Seismic pulse generator

A seismic pulse generator (1) has a chamber (4) pressurised with gas. The generator includes a solenoid valve (60) which can be actuated to cause the release to atmosphere of only a small amount of pressurised gas from a small control chamber (34) behind a main piston (30) to enable the piston to move away from a valve seat (32) around an outlet (26). This permits the abrupt release of the pressurised gas in the chamber (4) through the outlet and the "firing" of the generator. The arrangement maximises the use of the main pressurised gas for actual "firing" of the generator.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to seismic pulse generation and, more
 particularly, to seismic pulse generators for use, for example, in both
 on-shore and off-shore exploration.
 2. Description of the Related Art
 One kind of seismic pulse generator with which the invention is concerned,
 although not exclusively, is that which basically includes a two-chamber
 device in which a "driver" gas at relatively high pressure in one chamber
 is employed to generate a shock wave in the other, adjoining, chamber
 which contains a different gas which is at relatively low pressure. The
 chambers are not allowed to communicate with each other until the
 generator is operated. Communication between the two chambers may be
 prevented for example by means of a releasable shuttle valve member which
 in the closed position closes an opening between the chambers. When the
 shuttle valve member is abruptly released from this closed position, to
 allow communication, a shock wave is generated and propagates in the low
 pressure chamber as a result of the impact of the high pressure gas onto
 the low pressure gas. This kind of generator is also referred to as a
 seismic shock gun. In prior art devices the release of the shuttle valve
 member from the closed position mentioned above may be initiated by
 releasing a pressure release member to allow the release of a small
 portion of the high pressure gas rearwardly from the generator. The
 pressure release member may be, for example, a poppet-valve or a flap
 valve as described in Applicant's earlier GB patent specifications nos.
 2165945B and 2230860B, respectively.
 An object of the invention is to provide a construction of seismic pulse
 generator which facilitates repeatable firing.
 BRIEF SUMMARY OF THE INVENTION
 The present invention consists in a seismic pulse generator comprising a
 body having an internal gas-pressurisable charging chamber, an outlet, and
 a gas-operated piston valve which is slidably movable within an associated
 guide to a position to close the outlet when the charging chamber is
 pressurised with gas to attain a primed ready to fire state for the
 generator, the piston moving away from the closed position to permit an
 abrupt release of pressurised gas from the charging chamber through the
 outlet upon firing of the generator by release of a small portion of gas
 from a gas pressurised control chamber to atmosphere by means of a
 solenoid valve assembly mounted on the body, the solenoid valve assembly
 comprising an annular plunger valve slidably mounted on a valve stem so as
 to be movable within an annular channel defined between the valve stem and
 an annular wall, the valve stem having a generally longitudinal gas
 exhaust passage extending therethrough and terminating at a first opening
 at the end of the stem remote from the piston, via which first opening the
 passage communicates with atmosphere, and at least one second lateral
 opening in the side or peripheral wall of the stem for communicating with
 the interior of the control chamber, wherein the valve plunger is normally
 biased to a position on the valve stem whereat the valve plunger closes
 off the at least one second opening and wherein on actuation of the
 solenoid valve the plunger slides on the valve stem away from the closed
 position to allow the at least one second opening to be in communication
 with the control chamber, whereby the release of the small portion of the
 pressurised gas from the control chamber to atmosphere can occur.
 Conveniently, the passage, via the first opening, opens into a buffer
 chamber of relatively large volume and having an outlet which vents to
 atmosphere. This buffer chamber would act to disperse or buffer the effect
 of the sudden or violent release of the small portion of pressurised gas
 via the valve stem passage. The advantage of this is that there is a
 reduction in the pressure and the noise level of the suddenly or violently
 released gas.
 The buffer chamber may in part be defined by a plug or the like fitting in
 the body of the generator. Conveniently, the plug is adapted to be a
 removable f it on the body. For example, the plug may be a screw threaded
 fit in the body. In one embodiment the solenoid valve assembly may be
 removably secured to the plug, such that when the plug is a removable fit
 on the body the solenoid assembly can be removed from the body with the
 plug.
 Conveniently, the longitudinal part of the passage in the valve stem and
 the buffer chamber are substantially in alignment. Such alignment reduces
 the restriction to, or drag on, the flow of the small portion of gas
 exhausted from the control chamber to the outlet and thus facilitates the
 rapid release of the gas from body.
 The buffer chamber may serve as a conduit for a portion of an electrical
 power supply line for the solenoid valve and/or for a portion of a
 pressuring gas inlet line for the pressurisable control chamber.
