Conventionally, landmines are detected by means of metal detectors, similar to those employed in treasure hunting. A typical mine detector comprises a search head which is swept over the ground to be cleared of mines, the head including a current-carrying coil arranged to produce an electromagnetic field in the ground. A second coil arrangement is incorporated in the search head to detect eddy currents consequently induced in metallic objects in the ground, which eddy currents may indicate the presence of a landmine or other substantially metallic object.

A problem encountered with such mine detectors is that modem landmines may include a large proportion of plastics material and so the metal to be detected may be small, sometimes as little as one spring within a landmine. It has been proposed to increase the sensitivity of such detectors. However, a consequence of this is the increased number of false alarms generated as the detector encounters small pieces of battlefield scrap.

The invention provides a detector for detecting mines concealed underground, the mine detector comprising a head which can be scanned over a surface of the ground, the head including means arranged to remotely induce conduction currents in the ground

and means arranged to detect an electric field at the surface of the ground, an anomaly in the detected electric field being indicative of the presence of a mine.

The detection of an electric field permits electric insulators, such as landmines containing a high proportion of plastic, to be detected.

The principle behind the invention, namely that of inducing conduction currents in a region of a partially electrically conductive medium and detecting an electric field above the surface of the partially conductive medium, is not limited to mine detection.

Accordingly, a further embodiment of the invention provides a detector for detecting an object concealed within a medium, the apparatus comprising a head which can be scanned over a surface of the medium, the head including means arranged to remotely induce conduction currents in the medium and means arranged to detect an electric field at the surface, an anomaly in the detected electric field being indicative of the presence of an object.

The term object is used throughout this specification to denote any region in a medium having a different electrical conductivity from the surrounding medium, for example foreign bodies in foodstuffs, cracks in masonry etc. Further examples will be apparent to those skilled in the art.

Preferably, the detector includes a coil, or a pair of coils, arranged to induce

substantially straight and parallel currents in a region of the ground. Such currents facilitate the detection of plastic objects, which objects cause the currents to divert.

The currents induced may be alternating or pulsed.

Advantageously, a pair of electrodes is provided to facilitate detection of the electric field above the surface of the ground.

The coils may be arranged to be energised at frequencies between one and one hundred kilohertz. This is an important safety feature, as such low frequencies will minimise electric currents that may be induced in metallic parts of the mine, such as electric circuits connected to electro-explosive devices.

Means for producing an audio signal in dependence on the electric field detected may be provided and arranged to produce a different signal, such as a change in pitch, on detection of an electric field anomaly. In this manner, a user of the mine detector is alerted to the possible presence of a mine.

A handle may be provided, to facilitate manual usage, so that the user can sweep the head over and above the surface of the region to be scanned.

The invention will now be described by way of example, with reference to the accompanying drawings, in which:-

Figure 1 is a schematic diagram of a detector constructed according to a first embodiment of the invention; Figure 2 shows schematically internal features of the search head of the detector of Figure I when in use; Figure 3a is a plan view of a buried landmine with conduction currents and electric potentials induced by the apparatus of Figures 1 and 2; Figure 3b is a side view of the landmine, currents and electric potentials of Figure 3a; Figure 4 is a block diagram showing schematically the apparatus of Figures 1 and 2; Figure 5 illustrates schematically an alternative coil arrangement for the search head of Figure 2; Figure 6 is a schematic plan view of internal features of a search head of a second embodiment of the invention; Figures 7a and 7b are plan views of an object, with conduction currents induced by the apparatus of Figure 6; Figure 8 is a block diagram showing schematically apparatus associated with the search

head of Figure 6; and Figures 9a and 9b show schematically internal features of alternative search heads for the apparatus of Figure 1.

Like reference numerals refer to like parts throughout the specification.

Referring now to Figure 1, a detector of concealed objects, constructed according to the invention, is shown in the form of a landmine detector indicated generally by the reference numeral 1. The mine detector comprises an elongated handle 2 and a search head 3. The handle 2 conveniently is arranged so that an operator can sweep the search head 3 over the surface of a region of ground to be reconnoitred and cleared of mines.

The dimensions of the mine detector 1 are similar to those of conventional metal detectors. The search head 3 includes a coil arrangement which is illustrated, in use, in Figure 2.

