GB2311135A - Acoustic detection of underground pipes - Google Patents

Abstract

In order to detect the position of a buried pipe 10 carrying a liquid such as water, pressure pulses are set up in the liquid by a transmitter 13, which may contain a piston or a diaphragm, in contact with the liquid. The pressure pulses pass along the pipe and acoustic energy radiated outwardly from the pipe is detected by sensors 20, 21 each comprising a geophone and a ferrite rod (43, 44, fig. 3), and mounted close to the surface 12 of the ground remote from the transmitter in order to detect the pipe at that location. The oscillation source 15 preferably drives the transmitter at a frequency of at least 200Hz. Detectors 20, 21 are preferably aligned with the pipe 10 to determine the distance to the transmitter 13. The transmitter is preferably in a branch 11 of the pipe, and preferred transmitter constructions are described and shown in figs. 4 and 5.

Description

DETECTING UNDERGROUND PIPES The present invention relates to the detection of a pipe by means of pressure waves in the pipe.

US-A-5269335 disclose a method of detecting a buried pipe, in which pressure waves were generated in the pipe.

Those pressure waves passed down the pipe and outwardly of the pipe. They could therefore be detected by sensors adjacent to the ground surface. By providing two sensors, at spaced apart locations, it was possible to determine the location of the pipe relative to the sensors, and a reference sensor could be used to allow distance measurement along the pipe by monitoring the change in energy levels of the pressure waves detected.

In US-A-5269335, the pressure waves in the pipe were generated by a valve which controlled the flow of water in the pipe. Thus by causing the water to intermittently flow and be shut off, a water hammer effect was generated in the pipe, thereby generating the pressure waves.

The efficiency of such an arrangement is dependent on the rate of water flow, and is therefore not always suitable for detection of the pipe. For example, if the water flow is slow, or if the water is not moving in the pipe, then the pressure waves cannot be generated.

The present invention proposes that water or other liquid in a pipe be in communication with a piston or diaphragm, to which an oscillation is applied. The diaphragm then generates pressure waves, e.g. acoustic waves, in the water which can be detected at a point removed from the location of the diaphragm. An arrangement is similar to US-A-5269335 is proposed, but in which the pressure waves are generated by the diaphragm.

Preferably, the diaphragm is mounted on a stand pipe or other branch pipe as this enables the diaphragm to be attached to that branch pipe above the surface, but for the pressure waves then to pass down the branch pipe into the pipe whose location is to be detected at a remote point. It would be possible, however, for the diaphragm to be mounted in a wall of the pipe whose location at a point is to be determined.

Since the vibrations of the diaphragm generate vibrations of the water, there is no need for independent water flow. The present invention is thus applicable not only to arrangements in which water is moving in the pipe, but also in which the water is static. The use of acoustic waves is preferred, since the speed of sound in water is high compared with its speed in e.g. air.

Preferably, the frequency of vibration of the diaphragm is at least 200Hz.

An embodiment 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 a schematic view of the operation of a locator embodying the present invention; Fig. 2 shows in more detail the oscillation source of Fig. 1; Fig. 3 shows in more detail the detector of the embodiment of Fig. 1; Fig. 4 shows an example of the structure of the transmitter of the embodiment of Fig. 1; Fig. 5 shows an alternative example of the transmitter of the embodiment of Fig. 1.

Referring first to Fig. 1, a pipe 10 which is to be located at a remote site A is buried underground. A stand pipe 11 extends from the pipe 10 above the surface 12 of the ground at a transmission site B. A transmitter 13 is connected to the stand pipe 11, and contains a diaphragm. There may be a valve 14 in the stand pipe to prevent water escaping therefrom when the transmitter 13 is not in place.

With the valve 14 opened, a vibration is imparted to the diaphragm of the transmitter 13 by power from an oscillation source 15. The vibration of the diaphragm causes water in the stand pipe 11 to vibrate, and this vibration is transmitted to water in the pipe 10.

Pressure waves therefore move outwardly from the site B.

The main path of transmission is through the water in the pipe 10 but some vibrational energy will radiate outwardly from the pipe 10 along its length.

