Important Characteristics for Mine Detectors

Important Characteristics for Mine Detectors

Detection depth

Apart from optimising sensitivity and endurance, mine detectors require a well-known detection depth. Mines are usually placed at a depth of 20 to 50 mm below the surface, but subsequent movement of soil (or the mine) can increase the depth. Deminers would like a detection depth to exceed 200 mm for minimum metal content mines, but in practice have to be content with a depth of 10-13 cm. The depth capability must extend across the width of the detector head, preferably with little change in depth across the head. This reduces the chance of missing targets, or finding targets in a subsequent quality control check.

If the detector is too sensitive (i.e. detection limit is too deep), then more small targets may be detected. Since each may have to be checked, this can greatly increase the work required.

Metal detector coil generates a magnetic field (field direction lines shown). Detection sensitivity for a particular target decreases rapidly with distance away from the coil (shown in shades of blue-green). The detection limit for a particular target has been illustrated by the blue dashed line below the coil.

As a deminer sweeps the ground with the detector from side to side, he may assume that by moving ahead one coil width on each sweep he has not missed targets. The diagram shows that if the target was at position 'A' it would have been missed. If the detection limit is relatively flat across the coil width, this does not matter much.

If the detection limit varies considerably across the coil, this may cause problems for deminers, even if the minimum depth meets the required performance standard. After manual clearance, a quality control check requires that part of the minefield be checked with another metal detector. If there is a chance that targets will be detected, even though they are below the specified depth, this can cause unnecessary work in investigating the targets and possibly repeating the manual clearance task.

Localising capability

Deminers like to use the detector to localise the target as precisely as possible.

Having found a target with the coil horizontal, they may use the coil tilted at an angle, exploiting the much reduced width of the detection limit at an angle.

The detector may function in one of two ways:

1. The deminer moves the detector until the strongest continuous sound is heard - the sound continues while the detector is held still (The most common, almost universal with demining detectors).
2. The deminer has to keep the detector moving to hear a sound. If the detector remains still, the sound dies away. This type of detector compensates for ground effects by averaging the signal over 1 - 3 seconds. Any change lasting a short time is assumed to be a target and causes a sound. Holding the detector still causes the averaging circuit to assume that the target signal is part of the ground and the sound dies away.

Localising the target requires the deminer (in typical operating procedures) to mark the point where the detector sound is loudest, and mark another point on the near side of the target where the sound can just be heard. A further point may be marked 15 cm on the near side of this point where it is safe to start excavating the target. The detector may be responding to metal on the far side of the mine, so excavating the point where the sound is minimal may be dangerous.

Some kinds of metal detectors may have a non-circular detection limit when viewed from above. The sensitivity is concentrated in a narrow zone either side of the centreline.