WO1998054601A1

WO1998054601A1 - Identification of buried cables

Info

Publication number: WO1998054601A1
Application number: PCT/GB1998/001563
Authority: WO
Inventors: Andrew Biggerstaff Lewis
Original assignee: Radiodetection Limited
Priority date: 1997-05-30
Filing date: 1998-05-28
Publication date: 1998-12-03
Also published as: GB9711222D0; AU7666598A

Abstract

In order to detect a buried cable (12) carrying an alternating signal with a frequency of 10 Hz or less, the magnetic field generated by the signal is detected by a detector (10) having a plurality of magnetic field sensors (11) in a fixed array. The outputs of the sensors (11) are then compared to determine the relative position of the detector (10) and the buried cable (12).

Description

IDENTIFICATION OF BURIED CABLES BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to the location and identification of cable buried underground. SUMMARY OF THE PRIOR ART

The proliferation of networks and buried cables from many different utilities (electricity, telecommunications etc) has made it increasingly difficult to identify whether any particular cable, located by a suitable location technique, belongs to a particularly utility or not .

It is possible to identify buried cables by applying an audio frequency electrical signal to the cable, and then detecting the magnetic fields generated by that signal at the surface. However, such audio frequency signals may be transferred to other cables by induction and capacitive leakage. This leads to distorted magnetic fields, resulting in mis-location of the cable, or even the possibility of the wrong cable being identified, because induction results in the signal being carried by a cable other than that to which it is applied.

Therefore, in WO-A- 96-03664 we proposed that a very low or zero frequency current signal be applied to the cable, with the magnetic field generated by that current signal being detected by a suitable detector. Unlike known systems for detecting audio frequency magnetic fields, the frequency of the signal is low, normally less than 10Hz, preferably less than 1 Hz. Whilst it is possible tn operate at zero frequency (DC) there would then be the problem of distinguishing the signal from stray ground currents and anomalies in the earth's magnetic field.

As explained in WO-A-96-03664 as a result of the use of such a very low frequency, there was no significant coupling of the signal on the cable to adjacent cables, but the range of detection was short. This meant that the detector had to be proximate to the cable, and cable/detector separations of 15cm were suggested. SUMMARY OF THE INVENTION

It has now been realised that if a plurality of sensors are provided, and if those sensors are preferably arranged in a horizontally-extending array, then the sensors may be further away from the cable and still provide satisfactory identification and/or location.

The present invention also permits improved rejection of interference, and location of buried cables may be improved in situations where field distortion has been a problem when known systems are used.

If a plurality of sensors are used, it is possible to compare the outputs of the sensors and thus detect field distortions. Detection of the magnetic fields at low or zero frequency is affected by stray currents on adjacent cables, since these may generate magnetic fields. In WO-A-96-03664, proximity of detection was relied on to avoid such mis-identification. The present invention seeks to avoid this.

Moreover, by arranging the sensors in a horizontally-extending array, the lateral location error is reduced relative to depth. Thus, for a given location error, the sensor to cable depth may be increased. Hence, the sensor array formed by the plurality of sensors may be located on the surface of the ground, or above the ground, over the cable. Exact positioning relative to the cable is not required. The sensor array may then detect a low frequency signal on the cable, thus enabling the correct cable to be identified. Preferably, the array is positioned horizontally, and perpendicular to the cable, the cable having previously been located by a known location technique. It is important that the array is steady, since low magnetic fields are being detected and thus even small movements of the array may affect the result significantly. BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described in detail, by way of example, with reference to the accompanying drawing, in which:

Fig. 1 is a schematic view of a sensor array according to the present invention above a cable;

Fig. 2 shows in more detail the circuitry necessary for processing the outputs of the sensors of the array of Fig . 1 ; and

Fig. 3 shows in more detail the arrangements for generating a suitable output from a single sensor. DETAILED DESCRIPTION

Referring first to Fig. 1, an array 10 of sensors 11 is located above a buried cable 12. As shown in Fig. 1, the array 10 is positioned so that the sensors 11 extend generally horizontally, and perpendicularly to the direction of elongation of the cable 12. To achieve this, the location of the cable 12 will previously have been determined by a suitable location technique, and the present invention is then used to identify the cable, namely to determine that the cable that has been located is the desired cable, namely the one to which the low frequency current signal is applied. There may, for example, be many other cables in the immediate vicinity of the cable 12. When the low frequency current (normally 10Hz or less) is applied to the cable 12, there will be little or no induction of the signal on the cable 12 to any adjacent cables. On the other hand, the magnetic fields generated from the cable 12 will also be small. Nevertheless, since there are a plurality of sensors 11, and the spacing of those sensors 11 is determined by the array 10, it is possible for a device according to the present invention to identify even the weak magnetic fields that occur at some distance from the cable (more than 1m) . Note that the arrangements for applying the low frequency current signal to the cable may be the same as discussed in WO-A-96-03664 and will not be discussed in more detail now. Because the frequency is very low, there is no significant induction of signal into conductors adjacent cable 12, such as conductor 13 in Fig. 1. However, in the unlikely event of a direct short to such other conductor, such as conductor 13, the plurality of sensors 11 permit the resulting field distortion to be detected. As a result, an appropriate correction can be applied, and/or a warning provided to the user. The choice of whether a correction is made, or a warning provided, may be dependent on the severity of the field distortion.

As shown in Fig. 2, analog signals from the sensors 11 pass to an analog-to-digital converter (ADC) 20. Those analog signals depend on the magnetic fields detected by the respective sensors 11. The analog signals are then converted to digital signals by the ADC 20, with the digital signals then representing the instantaneous magnitude of the magnetic field detected. Digital signals from the ADC 20 then pass to a processor (DSP) 21. The ADC 20 and the DSP 21 may be a physical (wire) interconnection, or may be a telecommunications connection. In this case, the processing circuitry to the right of the DSP 21 in Fig. 2 may be at a site remote from the sensor array 10.

The DSP 21 carries out basic processing of the signals from the sensor. In particular, the DSP 21 captures the data from sensors 11, filters those signals to remove very low frequency noise, and interference from many potential sources, and also to buffer the data to reduce the data rate for further processing. In practice, the DSP 21 will operate in a series of frames, with each frame being data from each of the sensors. It should be noted that in Fig. 2 there are eight sensors, whilst in Fig. 1 there are four. The number of sensors is not critical to the present invention.

The DSP 21 is itself controlled by a micro controller 22 which controls the interaction of the DSP 21 and the ADC 20, and also may carry out higher lever processing on the signals from the sensors, to identify the cable 12.

Each sensor 11 is preferably a magnetometer with its own drive circuit 30, as shown in Fig. 3. The output sent from the sensor 11 passes via an amplifier 31 to the ADC 20. Note that the ADC 20 preferably contains a plurality of analog-to-digital conversion circuits, with one conversion circuit for each sensor. Preferably, the drives 30 for all the sensors 11 are controlled by a common clock (not shown) . This synchronises the outputs from the plurality of sensors 11, and so enables the DSP 21 to process their signals as a series of frames.

Claims

1. A method of detecting a buried cable carrying an alternating signal with a frequency of 10 Hz or less, comprising detecting the magnetic field generated by the signal using a detector having a plurality of magnetic field sensors in a fixed array, the output of the sensors being compared to determine the relative position of the detector and the buried cable.

2. A method according to claim 1, wherein the array is horizontal .

3. A method according to claim 1 or claim 2 , wherein the array extends perpendicular to the cable.

4. A method according to claim 1, wherein the comparison of the outputs of the sensors is carried out in a series of frames, with each sensor generating an output signal in each frame .