GB2471899A - An electrode assembly for an electrical discharge acoustic source. - Google Patents

Abstract

An electrode assembly for use in an electrical discharge acoustic source, comprises a conductive tip holder 54 that can be connected to a power supply (12 figure 1), a discharge tip 52 connected to the conductive tip holder 54, and an insulating collar 60 surrounding the connection between the tip 52 and the holder 56 and having an aperture through which the discharge tip 52 projects. The tip is held in place by a sealing O ring 58 which negates the need to make a groove in the tip thereby reducing the weaknesses a groove causes. A series of multiple pairs of such electrodes can be configured and a switching arrangement can be configured so that any capacitor can be connected to any one of the pair of electrodes. These electrodes can be used in a Plasma gun used over a large range of sea depths.

Description

Description

ELECTRODE SYSTEMS FOR AN ELECTRICAL DISCHARGE

ACOUSTIC SOURCE

Technical field

[0001]This invention relates o electrode systems for acoustic sources which use an electrical discharge o create the acoustic pressure wave in the surrounding environment. Such sources, such as spark or plasma sources find application as acoustic sources of the type used in the oil and gas industry as seismic signal sources or the like.

Background art

[OOO2]Acousic sources are used in a number of different industries and fields of technology. A variety of acoustic sources have been developed that have a range of oupu energies and frequency bands. One such source, commonly referred o as a sparker, discharges electrical energy that has been stored in capacitors across one or more pairs of electrodes immersed in an elecrolye. The discharge creates an arc between the electrodes that is associated with an acoustic oupu. Sparker type sources have been employed in a wide variety of applications such as seismic exploration for natural resources, measuring the elastic moduli of rocks both in-situ for civil engineering site invesigaion and in the laboratory, and in medical imaging. They may also be used for sonic cleaning, killing organisms and in medical applications known as lithoripsy o break up unwanted material within the body.

[0003]Sparkers have a number of limiaions that may be significant depending on their intended application, including: a. Limited output energy and frequency band.

b Rapid and erratic wear of the electrodes.

c. Unpredictable failure of the energy storage capacitors.

d Pressure dependence of the oupu and spark formation.

[0004]ft has been proposed o use sparkers as marine acoustic sources for geological imaging beneath the seabed using the refIecion seismic technique (e.g. US 3286226). Various designs using multiple electrodes have been proposed o improve the output: and reduce variations due o erratic erosion of the electrodes, (e.g. US3613823). One of these uses a consumable wire as an electrode (WO 92/02926). Another (US5228011) contains the spark in a region that is isolated from the electrodes o prevent erosion of the eIecrode by the spark. None of these designs appears o have been entirely saisfacory.

[0005]There are a number of factors which can affect the efficient conversion of electrical o acoustic energy. Overall, the most efficient conversion of electrical o acoustic energy takes place if a damped electrical discharge is achieved. In this case most of the electrical energy is dissipated in the creation and expansion of the plasma arc rather than being lose in the oscillation of the electrical circuit. The damping depends on the initial conduciviy of the elecrolye, circuit capacitance and inductance, exposed cathode area, the resistance of the arc, the electrode gap, the discharge voftage, the ambient temperature and the ambient pressure. under optimal conditions the dominant acoustic frequency is similar o the resonant frequency of the circuit.

[0006]ft is known that the amplitijde of an acoustic source may be increased by firing a number of sources a the same time according o the principal of superposition, provided the sources are greater than one wavelength apart. Firing a number of sources in a pre-deermined sequence can also be used o increase the overall frequency bandwidth of the source (us 4739858) without aftering the frequency of the individual discharge circuits. Methods of shaping the source oupu, commonly referred o as tuning, are also known (e.g. US 5398217).

[0007]Sparkers and airguns are used for marine seismic reflection imaging in exploration for hydrocarbons. These sources are positioned in water and generate a bubble of air from an airgun or water vapour from a sparker which produces a pressure pulse as the bubble expands, osciIlaes and collapses. The acoustic efficiency of such bubble sources, including explosives, is highly dependent on the ambient pressure and so these sources are typically only operated a a few metres below the surface.

