GB2192457A - Seismic motion sensing device - Google Patents

Description

1 GB 2 192 457 A 1 of the mass. However, this device also is sensitive

SPECIFICATION onlyto movement in one linear direction, and it cannot be made sensitive to acceleration in other Motion sensing device directions because placing stress sensors on 70 orthogonal faces of the mass would lead to frictional This invention relates to an acceleration sensing forces which would impede movement in directions device, particularlyfor use in seismic reflection parallel to thefaces.

surveying. An object of this invention isthereforeto provide The use of motion sensing devices iswell known in an acceleration sensing dev ' ice which can be of the field of seismic reflection surveying, in which, 75 relatively small size, mass and cost, and which vibrations, transmitted into the Earth from an reduces, at least partially, the problems listed above.

explosion or other means and reflected towardsthe Afurther object of this invention isto provide a surface bythe underlying rockstrata, are device, different examples of which would be subsequently sensed and recorded atvarious points capable of sensing acceleration orcomponents of distributed in a predetermined way relativetothe 80 acceleration in one direction ortwo orthree mutually source. The recordings of the reflected vibrations perpendicular directions whilst having only one maythen be analysed to yield information about the inertia] mass.

configuration of the strata. According to the presentinvention there is The standard motion sensorforthese applications provided an acceleration sensing device having.a 20. is the geophone, in which a cylindrical mass with an 85 closed cell having effectively rigid walls containing a attached electrical coil is suspended in a magnetic volume substantially filled with fluid with at least one field which is rigidly coupled to the Earth. When the pressure responsive means attached to a wall of the

Earth's surface moves due to the reflected vibration cell to respond to the pressure exerted bythe fluid-on the geophone produces a voltage proportional to the the particular wall to which the pressure responsive longitudinal velocity difference between the mass 90 means is attached, whereby signals are produced by and the geophone casing. the pressure responsive means representing an This device is, however, limited in the following acceleration of the device with substantially no respects:movement of the fluid relative to the cell.

1. Its size and weight are relatively large. To make The device may have two or more pressure the geophone considerably smaller would increase 95 responsive means located indifferent directions the cost, as it is an electromechanical device with relative to one or more other pressure responsive moving parts. Itwould also make it more difficuitto means,the outputs of the pressure responsive achievethe required specification in parameters means being combined differentially so asto such as resonantfrequency, sensitivity, dynamic respond to components of acceleration in those - range, and maximum amplitude. to providethe 100 directions. Alternatively, the pressure responsive performance necessary. means may be located in different directions relative 2. It is normally an axial sensor only. Vertical to a point of constant pressure of the fluid so thatthe component and horizontal component geophones outputs of those pressure responsive means require different suspensions and cases, with a risk respectively represeritthe components-of of differing response characteristics, and a three 105 acceleration in those directions.

component geophone is simplythree The output signals of two or more pressure normally-designed geophones packaged together responsive means may beweighted and combined and is of correspondingly largersize andweight. to producea signal representing acceleration ofthe 3. The cost of the geophone itself is quite high device in a direction defined bythe relative positions and many of them are needed to be used in a survey. 110 of the pressure responsive means ortheir positions A heavy-effort seismic field crew may have some relativeto the point of substantially, constant

30,000 geophones representing a large capital pressure of the volume of f luid and the weightings investment. A much less expensive sensorwould applied to the output signals of those means. This enable more to be used, improving signallnoise may also be done with differentially connected pairs ratios by improved array techniques, andlor routine 115 of pressure responsive means. Each pressure recording of three components of motion ratherthan responsive means may be a piezo-electric crystal onlyone. 1 which may be connected to a buffe amplifier having 4. The response characteristics of a geophone a very high input impedance. AlternatiVelythe sufferfrom significant (but known) phase and output of the crystal may be used directly by amplitude effects which are impressed upon the 120 connecting itthrough a capacitor to the viual recorded signi and need to be compensated. ground input of a charge amplifier having;d.c.

