Gichd

The detection of metallic objects is possible using electromagnetic sensors exploiting low frequency electromagnetic fields up to some hundred kHz roughly. These sensors are capable of detecting metallic objects buried in the ground at shallow depth, whilst indirectly providing "limited" information on their nature (depth, shape, size, etc.). Proximity of the sensor to the surface is usually required.

  • Magnetic Devices

    Magnetic devices rely on the influence of nearby ferromagnetic objects, either via induced or via residual magnetisation, on top of the Earth magnetic field. They are called magnetometers, or gradiometers when used in a differential arrangement. These very sensitive devices are usually employed to detect large ferromagnetic objects such as UXO and can be effective at depths of several meters, but do not react to non-ferromagnetic targets. They are only used in humanitarian demining when a real need exists (e.g. deeply buried UXO).

  • Metal Detectors (Electromagnetic Induction Devices) (See also detailed MD Patent Study)

    Electromagnetic induction devices, which are often referred to as "metal detectors", are active devices capable of detecting tiny amounts of metal (from a fraction of a gram onwards) at shallow depths. They are still to the best of our knowledge, apart from dogs, the only detectors really being used in the field, and are probably going to remain in use for some time. The vast majority of all deployed mines do indeed contain some amount of metal – the problem is usually in the high false alarm rate rather than in the detection capability. Frequency Domain systems have often represented the choice because they seem to work well especially for very small and nearby objects, but they are being more and more challenged by pulse systems, and not only where ground conditions are severe (e.g. sea-water or laterite soils).

    European metal detector manufacturers are well established at international level and include Ebinger, Förster and Vallon in Germany, Schiebel in Austria, and Guartel in the UK. Most of these companies are small, and jealously guard the secrets of the trade; technical and scientific documentation has unfortunately been rather rare up to now. Recently several systems that take into account the humanitarian demining needs have seen the light. Metal detectors designed for humanitarian demining usually share the following characteristics:

    • Weight: less than 2 kg.

    • Price: in the 2000-4000 EURO range.

    • Size:: round, oval or rectangular head. In the former case the diameter is between 20 and 30 cm, to achieve sufficient depth and a reasonable scanning surface and speed.

    • Operating depth: shallow, i.e. from flush (even with the surface) down to about 10-15 cm for minimum-metal mines, 20-30 cm for mines with an appreciable metallic content, and about 50-70 cm for large metallic objects such as UXO or metallic mines.

    • Electrical/Mechanical: capable of working with standard cell batteries for a long time (tens of hours), and usually simple to use. Many demining teams pay more attention to the ergonomics rather than to the pure performances of the detector itself.

    • Output: normally an audio signal, usually already the result of extensive internal data processing, from which an experienced operator can make some qualitative statement on the target and its position. When using manual methods as the primary procedure, each alarm is carefully checked until it has been fully understood and/or its source removed.

    To the best of our knowledge no current metal detector for humanitarian demining applications delivers some quantitative information on the object under analysis. This is astonishing at first view, since there are other disciplines like Non Destructive Testing where this is the case. It can probably be explained by the urgent priority to enhance detection performance through better background rejection (i.e. reduction of the metal detector false alarm rate) and achieving higher sensitivity, as well as by the need of being very precise whilst usually not having any a priori information on the object under analysis.

    A few large coil metal detectors, by Ebinger and Vallon for example, have been manufactured for the detection of larger metallic objects such as metallic mines or UXO. They can for example be employed as an alternative to magnetometers for the detection of ordnance that is not too deeply buried, or in cases where magnetometers can not be used (magnetic soil).

  • Metal Detector Arrays

    Most metal detector arrays, normally one to several meters wide, are derived from commercially available metal detector technology and are usually employed for vehicle platforms to rapidly scan large areas. Some of them can deliver information not only on the location of metallic objects but also on their depth and their approximate size, for example in the form of an "equivalent object volume" (which can be used to reduce the number of false alarms when looking for UXO for example). Some systems do also employ a special suspension system to make sure that the detectors are always parallel to the surface, and that a constant height is maintained.

    Apart from the use in combination with other sensors, metal detector arrays can be used on their own, possibly for Quality Control applications, and the set-up/maintenance of a data archive in order to compare previously executed searches with new searches (suggestions by TZN). Applications on road and road verge, or in combination with a magnetometer for the detection of UXO are also feasible (suggestions by Förster). They obviously strongly depend on the end user and its SOP.

    http:///images/MetalDetector1.jpg


Record updated on : 13 March 2014
Record id : 1

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QinetiQ  
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ERA Technology  
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Remote Sensing Laboratory  
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Croatian Mine Action Center (CROMAC)  
National Authorities and Centres

Ministry of Defence - Germany  
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RK Consulting  
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HarbourDom GmbH  
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Ebinger  
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Vallon GmbH  
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