[0008]Under optimum electrical conditions and with a suitable electrode configuration, the acoustic oupu of a sparker is due o a shock wave generated by the formation of a plasma arc between the electrodes rather than vaporisaion of the elecrolye. Under these optimum conditions sparkers do no produce a significant bubble and have been used effectively in boreholes a greater than 3km depth. This type of sparker that generates a high pressure plasma arc is referred o below as a Plasma Gun.

[0009] Borehole sparkers can suffer from poor and unpredictable capacitor reliability due o the very high charge densities that are required o provide sufficient energy from a small borehole tool volume. In addition the operating emperaure range may have o be relatively high due o the elevated temperatures a depth.

If a single capacitor fails the sparker muse be brought back o the surface for repair. This could occur after a few shoes or after many hundreds of shoes making i impossible o plan the duration and so cost of a survey.

[0010] Problems can also occur with the electrodes due o die extreme operating conditions in such system. The ip of the electrode needs o be able o withstand high emperaures. However, i also needs o be able o withstand the physical shocks generated during sparking.

[0011]Due o the enduring problems of reliability, repeaabiliy and the limited oupu of existing sparkers, the full poenial of sparkers does no appear o have been realised, particularly in exploration applications. Here, means are revealed to obtain a consisen, optimal spark under all ambient emperaure and pressure conditions. These means may be of benefit in all sparker applications including marine and borehole seismic applications, medical imaging and acoustic cleaning. In particular the invention provides a Plasma Gun for marine exploration that can be used over the full range of ocean depths.

Disclosure of the invention

[0012]A first aspect of this invention provides an electrode assembly for use in an electrical discharge acoustic source, comprising: -a conductive ip holder that can be connected o a power supply; -a discharge ip connected to the conductive ip holder; -an insulating collar surrounding the connection between the ip and the holder and having an aperture through which the discharge ip projects wherein the wall of the aperture in the ip holder is provided with formations holding sealing elements that seal around the discharge tip.

[0013]The discharge ip can be made of ungsen or a ungsen alloy.

[0014]The conductive ip holder can comprise an elongate rod with an axial hole a one end, in which the discharge ip can be located.

[0015]The insulating collar can be tapered around the aperture from which the discharge ip projects.

[0016] In one embodiment, a sleeve is provided around the conductive ip holder behind the insulating collar. In this case, a housing can be provided that embraces both the sleeve and the collar.

[0017]The discharge ip can consitue a cathode and the assembly can further comprise an anode spaced from the ip o define a spark gap. The anode and cathode can be mounted in axial alignment in a tool body.

[0018]The anode can include a hard insert a is tip. A drive mechanism can also be provided o adjust the axial position of the anode relative o the cathode.

[0019]The tool body can be filled with an insulating fluid. The tool body can also include a reservoir for the insulating fluid closed by a moveable piston that is exposed on one side o ambient pressure such that changes in ambient pressure are ransmitXed o the insulating fluid via the piston.

[0020]A flexible bladder can be provided around the anode and cathode for containing an elecrolye fluid. A reservoir with a piston arrangement can be provided o feed elecrolye o the bladder.

The bladder may also have a vent o allow elecrolye o be flushed from the bladder.

[002 1]The invention also provides an electrical discharge acoustic source comprising: -a power section; -a control section; -a bank of capacitors; and -an electrode arrangement comprising an electrode assembly according o the first aspect of the invention.

[0022]The electrode arrangement can comprise muftiple pairs of discharge electrodes. In this case, a switching arrangement can be configured so that any capacitor can be connected o any one pair of electrodes.

[0023]Other aspects of the invention will be apparent from the

following description.

