GB2163826A - Elongate magnetic member - Google Patents

Abstract

An elongate member (1) for installation under ground, such as an electrical or communications cable or a supply pipe for gas or water, is magnetised in a plurality of axially aligned, individually axially magnetised, equal length sections (2), each section preferably having a length (L) between 0.5 and 5.0 times the intended installation depth for the member (1), whereby detection and identification by means of magnetic field sensors (15, 16 Figure 12 not shown) is simplified. <IMAGE>

Description

SPECIFICATION Elongate magnetic member This invention relates to an elongate magnetic member for installation under ground, such as an electrical or communications cable or a supply pipe for water or gas.

One known method of detecting a buried or submerged cable is to apply an electrical signal to the cable and then detect the electric and/or magnetic field generated thereby.

Another known method is to detect residual magnetisation caused by the past magnetic hysterisis of the cable.

The latter method can be applied not only to a cable but also to a supply pipe for water or gas as long as it contains a magnetic substance.

However, these known methods have the following disadvantages.

The electric andior magnetic field generated by applying an electrical signal to a cable is not always strong enough for detecting the precise location of a cable when buried deep down under the ground or sea bed. Further, when a cable is broken it is impossible to apply an electrical signal thereto, and thus it will not be possible to retrieve such a cable for repair.

In detecting residual magnetisation due to magnetic hysteresis, the distribution pattern of the magnetic field may vary according to the type of cable and with position along the cable, and thus it is very difficult to detect the correct cable if other magnetic members such as wire ropes are nearby.

The situation is the same even when several cables of the same type exist near each other.

According to this invention there is provided an elongate member for installation under ground comprising a plurality of axially aligned, individually axially magnetised, equal length sections.

Preferably each section has a length between 0.5 and 5.0 times the intended depth at which the member is to be buried.

This invention will now be described by way of example with reference to the drawings, in which: Figure 1 is a diagram illustrating the magnetic flux distribution in a member according to the invention; Figures 2 and 3 illustrate the installation of members according to the invention; Figure 4 is a diagram illustrating a method of searching for a buried member according to the invention; Figure 5 is a diagram illustrating the distribution of the measured magnetic field from a cable according to the invention installed under the ground; Figure 6(a) illustrates a pipe according to the invention; Figures 7(a), 7(b) and 7(c) are cross-sectional views of three buried cables according to the invention; Figure 8 illustrates the construction of another member according to the invention;; Figure 9 illustrates a method of magnetically coding a member according to the invention; Figures 10 and 11 are schematic diagrams of two types of apparatus for magnetising members to provide members according to the invention; and Figure 12 is a schematic diagram of an apparatus for use in detecting a member accord:ng to the invention.

Figure 1 shows an elongate cable 1 comprising a plurality of axially aligned, individually axially magnetised, equal length L sections 2.

In Figure 1, the arrov s indicate the directions from the N pole to the 3 pole of each section 2. As shown, the directions of magnetisation of adjacent sections 2 are opposite to each other, that is adjacent ends of adjacent sections have the same magnetic polarity.

Either each section 2 has a constant magnetic flux strength along its length, or the magnetic strength varies along each section 2, for example in accordance with a sine or triangular wave with a period of 2L.

Reference numeral 3 shows the magnetic flux which is generated by the cable 1. By calling the N and S poles of the sections 2 positive and negative magnetic charges, respectively, the magnetic flux generated by each section 2 is represented using arrows passing from the positive magnetic charge to the negative one.

When poles of the same polarity (charges of the same sign) are placed adjacently, they will repel each other, and flux 3 will tend to extend in the radial direction. There is a relation in that the magnetic flux per unit section is proportional to magnetic field intensity. In other words, the magnetic field intensity at a point of high magnetic flux density is large and vice versa.

