GB1577742A

GB1577742A - Apparatus for and methods of electromagnetic surveying

Info

Publication number: GB1577742A
Application number: GB1808976A
Authority: GB
Original assignee: Electrolocation Ltd
Current assignee: Radiodetection Ltd
Priority date: 1977-05-04
Filing date: 1977-05-04
Publication date: 1980-10-29

Description

(54) IMPROVEMENTS RELATING TO APPARATUS FOR AND METHODS OF ELECTROMAGNETIC SURVEYING (71) We, ELECTROLOCATION LIMITED, of 30 Queen Charlotte Street, Bristol BS1 4HS, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to apparatus for and methods of electromagnetic surveying.

It is particularly, but not exclusively, concerned with surveying underground electrically conductive elements, and in this Specification it will be convenient to describe the invention in such terms, with the apparatus "above" the element to be detected and, if not directly above some part of the element, then also to one "side" of it.

Metallic pipelines and other subterranean conductors often have an insulating wrap ping, which is subject to deterioration and failure. The location of trouble spots without unearthing lengths of pipe or cable is obviously desirable, and it is an object of the invention to enable this to be done by an operator "walking the line", being kept to the line by some signals, while others reveal the condition of the wrapping.

According to one aspect of the present invention there is provided apparatus for surveying an underground pipeline or other elongated element which carries, or can be made to carry, AC, the pipeline or element then forming a conductor around which there is a substantially cylindrical electromagnetic field, the apparatus comprising a coil array with a first pair of axially parallel coils arrangeable, in use, one above the other with their axes horizontal and transverse to the conductor and a second pair of spaced coils which, with the first pair so positioned, are generally vertical and coplanar therewith; a phase comparator and indicator circuit for determining from said second pair whether the array is located over the conductor, a depth correction circuit including means responsive to the combined output of said first pair of coils to provide a correction factor dependent on the depth of the conductor below said array, and a variable gain amplifier whose output is that of one coil of said first pair modified by the correction factor in such manner that variations in depth alone of the conductor leaves said output substantially unaffected; and indicator means re sponsive to said variable gain amplifier output.

According to another aspect of the present invention there is provided a method of surveying underground pipelines or other elongated elements which are carrying AC generating a generally cylindrical electromagnetic field, wherein the apparatus defined above is traversed over the ground with the coil array arranged as specified, and the output of the phase comparator and indicator circuit is used to maintain the traverse along the conductor while the modified response to the variable gain amplifier is observed.

The phase comparator may be arranged to compare the phases of one of the first pair of coils and one of the second pair to determine on which side of the conductor the coil array lies. It may then act as a gate to pass to the indicator one or other of the signals from the second pair of coils, depending on the phase relationship between said one coils of the first and second pairs. The indicator is conveniently of the moving needle type, the needle being centred when the coil array is directly over the conductor. Excursions to either side warn the operator when he is off course and show the direction to correct. This may be augmented by audible warning means; for example, sound is generated if the operator is off course.

The output of the variable gain amplifier may also be applied to a visual display in the form of a moving needle indicator. It may also be integrated and an alarm ar ranged to respond to variations greater than a predetermined amount in the integrated output.

For a better understanding of the invention, one embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of apparatus for electromagnetic surveying.

Figure 2 is an illustration of an operator using the apparatus, traversing a buried cable; and Figure 3 is a graph associated with part of the circuit of Figure 1.

The apparatus comprises a coil aerial array 1, which may be of the kind described in out Patent No. 1,509,914. It has a first pair of parallel coils 2, 3 carried with their axes horizontal and a second pair of coils 4, 5 at right angles to the first pair and carried vertically, the coplanar coil axes defining the sides of a square or rectangle. Preferably the coils are ferrite cored.

The outputs of these coils are used to determine from a phase comparator and indicator circuit 6 whether the coil array is correctly located above a buried conductor carrying AC, such as a pipeline P (Figure 2). They are also used to determine whether the electromagnetic field has any anomalies, making allowance for changes in depth in the pipeline P. This is ascertained from a depth corrector circuit 7.