 Advantageously, a silencer is mounted on the body so that gas released from
 the pressurisable control chamber via the passage passes through the
 silencer to atmosphere. The silencer may be of known or traditional
 design. The provision of the silencer reduces the likelihood of the sound
 of the firing generator disturbing operators or other people nearby.
 Preferably, the silencer is in general alignment with the longitudinal
 part of the passage in the valve stem, and the buffer chamber if present,
 to further facilitate rapid release of gas from the control chamber.
 The solenoid valve assembly may include a gas inlet for communicating with
 the annular channel with the valve plunger at least in part defining
 passages which communicate with the control chamber. In one embodiment the
 annular valve plunger may comprise in its outer peripheral surface
 channels or grooves which extend generally longitudinally of the plunger
 and define with the annular wall the passages providing communication
 between the gas inlet and the control chamber. In an alternative
 arrangement the valve plunger may have through holes for providing
 communication between the gas inlet and the control chamber.
 The piston guide may be in the form of a cylinder located in the
 pressurisable chamber and aligned with the outlet associated with the
 piston valve. An end of the cylinder remote from the outlet may
 accommodate at least part of the solenoid valve assembly. The control
 chamber is defined by the cylinder and the opposing solenoid valve
 assembly and the piston valve when the latter is in its closed position
 closing off the outlet. The control chamber may communicate with the
 charging chamber via a bleed hole or resistance-to-flow through hole for
 example in the wall of the cylinder. Consequently, the supply of
 pressurised gas to the control chamber can be used to pressurise the
 charging chamber by virtue of the bleed hole.
 In the case where the plug defining a buffer chamber is provided, the
 cylinder may be secured to the plug. Thus, if the plug is removable from
 the body, the cylinder as well as the solenoid valve assembly may be
 removed with the plug when the latter is removed from the body. This
 facilitates access to the interior of the body and to the solenoid valve
 assembly, piston valve and cylinder.
 The solenoid valve assembly may comprise one or more body parts defining at
 least one passage which provides communication with the annular channel
 and with the passage in the valve stem when the solenoid valve is actuated
 and the at least one lateral opening in the valve stem is uncovered as the
 valve plunger moves from its closed position.
 In one embodiment of the generator, the pressurisable charging chamber
 adjoins a second chamber with the outlet which is closable by the piston
 valve opening into the second chamber such that when the generator is
 "fired" the piston valve moves away from the closed position to uncover
 the outlet to cause communication between the adjoining chambers and
 generation of a shock wave in the second chamber in response to the abrupt
 release of gas from the pressurised charging chamber, the second chamber
 having means to permit a pulse of energy to be transmitted from the second
 chamber in response to the shock wave generated in the second chamber.
 The means to permit a pulse of energy to be transmitted may comprise outlet
 means to permit the release of gas pressure and a shock wave from the
 second chamber. The outlet means may be a permanently open outlet from the
 second chamber, or may comprise a valve which is operable to open from a
 closed position whereat it closes an outlet from the second chamber. The
 outlet valve means may form part of a fitting adapted to be a releasable
 fit on the second chamber.
 The present invention also consists in a seismic pulse generator comprising
 a body having an internal gas pressurisable charging chamber adjoining a
 second chamber with an outlet means, a gas operated piston which when the
 charging chamber is pressurised is movable away from a position whereat it
 closes an opening to cause communication between the chambers, abrupt
 release of gas from the pressurised charging chamber through the opening,
 and generation of a shock wave in the second chamber, the gas operated
 piston being slidably movable within an associated cylinder located in the
 first chamber, a gas pressurisable control chamber being defined by the
 end of the piston remote from the opening when the piston is in the
 opening closed position, a section of the cylinder and a solenoid valve
 assembly mounted on the body at or adjacent the end of the cylinder remote
 from the opening, wherein when the control chamber is pressurised and the
 valve assembly is in a non-actuated state or condition a gas exhaust
 passage for connecting the control chamber to atmosphere is closed, and
 wherein on actuation the solenoid valve assembly operates to effect
 communication between the control chamber and atmosphere via the gas
 exhaust passage to allow the release of gas from the control chamber to
 atmosphere and the piston to move away from the opening-closed position
 and permit the abrupt release of gas from the pressurised charging
 chamber.