Brief description of the drawings

[0024] Figure 1 shows a schematic diagram of a source according o an embodiment of the invention; Figure 2 shows detail of a switching arrangement for use in the embodiment of Figure 1; Figure 3 shows one embodiment of an eIecrode assembly in accordance with the present invention; Figure 4 shows detail of an electrode consrucion in accordance with the invention; and Figures 5a and 5b show a three-electrode embodiment of the invent:ion.

Mode(s) for carrying out the invention [0025]This invention is based on an electrode design that can provide the characerisics of slow and progressive erosion of the electrode tips and a consisen acoustic oupu. In addition the electrode gap may be remotely adjusted.

[0026]The embodiment of a Plasma Gun is shown schematically in Figure 1, comprises a compuerised controller (the controller) 10, a surface power supply 12, a connecting cable 14 and a remote sparking device 16. The sparking device 16 typically comprises a separate housing that can be positioned under water or in a borehole extending underground in the formation o be invesigaed. The remote sparking device 16 comprises a power section 18 including one or more high voftage power supplies 20 and optional batteries 22, a bank 24 of energy storage capacitors connected via a switching arrangement 26 o a series of pairs of discharge electrodes 28, and a control system 30 including electronics and devices for monitoring and controlling the discharge of the pairs of electrodes 28.

[0027]An electrical charging circuit for a muftiple electrode and capacitor arrangement is shown schematically in Figure 2, in which: a. p indicates the number of electrodes b N indicates the number capacitors or groups of capacitors (for convenience each capacitor illusraed may be wo or more capacitors connected in parallel).

c. G1 o Gp are high voltage switches. These switches may be of the triggered spark gap type or other suitable design capable of holding off the required voltage and being remotely acivaed.

d E o E are electrode pairs across which the spark is formed e. D1 o D are high voftage diodes o prevent voltage reversals on the capacitors. Afternaively, D1 o DN diodes can be connected in series with the individual capacitors.

f. C1 o CN are capacitors. The capacitance of each capacitor or group of capacitors may be different o obtain a range of acoustic frequencies o be combined in the oupu.

g P1 o PN are isolaorswiches allowing each capacitor o be individually isolated from the rest of the circuit. The condition of the capacitors C1 o CN is monitored individually. Prior o charging, any failed capacitors can be deeced electronically and removed from the circuit by the switches P1 o PN.

h HV1 o HV are high voltage negative supplies. These voltages may be variable o achieve the required firing voftage and discharge energy and are compaibIe with the voftage raUngs of the capacitors, spark gaps and diodes con necked o each of the HV1 o HV supplies.

i. S11 o SNP are high voltage switches connecting each of the C1 o CN capacitors in parallel o one of the Ei o E electrode pairs. There should be no significant voftage across these switches when they are opened or closed.

j. R o RN are high voltage resistors o bleed off any residual charge remaining on the capacitors after firing.

k. H1 o H are variable inductors.

[0028] Operation of this system is as follows: a. Wait for HV1 o HVo decline o a level a which the switches S11 o SNPcan be acivaed. These voltages will decline due o the resistors R1 o RN.

b Identify any failed capacitors.

c. From the remaining capacitors, the controller selects which capacitors will be connected o each of the electrode pairs o obtain the required electrical discharge.

d The conroIler remotely operates each of the switches S<1 o S< o connect capacitor x in parallel with the required power supply HV where y is the electrode number.

e. The high voftage supplies HV1 o HV, that may have the same value or different values, are turned on by the controller. Depending on energy stored in each discharge circuit and the charging rates, HV1 to HV may be turned on a different times o achieve full charge of the capacitors a approximately the same time.

f. If a capacitor failure is deeced during charging the controller switches off the corresponding HV supply, switches in an afternae capacitor and resars charging of the new capacitor.

g When the controller detects that all the circuits are a the required discharge voftage, high sample rate monitoring of the discharge parameters is turned on. Subsequently the high voftage switches Gi o Gp are acivaed either synchronously or according o a pre-deermined sequence o achieve a ttned oupu.

h The high sample rate monitoring continues for a sufficient period o cover the time in which the capacitors discharge.