In Figure 1, if the length L of each section 2 is small, the distance between the positive and negative magnetic charges of each section becomes small, and the resulting magnetic flux 3 is enclosed in a narrow range, so that the magnetic field strength at a point a certain distance in the direction of depth D (Figure 2) becomes small. On the other hand, if the length L is large, the distance between the positive and negative charges at the ends of each section 2 becomes large, so that magnetic flux 3 is enclosed in a broad range. However, the magnetic field strength from the cable 1 will start to decrease at a certain distance therefrom when the length between the positive and negative charges of each section 2 is large.

Thus, there must exist an optimum vlaue of L when a member 1 buried at a depth D below the ground surface "ab", is to be detected using a detector 4 located at a distance H from the member 1, as shown in Figure 3.

A way of finding the optimum value will now be described with reference to Figure 4 which shows a cable 1 which is steel armoured and is magnetised in sections of length L of alternating magnetic polarity. A magnetic sensor 4 is moved along a sensing line b1-b2 (in the Y axis direction) which is perpendicular to the axis of the cable 1 and which lies in a plane ala2a3a4 whose vertical distance from the cable 1 is H.

In setting coordinate axes X, Y, and Z, the Z axis is taken as the upward direction normal to plane a1a2a3a4 from a point above the junction between two adjacent sections of the cable 1 on plane a1 a2a3a4; the X axis is the axial direction of the cable 1 on the plane a1a2a3a4; and the Y axis is on the plane a1a2a3a4 in the direction normal to the X axis.

First measure the value of the maximum absolute magnetic field generated by cable 1 Hymax on the line b1-b2 while moving the magnetic sensor 4 a distance y within the range from -3H to 3H along ty Y axis while keeping the X axis coordinate at an arblitary constant value x0. Then measure maximum values Hymax 1, Hymax 2 , Hymaxn by changing the position of the line b1-b2 n times in the X axis direction between (X-L) and (X-- L).

The minimum value must exist among the measured group of maximum absolute values. However, if Hymax exists at position x0 which is right above cable 1, Hymaxn will have a symmetrical set of values on both sides of point x0. Consequently there will be a pair of Hymin values, that is at points +L and -L on the X axis, where the absolute values of Hymaxn will be the same although the polarities will be different.

Thus, it can be said that the most unfavorable sensing line is the one on which a minimum value Hymin is obtained when detecting the magnetic field in a direction substantially orthogonal to the lengthwise direction of the cable 1. (This is called "worst sensing line" hereinafter).

However, while the magnetic field along the worst sensing line can take only the smallest value of intensity among the n measured maximum values, the magnetic field intensity may be changed to a larger one by changing the length L of each section of the cable as shown in Figure 5. In Figure 5, Hmin has a maximum value when the value of LH is approximately equal to 1. IL H is called L normalized by H). In the range where L H is equal to or less than 1, the value of Hmin will sharply decrease as the value of L;H decreases. Also, in the range where L H is larger than 1, it will decrease as the value LIH increase. Therefore, as shown in Figure 5, there exists an optimum value, which is approximately equal to 1.

Consequently, we find that a meaningful value range for L H is between 0.5 and 5.0 although the magnetic charge distribution in each section 2 of the cable 1 must be taken into consideration as an influential factor in any exact calculation.

The above experiment is for a submerged cable, but the result can also be applied to other types of elongate magnetic member.

It is preferable for the sensor 4 to be installed and moved at a position as close as possible to the ground, ocean floor, river bed, lake floor, etc, that is the height H of the magnetic sensor 4 from the cable 1 should be equal to or as close to installation depth D (Figure 3) as possible.

If cable 1 is composed of a plurality of axially aligned axially magnetised sections each of length L whose value is from 0.5 to 5.0 times the installation depth D of the cable 1, a magnetic field of maximum intensity will be generated in a periodic wave form with a known cycle at a point distant from cable 1 in the direction orthogonal to the lengthwise direction of cable 1. By detecting the intensity of the magnetic field with the magr etic sensor 4, the location of the cable may be found easily.

