer the highest sensitivity and are typically used for detection on industrial food processing lines.  Pulse detection is preferred for applications where vibration is a problem.  Vibration causes variations in the primary magnetic field, pulse methods are able to gate out the signal from the primary field and just look at the decay tail, which indicates the presence of a buried object as shown in the Figure. In this case an iron-rich meteorite buried at shallow depths within ice.

Pulse induction metal detector for the Lost Meteorites project is designed and built at University of Manchester in conjunction with the British Antarctic Survey (BAS).  The coil panels are built from sections of polythene sled used by BAS (see photo below).  A similar polythene sled system is used by BAS to transport fuel.  The coils are encapsulated in sled material.  For a system design the system aims to detect a 30 mm diameter (100 g) iron-rich meteorite at 300 mm depth (minimum target).  An array of typically 5 panels in parallel will be towed behind a skidoo – the design of this system is a key part of the project development to ensure that it is robust in the harsh Antarctic environment for a 5-6 week field season.

Sled
British Antartic Survey image of fuel tanks being dragged on polythene sledges.

The group at the University of Manchester leading the design of the metal detection technology are also expert in the research of advanced metal detection systems for humanitarian demining, security, screening of food, and scrap recycling.

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