The signals from coils 2, 3 are respectively applied through amplifiers 8, 9 to the circuit 7, while that of coil 2 is also applied, after amplification, via limiter 10 to circuit 6. The signals from coils 4, 5 are fed to respective amplifiers and limiters 11, 12 and thence to circuit 6. In that circuit a phase comparator 13 compares the various phase relationships of coils 2, 4 and 5 and governs the operation of a moving needle indicator 14 in the form of a centre-zeroing meter and an audible alarm 15.

A generally cylindrical electromagnetic field is caused by AC in the pipeline P and this current may be applied by a signal generator (not shown) connected between the pipeline and earth. The frequency is preferably between 500 Hz and 10KHz and the current in the range 10 to 2000 mA.

The horizontal and vertical components of this field are picked up by the coils. With the vertical coils 4 and 5 straddling the pipeline as shown in Figure 2, their signals are in antiphase. If the coils 4 and 5 move entirely to one side or the other of the pipeline they go in-phase and the comparator 13 will then cause the alarm 15 to sound, there being silence with the antiphase condition.

In order to distinguish to which side of the pipeline the coil array has strayed, comparison is made between the signals from one of the horizontal coils, coil 2, and from one of the vertical coils, say coil 5.

As coil 5 passes from one side of the pipeline to the other the phase sense of its output will change in relation to that of coil 2, which will only vary in amplitude. Thus a phase comparison between these coils will give a "left" or "right" indication of the location of the coil array in relation to the pipeline, and this is used to switch the polarity of the centre-zeroing meter 14. An indication not only of the side but also of the extent of the deviation is given by this moving needle indicator 14. As the coils move away from the central position so for a short distance does the strength of the signals from the vertical coils rise, and this can be reflected in the swing of the needle.

The signal applied to the indicator is derived from whichever vertical coil is nearest the pipeline P, the pases comparator acting as a gate that switches on sensing a phase change, as described above, selectively to connect the output of circuit 11 or 12 to the indicator 14. Thus the needle will inform the operator on which side and by how much he has deviated from the pipeline.

The amplified outputs of coils 2 and 3 are applied to a ratio computer 16 in the depth correction circuit 7. It can be shown that the ratio of signal strengths from two coils one above the other with their axes horizontal has a direct correspondence with the depth of an AC source directly below them.

Thus the output of the computer 16 will vary with the depth of the pipeline P. The output of the lower coil 3 is fed to a variable gain amplifier 17 whose gain is governed by the output of computer 16 in such manner that fluctuations in pipeline depth are allowed for and no apparent variation in signal strength occurs, provided that there are no anomalies in the pipeline wrapping causing current to leak.

If this correction were not provided, variations in depth would give false indications of such current leakage. The output of amplifier 17 is applied to display means, such as a moving needle indicator 1. If the current intensity shows a sudden variation as the aerials are traversed along the pipeline, then it can be assumed that there is a wrapping fault.

The graph of Figure 3 shows the correction factor slope for depths below coil 3 from 0 5 m to 4 5 m. Assuming that coil is carried 10 m above ground, on the operator's back, this will cover pipe depths to approximately 3 5 m. The slope can be extended for greater depths, if desired.

Tn addition to the automatic gain control provided by computer 16, there may also be manual gain control (not shown).

This can be adjusted so that different generator input levels can be set to convenient meter readings on the indicator 18.

It may be required to provide an indication of absolute pipeline current intensity.

In this case a "calibrate" facility is incorporated, whereby the manual gain control is disconnected and a fixed gain level is substituted. This fixed gain level would be established empirically for given pipecur- rent levels and the meter scaled accordingly. The only other variable that of the depth of the pipe-will still be compensated by the computer 16.

An integrating circuit may be incorporated to monitor the corrected current intensity signal and give an alarm if this varies by more than a predetermined amount over a given time. Thus the operator will be alerted to intensity changes as he walks thc line.

The circuit and meters can be incorporated in a conveniently small container, which may be strap hung in front of the operator, with the coils on his back.