Figure 6 shows two embodiments of cable 1.

Figure 6(a) shows a hol ow magnetic tube 5 (for example a water supple pipe or gas supply pipe) magnetised in sections of length L. Magnetic tube 5 can be made of any tope of magnetic material such as iron, iron alloy, rubber magnet material, plastics magnet material and so on. The external and, or internal surfaces of the magnetic tube 5 may be covered with another material. The inside of the tube 5 may be either hollow or filled with fluid, liquid or solid material. The cross-sectional shape of the tube 5 can be of any required shape.

Figure 6(b) shows a cable 1 (for example, a communication cable or power cable) which is covered externally by a jacket made of a bunch of magnetic wires 5, such as steel wires. Reference numeral 6 indicates the cable core which consists for example of conductive wire, insulation material, an optical fiber, a jacket, a tension member and so on. In additon, the jacket 5 may be coated with a film of non-magnetic material such as zinc or polyethylene. Things like a rubber magnet or a plastics magnet may be attached to it. Also, magnetic jacket 5 may be covered by a jacket which is made of non-magnetic material such as a polyethylene or of magnetic material such as rubber magnet material.

Figure 7 shows cross-sectional view of a communications cable 1 which has a magnetic wire jacket 5 as shown in Figure 6(b).

In Figures 7(a), 7(b) and 7(c), reference numeral 1 is the communications cable, reference numerals 5a and 5b are magnetic wires, reference numerals 7a and 7b are jackets, reference numeral 8 is an outer conductor, reference numeral 9 is an insul 3- tion layer, and reference number 10 is an inner conductor containing a tension member.

Figure 7(a) shows a construction in which may- netic wire 5a of a constant diameter and of a single kind of material is used.

Figure 7(b) shows a construction in which magnetic wires 5a and 5b, of different materials but having the same diameters are used.

Figure 7(c) shows a cosntruction in which magnetic wires 5a and 5b of different sizes and made of different materials are used.

For the constructions shown in Figures 7(b) and 7(c), if a material having a low coercive force and a high magnetic saturation point is used as wire 5b and wire 5b is placed next to wire 5a whose coercive force is high, the effect from the external magnetic field may be lessened.

If magnet material powder is spread over outer jacket 7b to make its coercive force higher than that of magnetic wire 5, the same effect as with magnetic wire 5b in the above case (Figure 7(b) and 7(c)) will result.

Figure 8 shows another construction of elongate magnetic member to be installed under the ground. In this figure, reference numeral 5 is a sheet of magnetic material in the form of a tape, reference numeral 6 is a communications cable or power supply cable, and reference numeral 7 is a jacket. Magnetic tape 5 can be metallic or of any other magnetic material.

In the constructions shown in Figures 1, 2, 3, 4 and 6, the elongate members consist of a plurality of sections of different polarity alternately, the length L of each section being between 0.5 and 5.0 times of the intended installation depth for the member.

However, other magnetisation patterns are possible.

Figure 9 shows a construction in which the sections are arranged so that they form a binary code to indicate the distance from the member to a certain point therealong, such as terminal station or repeater if a communications cable, a power generation/transfer station if a power supply cable, a water supply station if a water pipe, and a gas tank if a gas pipe.

The manner of arranging the polarity of the sections can be read as a binary number, by regarding, for example, the right and left arrow sections, as "0" and "1" binary digits, respectively. It could be coded as an octal number, hexadecimal number of Grey code, as well, if required. Thus, if such a method of arranging the sections is employed, it is possible to know not only the exact location of the member but also the distance from a certain point therealong, by measuring the change of intensity and direction of the magnetic field using a magnetic sensor 4 as shown in Figure 3.

A method of magnetising a cable to be installed under the ground will now be described with reference to Figure 10.

In Figure 10, assume that the cable 1 is moving from the left to the right, being driven by a winch or cable engine. Cable length measuring unit 11 gives a signal indicating the feed length of cable 1.

