GB2300267A - Location of underground objects - Google Patents

Abstract

A location system for locating a concealed conductor includes a tuned coil 40 directly or inductively coupled to the concealed conductor 1, the coupling not being a series coupling. An alternating signal is applied to the conductor and a receiver 5 is provided for detecting the field generated by the coil in response to the alternating signal. The coil may be directly connected to the conductor between the conductor and ground. Alternatively, the coil can be connected to the conductor via an inductive coupling (figure 6).

Description

LOCATION OF UNDERGROUND OBJECTS The present invention relates the location of concealed objects, such as pipes or cables buried underground.

When an underground obJect such as a buried pipe or cable is to be located, to permit tracing of the path of the pipe or cable, or to permit excavation in the vicinity of the pipe or cable without damage thereto, it is necessary to locate the position of that underground object. It is well known that if an alternating current is applied to the underground object, and the underground object is conductive, electromagnetic fields will be generated which can be detected from above the ground.

In simple arrangements, where e.g. the underground object is a cable carrying an alternating current, the field generated by that alternating current can be detected and the path of the underground cable traced. Such an arrangement is not suitable for use where there are many underground cables all carrying currents at the same or similar frequencies, and therefore systems have been proposed in which signals of a specific frequency or frequencies are applied to the underground object at e.g. a remote site, and for a tuned receiver then to detect the field generated by that signal along the underground object at places remote from the signals are applied to the underground object thus enables the underground object to which the signals are applied to be identified separately from other adjacent underground objects.

It is also well known to provide facilities for measuring the depth, signal current amplitude, and direction of the underground object relative to the locator (the latter using a receiver with a plurality of antennae).

However, apart from attenuation effects along the underground object, such known systems do not permit specific points along the path of the underground object to be distinguished from other parts of that object. It is thus not possible to identify the location of splices or joints in e.g. telephone or power cables, or joints or junctions in pipes.

Although the path of the underground object can be traced, the operator does not know whether there is a specific part of the underground object at any particular location.

Therefore, the present invention proposes that a tuned coil be fed from an a.c. signal applied to the underground object e.g. to locate and trace its path and be associated with one or more specific points along the underground object, e.g. at the or each splice or joint, and that a receiver then be used which is capable of detecting the field generated by the coil in response to the a.c. signal applied to the underground object.

At first sight, where the underground object is a conductive line, it may be thought that the coil can be connected in series with the line to cause localised fields to be generated at the site of the coil, thereby allowing that site to be identified separately from other parts of the conductor.

However, such an arrangement has the disadvantage that impedance is added to the line, due to the presence of the coils. Furthermore, resonance tuning of such an arrangement is impracticable as it would raise the impedance to unacceptably high levels. It is also difficult to detect the fields from the coils if the axis of the coil is parallel to the axis of the conductive line using a standard receiver.

Although the connection of the coil in series with the line is therefore impractical, the coil must still be coupled to the line so that the a.c. signal applied to the line generates a signal in the coil.

In principle, any non-series coupling is possible, and the coupling may be direct so that the coil is electrically connected to the underground conductor or may be indirect, such that the magnetic field generated by the a.c. signal in the underground object generates a current which then flows through the coil.

Then, in a first embodiment of the present invention, it is proposed that the coil be connected between the underground object and ground. It is then advantageous to tune the coil to the frequency of the a.c. signal applied to the underground object with a capacitor in parallel with the coil, to develop maximum magnetic field with minimum current drain so that the a.c. impedance to ground presented by the coil/capacitor connection is very high. Hence the loss of current from the underground object due to the ground connection of the coil can be made negligible.

Such a coil may be arranged in a generally horizontal plane perpendicular to the axis of the underground object. In this way it is possible to detect the field from that coil using a standard receiver. Alternatively, however, the coil may be arranged so its axis is vertical, permitting it to be detected by a suitably orientated receiver coil in the detector, with the field from the coil being distinguished from the field due to the signal on the underground object itself.

In either case, it is preferable that the coil is closer to the surface than the line, so that its field can be detected easily.