 The present invention further consists in a seismic pulse generator
 comprising a body having an internal gas-pressurisable charging chamber,
 an outlet, and a gas-operated piston valve which is slidably movable
 within an associated guide to a position to close the outlet when the
 charging chamber is pressurised with gas to attain a primed ready to fire
 state for the generator, the piston moving away from the closed position
 to permit an abrupt release of pressurised gas from the charging chamber
 through the outlet upon release of a small portion of gas from a gas
 pressurised control chamber to atmosphere, wherein the small portion of
 released gas passes through a buffer chamber associated with the generator
 before passing to atmosphere.
 The present invention still further consists in a seismic pulse generator
 comprising a body having an internal gas-pressurisable charging chamber,
 an outlet, and a gas-operated piston valve which is slidably movable
 within an associated guide to a position to close the outlet when the
 charging chamber is pressurised with gas to attain a primed ready to fire
 state for the generator, the piston moving away from the closed position
 to permit an abrupt release of pressurised gas from the charging chamber
 through the outlet upon release of a small portion of gas from a gas
 pressurised control chamber to atmosphere, wherein the small portion of
 released gas passes through a silencer connected to the generator.

DETAILED DESCRIPTION OF THE INVENTIONS
 With reference to the drawings, a seismic pulse generator 1 of the `shock
 gun` kind comprises a first body part 2, the lower portion 2a of which
 provides a first charging chamber 4 for relatively high pressure gas and
 adjoins a second generally tubular body part 6 which defines a second
 chamber 8 for relatively low pressure gas.
 As viewed in FIG. 1, the upper portion 2b of the first body part 2 is above
 the charging chamber 4 and defines an internally threaded opening 10 into
 which is screwed an externally threaded plug 12 having an axial buffer
 passage or chamber 14 extending therethrough. The lower end 16 of the plug
 12 is recessed and shaped to accommodate the upper end part 18a of a
 solenoid valve assembly 18. The lower part 18b of the valve assembly 18 is
 accommodated in the upper end portion 20a of a piston valve guide 20 which
 is in the form of a cylinder and which extends downwardly towards the
 second chamber 8.
 The upper end of the cylinder 20 has an outwardly extending flange 22 via
 which the cylinder is secured as at 24 to the plug 12. Opposing portions
 of the secured together plug 12 and cylinder 20 sandwich the solenoid
 valve assembly 18 and locate it in position therebetween. The lower end of
 the piston valve cylinder 20 terminates short of an outlet 26 from the
 charging chamber 4, the outlet 26 also serving as an opening to provide
 communication between the two chambers 4 and 8. A piston valve 30 is
 slidable in the cylinder 20. The piston valve may be of substantially
 cylindrical form and made of a suitable material, for example plastics or
 steel. The lower end face 30a of the piston valve is engagable with a
 sealing ring 32, which surrounds the outlet 26, to close the outlet. The
 dimension of the lower end face 30a is such that when it engages the
 sealing ring 32 the outer periphery of the face 30a is beyond the sealing
 ring 32 and an outer peripheral portion of the face 30a is exposed to gas
 in the charging chamber 4. As may be seen from FIG. 1, when the piston
 valve 30 is in engagement with the sealing ring and closes the outlet a
 chamber 34, referred to as the control chamber, is defined by the cylinder
 20 between the lower end of the solenoid valve assembly 18 and the top of
 the piston valve 30.
 The solenoid valve assembly 18 which is shown on an enlarged scale in FIG.
 2 comprises a generally annular housing 36 having upper, intermediate and
 lower parts 36a, 36b and 36c secured together.
 A recess or channel 37 extends from the bottom of the housing 36 part way
 up the outer periphery thereof and together with the wall 38 of the
 cylinder 20 defines a passage 39 which communicates with a
 resistance-to-flow through hole 40 in the wall 38. The resistance-to-flow
 through hole 40 and the passage 39 provide communication between the
 control chamber 34 and the annular charging chamber 4 surrounding the
 piston valve 30 and cylinder 20 and substantially defined between the wall
 of the lower portion 2a of the first body part 2 and the cylinder 20.
 Extending centrally downwardly from the upper part 36a into the interior of
 the housing 36 is a valve stem 42 having an axial passage 44 therein which
 is aligned with and communicates with the chamber 14 in the plug 12 via a
 first opening 46 at the top of the valve stem. The bottom end of the valve
 stem 42 is closed. Adjacent the bottom end of the passage 44 the
 peripheral wall of the lower part 48 of the valve stem is provided with a
 plurality of lateral openings or through-holes 50 which communicate with
 the passage 44.
