GB2174203A

GB2174203A - Underground cable detectors and methods of detecting such cables

Info

Publication number: GB2174203A
Application number: GB08510063A
Authority: GB
Inventors: Christopher Keith Richardson
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1985-04-19
Filing date: 1985-04-19
Publication date: 1986-10-29
Also published as: WO1986006501A1; GB2174203B; GB8510063D0; JPS62502566A; EP0218669A1; GR861025B

Abstract

A method of detecting underground cables comprises detecting a vertical component of magnetic field produced around the cable as a result of an EMF induced in the cable, the EMF being induced consequent upon the magnetic field of a remotely generated signal cuting the longitudinal axis of the cable and the magnetic field of the signal being substantially horizontally polarised above ground and, due to diffraction effects, substantially vertically polarised below ground. An underground cable detector operative to detect cables according to this method comprises and aerial (10) which is arranged to be substantially vertical when in use and a cable detection means for generating a signal indicating the presence of a cable when the aerial passes through the vertical component of magnetic field produced around the cable.

Description

SPECIFICATION Underground cable detectors and methods of detecting such cables This invention relates to underground cable detectors and methods of detecting underground cables.

A previously known method of detecting underground cables involves the use of a metal detector which radiates electromagnetic signals. When the metal detector is close to an underground cable some of the radiated electromagnetic energy is reflected by the cable and this can be detected by the metal detector thereby indicating the presence of the cable.

This method is disadvantageous in that the detector cannot discriminate between underground cables and non-cable metal objects.

Further, such detectors have a short range.

The present invention is directed to providing an improved underground cable detector as well as a method of detecting such cables at relatively long ranges compared with known methods, and a method of determining the direction in which the cable lies.

According to the present invention there is provided a method of detecting underground cables comprising detecting a vertical component of magnetic field produced around the cable as a result of an EMF induced in the cable, the EMF being induced consequent upon the magnetic field of a remotely generated signal cutting the longitudinal axis of the cable and the magnetic field of the signal being substantially horizontally polarised above ground and, due to diffraction effects, substantially vertically polarised below ground.

According to the present invention there is also provided an underground cable detector operative to detect underground cables according to the method defined above, wherein the detector comprises an aerial, arranged to be substantially vertical when in use, having a coil in which signals are induced when the coil cuts the field lines of the vertical component of magnetic field, and a cable detection means for generating a detection signal indicative of the presence of the underground cable in response to the signals induced in the coil as a result of the coil cutting the field lines of the vertical component of the magnetic field.

The aerial may be in the form of a loop aerial or a ferrite rod aerial.

The aerial may comprise a second coil, and the cable detection means may comprise a field detector for generating an output signal in response to the signals induced in the coil and a vertical detection receiver, coupled to the second coil, tuned to detect very low frequency (VLF) signals and to provide an output signal when the detected VLF signal is substantially zero, thereby indicating that the aerial is substantially vertical, the output signal of the field detector being fed to a sample and hold circuit which is enabled by the output signal from the vertical detection receiver so that the circuit can produce an output signal indicative of detection of the vertical component of the magnetic field only when the aerial is substantially vertical.

Alternatively, the cable detector means may comprise an envelope detector for providing a signal dependent upon the amplitude of the signals induced in the coil, and a minimum detector for providing an output when the amplitude is a minimum.

In this case, the cable detection means may further comprise a frequency discriminator which provides an output dependent upon the frequency distribution of the signals induced in the coil thereby providing means enabling discrimination between a plurality of cables.

The signal provided by the envelope detector and the output provided by the frequency discriminator are combined by the minimum detector to provide an output indicative of the presence of one or more cables.

The output from the minimum detector may be fed into a voltage to frequency converter which provides an audible tone which varies with pitch according to frequency and strength of the received signals.

A cable direction determining means may be provided to be operative to establish the directions in which the longitudinal axis of the cable lies by determining the direction of origin of the remotely generated signal which gives rise to the strongest vertically polarised magnetic field.

The cable direction determining means may comprise a switch means operative to change the tuned frequency of the ferrite rod aerial so that different remotely generated signals can be detected successively, the direction being determined according to the relative strength of signals detected.

The method of detecting an underground cable may further include the step of determining the directions in which the longitudinal axis of the cable lies by determining the direction of origin of the remotely generated signal which gives rise to the strongest vertically polarised magnetic field detected.

The invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a ferrite rod aerial mounted in a gimbal; Figure 2 is a block diagram of a cable detector according to a first embodiment of the present invention; and Figure 3 is a block diagram of a cable detector according to a second embodiment of the present invention.