This period depends on the resonant frequencies of the circuits. For a frequency of 100Hz a period of from lOs of milliseconds before sparking o 5Oms after sparking could be used.

[0029] Based on the data collected during the discharge and the ambient conditions the operator may select afternae electrical parameters and revise the firing sequence o opimise the acoustic oupu. Over time, the acoustic oupu for an increasing range of ambient conditions and discharge parameters can be collected. By reference o this library of existing data i will be possible o select the electrical parameters o achieve a near optimum acoustic oupu that may be further refined for the specific conditions if necessary.

[0030]A cross section through an electrode assembly according o an embodiment of the invention is shown schematically in Figure 3.

This figure illusraes the axial arrangement of the cathode and anode electrodes, 32 and 34 The anode 34 contains a large ungsen insert 36 that provides a low rate of erosion. The flexible bladder 38 containing the elecrolye, such as a copper sulphate or sodium chloride solution, surrounds the electrodes 32, 34. An electric motor or hydraulic means 40 is used o adjust the gap between the anode 34 and cathode 32 by moving the anode 34 axially. Connection between the anode 34 and ground, with a return path through the body of the tool, is provided by a flexible connection 42 The internal pares of the assembly may be filled with an insulating fluid, such as silicone oil. In this case the internal pressure and ambient pressure outside the tool may need o be balanced. This can be achieved by a piston 44 that is exposed on one side o ambient pressure and on the opposite side is exposed o internal tool pressure. A further piston arrangement 46 is revealed for continuously flushing elecrolye from a reservoir 48 through the bladder 38 and venting a the op of the bladder 50. The elecrolye reservoir 48 may be significantly larger than shown. Flushing the bladder 38 removes ionised elecrolye and electrode erosion produces from the bladder in order o maintain a clean and consisen elecrolye for arcing.

[003 1]The elecrolye contained in the flexible bladder 38 can be, for example, copper sulphate. Flexible bladders have been proposed previously for borehole tools and can be made of a flexible material such as rubber or Viton. A sensor can be provided o monitor the conduciviy of the elecrolye. To achieve an optimum electrical discharge, the conduciviy of the elecrolye can be increased or decreased by electrolysis (using separate electrodes o those used for sparking). The gap between the electrode tips is also variable as is discussed above and may be remotely se by the controller o opimise the electrical discharge.

[0032] Figure 4 shows an electrode according o an embodiment of the present invention. The cathode consists of a ungsen (or similar high mefting point, hard metal, alloy or compound) electrode contained within a close fiWng insulator made of high emperaure plastic such as PEEK (other, similar hard, high emperaure insulating materials can be used).

[0033]The use of ungsen provides a high emperaure capability for sparking and increased resistance o erosion compared o softer materials such as steel. The ungsen ip a the cathode has a small diameter and small un-insulated length o minimise the exposed electrode area whilst retaining sufficient strength o witiisand sparking without fracwring.

[0034]As shown in Figure 4, a ungsen electrode 52 is se within a metal holder 54 that contains a secondary sealing 0 ring 56.

Depending on the specific ip material used, manufacturing requirements, or other considerations, the holder 54 can be produced in steel or an afternae metal such as brass. A primary O ring seal 58 is fitted over the ip 52 a the point where the ip 52 meets the holder 54. This eliminates the need o cu an 0 ring groove in the ungsen that would be a significant weakness and liable o shear during sparking. ft has also been observed that sparking produces small spheres of mefted metal on the tips of the electrodes. These spheres produce high current densities and so promote spark formation.

[0035]A machined insulator collar 60 is provided around the ip 52 and holder 54. Following machining of the insulator i is useful o relieve the stress in the material by following a heat reamen procedure. This greatly reduces the poenial for fracturing of the insulator during sparking. The design of the cathode is such that the insulator can be a relatively simple piece without obvious stress concenraions that would weaken i. A sleeve 62 is provided behind the collar 60 and both pares are located in a housing 64.