By presetting a constant value of length L, the value of L and an output signal will be reset to zero.

For example, the working principle of cable feed length measuring unit 11 is as follows.

A roller is placed so as to be pressed against cable 1. The number of roller revolutions is measured by a rotary encoder as the cable is fed. The output signal from the rotary encoder is sent to a counter unit equiped with a resetting function. The counter reading is displayed and a signal corresponding to the reading is transmitted and resets itself to zero whenever a reading reaches L.

Reference number 12 is a control circuit, which controls intensity and polarity of the output current of a power supply unit 13 as part of the magnetising unit, according to the output signal from cable length measuring unit 11. Reference numeral 14 is a cable magnetising unit equipped with an electromagnet whose shape is designed so that it imposes a magnetic field in the axial direction of the cable 1.

A suitable electromagnet is the shape of a U with a hole in each arm for letting cable 1 go through.

The process of magnetising the cable is as follows. First, set required length L for each section of the cable on the measuring unit 11, L having a value between 0.5 and 5.0 times the intended depth of installation of the cable. Then, set the control circuit 12 so as to magnetise the cable using periodic magnetic distribution such as in a sine (cosine) wave or triangular wave which has a cycle time of twice the length L, and feed the cable 1 through the magnetising unit 14, with the roller of cable length measuring unit 11 pressed against the running cable, to magnetise the cable according to the information set on control circuit 12, with a polarity reversal at the end of each section of length L.

Figure 11 is a schem Itic diagram of another magnetising unit. Refelence numberal 1 is the cable and reference numeral 14 is the magnetising unit. Reference numeral 11 is a cable length measuring unit. This construction differs from that shown in Figure 10 in that a signal is output only when the counter reading becomes equal to L. Reference numberal 13 is the power supply unit of magnetising unit 14, and it also has the function of changing the polarity of the current to be fed to magnetising unit 14 according to the output signal from cable length measuring unit 11. Using the magnetising unit shown in Figure 11, a cable 1 can be magnetised into an alternating polarity sequence of magnetic sections each of which has a constant length L and constant magnetisation.

Figure 12 is a schematic diagram of an elongate magnetic member retrieving unit. In Figure 12, 15 and 16 are magnetic sensors to detect a static magnetic field, and 17 is a circuit to process the outputs from sensors 15 and 16. Reference numeral 18 is a carrier to move the magnetic sensors 15 and 16 and processing circuit 17 around on a surface ab such as the ground. Reference numeral 19 is a display! processing unit located at a point remote from 18, to process and display the output from processing unit 17.

Magnetic sensor 15 is fixed onto the carrier 18 at a positon as close to the ground surface ab as possible, while magnetic sensor 16 is fixed at a position as far from the ground surface ab as possible.

Magnetic sensors 15 E nd 16 must have high sensitivity and almost the same sensitivity to a magnetic field, but they can be either directional or omnidirectional. For example, proton magnetometers or flux gate magnetometers may be used as the sensors.

If a pair of flux gate magnetometers which are directional are used, the two magnetometers must be installed on carrier 18 so that the maximum sensitivity directions coincide. Preferably the sensors 15 and 16 are fixed on the carrier so that their maximum sensitivity direction coincides with the most frequent direction of carrier movement or perpendicular to the ground surface.

Processing circuit 1' sends an output signal corresponding to the difference in measured magnetic fields between sensors 15 and 16 to display processing unit 19. Otherwise the outputs from the sensors 15 and 16 can be individually sent to the display processing unit 19 which operates to calculate and display the difference therebetween.

The location of the cable 1 under the ground is found as follows. Set a sensing line in a certain direction by placing magnetic sensor 15 as close to the ground surface ab as possible. Read the difference in output values between the sensors 15 and 16 displayed by display processing unit 19 while moving carrier 18 along the sensing line.

