Metal Detector Description

A typical metal detector used for detecting buried coins, gold, or landmines consists of a circular horizontal coil assembly held just above the ground.

A pulsed or alternating current generates a time-varying magnetic field around the coil, as shown in figure 1. This field induces currents in a nearby metal object which, in turn, generate a time-varying magnetic field of their own. These fields induce a voltage in the receive coil which, when amplified, reveal the presence of the metal object or target.

Figure 1: Twin coil detector - outer coil is receiver and inner coil is transmitter. Transmit coil field generates eddy currents in buried metal target which magnetically induce voltage in receiver coil.

There are two broad types of metal detector, classified by the type of magnetic field generated by the transmit coil.

1. Pulse induction (PI) detectors typically generate a transmitter current which is turned on for a time, and is then suddenly turned off. The collapsing field generates pulsed eddy currents in the target, which are then detected by analysing the decay of the pulse induced in the receiver coil.
2. Continuous wave (CW) detectors generate a transmitter coil current which alternates at a fixed frequency and amplitude. Small changes in the phase and amplitude of the receiver voltage reveal the presence of metal targets.

Most metal detectors amplify the differences in the receiver coil voltage caused by nearby metal targets and generate a sound signal audible to the operator (aural display) when a target is detected.

The following plots illustrate the different concepts by showing graphs of received signals with time on the horizontal axis. The scales are notional as the intention is to illustrate the principles of the two different kinds of detector.

Signal received by PI detector (blue) has change in decay rate compared to reference signal (red) when passing over a target at about 10 on the horizontal scale.

Signal received by CW detector (blue) has change in phase and amplitude relative to transmitted signal (red) when passing over target at about 10 on horizontal scale.

The main practical issues in designing a useful metal detector are as follows:

Sensitivity and detection depth

The changes caused by the target are small, so sensitive electronic circuits are needed to detect the changes which occur when a target is present. The transmitter coil requires considerable power and the nature of the transmitted signal must be controlled precisely to detect small changes in the receiver coil output due to nearby targets.

Closely related to sensitivity is the detection depth. It is desirable to be able to locate targets deep under the ground. Usually the diameter of the coil is the major determinant of detection depth.

Robustness and reliability

Metal detectors are often used far from support and repair facilities. Therefore reliability is vital, and self-calibration is needed as afar as possible to avoid complex adjustments in field use. The controls need to be well protected from accidental damage or actuation, waterproof (if possible), and resistant to UV in sunlight.

The electronics unit needs to be an integral part of the detector if it is to be set down on the ground and picked up often. This means that it must be very light, with batteries. A body-mounted electronics unit must be easily dis-connectable if used.

External wiring is a major vulnerability and needs to be eliminated if possible.

Endurance

The transmitted power must be as small as possible to enable the detector to be used for as long as possible on a given battery. Recharging from a car or truck is an essential part of a re-chargeable battery design. Demining agencies often prefer dry cell battery designs because re-chargable batteries require careful handling. Careless charging (over-charging, leaving them uncharged for a long time) can damge older types of batteries. However, modern Li-Ion designs have overcome most of these deficiencies.

Immunity to electro-magnetic interference

The detector should be disturbed as little as possible by electromagnetic fields generated by nearby detectors, nearby power lines, radios etc.

Ground effect

The soil will affect the receiver coil output. Some kinds of soil, particularly those containing a high iron content (often known as mineralised soil), affect the output strongly enough to indicate the presence of a metal target with certain kinds of detector. Most detectors provide a means for compensating the output for the ground effect. This usually requires the operator to position the detector near the ground (but not near a metal target) and adjust a control until the target signal disappears.

There are several different techniques for compensating the detector for ground effects.

Ground noise

Small variations in the soil characteristics and stones (particularly those containing iron compounds) can cause small changes in the detector output. Often these changes cause small target-like signals, often known as 'ground noise'. These can confuse the operator as they sound like small targets, but the target signals cannot usually be associated with a ground location.

Target localisation

It must be possible to find the location of a target. Buried targets have to be checked by subsequent probing through the ground or, more commonly, excavation to expose the target for visual inspection. The operator must be able to locate the target accurately enough to avoid unnecessary digging effort.

Target discrimination

It is desirable to enable the operator to detect a different sound when the target is shallow or deep, when the target is large or small, or if the target is a particular kind of metal. This has great practical significance for a gold prospector who would like to ignore all metal targets other than gold.