[0036]The exposed area of the anode is no so critical, so the design of the anode can be very flexible. In this case a relatively large diameter ungsen insert, compared o the cathode, can be used o obtain a long life expectancy bu many other designs could be employed.

[0037]The spark gap between the cathode and anode affects the efficiency of the conversion of electrical o acoustic energy. As the cathode is a an elevated voltage i is most practical o adjust the electrode gap by moving the anode. As is discussed above in relation o Figure 3, this can be achieved by use of a motor. The elecftode gap may be remotely controlled from the surface by shifting the anode. For example, the gap will increase during sparking due o electrode wear and so for prolonged operations the anode will need o be moved towards the cathode periodically o maintain the acoustic oupu. Afternaively, if the spark gap is moved o a region of lower pressure the anode may be moved away from the cathode o opimise the acoustic output.

[0038]The electrode arrangement shown in Figure 3 is scaleable and can be smaller for high frequency/low energy discharges and larger for higher energy discharges. A muftiple electrode source is ilIusraed in Figures 5a and Sb comprising three electrode pairs 66, 68, 70 mounted around a central core 72. An outer housing 74 with windows 76 can be provided around the electrode pairs 66-70. Other embodiments of muftiple electrodes using the design shown in Figure 3 may be conceived depending on the application or other consrains.

[0039] In use, surface controller 10 receives data from the remote sparking device 16. These data are processed o determine the optimum parameters o achieve the desired acoustic oupu. The controller 10 sends insrucions back o the sparking device 16 o update the electronic and physical parameters relating o sparking o opimise the output. The parameters that may be

adjustable include:

Electronic parameters, such as Discharge voftage Circuit inductance, and Switching failed capacitors ou of circuit; and Physical parameters, such as Number of spark gaps fired, Firing sequence and timing, Electrode gap, and Elecrolye conduciviy.

[0040]Sensors exposed o the environment surrounding the sparking device 16 monitor the ambient temperature and pressure conditions. A high frequency pressure sensor, such as a hydrophone, can be provided o monitor the source pressure versus time signature. Within the sparking device further sensors monitor the temperature a critical points such as the capacitors and high voftage power supply o ensure the tool is operated within is specification. There may also be a motion deecor, such as a geophone or a pressure detector such as a hydrophone, o monitor the source oupu.

[0041]Elecrical power and communication with the sparking device 16 are ransmitXed via the connecting cable 14. The choice of connecting cable depends on the application and suitable cables for borehole or marine use, are well known. In some applications, such as borehole tools, the rate a which the tool can be charged and fired may be limited by the current and voltage ratings of the connecting cable. In this case, the optional batteries 22 can be included in the tool. The batteries 22 can be used o rapidly charge the capacitors 24 o minimise the time between shoes. Whilst the tool is inactive, for example during movement o another tool depth and between shoes, the batteries may be re-charged via the connecting cable.

[0042] Methods of digital communication of data via the connecting cable between the controller and sparking device are well known.

During sparking relatively small data sample intervals of lOps are useful o adequately resolve the high frequency parameters.

High frequency parameters that can be monitored include the light oupu deeced by a sensor within the bladder, the voftage across the electrodes, the current flow in the discharge circuit and the acoustic pressure or motion deecor. Greater data sampling intervals of is or more are sufficient for monitoring the ambient parameters of internal emperaures a various points within the tool, external temperature, external pressure, elecrolye conduciviy, condition of critical electronic components, charging voftage and charging current.

[0043]The condition of each capacitor can be monitored electronically.

If a capacitor failure is deeced this is communicated o the surface controller 10. An insrucion o the sparking device 16 o connect a spare capacitor into the circuit may be generated auomaically or may be given manually by the operator and the failed capacitor is remotely removed from the circuit.