 The interior surface of the annular intermediate housing part 40b is
 recessed as at 52. The recess 52 locates a solenoid coil or winding
 assembly 54. The inner periphery 56 of the assembly 54 provides an annular
 wall which, together with the valve stem 42, defines an annular channel 58
 therebetween. An annular plunger valve 60, made for example of soft iron,
 is slidably mounted on the valve stem 42 so as to be movable within the
 channel 58.
 An impact washer 62 for the plunger valve 60 is located at the upper end of
 the annular channel 58.
 A sealing ring 64 for engagement by the plunger valve 60 is located around
 the valve stem 42 at a location a little below the through-holes 50, as
 viewed in FIG. 2.
 The bottom of the valve stem 42 terminates short of the lower part 40c of
 the housing so that there is a gap 66 therebetween which communicates with
 the annular channel 58.
 The bottom of the lower part 36c of the housing 36 has a plurality of
 downwardly extending holes 68 which provide communication between the
 annular channel 58 (and the gap 66) and the control chamber 34 between the
 solenoid valve assembly 18 and the top of the piston valve 30.
 The longitudinally extending chamber 14 in the plug 12, which communicates
 with the axial passage 44 in the valve stem 42, vents to atmosphere via an
 outlet 70 in an end cap or flange 72 secured to the top of the plug 12.
 A silencer 74 is fitted on the outlet 70 at the end cap.
 An electrical power supply line 76, for supplying electricity to the
 solenoid winding 54 into order to actuate the solenoid valve (as will be
 described below), extends through the end cap 72 in a sealed manner and
 then through the chamber 14 in the plug to a power connector 78 fitted in
 an aperture 80 in the top of the upper housing part 36a. The connector 78
 is connected to the winding 54 via wiring 82 which extends through a
 passage 84 in the upper housing part.
 Pressurised gas, e.g. helium, for the charging chamber 4 is supplied from a
 source (not shown) via a gas inlet line 86 which also extends through the
 end cap 72 in a sealed manner and then through the chamber 14 in the plug
 12 to a connection piece 88 fitted in an aperture 90 in the top of the
 upper housing part 36a. A passage 92 in the upper housing part extends
 from the connection piece 88 to an opening 94 in the impact washer 62 to
 provide communication to the portion of the annular channel 58 above the
 annular valve plunger 60. The valve plunger comprises in its outer
 peripheral surface 60a a plurality of spaced channels 60b which extend
 longitudinally of the plunger valve and define with the surrounding
 annular wall 56 passages 96 which allow communication between the gas
 inlet line 86 and the control chamber 34 and the charging chamber 4, since
 the annular channel 58 communicates with the control chamber 34 via the
 holes 68, and the control chamber 34 communicates with the charging
 chamber 4 via the passage 39 and the through-hole 40.
 It will be appreciated that the generator should be substantially gas- or
 air-tight to prevent undesired leakage of gas or air when the generator is
 pressurised for use. Consequently, the valve plunger 60 carries an annular
 or shaft seal 98 to provide a sliding seal between the plunger 60 and the
 valve stem 42, and O-ring seals are also provided between various
 connected parts of the generator as at 100, 102, 104, 106, 108, 110 and
 112.
 The embodiment of shock gun presently being described is arranged, as will
 be readily appreciated by a person skilled in the art, such that when the
 valve plunger 60 is in its usual (non-actuated) position (as shown in full
 lines in FIG. 2) it is biased or urged by the pressure of the gas from the
 gas inlet line to the position on the valve stem 42 whereat the lower edge
 60c of the valve plunger engages the sealing ring 64 and closes off the
 lateral openings 50 and thereby prevents communication between the chamber
 34 and atmosphere. On actuation or energisation of the solenoid valve the
 plunger 60 is caused to slide up the valve stem 42 to a position (as shown
 in broken lines in FIG. 2) so that the openings 50 are uncovered and the
 chamber 34 is vented to atmosphere.
 A method of operating the above seismic pulse generator will now be
 described.
 Conveniently, the generator 1 is used in a generally upright position, as
 viewed in FIGS. 1 and 2. The generator may be located in position for use
 off-shore with the lower end of the second chamber 8 beneath the surface
 of the water. In the present embodiment the lower end of the second
 chamber has a permanently open outlet 8a which is submerged in the water.
 With the solenoid valve in the non-energised state and thus the annular
 plunger 60 engaging the sealing ring 64 and closing off the lateral
 openings 50 the generator is ready to be primed or "set" for firing or
 producing a pulse. This is achieved by pumping a gas of relatively high
 sound speed, preferably helium, through the gas supply line 86 into the
 control chamber 34 and into the charging chamber 4. As the gas passes
 through the holes 68 the piston valve 30 is forced down to seal against
 the sealing ring 32 to close the outlet 26. The relatively high
 resistance-to-flow through-hole 40 in the wall 38 of the piston valve
 cylinder 20 allows gas to pass into the charging chamber.
