The present invention relates to an electro¬ magnetic scanning system which may be used for surveying or inspecting the earth's crust, machines, vehicles, craft of all types and other items and materials capable of having eddy currents induced in the bodies of such items or materials, or in components thereof. For example, the invention is particularly suitable for conducting deep geological surveys of the earth's crust. The present invention consists in an electro¬ magnetic scanning system, comprising a transmitter for producing a beam of low frequency electromagnetic pulses which are directed towards and/or through an item or material to be scanned, a receiver for detecting electromagnetic signals produced as a result of eddy currents induced in the body of the item or material, or conductive components thereof, by the electromagnetic pulses produced by the transmitter, analog-to-digital converter means for converting the received analog signals into a digital format suitable for analysing, and a synchronising pulse generator coupled to the transmitter and receiver for synchronising operation of the receiver with the generation of the electromagnetic pulses by the transmitter.

In the operation of the invention, the beam of electromagnetic pulses is directed towards and/or through an item or material to be scanned and these pulses induce eddy currents within the body of the item

or material, or within conductive components thereof, which, in turn, generate electromagnetic signals which can be detected by the receiver. Eddy currents are induced within the body of a conductor which is in a region where there is a change in magnetic flux and, with the invention, such a region of periodic magnetic flux change is produced by the transmitted train of electromagnetic pulses. Furthermore, conductive items or materials, or items or materials containing conduct- ive components, have specific electromagnetic "signa¬ tures" resulting from induced eddy currents and it is these electromagnetic signatures which are detected by the receiver and which can be processed and analysed so as to produce a survey of the composition of an item or material. Where the signatures of specific items or materials have been prediagnosed, the invention may be utilised to detect any modifications to said items or materials or changes in the contents thereof.

Synchronising pulses for the invention may be derived from a satellite navigation system, such as, that marketed under the trade name NAVSAT, which provides instantaneous and highly accurate three- dimensional location for air, sea and land vehicles equipped with satellite receivers. The information derived from the satellite navigation system may be used to position the scanning equipment by longitude and latitude. Ground/air speed is calculated by pulses derived from an odometer cable and a pulse train of data is transmitted via the output port to a computer once per second.

The present invention also consists in a trans¬ mitter suitable for use in the above-described scanning system, comprising a pulse generator for producing a low frequency train of electrical pulses, an amplifier for amplifying .the electrical pulses, and an array of at least four electrical coils disposed about an axis of the array with the axes of the coils substantially parallel to the array axis, said coils being connected to the output of the amplifier. When the array of coils is energised by the train of amplified electrical pulses supplied by the ampli¬ fier, a beam of electromagnetic pulses is generated along the axis of the array and, by suitable adjustment of the array, may be directed towards and/or through an item or material to be scanned. Suitable screening may be provided at one axial end of the array so that the beam is propagated from only the opposite end of the array. In order to optimise the power of the pulsed or periodic electromagnetic signal, the coils may be conn- ected in. parallel to the amplifier output. Furthermore, the impedance of the coils should be matched to the out¬ put impedance of the amplifier.

The mark-space ratio of the pulses produced by the pulse generator and, hence, of the pulsed or period- ic electromagnetic beam will depend on the electromagnet ic attenuation produced by the item or material being scanned. For example, when utilising the invention to conduct geological surveys of the earth's crust, the mark-space ratio of the pulses will be increased with the depth to which the crust is to be scanned.

Conveniently, the electrical coils are printed circuit coils. Each coil may comprise a double con¬ centric winding printed on an insulating substrate with the turns of the windings being arranged alternately. The turns may be of rectangular configuration. The amplifier may be a stereo power amplifier with the two windings of each coil connected respectively to the two different outputs of the amplifier and the windings connected to each output being mutually inter-connected in parallel.

The coil array may comprise at least four stacks of coils disposed about the axis of the array, each stack comprising a plurality of generally coaxial coils arranged with the axis of the stack substantially parallel to the array axis. For example, each stack may comprise a plurality of superimposed printed circuit coils, each printed circuit coil consisting of double concentric windings, as described above. The individual windings of each printed circuit coil are connected to different outputs of the stereo amplifier and in parallel with the windings of other coils of the stack. • The pulse generator is adapted to produce electr¬ ical pulses at a frequency within the long wave frequency range of the electromagnetic spectrum and, preferably, in the range from 0.33 Hz to 150 Hz.

Preferably, the pulse generator is a stabilised pulse generator which produces square pulses having precisely defined starting edges.

The present invention further consists in a receiver suitable for use in an electromagnetic

scanning system of the type described above and for detecting electromagnetic signals produced by eddy currents induced in items or materials, or conductive components thereof, comprising an electrical coil detector comprising one or more substantially coaxial windings for positioning with their axis directed towards the item or material from which the electro¬ magnetic signals are to be detected, an amplifier for amplifying detected signals, analog-to-digital converter means for converting detected analog signals into a digital format, and means for storing and/or processing the digital signals.