As a result, sensors 20 and 21 at the detection site A will be able to sense vibrational energy transmitted from the pipe 10 through the ground. The sensors 20, 21 may thus be mounted close to the surface 12 of the ground. The sensors 20, 21 pass signals to a detector 22, and the analysis of those signals enable an operator to determine whether the pipe has been located correctly.

Whilst the presence of the pipe carrying the vibrational energy can be detected by one sensor 20, 21, the use of two sensors enables the location of the pipe 10 at the detection site A to be determined more accurately. If it is found that the sensors 20, 21 are aligned with the pipe, and spaced along it, it may also be possible to determine the distance between the detection site A and the transmission site B by the change in vibrational energy levels between the sensors 20, 21. Alternatively, a further sensor (not shown) may be provided connected at detector 22, but spaced between the detection site A and the transmission site B.

The vibration applied to the diaphragm of the transmitter 13 may be a simple sine wave. However, desired, a modulation may be applied to enable different pipes to be identified. In such circumstances, it may be useful for the oscillation source 15 to signal its modulation to the detector, shown by arrow 23.

Fig. 2 shows the oscillation source 15 in more detail. The oscillation source 15 has a battery 30 forming a power source which drives an oscillator 31 and an power amplifier 32. The path from the battery 30 to the oscillator 31 and the power amplifier 32 passes via a control switch 33 which enables the operator to trigger the generation of oscillations by the oscillator 31.

Those oscillations are amplified by the amplifier 32, and then passed through the transmitter 13. In order to charge the battery 30, a charger circuit 34 is provided which is connected to a charge power input 35.

Indicators 36 indicate the charge state of the battery 30.

Preferably, the frequency which is passed to the transmitter 13 is at least 200Hz. This means that the water in the stand pipe 11 couples to the diaphragm of the transmitter 13 as a pure mass load. This results in the diaphragm movement involving high forces and low velocities. Therefore, it is preferable that the diaphragm is driven by variable reluctance drive mechanism, rather than a moving coil. The diaphragm also preferably has a compliant backing, because it is not normally practical to load the rear surface of the diaphragm.

Referring now to Fig. 3, the detector 22 has two analog inputs 41, 42 connected to each of the sensors 20, 21. Each sensor 20, 21 comprises a geophone 43 and a ferrite rod 44 above the geophone 43. Only one geophone 43 and one ferrite rod 44 are shown in Fig. 3, but there will be one for each sensor 20, 21. The geophone 43 passes signals to the input 41, and from there to a switching unit 45. In a similar way, signals from the ferrite rod 44 are passed by the input 42 through the switching unit 45. The switching of the switching unit 45 is controlled by a processing unit 46, to pass signals from either of the two inputs 41, 42 the processing unit 46. Power for the detector 22 is obtained from a battery pack 47, power from which is passed by a power control unit 48 to the switching unit 45, and thence to the processing unit 46. The processing unit 46 also controls the power control unit 48. The processing unit 46 uses a program stored in a memory 49 to analyse these signals from the geophone 43 and ferrite rod 44, to generate a display which displayed on a display unit 50 to indicate the results of the detection of the acoustic waves. A key pad 51 permits the user to input commands to the processing unit 46 and to the power control unit 48.

Fig. 4 illustrates in more detail one example of the transmitter 13 of Fig. 1. The transmitter has a diaphragm 60 connected to a piston rod 61, with piston rod 61 passing through a core 62 to an armature 63. The core 62 is E-shaped, and has a coil 64 around the central arm of the core 62. Current through that coil 64 causes the core 62 to develop a field which attracts or repels the armature 63. As this happens, the diaphragm 60 is moved in a bore 65 of the transmitter 13. That movement causes pressure waves to develop in any water in the bore 65. The bore 65 is in fluid communication with the stand pipe 11 (Fig. 1) so that the pressure waves pass through the stand pipe 11 to the pipe 10 It can be seen from Fig. 4 that the pressure applied to the liquid in the bore 65 increases as the armature 63 moves towards the core 62. When the diaphragm 60 and the armature 63 is in the position shown in Fig. 4, the pressure of the liquid in the bore 65 is then at ambient pressure. In practice, a peak pressure of approximately + 8 bar relative to the ambient pressure is needed. In order to achieve this, it is possible for the armature 63 to move to contact the core 62. However, if this were to happen, there would be an inverse square law relationship between the force applied and the gap between the core 62 and the armature 63. This would provide a non linear effect which would introduce a significant amount of harmonic distortion to the vibrations of the liquid in the bore 65. This harmonic distortion would mean that some acoustic energy will not be easy to detect, and therefore such an arrangement is undesirable. On the other hand, if the minimum gap between the core 62 and the armature 63 is large, large currents need to be supplied to the coil 64. Thus, the minimum gap between the core 62 and the armature 63 needs to be a compromise between the two considerations.

