GB2116723A - Electromagnetic detection of underground objects - Google Patents

Abstract

An underground tunnelling device with a chisel head 6 is powered by an internal fluid-powered percussion hammer which drives it forward by striking anvil 5 normally in a straight line, from a starting position towards a target position. Deviations from this route are detectable by an instrument above ground. For this purpose, a coil (8) is wound around a leading end portion (2) of the device and energised with AC from a small battery and oscillator housed within the device. The coil is recessed below the outer surface of the body and protected by a sleeve (12). <IMAGE>

Description

SPECIFICATION Improvements relating to the direction of under ground objects This invention relates to the direction of underground objects.

There are coming into increasing use tunnelling devices which are essentially pneumatic or hydraulic hammers which work their way through the ground trailing a fluid supply hose behind them. Thus, holes can be dug at opposite sides of a road, and such a device can be "launched" from one hole towards the other, eventually to emerge having formed a tunnel through which a pipe or cable can then be drawn.

This technique avoids the expense and inconvenience of digging up the road. Likewise, a tunnel can be formed through a road or railway embankment, but without preliminary holes.

These tunnelling devices, or moles as they will now be referred to, generally have long cylindrical bodies which tend to keep them naturally in alignmenu with the set direction. They are of course a close fit in the tunnels which they form. However, there are inevitably occasions when the mole meets a particularly hard obstruction or where some other anomaly in the ground causes itto deviate. If it is allowed to continue on the wrong course, it can bury itself beyond hope of retrieval. It is therefore desirable to have some means of tracking the progress of such a mole.

Reference will be made later to the orientation, which will mean the angle in a horizontal plane between the intended course and the direction in which the mole is actually pointing, and to the inclination which will mean the vertical angle between the longitudinal axis of the mole and the horizontal plane.

One obvious way of tracking might be to use the percussion of the hammer head and an acoustic listening device at the surface. However, this is not sufficiently precise. It can sometimes give a reasonable overhead position, but the really important information is the depth, orientation and inclination of the mole and it cannot reliably determine these.

It has also been proposed to provide in conjunction with the mole a coil with a source of A.C. for it, and in order to obtain an adequate signal it was thought necessary to have a coil wound in the usual fashion on a ferrite core. The only practical position for this was taped to the fluid hose just behind the mole. However, if the mole was going off course, it would be at a different angle to the hose it was trailing, and therefore the information from such a coil would be out of date. It was considered that having a coil mounted on the mole itself was not practical, since an efficient core could not be provided, and it would be very much subject to damage either from the tunnel walls or from the violent vibration. However, unexpectedly, we have found that these anticipated problems can be overcome.

This invention therefore relates to an underground tunnelling device of the kind having an elongate generally cylindrical body, having a hydraulic or pneumatic hammer which imparts repeated blows on a chisel head at the leading end and thereby drives the device through the ground, the fluid supply being through a trailing hose, and one aspect of the present invention comprises having a coil co-axially wound around such a device in an annular recess, and means for generating an A.C. signal in the coil, these means being housed within the body of the device.

According to another aspect of the present invention there is provided a method of tracking a tunnelling device of the kind described and equipped with the coil and A.C. generating means, wherein the electromagnetic field created by the A.C. in the coil is detected by an overground coil array from which can be determined the position, depth, orientation and inclination of the device.

Such a coil array is provided by our instrument known as the GPR 404.

For a better understanding of the invention, one embodiment will now be described, by way of example, with reference to the accompanying draw ing in which the single Figure is a diagrammatic exploded longitudinal section of the leading end of a mole tunnelling device.

The device has a main body 1 in the form of a cylinder which houses a hydraulic or pneumatic hammer (not shown). A nose piece 2 screws into the leading end of the body 1 and has a through passage 3 to receive a shaft 4. At the rear end, this shaft has an anvl 5 on which repeated blows are imparted by the hammer. At the leading end it projects beyond the nose piece 3, and is screw-threaded for receiving a chisel head 6. This is of generally conical form, but stepped rather than smooth flanked.

The main portion of the nose piece 3 is a massive ring 7 whose outside diameter corresponds to that of the main body 1. However, to the rear it has an annular recess 8 in which is wound a coil 9. The ring can be further recessed at 10 and 11 to house a battery and an oscillator circuit for energising the coil with A.C. The battery need only be very small, of the type used for hearing aids for example, and the oscillator circuit can be embodied in a very compact chip. The ring 7 is normally drilled or recessed for engagement by a spanner or wrench for assembing or dismantling the device, and these further portions which are now cut away compare in size and will not seriously weaken the structure. To protect the coil, a plastics sleeve 12 surrounds it, the outside diameter of the sleeve being slightly less than that of the ring 7 so that it will not rub against the tunnel wall.

The coil has been found to give a signal of adequate power at 31/2 Khz for detection on the ground several feet above. Our instrument GPR 404 can readily determine the position and depth, and the orientation can also easily be gauged by twisting the instrument about a vertical axis to find the strongest signal. It will not be so sensitive to inclination, as it relies on coils with horizontal axes. If the sensitivity in this respect needs to be improved, then a vertical coil would give better results. However, a serious deviation in inclination can be detected even with horizontal coils. The instrument is swung about a horizontal axis transverse to the orientation of the mole, and the position of the strongest signal gives the inclination.

CLAIMS (Filed on 16/3/83) 1. An underground tunnelling device of the kind described, having a coil co-axially wound around it in an annular recess and means for generating an AC signal in the coil, these means being housed within the body of the device.

2. A device as claimed in claim 1, wherein the coil and generating means are carried by a nose piece mounted at the forward end of said body.

3. A device as claimed in claim 2, wherein the nose piece has an outer surface co-cylindrical with that of the body, the recess for the coil being formed by an annular rebate at the rear end of that surface.

4. A device as claimed in any preceding claim, wherein a protective sleeve surrounds the coil, the sleeve being wholly within said recess.

5. An underground tunnelling device substantially as hereinbefore described with reference to the accompanying drawing.

6. A method of tracking a tunnelling device of the kind described, and as claimed in any preceding claim, wherein the electromagnetic field created by the AC in the coil is detected by an overground coil array from which can be determined the position, depth, orientation and inclination of the device.

7. A method of tracking a tunnelling device substantially as hereinbefore described with reference to the accompanying drawing.

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

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. gives the inclination. CLAIMS (Filed on 16/3/83)

1. An underground tunnelling device of the kind described, having a coil co-axially wound around it in an annular recess and means for generating an AC signal in the coil, these means being housed within the body of the device.

2. A device as claimed in claim 1, wherein the coil and generating means are carried by a nose piece mounted at the forward end of said body.

3. A device as claimed in claim 2, wherein the nose piece has an outer surface co-cylindrical with that of the body, the recess for the coil being formed by an annular rebate at the rear end of that surface.

4. A device as claimed in any preceding claim, wherein a protective sleeve surrounds the coil, the sleeve being wholly within said recess.

5. An underground tunnelling device substantially as hereinbefore described with reference to the accompanying drawing.

6. A method of tracking a tunnelling device of the kind described, and as claimed in any preceding claim, wherein the electromagnetic field created by the AC in the coil is detected by an overground coil array from which can be determined the position, depth, orientation and inclination of the device.

7. A method of tracking a tunnelling device substantially as hereinbefore described with reference to the accompanying drawing.