The coil detector may comprise a stack of coaxial coils connected in series. Each coil may be a printed circuit coil and comprise a double concentric winding with the turns of its winding arranged alternately. With such an arrangement, the individual windings of each printed circuit coil may be connected respectively to the two inputs of a two channel amplifier, the windings connected to each channel being mutually interconnected in series. The printed circuit coils are stacked coaxially in a spaced array with adjacent substrates being at least one inch apart.

The digital signals derived from the receiver may be processed and analysed in situ to produce survey or inspection results or may be stored for subsequent processing and analysis.

In order that the present invention may be more readily understood, reference will now be made to the accompanying drawings, in which:-

Figure 1 is a block schematic diagram illust¬ rating a transmitter according to the invention,

Figure 2 is a schematic side elevation of the coil array of Figure 1 , and Figure 3 is a block schematic diagram of a receiver according to the invention.

Referring to Figures 1 and 2 of the drawings, the transmitter is connected to the receiver 1 of a satellite navigation system, such as, a NAVSAT receiver, for supplying synchronising pulses at 1 Hz. The transmitter comprises a low frequency pulse generator 2 to which the synchronising pulses are supplied by the NAVSAT receiver, a stereo power amplifier 3 and a coil array 4 for propagating a beam of electromagnetic pulses. The output of the pulse generator 2 is connected to the two inputs 5 of the stereo amplifier. The coil array 4 comprises four stacks of printed circuit coils disposed about a central axis of the array. Each stack comprises a plurality of superimposed printed circuit coils 7 arranged substantially coaxial with their axes disposed substantially parallel to the central axis of the array. Each coil 7 comprises a double concentric winding 8 of rectangular configuration with the turns of. eagih winding being arranged alternately. The two windings of each printed circuit coil are connected respectively to the two outputs 9 of the stereo ampli¬ fier 2 ^an ^' i ⁿ each stack, the windings 8 connected to each output of the amplifier are interconnected in parallel.

The low frequency pulse generator 2, synchronised by the NAVSAT receiver 1 , is adapted to supply a train of electrical pulses at, for example, 1 Hz, to the power amplifier 3 which may, for example, have an output of 1000 watts per channel. As is apparent from the foregoing, each channel is used to drive one half of the transmitter coil array 4.

The impedance of each set of parallel connected windings 8 is matched to the output impedance of the amplifier 3 at a nominal 4-8 ohms direct current resistance per channel.

As illustrated in Figure 3, the receiver comprises an electrical coil detector 10, a two channel preamplifier 11, a computer 12 with analog-to-digital conversion facilities, a computer 13 with storage on hard disk drives, and a computer 14 with high resolution display capabilities. The operation of the receiver is synchronised with that of the transmitter by synchronising pulses supplied to the computer 12 by the NAVSAT receiver 1. Each of the computers is connected to a display monitor 15,16,17.

The coil detector 10 comprises a stack of substantially coaxial printed circuit coils 18. Each coil 18 comprises a double concentric winding, with the turns of its windings arranged alternately, and the individual windings of each printed circuit coil are connected respectively to the two input channels 19 of the preamplifier. The windings connected to each amplifier channel are mutually interconnected in series. The substrates of the printed circuit coils 18

-8-

are spaced apart in the stack by at least one inch. Except for the printed coils, the stack 10 should be entirely constructed of insulating material.

Wfeen used for conducting deep geological surveys of the earth's crust, the transmitter coil array 4 is disposed with its axis generally perpendicular to the ground, and the pulse generator 2 is operated at low frequency, for example 1Hz, established by the synchronising pulses derived from the NAVSAT receiver 1 so that the starting edge of each pulse is precisely defined. The coil array 4 consequently generates a beam of low frequency electromagnetic pulses which propagate in the earth's crust and induce eddy currents of conductive components within the earth's crust. The mark-space ratio of the pulses is adjusted depending on the depth to which the survey is to be carried out. Hence, the deeper the survey, the larger the mark-space ratio. The eddy currents induced in the conductive components generate electromagnetic signals which are detected by the stack of receiver coils 10 which are also arranged with their axis generally perpendicular to the ground. The computer 12 reads the resulting analog data from the preamplifier 11 , changes the analog signals into digital words and transmits digital data via input/output ports to the other computers 13,14 for processing and analysing to provide data identifying the composition of the volume of the earth's crust scanned by the beam of electromagnetic pulses produced by the transmitter. The computer 12 displays the analog data on an associated monitor 15.

The operation of the computer 12 is synchronised with the production of pulses by the transmitter so that the digital data derived from electromagnetic signals detected by the receiver coils 10 is precisely timed. The computer 13 reads the digital data supplied from the computer 12 via input/output ports, adds date and time to the data and stores the combined data on hard disk storage. The incoming digital data is displayed on its associated monitor 16. The computer 14 also reads the digital data from the computer 12 transforms the digital data for PGA display and displays it on the associated multisynchronous monitor 17 in real time mode.

Whilst a particular embodiment has been desc- ribed, it will be understood that modifications can be made without departing from the scope of the invention.