The coil arrangement comprises two, D-shaped coils 4,5 arranged in a common plane with their straight regions adjacent and substantially parallel. The coils 4 and 5 are shown in Figure 2 as single wires for clarity, although a plurality of turns, for example of copper, may be employed.

Figure 2 also illustrates conduction currents, indicated generally by the reference numeral 7, induced in a uniform patch of soil by the electromagnetic field generated

by the D coils 4 and 5. Counter-rotating currents are made to flow through the D coils 4,5, at a frequency of typically 50 kHz, which currents generate an annular magnetic field, indicated by the broken lines 6, the axis of the annulus being substantially in the plane in which the coils lie. The annular magnetic field 6 passes into the soil beneath the search head and induces in the soil an electric field which causes concentric, D- shaped circuits 7 of counter-rotating currents to flow. The straight parallel sections of the current circuits 7 give rise to substantially uniform current flow through the region of interest, i. e. the region below the search head 3. The search head 3 of Figure 2 also incorporates a pair of electrodes 8 and 9, the purpose of which will be described later in the specification.

Figures 3a and 3b illustrate the paths of the conduction currents in a region of soil incorporating a cylindrical plastic landmine 10. The broken lines of Figures 3a and 3b denote the paths of electric current in the absence of a mine. The conductivity of plastics material is typically in the region of 10-12 S/m to 10-15 S/m, whereas the conductivity of soil is in the range 1 S/m (salt marsh) to 10-4 S/m (arid desert). Thus, electric current in the region is forced to divert around the plastic mine 10. The diversion of electric current does not occur suddenly at the interface between the mine and surrounding soil, and at the interface between the soil and the atmosphere, as this would create infinite current densities at the interfaces. Instead, the current flow tends to diverge at a distance from the interfaces, as depicted in Figures 3a and 3b, due to an additional electric field created by the build-up of surface charge, as depicted at 11 and 12, at the interfaces. The energisation frequency of the coils 4,5, and hence the

frequency of the induced currents, is maintained below the relaxation frequency of the soil medium to ensure that build-up of surface charge occurs. The relaxation frequency of soil is in the range 100 kHz to 1 GHz, subject to the soil conditions.

Hence, both the landmine 10 and the ground immediately above the mine/soil interfaces become electrically polarised. Thus, an electric field anomaly 13 is created above the surface 14 of the soil above the landmine 10, which anomaly is detectable by means of the electrodes 8 and 9 of Figure 2, incorporated in the search head 3.

As the search head 3 is swept over land containing a landmine 10, the electrodes 8,9 detect a significant change in electric field corresponding to the aforementioned electric field anomaly. The signal from the electrodes 8,9 is input to an amplifier 15, as shown in the block diagram of Figure 5.

The amplifier 15 may be of the type having a high impedance input, for instance an electrometer, which amplifier picks up and amplifies voltage. The voltage picked up by the amplifier 15 is directly proportional to both the anomalous electric field and the separation of the electrodes 8 and 9. Alternatively, the amplifier 15 may be of the type having zero impedance, in which case current is detected. The current induced in the electrodes is directly proportional to the anomalous electric field, the separation of the electrodes and their external mutual capacitance.

The output of amplifier 15 is fed into a phase-sensitive detector 16, which receives a

phase reference from oscillator 17, suitably phase-shifted at 18, and a further amplifier 19. The oscillator 17 is that which is employed to energise the coils 4,5 at safe frequencies, that is to say frequencies that will not induce significant current in any electric circuits that may be present in the mine. This is an important safety feature, as conventional landmines may incorporate electro-explosive circuitry.

The amplifier 19 has a low bandwidth response ranging from DC to a few hertz. The combination of the phase-sensitive detector 16 and amplifier 19 is employed to ensure that the bandwidth of the detection system is kept to a minimum and preferably is matched to the speed at which the search head 3 is swept manually. The purpose of minimising the bandwidth is to reduce the effect of atmospheric noise.

The detected and amplified signal is converted to an audio signal and is applied to headphones 20, or a loudspeaker, in a manner similar to that employed in metal detectors so that an operator can hear a change in audio signal when the search head is swept over ground containing a plastic landmine.