Figure 1 shows a ferrite rod aerial 1 which is mounted in gimbal 2. The gimbal 2 is a universal bearing which allows turning movement in any direction and comprises a pair of concentric annular rings 3 and 4. The ring 3 supports the ferrite rod 1 so that it can rotate about an axis A, and the ring 4 supports the ring 3 so that it can rotate about an axis B orthogonal to the axis A. The centre of gravity of the ferrite rod 1 is slightly below the plane of the axes A and B so that the gimbal 2 permits the ferrite rod 1 to rest in a vertical position. One or more coils (not shown) are arranged around the ferrite rod 1 so that they are sensitive to only vertical components of magnetic field when the ferrite rod 1 is in a vertical position.

A small weight 5 having a mass which is small compared with the mass of the ferrite rod aerial 1 is positioned at the bottom of the ferrite rod. The presence of the weight 5 reduces the susceptibility of the gimbal 2 and ferrite rod aerial 1 to vibration.

The coil or coils positioned on the ferrite rod aerial 1 are sensitive to vertically polarised magnetic fields produced around underground cables in response to EMFs induced in the cables by broadcast radio signals.

Radio signals broadcast in the frequency range 600 KHz to 1.6 MHz each have a vertically polarised electric field and a horizontally polarised magnetic field. At more than about 1 metre above flat ground, the magnetic field component of the broadcast signals is very accurately located in the horizontal plane and the vertical components of the magnetic field can be typically between 40 and 60 dB less than the level of the horizontal components of magnetic field.

At ground level and below, the electric field becomes horizontally polarised and the magnetic field becomes vertically polarised due to diffraction effects at the air/ground interface.

If there is a cable at or below ground level, then the vertically polarised magnetic field cuts the cable and induces an electro-motive force (EMF) in the cable. This EMF gives rise to a magnetic field around the cable, the magnetic field lines of force of which lie in planes perpendicular to the longitudinal axis of the cable.

Consequently, by detecting the presence of the magnetic field produced by the EMF induced in the cable, the presence of the cable can be deduced.

The magnitude of the EMF induced in the cable is highest, for a given broadcast signal, when the longitudinal axis of a cable is in line with the direction from which the broadcast signal originates. Hence, a cable direction determining means, not shown, may be provided in the underground cable detector to determine the direction of origin of the remotely generated broadcast signal which gives rise to the strongest induced magnetic field detected, or to compare the relative magnitudes of induced magnetic fields corresponding to different broadcast signals,so as to determine the direction in which the cable lies.

Figure 2 shows a block diagram of an underground cable detector which comprises a ferrite rod aerial 10 mechanically coupled to a mechanical oscillator 11. The ferrite rod aerial 10 may be mounted in a gimbal such as that described with reference to Figure 1. The mechanical oscillator 11 is arranged to swing the ferrite rod aerial 10 through a vertical position.

The ferrite rod aerial 10 has wound thereon a pair of coils 12 and 13 which are coupled to a cable detection means which comprises a field detector 14 and a vertical detection receiver 15, respectively. The coil 12 and the field detector 14 detect the presence of vertical components of magnetic fields when the ferrite rod 10 is in a vertical position or they detect horizontal and vertical components of magnetic fields when the rod 10 is swung off vertical. In response to this detection, the field detector 14 produces a signal indicative of the presence of a magnetic field which signal is passed to a sample and hold circuit 16.

The second coil 13 and the vertical detection receiver 15 are arranged to detect very low frequency (VLF) e.g. 16 KHz broadcast signals. Such VLF signals have too long a wavelength to induce an EMF in the underground cables and so no corresponding vertical magnetic field is generated around the cable. Hence, the second coil 13 and the vertical detection receiver 15 will detect no vertical magnetic field components of any VLF signals when the ferrite rod 10 is in a vertical position. When this condition is satisfied the vertical detection receiver 15 produces a signal which enables the sample and hold circuit 16. In response to this enabling signal, the sample and hold circuit 16 looks at the signal which has emerged from the field detector 14 and permits it to be fed to an indicator-monitor 17 where it is displayed and/or recorded.

By sampling the signal from the field detector 14 only when the ferrite rod aerial 10 is in a vertical position, the cable detector only detects vertical components of magnetic field and hence only magnetic fields generated due to the presence of an underground cable.

If there is no underground cable present, there will be no vertical component of magnetic field and hence, the field detector 14 will produce no signal, thereby indicating that an underground cable is not present.

Preferably, the vertical detection receiver 15 is tuned to a plurality of VLF broadcast signals originating from different directions. This improves the ability of the vertical detection receiver 15 to provide an enabling signal when the ferrite rod 10 is vertical.

Figure 3 shows a block diagram of an alternative cable detector. A ferrite rod aerial 20, which is supported by gimbal for example, similar to the one described with reference to Figure 1 comprises a single coil 21 coupled to an amplifier 22. The gimbal allows the aerial 20 to swing freely through the vertical position so that the coil 21 picks up horizontal and vertical magnetic field components when off vertical. Signals corresponding to magnetic field components picked up by the coil of the aerial 20 are fed to a cable detector means which comprises an amplifier 22 and a 600 KHz to 1.6 MHz band pass filter 23 respectively which amplifies and filters the signals before they are fed to an envelope detector 24.

