GB2231152A - Positioning transducers in boreholes - Google Patents

Abstract

Apparatus for use in positioning a transducer (38, 40) in a borehole (44) comprises support means (12, 14, 16) for supporting the transducer, a flexible impervious envelope (18) sealed to the support means (12, 14) and enclosing the transducer (38) and means (20) for filling the envelope (18) with liquid. The transducer is a spark discharge means powered via a cable (34) and the envelope is filled with an electrolyte liquid (48), the spark being discharged between electrodes (38, 40). The transducer may, however, be a sensor e.g. to detect compression waves transmitted through the ground. The filled envelope (18) makes good acoustic contact with the walls of the borehole (44). The head (10) is secured to a rod (24) by which the head can be moved to a desired position lengthwise of the borehole (44). The positioning apparatus is particularly suitable for use in horizontal boreholes which are dry. <IMAGE>

Description

POSITIONING TRANSDUCERS IN BOREHOLES This invention is concerned with apparatus suitable for use in positioning a transducer in a borehole and a method of positioning a transducer.

Seismic surveying techniques are known for various purposes. In order to generate a seismic compression wave, various techniques have been used for example small explosions or powerful sparks. These have commonly been used in vertical boreholes and data collected by suitable detecting devices placed on the ground surface around the boreholes at suitable locations and/or in one or more further vertical boreholes appropriately positioned in relation to the borehole in which a compression wave is to be generated. A spark discharge apparatus and apparatus for providing the electrical energy to cause a spark discharge for use in vertical boreholes are described in our copending Patent Application No.

8812594.3 Whilst such apparatus has proved to be effective with vertical boreholes, it has not been conveniently possible to use such spark discharge apparatus in horizontal, or approximately horizontal boreholes.

When surveying ground for the drilling of a tunnel, it is desirable to make a detailed survey of the ground through which the tunnel is to be drilled as the tunnel progresses. This has been achieved previously by drilling a number of vertical bore holes on the approximate line of the tunnel hut this is not convenient in some circumstances, for example where the tunnel is to pass beneath a body of water. It has been proposed to bore, from the leading end of the tunnel, relatively small diameter holes which are parallel or substantially parallel to one another and which extend generally in the direction of the tunnel.By creating a compression wave using suitable apparatus in one of these boreholes and a detecting device in the other borehole it is possible to monitor the speed of travel of the compression waves from the first bore hole to the detector device or the amplitude of the waves received. Renown techniques may be used to analyse the results of this type of survey.

One of the objects of the present invention is to provide improved apparatus suitable for use in positioning a transducer in a borehole.

In one aspect the invention may be considered to provide apparatus suitable for use in positioning a transducer in a borehole comprising support means for supporting the transducer, a flexible impervious envelope sealed to the support means and enclosing the transducer and means for filling the envelope with liquid.

Preferably apparatus in accordance with the invention comprises means, for example comprising a pipe, by which liquid may be supplied to the interior of the envelope and means, for example comprising a further pipe, by which liquid may be removed from the envelope.

Preferably apparatus in accordance with the invention comprises a pressure relief valve situated to vent pressure from within the envelope. In some circumstances, gas may be generated within the envelope and conveniently a plurality of pressure relief valves are provided situated in a position to vent gas from the liquid within the envelope1 irrespective of the general orientation of the envelope.

In a preferred apparatus in accordance with the invention the support means comprises a first member mounting the transducer, a second member remote from the first, and connecting frame means extending between the first and second members to retain the members in spaced relation. The envelope is suitably provided by a tubular sleeve sealed at one end to the first member and at the other end to the second member, and enclosing the frame means. Where pressure relief valves are provided, these may be mounted at appropriate positions on the first and second members.

Apparatus in accordance with the invention preferably further comprises mounting means by which the support means is mounted for movement in the borehole.

Suitably the mounting means comprises a stiff rod to one end portion of which the support means is secured, the other end portion of the rod comprising means by which the rod can be connected to a further rod. Suitably the means by which rod can be connected to a further rod comprises a collar secured to the end portion of the first rod, adapted to receive and retain an end portion of a further rod, for example by means of internal screw threads in the collar and external threads on the end portion of the further rod. In order to move the illustrative apparatus along a considerable length of borehole, a plurality of further rods may be secured in similar fashion, one to the next.