Anothertype of motion sensor used in seismic - negative feedback.

reflection surveying is the accelerometer. In this In one example, the pressure responsive means device, a pair of piezo-electric stress sensors are are respectively responsive to components of fixed between respective opposite faces of a metal 125 acceleration in two or three mutually perpendicular mass and a f rame rig idly attached to the Earth. directions. For exam pie, the cel 1 may be in the form Vibrations in the Earth are transmitted via the of a cube, having pressure responsive means in the piezo-electric crystals to the metal mass, and the middle of each face of the cu be, the'pressure resulting inertial forces stress the crystals. thereby responsive means on opposite faces bei rig producing a voltage proportional to the acceleration 130 connected together so that the difference between 2 GB 2 192 457 A 2 their outputs yields directly the components of screw provided on the exterior of the device, or acceleration along axes normal to the pairs of faces. alternatively by adjusting the tightness of fit of a Alternatively, the cell maybe of tetrahedral form, cover plate and associated rubber gasket. The bias having three-orthogonal faces with pressure pressure should be sufficient to prevent cavitation of responsive means on each,and a fourth diago-nally 70 thefluid atthe highest expected acceleration.

intersecting face completing the cell. The device may be provided with meansfor rigidly In another example,the outputsfrom the pressure coupling the device to the objectwhose acceleration responsive means may be combined in an electric al is to be measured. This means may comprise a summing circuitto produce outputs respectively threaded stud orotherfixing means provided on the representing the components of acceleration in two 75 outercasing ofthe device, to which a spike, clamp or orthree orthogonal directions. In this case,the cell othercoupling device may befixed.

may have any polyhedral orcurved shape, but itwill An example of the invention will now be described preferably be shaped so asto permitthe acceleration with referencetothe accompanying drawings in of the object beingsurveyed to be resolved inthe which:- desired directionsby simple summation ofthe 80 Figure 1 shows a disassembled perspectiveview sensor outputs. F6rexample, the cell may be a of the example of the invention, having a tetrahedral regulartetrahedron with pressure responsive means cell; in the middle of each of thefaces. Asimple capacitor Figure2 shows an electrical networkfQr usewith.

network connected to charge amplifiers would1hen the above-embodiment of the invention; and -be sufficient to resolve the outputs of the four 85 Figure3 shows the example of the invention with -pressure responsive means in three mutually an attachmentfora ground spike.

perpendicular directions, although moreelaborate Referring to Figure 1, a motion sensing device summing means may be used. A resistor network comprises an effectively rigid cubic case 1 made of could alternatively be used. steel, for example, having relatively thickwalls, a 2& In.another example, if it is desired.to measure 90 flexible, fluid- containing envelope 2, piezo-electric rotational acceleration, the cell maybe provided with crystal transducers 3, a plug 4, a gasket 5 and an end one or more pressure responsive means radially plate 6. The envelope 2 is filled with mercury and is displaced from the axis of rotationfor detecting the formed into the shape of a regular tetrahedron by inertial forces acting on a fluid mass in a direction welding two crimps 7,7'along mutually tangential tothe axis of rotation. For-example, the 95 perpendicular diameters of a flexible, mercury-filled, cell may be in the form of an arcuate tube with the cylindrical tube, such that, afterwelding, the pressure responsive means mounted on the.end -centre-points of the crimps areseparated by a faces of thetube, perpendiculartothe arcuate axis of - distance equal to 7rr/N/2,where r isthe radius ofthe thetube. tube. Thetube may be of a plastics material such as Thefluid may be mercury or any othersubstance 100 polyethylene or polypropylene. Piezo-electric which is a liquid atthe desired operating transducers on annular mounts 3 are attached to the temperatures butwhich is preferablyof high density centres of each of thefourfaces of the envelope.