 The control chamber and the charging chamber may be pressurised to a
 pressure of, for example, 50 to 120 bar.
 If the lower part of the lower second chamber 8 has merely been lowered and
 submerged in water then the second chamber, which is closed off at the
 upper end by the engagement of the piston valve 60 with the sealing ring
 32 and at the lower end merely by the water, contains air (which is a gas
 of relatively low sound speed) at a pressure of approximately 1 bar.
 The generator is now primed. In order to `fire` the primed generator, the
 solenoid valve is momentarily energised i.e. for about 25 ms, from a
 remote location. This causes the plunger valve 60 to slide rapidly up the
 valve stem 42 and move away from the position whereat it closes the
 lateral openings 50 so as to uncover these openings and allow gas in the
 control chamber 34 to be rapidly released or vented to atmosphere via
 holes 68, the lower portion of the annular channel 58, the lateral holes
 50, the axial passage 44, the chamber 14, the outlet 70 and the silencer
 74. While this gas is being rapidly released to atmosphere, the relatively
 high resistance-to-flow hole 40 prevents the gas pressure in the charging
 chamber 4 falling appreciably over the same time period. This results in
 an imbalance in pressure between control chamber 34 and the charging
 chamber 4. The considerable drop in pressure in the control chamber, i.e.
 above the piston valve 30, enables the pressurised gas in the charging
 chamber 4 which is acting on the peripheral edge of the lower face 30a to
 move the piston valve upwards and cause it to become rapidly or abruptly
 disengaged from the sealing ring 32 thereby allowing gas from the
 relatively high pressure chamber 4 to flow rapidly through the outlet 26
 into the lower chamber 8. A shock wave develops and propagates into the
 air in chamber 8 and exits from the latter via the submerged outlet 8a.
 The rapidity of the release of the small amount of gas to atmosphere via
 the silencer 74 is important in order to obtain the desired abrupt release
 of gas through the outlet 26. In the present embodiment it is believed
 that the rapid release of the small amount of gas is facilitated or
 enhanced by the passage 44 and 14 in the valve stem, the buffer chamber 14
 in the plug and the silencer 74 being generally aligned and only a small
 deviation (or baffling effect?) of the gas from the control chamber 34
 being necessary to direct the gas into the passage 44, i.e. via the
 lateral openings 50.
 Applicants experiments using a generator as referred to above have thus
 shown that the desired rapid disengagement of the piston valve 30 from the
 outlet 26 using a compact solenoid arrangement and that the desired high
 rate of release of only a small amount of gas from immediately above the
 piston valve 30 (i.e. from the control chamber 34) can be achieved.
 After the generator has `fired` the gas is again pumped into the control
 chamber and charging chamber as before, and the non-energised solenoid
 valve condition means that the plunger valve 60 returns to its closed
 position to close the lateral opening 50 and the piston valve 30 is able
 to re-seat or re-engage with the sealing ring 32 and close the outlet 26.
 By repeating the sequence of operations described above the generator can
 automatically be repeatedly primed or `set` for firing.
 In a modified embodiment, the permanently open outlet end 8a of the second
 chamber 8 is replaced by an outlet means 114 comprising a valve 116 such
 as a gate valve which is operable to open from a closed position whereat
 it closes the outlet from the second chamber. The valve 116 would be
 controlled remotely (not shown) in timed relationship with the actuation
 of the solenoid valve arrangement and/or the opening of the outlet 26 in a
 manner which would be apparent to a person skilled in the art.
 The valve 116 may form part of a fitting 118 adapted to be a releasable fit
 on the second chamber. The second chamber may be adapted (not shown) so as
 to be pressurisable. The second chamber could then contain, for example,
 air at significantly more than 1 bar. With this modification, there is not
 the restriction of the second chamber having to contain gas or air only at
 about 1 atmosphere.
 Whilst a particular embodiment and modification of the embodiment have been
 described above, it will be understood that various alterations may be
 made without departing from the scope of the invention. For example, the
 control chamber 34 need not be in communication with the charging chamber
 4. In the absence of communication between these two chambers, it will be
 appreciated that the charging chamber must have a separate gas inlet via
 which pressurised gas can be supplied to the charging chamber. It will
 also be appreciated that the generator may be used in a medium or fluid
 other than water. For example, the generator may be fired into drilling
 mud or just into air.