In one preferred apparatus in accordance with the invention, the transducer comprises a spark discharge means. Suitably the spark discharge means comprises a first electrode mounted by, but electrically insulated from, the said first member of the support means, and the frame means provides a further electrode. An electrolyte liquid, for example sodium chloride solution, fills the envelope.

In another apparatus in accordance with the invention the transducer may be a sensor adapted to sense compression waves. Suitably the sensor may be a hydrophone and the envelope may be filled with water, or other suitable liquid.

In another aspect the invention may be considered to provide a method of positioning a transducer in a borehole comprising mounting the transducer in apparatus according to the invention, introducing the apparatus with the envelope in an empty or only partially filled condition into the borehole, moving the support means and envelope to a desired position lengthwise of the borehole, and filling the envelope with liquid whereby to force the envelope firmly into contact with the wall of the borehole with part, at least, of the transducer immersed in the liquid.

Apparatus in accordance with the invention is especially suitable for use in carrying out a method in accordance with the invention where the boreholes are dry. Inflation of the envelope so that the exterior of the envelope is in firm contact with the wall of the borehole provides good acoustic coupling to the surrounding borehole wall. This apparatus and method are especially suitable for use in horizontal or substantially horizontal boreholes.

These now follows a detailed description to be read with reference to the accompanying drawings, of apparatus and a method embodying the invention. It will be realised that this apparatus and method have been selected for description to illustrate the invention by way of example.

In the accompanying drawings: Figure 1 is a diagrammatic view in section showing apparatus embodying the invention; Figure 2 is a diagrammatic view showing the use of apparatus embodying the invention in carrying out a seismic survey; Figure 3 is a further view showing diagrammatically the circuitry involved in carrying out the seismic survey method shown in Figure 2; Figure 4 is a diagrammatic view showing the mounting means of the illustrative apparatus; and Figure 5 is a diagrammatic view on an enlarged scale of part of the illustrative apparatus shown in Figure 4.

The illustrative apparatus embodying the invention, is suitable for use in positioning a transducer in a borehole, especially a horizontal or substantially horizontal borehole. The illustrative apparatus comprises a head 10 shown in Figure 1 which comprises support means for supporting a transducer (in the head shown in Figure 1 the transducer is a spark discharge means). The support means comprises a first member 12 mounting the transducer and a second member 14 remote from the member 12 and connected to the member 12 by frame means 16 extending between the first and second members 12, 14 to retain the members in spaced relation.

The head 10 further comprises a flexible impervious envelope 18 which in the illustrative apparatus is provided by a tubular sleeve sealed at one end portion to the first member 12 and at the other end portion to the second member 14; as can be seen, the transducer is enclosed by the envelope. The envelope 18 is sufficiently strong to withstand the and other damaging treatment to which it might be subjected during insertion into a borehole while yet being relatively flexible and impervious. A suitable heavy rubber sleeve may be used; alternatively other flexible impervious material of sufficient strength and abrasion resistance may be used.

The illustrative apparatus further comprises means, namely a pipe 20 for filling the envelope 18 with liquid.

The apparatus comprises a further pipe 22 by which liquid may be removed from within the envelope.

The head 10 further comprises a plurality (conveniently six) of pressure relief valves (not shown) situated to vent gaseous material from within the envelope. The six valves are positioned so that any gas build-up within the envelope can be vented thus whatever the orientation of the head 10. The illustrative apparatus further comprises mounting means comprising a plurality of stiff rods. The first member 12 of the support means is secured to an end portion of a first rod 24. The rod 24 comprises coupling means 26 at the opposite end from the head 10 by which the rod 24 can be connected to a further rod 28 having at its end portion remote from the rod 24 a further coupling means 26 by which the rod can be connected to a further rod 30 likewise provided with coupling means 26 for connecting the rod 30 to further rods (not shown).Suitably the rods 24, 28, 30 are about 2 cms in diameter and each about 4 metres long. By joining together a plurality of rods 24, 28, 30 mounting means of sufficient length to propel the head 10 along a borehole for considerable distance may be provided.

The coupling means 26 is conveniently provided by a collar 32 secured to one end portion of a rod 28 (see Figure 5) and of somewhat greater diameter than the rod 28. The collar 32 may conveniently be internally threaded to receive in threaded engagement a threaded end portion of a next adjacent rod for example the rod 30.