[0044]The controller monitors the charging voltage and current o determine when the required discharge voltage has been reached. A this time the controller can either auomaically instruct the sparking device o fire or await a manual command o fire from the operator. As is discussed above, the sparking device 16 may contain a number of pairs of electrodes that can be fired together or sequentially. Firing a number of electrodes synchronously enhances the acoustic amplitude. Sequential firing a consan or variable intervals can be used o control the amplitude spectrum of the oupu.

[0045]The energy discharged across an individual electrode pair can be controlled by adjusting the discharge voftage and switching the number of capacitors connected o the electrodes. The acoustic frequency of each electrode pair is related o the circuit capacitance and inductance and can also be controlled remotely by changing the inductance. In seismic imaging applications the image resolution and range of invesigaUon are related o the source frequency. Dynamically controlling the acoustic oupu enables a range of imaging applications o be covered with one source.

[0046]The design of the electrodes can be imporan in obtaining a consisen and efficient discharge. A large range of electrode designs have been proposed. Dyer B.C. and Baria R., 1996 Development of the CSMA mark II high emperaure borehole sparker source. Scientific Drilling, Vol 5, No 6, pp243-248 describe electrode parameters that affect the efficiency of the acoustic oupu. These include minimisation of the exposed cathode area and the electrode gap that is se o achieve a damped electrical discharge. Further, the elecrolye conduciviy and/or the discharge voftage muse be increased with increasing ambient pressure o allow a spark o form.

[0047]A wide variety of applications of sources according o the invention exist, including: a. Velocity caIibraion shoes for example check shoes in microseismic velocity esimaion b Sonic cleaning of pipes c. Killing aquatic organisms d Crosshole seismic imaging e. Reverse Vertical Seismic Profiling f. Single well surveying g Very long spaced sonic logging [0048]Three particular applications of the plasma gun are proposed: deep marine seismic imaging a source depths of lOs o l000s of metres, generating directional waves from within a borehole and back off shooting in drilling.

[0049]Convenional marine seismic reflection surveying and VSP surveys are typically performed using an airgun or other source a a few metres depths. For deep water surveys in water depths of lOs o a few 1000 metres the source amplittjde decreases in proportion o the distance the source energy is ransmitXed through the water. Increasing water depth also reduces the poenial lateral resolution of the seismic image. Conventional seismic sources become increasingly inefficient with increasing depth and so are typically operated within a few metres of the surface only. In comparison the Plasma Gun can be operated a depths in excess of 3000m. Using an array of Plasma Guns, fired in a timed sequence or ogeLher, a suitably large acoustic pulse may be generated for seismic imaging beneath the sea bed. The seismic receivers can be deployed in any of the conventional configurations such as a linear, horizontal array in the water, a sea bed array or a string of receivers within a borehole, depending on the imaging objectives.

[0050] In a borehole, a Plasma Gun with a number of vertically spaced electrodes as illusraed in Figures 5a and Sb may be contained within a single tool. These electrodes may be fired in a timed sequence or simuftaneously o generate elastic waves in the rock. Using acoustic monopole sources Che X-H and Qiao W-X., 2004. Acoustic Field in Formation Generated by Linear Phased Array of Transmitters in Fluid-Filled Boreholes. Chinese Journal of Geophysics, Vol. 47, No. 4, 2004, pp:830-836, describe how the source energy may be steered into preferred directions in the rock by firing the sources in a suitable sequence. The Plasma Gun can be controlled in the same way o generate energy propagating in a preferred direction that is optimal for the survey configuration and the imaging arge.

[0051] Whilst drilling deep boreholes, the drill bi may become stuck such that i is no possible o withdraw the drill bi and drill pipe from the borehole. In this instance i may be necessary o separate the drill pipe as close as possible o the drilling bi and leave the drill bi downhole by a method referred o as backoff shooting. The backoff shoe is an explosive charge that is fired as close as possible o the drilling pipe joint a which the drill pipe is o be separated. This may take a number of backoff shoes o achieve which is very time consuming as only one shoe can be fired a a time and o recharge the backoff tool i muse be brought back o the surface. Using the Plasma Gun instead of the backoff shoe i would be possible o fire many shoes in a single deployment unUl the pipe string separates.