The invention has been described in relation to the detection of plastic landmines, however the mine detector is also suitable for detecting metallic mines. A buried metallic mine in electrical contact with the soil will attract the conduction currents induced by coils 4 and 5 and thus the electric charges present will be of opposite polarity to those generated in respect of a plastic landmine. Therefore, the polarity of the electric field anomaly will indicate whether the buried object is plastic or metallic.

This information may be transmitted to the operator by the generation of different audio signals for the different polarities.

Landmines having internal metallic parts not in contact with the soil may be detected magnetically as in the prior art, which prior art detection means may be incorporated in the mine detector of the invention.

The mine detector of the invention may be powered by a battery, for example a 9V battery, incorporated in the search head 3 or handle 2 of Figure 1.

A single coil may be substituted for the pair of coils 4 and 5, the coil being arranged with two D-shaped sections in order to induce a similar current pattern to that illustrated in Figure 2. A suitable single coil arrangement is shown schematically in Figure 5. The D-shaped sections are indicated by the reference numerals 4'and 5'.

Alternatively, a conventional circular or square coil may be employed, whereupon alternate polarity electric field anomalies will occur as the search head 3 approaches and recedes from the mine.

Internal features of an alternative search head are shown schematically in the plan view of Figure 6. In this embodiment of the invention, the coil arrangement comprises the first pair of D-shaped coils 4,5 and a second pair of D-shaped coils 21,22 arranged in a common plane. The straight regions of the first pair of coils 4,5 are adjacent and substantially parallel, as are those of the second pair 21,22. The coils are shown in this

drawing as single wires for clarity, although a plurality of turns, for example of copper, may be employed. Of course, a single coil, such as that illustrated in Figure 5, may be substituted for each pair of coils, each coil being arranged with two D- shaped sections.

The coils are arranged so that the straight regions of the coils 4,5 are substantially perpendicular to the straight regions of the coils 21,22. The pairs of coils 4,5 and 21,22 are driven in phase quadrature by means of an oscillator 23. The currents flowing in the coils generate an annular magnetic field, the axis of the annulus being substantially in the plane in which the coils lie. Owing to the pairs of coils being driven in phase quadrature, the magnetic field is a rotating annulus. This arrangement is particularly advantageous when attempting to detect objects having a large or small aspect ratio, for example a crack 24 in masonry such as that shown in Figures 7a and 7b.

These drawings also illustrate a portion of the currents flowing through the masonry. These currents are caused by the electric field induced in the masonry by the magnetic field, in a similar manner to that previously described in relation to mine detection. However, owing to the rotation of the magnetic field, the direction of current flow varies in accordance with the changing magnetic field.

Figure 7a illustrates an instance wherein conduction currents, indicated generally by the reference numeral 25, are flowing in a direction approximately parallel to the

length of the crack 24. In this instance, the necessary diversion of electric current around the crack is minimal. Consequently, the boundary of the crack is only slightly electrically polarised. Thus, the electric field anomaly at the surface of the masonry above the crack 24 will be barely detectable.

Figure 7b shows a portion of induced currents 26 flowing through the masonry a moment later. The currents are now flowing in a direction approximately perpendicular to the length of the crack 24. The electric current in the region is forced to divert around the crack. The diversion does not occur suddenly at the interface between the crack and surrounding masonry, and at the interface between the masonry and the atmosphere, as this would create infinite current densities at these interfaces.

Instead, the current flow tends to diverge at a distance from the interfaces, due to additional electric fields created by the build-up of surface charge at the interfaces.

The build-up of surface charge at the interface between the crack and surrounding masonry is indicated in this drawing by the numeral 27. Hence, the masonry around the crack 24 becomes electrically polarised and an electric field anomaly is created above the surface of the masonry over the crack. It will be noted that this anomaly is a maximum when the currents are flowing in a direction perpendicular to the length of the crack.

It is possible that the search head of Figure 2 would miss such a crack if oriented so that the current flow was similar to that of Figure 7a, unless the operator of the detector of Figure 1 physically moved the search head 3 over the same piece of ground

in many different directions. Such a time-consuming action is not necessary with the apparatus of Figure 6, wherein the rotating magnetic field causes currents to flow in different directions.