The two pipes 20, 22, extend along the rods 24, 28, 30 externally thereof and electrical communication with the head 10 is provided by a suitable cable 34 which likewise passes along the outside of the rods 24, 28, 30.

In order to reduce the risk of the pipes 20, 22 or cable 34 snagging as the illustrative apparatus is inserted into a borehole, the pipes 20, 22 and cable 34 are retained in a fairly taut condition (whilst taking care not to cause damage by over-tightening) and are secured in place by a suitable self-adhesive tape 36 wound around the pipes 20, 22 and cable 34 where they pass the collar 32, the self adhesive tape being sufficiently wide to cover the pipes 20, 22 and cable 34 at either side of the collar 32 to some extent. Thus, as the illustrative apparatus is inserted into a borehole, the tape 36 will provide protection against damage as well as securing the pipes 20, 22 and cable 34 in position.

The transducer shown in Figure 1 is a spark discharge means and comprises a first electrode 38 mounted by, but electrically insulated from the first member 12. The frame means 16 comprises two parallel metal rods 40 which, as well as providing the frame means, provide a further electrode of the spark discharge means. The first electrode 38 is disposed generally centrally of the head 10 extending generally axially thereof parallel with the rods 40. In the apparatus shown in Figure 1 the cable 34 is a coaxial power cable of which the core is connected to the first electrode 38 to provide a high voltage thereto, while the rods 40 provide an earth return and are connected to the first member 12 which is connected to the earth return sheathing of the cable 34. The second member 14 is also metal and in good electrical communication with the rods 40.

Although the transducer shown in Figure 1 is a spark discharge means, the transducer of apparatus in accordance with the invention generally similar to the illustrative apparatus may be a sensor, for example adapted to sense compression waves generated by the spark discharge means shown in Figure 1. A suitable sensor is mounted on frame means 16 which may, in the case of apparatus comprising a sensor, be provided by a stiff metal plate instead of the rods 40. Suitably the sensor comprises a hydrophone detector 42 (see Figure 3) of known construction.

In carrying out a method in accordance with invention, the heads 10, 11 of apparatus embodying the invention with the envelope 18 in an empty or only partially filled condition (in which the head 10 is shown in Figure 4) are introduced into boreholes 44, each being pushed along the borehole to a desired position lengthwise of the borehole by an associated set of rods 24, 28, 30, a plurality of rods being connected together by coupling means as required until the heads 10, 11 reach a desired position lengthwise of the bore hole 44.

For each head 10, 11 suitable liquid is then introduced through the pipe 20 to fill the envelope 18 and to force the envelope 18 firmly into contact with the wall of the borehole 44 making good acoustic contact.

As shown in Figure 2, the boreholes 44 are conveniently drilled from the end of a tunnel 46, generally along the line of the tunnel but diverging slightly. The heads 10, 11 can be pushed using the rods 24, 28, 30 to desired positions lengthwise of the boreholes 44 and readings taken at the various positions to build up an image of the ground G between the boreholes.

When it is desired to move the heads 10, 11 to a different position. some of the liquid is removed from within the envelope 18 through the pipe 22 to deflate the envelope 18 sufficiently that there is little or no risk of the envelope 18 being damaged by the walls of the boreholes 44. When a new position at which readings are to be taken is reached, the envelope 18 is reinflated.

The envelope 18 of the head 10 is filled with a suitable electrolyte liquid 48, for example sodium chloride solution and the spark discharge means is powered by spark generation apparatus 50 substantially as described in our copending Patent Application No.8812594.3 or by any other suitable generation apparatus. After having made a number of electrical discharges creating sparks of sufficient power for use in the seismic survey, there is a tendency for the electrolyte to degrade gradually: when degradation of the electrolyte occurs, the envelope 18 may be emptied through the pipe 22 and further, fresh electrolyte liquid 48 introduced through the supply pipe 20. Gas dissolved in the liquid 48 will be carried out when liquid is removed but should any significant volume of gas build up in the head 10, this will usually be vented through one of the pressure relief valves mentioned previously.