[0052]As the electrical energy discharged by a Plasma Gun may no be primarily dissipated in producing a bubble, the acoustic oupu does no necessarily decline due o the effects of pressure on the bubble in the same way as for an airgun. This also means that there may effectively be no secondary acoustic oupu from the Plasma Gun due o oscillation and collapse of the bubble. There are also very significant advantages in conducting reflection surveys using a suitable Plasma Gun source close o the seabed, particularly if the receivers are also towed close o the seabed or are laid on the seabed. These advantages include reduced energy losses from transmission through the water column, reduced interference from the sea surface refIecion, commonly referred o as the surface ghost, improved lateral resolution due o a reduction in the Fresnel radius and reduced acoustic interference with near surface swimming marine mammals.

[0053]Otiier variants and uses within the scope of the invention are envisaged.

Claims (18)

1. Claims 1. An electrode assembly for use in an electrical discharge acoustic source, comprising: -a conductive tip holder that can be connected to a power supply; -a discharge tip connected to the conductive tip holder; and -an insulating collar surrounding the connection between the tip and the holder and having an aperture through which the discharge tip projects; wherein the wall of the aperture in the tip holder is provided with formations holding sealing elements that seal around the discharge tip.
2. 2. An electrode assembly as claimed in claim 1, wherein the discharge tip is made from tungsten or a tungsten alloy.
3. 3. An electrode assembly as claimed in claim 1 or 2, wherein the conductive tip holder comprises an elongate rod with an axial hole at one end, in which the discharge tip can be located.
4. 4. An electrode assembly as claimed in claim 1 or 2, wherein the insulating collar is tapered around the aperture from which the discharge tip projects.
5. 5. An electrode assembly as claimed in any preceding claim, wherein a sleeve is provided around the conductive tip holder behind the insulating collar.
6. 6. An electrode assembly as claimed in claim 5, wherein a housing is provided that embraces both the sleeve and the collar.
7. 7. An electrode assembly as claimed in claim 6, wherein the discharge tip constitutes a cathode and the assembly further comprises an anode spaced from the tip to define a spark gap.
8. 8. An electrode assembly as claimed in claim 7, wherein the anode includes a hard insert at its tip.
9. 9. An electrode assembly as claimed in claim 7 or 8, wherein the anode and cathode are mounted in axial alignment in a tool body.
10. 10. An electrode assembly as claimed in claim 9, wherein a drive mechanism is provided to adjust the axial position of the anode relative to the cathode.
11. 11. An electrode assembly as claimed in claim 9 or 10, wherein the tool body is filled with an insulating fluid.
12. 12. An electrode assembly as claimed in claim 11, wherein the tool body includes a reservoir for the insulating fluid closed by a moveable piston that is exposed on one side to ambient pressure such that changes in ambient pressure are transmitted to the insulating fluid via the piston.
13. 13. An electrode assembly as claimed in any of claims 9-12, wherein a flexible bladder is provided around the anode and cathode for containing an electrolyte fluid.
14. 14. An electrode assembly as claimed in claim 13, wherein a reservoir with a piston arrangement is provided to feed electrolyte to the bladder.
15. 15. An electrode assembly as claimed in claim 14, wherein the bladder has a vent to allow electrolyte to be flushed from the bladder.
16. 16. An electrical discharge acoustic source comprising: -a power section; -a control section; -a bank of capacitors; and -an electrode arrangement comprising an electrode assembly according to any preceding claim.
17. 17. An electrical discharge acoustic source as claimed in claim 16, wherein the electrode arrangement comprises multiple pairs of discharge electrodes.
18. 18. An electrical discharge acoustic source as claimed in claim 17, comprising a switching arrangement configured so that any capacitor can be connected to any one of a number of pairs of electrodes.