and lowviscosity. The case 1 has, opening onto the upperface, a The fluid may be sealed directlywithin the cell, or it square cross- section cavity 8 of side 7rr/V2 the lower may be contained within a flexible envelope 105 portion of which is adapted to the shape of the two matched to the shape of the cell. The envelope may lowerfaces of the tetrahedral envelope 2. The square be of a plastics material such as polyethylene or plug 4, also of side 7rr/V2, is adapted for sliding fit polypropylene. within the cavity 8, and is shaped at its lower end 4a The pressure responsive Means may be mounted such thatwhen placed within the cavity 8 a directly upon thecell walls, orin the case of a cell 110 tetrahedral closed cell 9, matched to the size and with an envelope, sandwiched between the envelope shape of the envelope 2 is formed.The shaping of and the cell walls, the pressure respon. sive means the lower portion of thecavity 8 may be achieved by being mounted on either of the surfaces. The. fitting a second plug similarto the plug 4 into the pressure responsive means may alternatively be lower end of a square hole almost or completely contained inside the envelope, orencapsulated 115 through the case 1, the second plug being orientated withinthewal Is thereof. The pressure responsive at right angles to theplUg 4. The corner edges 10 of means may be piezo-electric crystal transducers, or the. plug 4 are bevelled such thatwhen the envelope any other suitable pressure sensitive devices. High 2 is placed within the cell 9, and theplug 4 is inserted input impedance buffer amplifiers may be provided. in the cavity 8. the wires 3a of the transducers 3 may to isolate piezo-electric transducers from the loading 120 pass to the exterior of the case 1 along the triangular of a resistor summi ng network,for example. - holes remaining at the corners. Both the cell 9 and The cel I may be provided with means for applying the plug 4 are provided with grooves 11,1 ? to a bias pressure to the fl uid, which may be at the accom modate the end seams 7,7'of the epvelope 2.

substantially constantpressure point. This bias The transducers 3, envelope 2 and plugA are held pressure means may comprise a movable plug 125 in place in the cavity 8 by the square rubber gasket5 forming part of the walls of the closed dell, the -and the cover plate 6. The cover plate is secured by movement of said plug affecting the enclosed - four screws 13, which provide a bias pressure to the volume of the cell, and thereby creating, a bias fluid in the envelope 2 byforcing the plug 4, via the pressure within the fluid. The position of the rubber gasket 5, down upon the envelope 2.

movable plug jnaybe adjusted by means of a set 130 Alternatively, a setscrew (not shown) maybe 3 GB 2 192 457 A - 3 provided screwed through the cover plate 6 to press generated by each crystal is split almost equally on a plate outside the gaskets opposite the plug 4 between the three paths. Each charge amplifier and provide an adjustable bias pressure. The bias receives one-third of the output of each crystal in pressure should be suff icientto prevent cavitation of parallel, in the appropriate sense (bearing in mind thefluid atthe highest acceleration expected. 70 that pressure on one face implies acceleration -away Instead of providing a separate envelope to from thatface) to yield the appropriate axial contain the mercurythe walls of the cell 9 may be component.

coated with a flexible lining, e.g. a plastics material That is: the East component is given bythe sum of which is formed into an envelope either by being the Lower South-West and the Upper North-West introduced into the cell in liquid form or by bonding 75 lessthe Lower North-East and the Upper South-East together separately coated parts using heat or glue. contributions; the North component is given bythe The mercury could be introduced afterthe cell sum of the Lower South-West and the Upper coating is completed through a suitable bore which South-East less the Lower North-East and the Upper isthen plugged. North-West contributions; and the Upwards The case 1 may be made of a.suitably rigid material 80 component is given by the sum of the Lower which does not reactwith mercury in which casethe south-West and the Lower North-East lessthe Upper plastics envelope or plastics coating may be omitted. North-West and the Upper South-East contributions.

Care should betaken to ensure that the outputs of the Preferably, the crystal transducers 3 are connected transducers 3 are not short-circuited bythe mercury. via respective buffer amplifiers (not shown in Figure) The transducers 3 may be attached directlyto the 85 to the resistors of the network. The bufferamplifiers walls of the case 1 by an adhesive or by mechanical may have very high input impedances so as to draw arrangements such as screws or clips so thatthey onlyvery small currents from the transducers. This are squeezed by the pressure in the mercury atthe would maintain the outputs from the transducers at locations of the transducers. As an acceleration of lowfrequencies.

the mercury produces a uniform pressure gradient in 90 Alternatively, the outputs of the crystal it, it is the directions of the transducers relative to a transducers cou Id be connected directly to a network point of constant pressure of the mercury which of equal capacitors of the same configuration as the determine the components of the acceleration to network of resistors. Each charge amplifer would be which they respond. The location of such a point provided with d.c. negative feedbackto produce a may be determined by the meansfor applying the 95 virtual ground at its input. This arrangement would bias pressure which will be sensibly constant. have a lower cut-off frequency determined bythe Alternatively, the transducers may be arranged in time constant of the feedback resistor and the pairs responding in opposite senses to the same capacitance at its input.