Head 11 comprising a hydro-phone detector 42 is suitably filled with water and if desired the water may similarly be exchanged through the pipes 20, 22 of the head 11. The sensor mounted in the head 11 is arranged to transmit appropriate signals, upon detection of compression waves generated by the spark discharge means of the head 10, and transmitted through the ground between the two boreholes, to a suitable recorder 52. In the illustrative apparatus the recorder 52 is a digital seismograph, signals to which are usually provided by a geophone which is a moving coil device of known construction.The preferred hydrophone of the head 11 comprises a ceramic transducer mounted on the frame means 16 but the output signals from a hydrophone are not readily recorded by a digital seismograpoh; accordingly, the frame means 16 of the head 11 also mounts electronic apparatus 54 adapted to amplify the signal provided by the hydrophone 42 and to make a voltage to current conversion in order to convert the signal provided by the hydrophone 42 to a signal simulating that produced by a geophone which can readily be handled by the digital seismograph recorder 52. Furthermore, the electronic apparatus 54 is adapted to suppress transients generated in the sensing apparatus, especially the head 11 and associated cable 34, by electromagnetic radiation which arises when the spark discharge occurs in the head 10. The electronic apparatus 54 uses known techniques to achieve these ends.

The digital seismograph recorder 52 is a readily available piece of equipment and is powered by only a 12 volt battery. The spark generation appartus described in our co-pending Patent Application is powered by a 24 volt battery. Thus the whole apparatus is sufficiently compact and manoeuvrable to be readily transported into tunnel workings as it does not require any substantial generator of electricity to power the apparatus.

Furthermore, the illustrative apparatus is relatively safe to use in confined tunnel workings. The heads 10, 11 may be propelled by the relatively stiff rods 24, 28, 30 substantial distances along the boreholes 44 for example of up to about 90 metres from the tunnel face from which the boreholes 44 are drilled. The rods being sufficiently rigid to prevent buckling.

The illustrative apparatus provides good acoustic coupling between the transducer and the surrounding ground; furthermore, in the case of the spark discharge head 10, the liquid filling the envelope as well as providing the acoustic coupling also provides the electrolyte for the spark discharge.

Claims (13)

1. Apparatus suitable for use in positioning a transducer in a borehole comprising support means for supporting the transducer, a flexible impervious envelope sealed to the support means and enclosing the transducer and means for filling the envelope with liquid.

2. Apparatus according to Claim 1 comprising means by which liquid may be supplied to the interior of the envelope and means by which liquid may be removed from the envelope.

3. Apparatus according to either one of Claims 1 and 2 comprising a pressure relief valve situated to vent pressure from within the envelope.

4. Apparatus according to any one of the preceding claims wherein the support means comprises a first member mounting the transducer, a second member remote from the first, and connecting frame means extending between said first and second members to retain the members in spaced relation, and wherein the envelope is provided by a tubular sleeve sealed at one end to the first member and at the other end to the second member, and enclosing the frame means.

5. Apparatus according to any one of the preceding claims comprising mounting means by which the support means is mounted for movement in a borehole.

6. Apparatus according to Claim 5 wherein the mounting means comprises a stiff rod to one end portion of which the support means is secured, the other end portion of the rod comprising means by which the rod can be connected to a further rod.

7. Apparatus according to any one of the preceding claims wherein said transducer comprises a spark discharge means.

8. Apparatus according to Claim 7 when dependent on Claim 4 wherein the spark discharge means comprises a first electrode mounted by, but electrically insulated from, said first member and wherein the frame means provides a further electrode.

9. Apparatus according to either one of Claims 7 and 8 comprising an electrolyte liquid filling the envelope.

10. Apparatus according to any one of Claims 1 to 6 wherein said transducer is a sensor adapted to sense compression waves.*

11. Apparatus suitable for use in positioning a transducer in a borehole constructed arranged and adapted to operate substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

12. Apparatus suitable for use in positioning a transducer in a borehole constructed arranged and adapted to operate substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

13. A method of positioning a transducer in a borehole comprising mounting the transducer in apparatus according to any one of the preceding claims introducing the apparatus with the envelope in an empty or only partially filled condition into the borehole, moving the support means and envelope to a desired position lengthwise of the borehole, and filling the envelope with liquid whereby to force the envelope firmly into contact with the wall of the borehole with part, at least, of the transducer immersed in the liquid.