A further consideration is that there may be variations in ambient pressure of the liquid in the bore 65. One way of dealing with these variations, in the arrangement shown in Fig. 4, is for them to be accommodated by mechanical compliance behind the diaphragm 61. This would then impose a further limitation on the mean gap between the coil 62 and the armature 63. An alternative, however, is shown in the arrangement of Fig. 5. The arrangement of Fig. 5 is similar to that of Fig. 4, and similar reference numerals are used to indicate corresponding parts. However, in the arrangement of Fig. 5, there is a pressurecompensating passage 70 extending around the diaphragm 60. This passage 70 has a sufficiently narrow crosssection that it resists rapid fluid flow therethrough.

As a result, when the diaphragm 60 moves due to vibrations imposed by the interaction of the core 62 and the armature 63, liquid does not pass through the passage 70 to any significant extent, and the movement of the diaphragm 60 is thus communicated as pressure waves through the liquid in the bore 65. On the other hand, relatively slow changes in ambient pressure in the liquid in the bore 65 communicate through the passageway to the back of the diaphragm 60, thus equalising ambient pressure on both sides of the diaphragm 64.

In the arrangement of Fig. 5, however, there needs to be some air or other gas at the back of the diaphragm 60, to permit the diaphragm 60 to move. That gas also increases the de-coupling of the passage 70. If that gas were free, it could pass through the passage 70 and escape. Therefore it is preferable that the gas is contained behind the diaphragm 60, e.g. in a block 71 of closed-cell foam synthetic rubber. Much of the volume of the block 71 will then be gas which is permanently trapped. Note that the block 71 may also be provided in the arrangement of Fig. 4. The block 71 does not need to be attached to the diaphragm 60.

As mentioned above, the passage 70 must be small.

In practice, it is preferable that it has a section which is not more than a tenth of the diameter of the diaphragm 60, and should be at least 20 times its own diameter in length.

Claims (9)

1. A method of detecting a buried pipe, in which pressure pulses are generated in a liquid in the pipe at a first location, pass to a second location remote from the first, and acoustic energy due to the pressure pulses at the second location is detected, thereby to detect the pipe at the second location, the pressure pulses being generated by oscillatory movement of a piston or diaphragm in contact with the liquid, which causes corresponding oscillation of the liquid at the first location.

2. A method according to claim 1, wherein the frequency of the oscillation is at least 200Hz.

3. A method according to claim 1 or claim 2, wherein the piston or diaphragm is in a branch of the pipe.

4. A method according to any one of the preceding claims wherein the acoustic energy is detected by a pair of sensors in contact with the ground proximate the second location.

5. A method according to claim 4, further including aligning the sensors with the pipe after detection thereof, and subsequently determining the distance between the sensors and the first location on the basis of the difference in acoustic energy detected by each sensor.

6. A pipe detection system comprising a piston or diaphragm in contact with liquid in a buried pipe at a first location, means for causing the piston or diaphragm to oscillate, thereby to cause a corresponding oscillation of the liquid at the first location in the pipe and thus generate pressure pulses in the liquid in the pipe, and at least one acoustic sensor for detecting acoustic waves from a second location of the pipe remote from the first location.

7. A system according to claim 6, wherein there are two sensors.

8. A method of detecting pipes substantially as any one herein described with reference to the accompanying drawings.

9. A pipe detection system substantially as herein described with reference to and as illustrated in the accompanying drawings.