acceleration and connected differentially. Other tetrahedral shapes may be used. For In one examplethe external faces of the case 1 100 example, the tetrahedron maybe defined by three - form a cube of edge length 2 cm. and the orthogonal planes, normal to the axial direction with cross-section of the cavity 8 in it is a square having a fourth diagonal plane completing it. The constant sides of length 1 cm. pressure point should be located in the centre of the As shown in Figure 3, the case 1 may also be diagonal plane. Crystal transducers may be fitted to provided with a threaded stud 14forthe attachment 105 the three orthogonal faces of the tetrahedron, and of a spike, or any other means for coupling the device since these can be located so as to yield thethree to pick up vibrations from the Earth. axial components of acceleration directly.

The outputsfrom the crystal transducers 3 arefed In another embodiment of the invention,the bywires 3a to the inputs 15 of the resistor network device is provided with a cubical cavity and mercury shown in Figure 2. This network resolves the 110 envelope, and six crystals, one on each face of the acceleration of the fluid mass as measured bythe cube, with those on opposite faces connected transducers 3 into three orthogonal directions, as differentially. The crystals'outputs are connected described below. through capacitors to the respective charge Assumethatthe three axes required are in the amplifiers forthe three axes, in parallel with any directions East, North, and Up, and thatthe device is 115 other contributors with which they areto be placed plug upwards with the external faces facing combined or summed.

the cardinal points, the output currents of the crystal The axial components are given in therSiX crystal transducerswill be proportional to the pressure example by combining the outputs from pairs of changes on the South-West Lowerface, the oppositefaces in opposing polarities. That is: the North-East Lower face,,theSouth-East Upperface, 120 Westface output lessthe Eastfaco'outpui gives a and the North-West Upperface of the envelope 2 signal proportional tothe Easterly comp9i nent;.th.p respectively. South face output less the North face outout gives a Each terminal of each crystal transducer is signal proportional to the Northerly component; and connected via three identical small value resistors 16 the Lower face output less the Upperface output to the inputs of charge amplifiers 17,18 and 19 for 125 gives a signal proportional to the Upwards summing the East, North and Up components component.

respectively. The charge amplifier input impedances Forthe same external dimensions, the cubical are arranged to be very low 6ompared to these example uses three times the quantity of fluid that resistors, and the impedance of the crystals to the tetrahedral design requires, and avoiqsthe need externally supplied current are very high, the current 130 for a resistive network. Thus it is considerpbly more 4 GB 2 192 457 A 4 sensitive than the tetrahedral design. acceleration of the devicewith substantially no Othershapesof cavity may be used. Forexample, movementof thefluid relativetothe cell.

if itis requiredto measure rotational accelerationsa 2. A device according to. claim 1, having two or semi-cylindrical or semi-annular cavity could be more pressure responsive means located in different used using differentially the outputs from two 70, directions relative to a point of substantially constant transducers on opposite sides of the axis on the pressure of the volume of fluid for detecting diametral face or faces. Other sector shapes could components of acceleration in those directions.

also be used although compensation would be 3.. A device according to claim 1 having at least required for the effects of non-rotationa! one pair of pressure responsive means located at accelerations. 75 opposite positions relative to the volume of fluid In all of these embodiments of the device, mercury with the signals from the pair of pressure responsive is used to form an inertial mass filling arigidly means combined differentially to produce an output defined volume. It is contained in a closed cell with representing the component of acceleration along stress sensors lining or partly-lining the walls ofthe the linefrom one pressure responsive means to the cell, mounted so as to reactto changes of pressure in -80 other.

the mercury due to acceleration of the mercury as a 4. A device according to claim 3 having several resultof acceleration of the cell. There is no relative pairs of pressure responsive means wherein the line movementwithin the mercury itself. from one pressure responsive means to the other of - In the absence of rotational effects,the each pair lies in a different direction.