The search head includes two pairs of electrodes 8,9 and 28,29. The electrodes together detect the changes in the electric field corresponding to the aforementioned electric field anomalies. The signals from each pair 8,9 and 28,29 of electrodes are input to respective amplifiers 30,31, as shown in Figure 8. The in-phase and quadrature signals from the amplifiers 30,31 are input to respective phase-sensitive detectors 32-35. The phase-sensitive detectors 32-35 receive phase references from the oscillator 23 employed to energise the coils 4,5,21,22. The phase-sensitive detectors 32 and 35 receive in-phase signals from the amplifiers 30,31 respectively and an in- phase phase reference from oscillator 23. Similarly, the phase-sensitive detectors 33 and 34 receive quadrature signals from the amplifiers 30,31 respectively and a quadrature phase reference from oscillator 23. The phase reference is suitably phase shifted and amplified at 36. The signals from the phase-sensitive detectors 32-35 are input to a processor 37, which determines maximum and minimum detected anomalies so that the aspect ratio and orientation of the crack or object detected can be indicated to an operator of the detector. Of course, the phase-sensitive detectors 32-35 could be incorporated in the processor 37.

In regions of rough terrain, it is possible that variations in the surface level of, for example, ground being reconnoitred, themselves produce anomalies in the detected

electric field. To this end, means to compensate for variations in the surface of a medium may be provided. The proposed methods and apparatus for compensating for variations in a surface will be described with reference to the search head of Figure 2, in order to retain clarity in the drawings. However, it will be apparent to the person skilled in the art the means by which these methods could be implemented in the search head of Figure 6 employing four coils and four electrodes.

With reference to Figure 9a, the search head 3 further comprises an auxiliary pair of electrodes 38,39, located in the search head so as to be spaced from each other a greater distance than the first pair of electrodes 8,9. The auxiliary pair of electrodes 38,39 is spaced at such a distance so as to not detect electric field anomalies caused by objects located below the first pair of electrodes 8,9. The auxiliary pair of electrodes merely picks up electric field anomalies caused by variations in the level of the surface of the medium. In this manner, such electric field anomalies can be compensated for in the signals from electrodes 8 and 9 by means of a digital processor. The processor may be programmed to account for the fact that the first electrode pair 8,9 and the auxiliary electrode pair 38,39 detect the electric field over different regions of the medium; the surface anomaly detected by electrodes 38 and 39 may not be exactly that being experienced by electrodes 8 and 9. Of course, this processor may be incorporated in the processor 37 of Figure 8, when this method is implemented in the search head having four coils.

The search head 3 may further comprise a planar Faraday screen (not shown),

interposed between the coil plane and the electrode plane. The Faraday screen serves the purpose of shielding the electrodes from the electric potentials of the coils 4,5.

Figure 9b illustrates an alternative method of compensating for variations in the surface of the medium. In this instance, the auxiliary pair of electrodes 38,39 are arranged to be in the same plane as, but perpendicular to, the first pair 8,9. Anomalies in the electric field at the surface of the medium are detected and processed as described above.

Further variations may be made without departing from the scope of the invention. For instance, the search head 3 need not be flat. It may be necessary to have a contoured search head to permit e. g. reconnaissance of trenches, investigation of recessed walls etc.

The electric field anomaly need not be communicated audibly to the user of the detector. A visual indicator, such as a light or a meter, may be employed.

The detector need not be manually sweepable. The apparatus incorporated in the search head may be mounted and moveable mechanically, for example on the underside of a vehicle to permit reconnaissance of road tracks, on a robot to permit remote mine detection, or even on a factory assembly line to locate foreign bodies in manufactured goods.

A plurality of search heads may be provided, for example across the width of the underside of a vehicle.

In some circumstances, the contours of a concealed object may be known. For example, when detecting plastic landmines such as that shown in Figures 3a and 3b, an operator may know that the mine has a circular cross-section in plan view. Thus, the mine detector may be arranged so that the electrodes are curved or crescent-shaped, to match the contours of the edge of the landmine, thereby permitting more efficient detection of landmines.

It has been found that, when a landmine detector constructed according to the invention was used in wet grass, moisture collected on the electrodes and changed the reading of the detected electric field. This effect has been minimised by providing a protective cover for the electrodes. However, the inventor has realised that this effect could be employed beneficially. For example, a detector could be built to detect rising damp on walls.