The envelope detector 24 provides a signal dependent upon the amplitude of the received signals, (i.e. the signals included in the coil 21), and this signal is fed to a minimum detector 26 which provides a minimum output signal when the amplitude is a minimum.

The signal received by the envelope detector 24 varies as the aerial 20 swings through the vertical, since when completely vertical, the aerial detects only vertical magnetic fields and hence the signal reaching the envelope detector 24 is a minimum. The minimum detector 26 detects that the output of the envelope detector 24 is at a minimum and, if it is, permits it to pass to an indicating means for indicating the presence of a cable. The indicating means may comprise a tone generator 27 in the form of a voltage to frequency converter which provides a signal which is amplified by an amplifier 28 and then drives a loudspeaker 29.

When no cable is present, the signal received by the envelope detector 24 drops to substantially zero since there is no vertical magnetic field in the absence of a cable.

As the cable is approached, the magnitude of the induced vertical magnetic field increases and hence the magnitude of the minimum detected signal increases. This gives rise to an increase in pitch from the loudspeaker. When the cable detector is immediately above the cable, the vertical components of magnetic field disappears and there is a marked dip in pitch owing to the absence of the vertical component at this point.

The detected signals are also fed to a frequency discriminator 25 which produces an output voltage which is dependent upon the frequency distribution of the received signals.

This voltage is fed to the minimum detector 26 and is combined with the output of the minimum detector when the minimum part of the envelope is detected. Consequently the tone generator frequency varies according to the magnitude of the vertical magnetic field detected and the frequency distribution of the received signals. This enables you to locate two or more cables which are going in different directions since different broadcast signals will be generating different vertical magnetic fields in each.

The frequency and strength of signals reflected from the cable and therefore received by the detector is determined by four major factors; the length of the cable, direction of the cable, the depth of the cable, and the distribution in frequency and strength of the radio signals.

Instead of a ferrite rod aerial, a loop aerial could be used with an annular coil.

Claims

1. A method of detecting underground cables comprising detecting a vertical component of magnetic field produced around the cable as a result of an EMF induced in the cable, the EMF being induced consequent upon the magnetic field of a remotely generated signal cutting the longitudinal axis of the cable and the magnetic field of the signal being substantially horizontally polarised above ground and, due to diffraction effects, substantially vertically polarised below ground.

2. An underground cable detector operative to detect underground cables according to the method defined in claim 1, wherein the detector comprises an aerial, arranged to be substantially vertical when in use, having a coil in which signals are induced when the coil cuts the field lines of the vertical component of magnetic field, and a cable detection means for generating a detection signal indicative of the presence of the underground cable in response to the signals induced in the coil as a result of the coil cutting the field lines of the vertical component of the magnetic field.

3. An underground cable detector according to claim 2 wherein the aerial is in the form of a loop aerial or a ferrite rod aerial.

4. An underground cable detector according to claim 2 or claim 3, wherein the aerial comprises a second coil, and the cable detection means comprises a field detector for generating an output signal in response to the signals induced in the coil and a vertical detection receiver, coupled to the second coil, tuned to detect very low frequency (VLF) signals and to provide an output signal when the detected VLF signal is substantially zero thereby indicating that the aerial is substantially vertical, the output signal of the field detector being fed to a sample and hold circuit which is enabled by the output signal from the vertical detection receiver so that the circuit can produce an output signal indicative of detection of the vertical component of the magnetic field only when the aerial is substantially vertical.

5. An underground cable detector according to claim 2 or claim 3, wherein the cable detection means comprises an envelope detector for providing a signal dependent upon the amplitude of the signals induced in the coil, and a minimum detector for providing an output when the amplitude is a minimum.

6. An underground cable detector according to claim 5, wherein the cable detection means further comprises a frequency discriminator which provides an output dependent upon the frequency distribution of the signals induced in the coil thereby providing means enabling discrimination between a plurality of cables.

7. An underground cable detector according to claim 6, wherein the signal provided by the envelope detector and the output provided by the frequency discriminator are combined by the minimum detector to provide an output indicative of the presence of one or more cables.

8. An underground cable detector according to any one of claims 2 to 7 further comprising a cable direction determining means operative to establish the direction in which the longitudinal axis of the cable lies by determining the direction of origin of the remotely generated signal which gives rise to the strongest vertically polarised magnetic field.

9. A method of detecting an underground cable according to claim 1, the method further including the step of determining the direction in which the longitudinal axis of the cable lies by determining the direction of origin of the remotely generated signal which gives rise to the strongest vertically polarised magnetic field detected.

10. A method of detecting underground cables substantially as herein described.

11. An underground cable detector substantially as herein described with reference to Figure 2 or Figure 3 of the accompanying drawings.