acceleration of the mercury in any direction. 85 5. A device according to claim 1 having two produces a uniform pressure gradientwithin the groups of pressure responsive means, each group mercury in that direction of a magnitude having at leasttwo pressure responsive means, proportional tothe acceleration, andthe position of a wherein the signaisfrom the pressure responsive transducer relativetothe constant pressure pointof means of the groups are additively combined in the the mercury determines the resolved part oft he 90 same wayfor both groups to produce respective pressure gradieritto which it responds. output signals from the groups, and the output The use of a fluid forthe mass, because it signal of one group is differentially combined with - attenuates shear forces strongly, permits the output signal from the other group to produce a transducers to be used in more than one orientation total output representing the component of on the mass simultaneously. If stress sensors were 95 acceleration in a direction from a point defined by placed on a solid mass, for example on three - the additive combination of the other group.

orthogonal faces of a cube, then motion along one of 6. A device according to claim 5 wherein the the axes would be impeded by friction against the same plurality of pressure responsive means is sensors:forthe other two. In many cases these other divided into pairs of groups and their outputs two sensors would also produce unwanted readings 100 combined in two or more differentwaysto produce as a result of the frictional transmission of shear different outputs representing components of forces. acceleration in different directions.

The cell and the sensors are shaped in a manner 7. A device according to any one of claims 2,4,5 such asto permitthe integration to be performed and 6 in which the pressure responsive means are relative to the desired axes by simple weighted 105 respectively represent components of acceleration summation of the sensor outputs, being in.these- in two orthree mutually perpendicular directions.

embodiments cubical with faces perpendicularto 8. A device according to anyone of claims 2,4,5 the axes, or regular tetrahedral with edges and 6 having an electrical circuitfor the weighted perpendiculartothe axes. summation of the outputs from the pressure Thewhole deviceis rigidly coupled tothe Earth's 110 responsive meansto produce a signal representing a surfaceor other object whose motion isto be component of acceleration in anotper direction.

measured.The axial force measurementfrom the 9. Adevice according to any preceding claim, in device, less the gravitational weight of the fluid which the fluid is contained within a flexible mass, divided:by the value of the mass, gives the envelope which is rnatched to the shape of the closed acceleration measurement along each axis. In the 115 cell.

case of crystals being used as stress sensors, the 10. A device according to claim 9, in which the weight correction disappears. as the crystal output is pressure responsive means are sandwiched proportional to the derivative of stress, and between the respective walls of the cell and the integration gives a force measirrement without bias. envelope.

120 11. Adevice according to anyof claims 1 to 8in

Claims (1)

1. CLAIMS which the walls of the cell are coated with a flexible lining to

   form a closed envelope within the cell.

   1. An acceleration sensing device having a 12. A device according to any, precedlng claim, in closed cell having effectively rigid wal Is containing a which the pressure responsive means are volume substantiallyfilled with fluid with at least one 125 piezo- electric crystal transducers.

   pressure responsive means attached to a wall of the 13. A device according to claiM 12 wherein each cell to respond to the pressure exerted by the fluid on piezo-electric crystal transducer is connected the particularwall to which the pressure responsive through a.capacitorto the virtual ground input of a means is attached, whereby signals are produced by charge amplifierfrom which the output signal from the pressure responsive means representing an 130 the transducer is obtained.

   i I GB 2 192 457 A 1 5 14. A device according to any preceding claim, in which the closed cell is formed bythe insertion of one or more closely-fitting plugs into a suitably shaped cavity in a rigid case.

   15. A device according to any preceding claim, in which a means is provided for coupling the device to the object whose acceleration is to be sensed.

   16. A device according to any preceding claim, having means for applying a bias pressure to the 10 fluid.

   17. A device according to any preceding claim wherein the fluid is mercury.

   18. A device according to any preceding claim, particularly adapted for use as a seismic surveying 15 instrument.

   19. An acceleration sensing device substantially as described herein, with reference to, and as illustrated by, the accompanying drawings, or modified as described herein.

   Printed for Her Majesty's Stationery Office by Croydon Printing Company (UK) Ltd, 11/87, D8991685. Published by The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies maybe obtained.

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