In a second embodiment of the present invention, the coil is not connected directly to the underground object, but is connected via an inductive coupling as this avoids the need for any ground connection, provided the signal on the underground object is sufficiently strong to generate a current in that coupling. Again, a capacitor may need to be provided in parallel with the coil, and it is again preferable that the coil is closer to the surface than the underground object.

By providing such a coil at any site along the underground object which needs to be identified, an operator with a suitable receiver may pass along the path of the underground object, with that path being determined the known methods discussed above.

However, at the specific site or sites at which the coil is provided, the receiver will detect a change in the field pattern corresponding to that coil, and therefore the operator will be able to identify the or each site. The provision of such a coil will not interfere with the ability of the receiver to monitor line conditions or faults by signal current amplitude and direction measurement, in the known way, but will provide the operator with additional information.

Embodiments of the present invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Fig 1 shows the application of a signal to a line being an underground object; Fig 2 shows a possible use of that signal to energise series coils; Fig 3 illustrates a first embodiment of the present invention; Fig 4 illustrates a modification of the embodiment of Fig 3; Fig 5 shows a second embodiment of the present invention and; Fig 6 shows a third embodiment of the present invention.

Before describing embodiments of the present invention, the general principles of location of underground objects using signals applied thereto will be discussed with reference to Fig 1. In Fig 1, an underground object in the form of a conductive line 1 is connected to a transmitter 2 which applies signals of a specific frequency to the line 1. The application of those signals results in signal currents I, flowing along the line 1. The line 1 may be considered to have capacitive couplings C9 along its length which cause progressive decay in the signal amplitude. If there is a fault e.g. at point 3 along the line 1, this will generate a ground current If which will cause an abrupt change in the current amplitude and/or direction of the signal appearing on the line. Thus, by passing a suitable receiver along the line, the signal applied by the generator 2 can be detected, and the location of the fault 3 can also be identified.

However, except for the fault at point 3, at which there is an abrupt change in the signal, it is not possible to identify specific points along the line, except possibly by measuring the progressive decay in amplitude. Thus, if the line has joints or splices at points A, B and C respectively, it will not be possible to identify the location of those points with accuracy.

Therefore, as shown in Fig 2, coils 4 may be connected to the line 1 at those points A, B and C.

The coils 4 cause localised fields to be developed at those points A, B and C and thus when the signal is applied to the line 1 from the generator 2, there is an abrupt change in the field distribution at those points A, B and C which can be detected by a tuned receiver 5 passed along the path of the line 1.

However, if the axis of the coils 4 is aligned with the line 1, as on Fig 2, the fields 20 are generally perpendicular to the fields 21 produced by the signal applied to the line 1 from the generator 2.

This makes those fields 20 difficult to detect, since orientating the receiver 5 so that its horizontal detection coils 22 are arranged to detect the fields 21 means that the coupling of the fields 20 to those coils 22 is minimal. Thus, the operator would continuously have to rotate the receiver 5, which is a disadvantage. Moreover, the coils 4 add impedance to the line, particularly if they are tuned by a parallel capacitor (this is not shown in Fig 2) to increase the fields 20. This could affect the line monitoring and is thus both disadvantageous and impractical.

Therefore, as shown in Fig 3, a first embodiment of the invention proposes that a coil 40 is connected between the line and ground. Furthermore, the axis of the or each coil 40 is perpendicular to the line 1, normally in a horizontal plane so that the fields generated by the coil 40 may be detected by the coils 22 of the receiver 5 when that receiver 5 is in the orientation for detecting the fields 21 from the signal applied by the generator 2 to the line 1.

The coil 40 may be tuned to the frequency of the signal applied by the generator 2 to the line 1 by connecting a capacitor 41 in parallel with the coil 40, and the field generated by the coil 40 may be further increased by providing a ferrite or ferromagnetic core 6 within the coil 40. Such a coil 40 may then be provided at each of the sites A, B and C to permit those sites to be identified.

The presence of the capacitor 40 in parallel with the coil 40 means that the a.c. impedance to ground of the coil/capacitor connection is very high, so that the leakage of the signal from the line 1 via that connection is minimal. If desired, however, the resistance to ground may be increased by providing a resistor 42 between the coil 40 and ground. DC isolation may be provided by providing a capacitor 43 in series with the coil 40 and the resistor 42.