When the carrier 18 moves far from the cable to be found there is no difference in the readings of the sensors 15 and 16, since only an environmental magnetic field such as terrestrial magnetism is detected, and a value zero will be displayed on display/processing unit 19. When the carrier 18 moves close to the cable, the magnetic field from the cable will give a higher reading for magnetic sensor 15 than for magnetic sensor 16. As the carrier 18 gets closer te the cable being searched for, the difference in readings will increase, and a value proportional to the magnetic field from the cable 1 will be displayed on display processing unit 19.Thus, when a point is discovered where the absolute value of the magnetic field displayed on display, processing unit 19 is a maximum while moving the carrier 18 along the sensing line, it will be known that the cable searched for is just under the sensor 15.

Secondly, repeat the same process along another sensing line.

Thirdly, set a third sensing line as a line connecting the two points where the maximum difference in the measured magnetic field from cable 1 sensed by sensors 15 and 16 is observed after completing the first two sensing lines. This third sensing line is presumed to run along and just above cable 1 or a bunch of unidentified cables including cable 1.

Repeat the same process as the above along the third sensing line, displaying the difference between the readings of sensors 15 and 16 on displays processing unit 19. It may be easily identified if the magnetic body in question is the one being searched for, since the displayed data on display processing unit 19 shows the magnetic field changing in the period of twice the constant section length L. On the other hand, if the magnetic body in question is other than the cable being searched for, for example a common wire rope, no periodic change in the magnetic field will be observed. Therefore, it is possible to distinguish easily the cable being searched for from others. Thus, if only one magnetic member is installed under the ground, there would be no trouble in locating it.

If a number of cables are installed near each other under the ground, the cable being searched for can be identified as follows. It is assumed that there are n cables and that the cables have been magnetised with different section lengths L1, L2 ..... , Ln.

First, set a first sensing line and measure the magnetic fields from the cables to record the point of maximum value for each cable on an unscaled map. Secondly, repeat the operation for a plurality of further sensing lines near the first one and in the same direction as the first one, and estimate the installation route of each cable based on the measured data. Thirdly, track the installation route of each cable and check the periodicity of the magnetic field along the cable to measure the cycle period length thereof.

If the value of the period length is exactly or approximately equal to twice the already known constant section length L for a cable being searched for, the cable tracked is identified as the cable being searched for.

Claims (11)

1. An elongate member for installation under ground, comprising a plurality of axially aligned, individually axially magnetized, equal length sections.

2. A member as claimed in Claim 1, in which each section has a substantially constant magnetic flux strength along its length.

3. A member as claimed in Claim 1, in which the magnetic flux strength varies along each section.

4. A member as claimed in any preceding claim, in which adjacent ends of adjacent sections have the same magnetic polarity.

5. A member as claimed in any one of Claims 1 to 3, in which the mag letic polarity of the sections is in accordance with a predetermined binary code.

6. An elongate member substantially as hereinbefore described wi.h reference to Figure 6, 7, 8 or 9 of the drawings.

7. An elongate member substantially as hereinbefore described with reference to Figures 1 and 2 of the drawings.

8. Apparatus for use in manufacturing a member as claimed in Claim 1, comprising a magnetising unit; means to pasc a member through the magnetising unit a section at a time under the control of a feed length control unit; and a control unit controlling the intensity and polarity of the magnetisation applied to each section by the magnetising unit as the member pa;ses therethrough.

9. Apparatus as clai ned in Claim 8 substantially as hereinbefore described with reference to Figure 10 or Figure 11 of the drawings.

10. Apparatus for detecting a member as claimed in any one of Claims 1 to 7, comprising a carriage for movement over a surface beneath which the member is located; a first magnetic field sensor carried by the carriage as close to the surface as possible; a second magnetic field sensor carried by the carriage at a position remote from the surface; and processing and display means adapted to display an output dependent upon the difference between the readings of the two sensors.

11. Apparatus as claimed in Claim 10, substantially as hereinbefore described with reference to Figure 12 of the drawings.