WHAT WE CLAIM IS: - 1. Apparatus for surveying an underground pipeline or other elongated element which carries, or can be made to carry, AC, the pipeline or element then forming a conductor around which there is a substantially cylindrical electromagnetic field, the ap paratus comprising a coil array with a first pair of axially parallel coils arrangeable, in use, one above the other with their axes horizontal and transverse to the conductor and a second pair of spaced coils which, with the first pair so positioned, are generally vertical and co-planar therewith; a phase comparator and indicator circuit for determining from said second pair whether the array is located over the conductor, a depth correction circuit including means responsive to the combined output of said first pair of coils to provide a correction factor dependent on the depth of the conductor below said array, and a variable gain amplifier whose output is that of one coil of said first pair modified by the correction factor in such manner that variations in depth alone of the conductor leaves said output substantially unaffected; and indicator means responsive to said variable gain amplifier output.

2. Apparatus as claimed in claim 1.

wherein the phase comparator is arranged to compare the phases of one of the first pair of coils and one of the second pair to determine on which side of the conductor the aerial array lies.

3. Apparatus as claimed in claim 2, wherein the phase comparator is adapted to act as a gate to pass to the associated indicator one or other of the signals from said second pair of coils, depending on the phase relationship between said one coils c the first and second pairs.

4. Apparatus as claimed in any preceding claim, wherein the indicator associated with the phase comparator is of the moving needle type, the needle being centred when the coil array is directly over the conductor.

5. Apparatus as claimed in any preceding claim, wherein the phase comparator and indicator circuit further includes audible warning means arranged to operate when the coil array strays from registry with the conductor.

6. Apparatus as claimed in any preceding claim, wherein said indicator means responsive to the variable gain amplifier is a moving needle indicator.

7. Apparatus as claimed in any preceding claim, wherein integrating means are provided for the variable gain amplifier output, and an alarm is arranged to respond to variations greater than a predetermined amount in the integrated output.

8. Apparatus for surveying an underground pipeline or other elongated element substantially as hereinbefore described with reference to the accompanying drawings.

9. A method of surveying underground pipelines or other elongated elements carrying AC generating a substantially cylindrical electromagnetic field, wherein apparatus as claimed in any preceding claim is traversed over the ground with the coil array arranged as specified, and the output of the phase comparator and indicator circuit is used to maintain the traverse along the conductor while the modified response to the variable gain amplifier is observed.

10. A method of surveying an underground pipeline or other elongated element substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. This can be adjusted so that different generator input levels can be set to convenient meter readings on the indicator 18. It may be required to provide an indication of absolute pipeline current intensity. In this case a "calibrate" facility is incorporated, whereby the manual gain control is disconnected and a fixed gain level is substituted. This fixed gain level would be established empirically for given pipecur- rent levels and the meter scaled accordingly. The only other variable that of the depth of the pipe-will still be compensated by the computer 16. An integrating circuit may be incorporated to monitor the corrected current intensity signal and give an alarm if this varies by more than a predetermined amount over a given time. Thus the operator will be alerted to intensity changes as he walks thc line. The circuit and meters can be incorporated in a conveniently small container, which may be strap hung in front of the operator, with the coils on his back. WHAT WE CLAIM IS: -

1. Apparatus for surveying an underground pipeline or other elongated element which carries, or can be made to carry, AC, the pipeline or element then forming a conductor around which there is a substantially cylindrical electromagnetic field, the ap paratus comprising a coil array with a first pair of axially parallel coils arrangeable, in use, one above the other with their axes horizontal and transverse to the conductor and a second pair of spaced coils which, with the first pair so positioned, are generally vertical and co-planar therewith; a phase comparator and indicator circuit for determining from said second pair whether the array is located over the conductor, a depth correction circuit including means responsive to the combined output of said first pair of coils to provide a correction factor dependent on the depth of the conductor below said array, and a variable gain amplifier whose output is that of one coil of said first pair modified by the correction factor in such manner that variations in depth alone of the conductor leaves said output substantially unaffected; and indicator means responsive to said variable gain amplifier output.

2. Apparatus as claimed in claim 1.