As shown in Fig 3, the coil 40 is closer to the surface 44 of the ground than the line, which increases the ability of the receiver 5 to detect it.

Indeed, it is possible for the coil 40 to be significantly closer to the surface 44 of the ground than the line 1 if desired.

It is also possible to protect the arrangement against lightening strikes by known parallel surge suppressors.

The embodiment of Fig 3 uses a coil 40 which is horizontal, so that it can be detected by the horizontal detection coils 22 of the receiver 5. If, however, the receiver 5 is provided with a vertical coil 50, as in Fig 4, then such a coil will give a null output when directly over the line 1 in the absence of any other field but that generated by the line 1 itself. If then a vertical coil 51 is connected between the line 1 and ground at the points A, B and C, there will be maximum coupling with the coil 50 when the receiver 5 is directly above the coil 51, i.e. the points A, B and C can be identified with precision, since the detection of the underground coil 51 by the coil 50 of the receiver 5 is not affected by the signals carried by the line 1 itself.

Again, in the embodiment of Fig 4, a capacitor 52 is connected in series with the coil 51, and the coil 51 is closer to the surface 44 of the ground than the line 1, to increase the coupling to the coil 50 of the receiver 5.

Another embodiment is shown in Fig 5, in which the detector 5 has two horizontal and mutually perpendicular aerials 60,61. When the detector 5 is directly above the line 1, and in the absence of other fields, the aerial 61 which is perpendicular to the line 1 will have maximised output, and the aerial 60 which is parallel to the line 1 will have no output.

If the coil 40 in the embodiment of Fig 3 has its axis parallel to the line 1, a signal will be detected by the aerial 60 only when the locator 5 is directly above the aerial 40.

In the embodiments of Figs 3 and 4, the coils 40,51 are connected between the line 1 and ground.

Fig 6 shows a further embodiment where there is no ground connection. Instead, a signal clamp 70 is positioned around the line 1 at the sites to be identified (e.g. points A, B and C). The clamp 70 is ferromagnetic and has a secondary winding 71 thereon.

A coil 72 is then connected to that secondary winding 71 and will thus generate a signal which can be detected by the detector 5. Again, the signal generated by the coil 72 may be tuned by a parallel capacitor 73.

With the present invention, it becomes possible to identify specific sites along an underground object. The line 1 may be a simple conductive line, but it is also possible to apply the present invention to other arrangements. For example, telephone transmission is increasingly achieved by optical fibres, but such optical fibres are provided with a conductive sheath. Then, the generator 2 may apply the signal to the sheath and the coil of the present invention be connected to the sheath, to enable the same effects to be achieved as in the embodiments described above. Since the coil is not connected in series with the line, there is no significant additional line impedance, and there is no interference with the ability of a locator operating in a standard way to monitor line conditions or faults.

Claims (13)

1. A location system for locating a concealed conductor, comprising at least one tuned coil electromagnetically coupled to the concealed conductor, the coupling not being a series coupling, means for applying an alternating signal to the conductor, and a receiver for detecting the field generated by the coil in response to the alternating signal.

2. A system according to claim 1 wherein the coil is directly connected to the conductor.

3. A system according to claim 2, wherein the coil is connected between the conductor and ground.

4. A system according to claim 1, wherein the coil is inducting coupled to the conductor.

5. A system according to any one of the preceding claims, wherein the coil is vertical.

6. A system according to any one of claims 1 to 4, wherein the coil is horizontal.

7. A system according to any one of the preceding claims, wherein there is a capacitor in parallel with the or each coil.

8. A system according to any one of the preceding claims wherein the conductor is, or is part of, an object buried underground.

9. A system according to claim 8, wherein the coil is closer to the surface than the conductor.

10. A system according to any one of the preceding claims, wherein a plurality of tuned coils are provided at spaced apart locations along the conductor.

11. A method of locating a concealed conductor, which conductor has at least one tuned coil electromagnetically coupled thereto, the coupling not being a series coupling; in which an alternating signal is applied to the conductor and a receiver detects the field generated by the coil in response to the alternating signal.

12. A location system substantially as herein described with reference to and as illustrated in the accompanying drawings.

13. A method of locating a concealed conductor substantially as any one herein described with reference to the accompanying drawings.