Abstract:
A vibration transducer such as a geophone, comprising a central pole piece ( 110 ) with a magnet ( 112 ) and coil ( 118 ) concentrically arranged around it. The position of the magnet ( 112 ) is fixed relative to the pole piece ( 110 ) and the coil ( 118 ) is movable relative to the magnet ( 112 ). A method of manufacturing a vibration transducer is characterised in that a bobbin carrying the coils is formed from a substantially tubular body which is positioned on a mandrel and at least one coil is wound around its outer surface, the mandrel being removed from the bobbin when the coil is complete. Another method is characterised in that the coil is formed separately and the bobbin is formed from a substantially tubular body which is positioned inside the coil and expanded to contact the coil when in position.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to geophones, devices for sensing vibrations in earth formations. The invention may be applicable to other types of vibration transducers, either in sensing or transmitting operation.  
         BACKGROUND AND PRIOR ART  
         [0002]    In seismic exploration, the vibrations in the earth resulting from a source of seismic energy are sensed at discrete locations by sensors and the output of the sensors used to determine the nature of the underground formations. The source of seismic energy can be natural, such as earthquakes and other tectonic activity, subsidence, volcanic activity or the like, or man-made such as acoustic noise from surface or underground operations, or from deliberate operation of seismic sources at the surface or underground. Sensors fall into two main categories; hydrophones which sense the pressure field resulting from a seismic source, or geophones which sense vibration arising from a seismic source.  
           [0003]    A prior art form of geophone is shown in FIG. 1. The geophone  10  consists of a moving coils  12 ,  13  mounted on a bobbin  14 , a magnet  15 , a pair of pole pieces  16 ,  18  with suspension springs  20 ,  22  and a housing  24  as shown in FIG. 1. The pole pieces  16 ,  18  and housing  24  are made of magnetically permeable material and form a magnetic field in which the moving coils  12 ,  13  are suspended.  
           [0004]    When the earth moves due to the seismic energy propagating either directly from the source or via an underground reflector, the geophone, which can be located at the earth&#39;s surface or on the wall of a borehole which penetrates the earth, moves in the direction of propagation of the energy. If the axis of the geophone is aligned with the direction of motion, however, the moving coils mounted on the springs inside the geophone stay in the same position causing relative motion of the coils with respect to the housing. When the coils move in the magnetic field, a voltage is induced in the coils which can be output as a signal. The response of a geophone is frequency dependent and can be expressed as  
               e   g     =         Bl        (     ω     ω   0       )       2           {     1   -       (     ω     ω   0       )     2       }     +       (     2                 ζ        ω     ω   0         )     2                         tan        (   ϕ   )       =       1   -       (     ω     ω   0       )     2         2                 ζ        ω     ω   0                         ω   0     =       k   m                                   
 
           [0005]    Where  
           [0006]    e g :induced voltage  
           [0007]    B:magnetic flux density  
           [0008]    l:length of the moving coil  
           [0009]    ω:velocity of motion  
           [0010]    ω 0 :natural frequency  
           [0011]    k:spring constant  
           [0012]    m:moving mass  
           [0013]    ζ:damping factor  
           [0014]    The internal damping is usually designed to be low and the total damping factor is adjusted by the use of a shunt resister externally attached and the factor is usually set to be about 70%.  
           [0015]    One problem encountered with this design is how to increase sensitivity without dramatically increasing the size of the sensor, especially its diameter when considering use as a borehole sensor. Most prior art geophones use alnico magnets. To increase the sensitivity, a better magnetic material is needed. It is know that rare earth cobalt and/or neodimium iron boron (neogium) magnets produce larger magnetic flux than alnico; however, they have different characteristics and to obtain optimum flux density, the shapes of magnets need to be different for the different materials. A suitable shape for an alnico magnet is a relatively tall cylinder, whereas a rare earth cobalt magnet is preferably a relatively flat disc. To overcome the shape problem, a dynamic accelerometer was proposed as described in Japanese Patent Application No. 2-419184 and shown in FIG. 2. Flat, rare earth cobalt magnets  30 ,  32  are mounted face-to-face on a yoke  34  connected to the sensor housing  36  to achieve large flux density. A centre pole piece  38  is located in the space between the opposed magnets  30 ,  32  and a moving coil  40  is mounted on springs  42 ,  44  around the central pole piece  38 . In this design, the natural frequency was chosen to be in the middle of the seismic frequency band and large damping is achieved by using imaginary short circuit connected across the coil output “ 40 ” of an operational amplifier  50  with appropriate resistors R 1 , R 2  as shown in FIG. 3. While it is possible to attain a suitable size for such a sensor, the assembly cost has proved to be high.  
           [0016]    Another problem with the prior art design of FIG. 1 is that the bobbin  14  should preferably be as light as possible. However, in the past this bobbin has been machined from metal which has proven both expensive and difficult to achieve the small thickness desired for size and mass limitations.  
         SUMMARY OF THE INVENTION  
         [0017]    A first aspect of the invention comprises a novel vibration transducer design which finds particular utility as a geophone for seismic measurments. The transducer according to this aspect of the invention has a central pole piece with a magnet and coil concentrically arranged around it. The position of the magnet is fixed relative to the pole piece and the coil is movable relative to the magnet.  
           [0018]    In accordance with one embodiment of the first aspect of the invention, there is provided a vibration transducer, comprising a housing; a central pole piece located inside the housing; a magnet mounted on an inner surface of the housing so as to extend around the pole piece; and a coil located between the magnet and the pole piece and resiliently mounted with respect to the magnet.  
           [0019]    In accordance with a second embodiment of the first aspect of the invention, there is provided a vibration transducer comprising a housing, a central pole piece located inside the housing, a magnet mounted on an outer surface of the pole piece so as to extend substantially completely around the pole piece, and a coil located between the magnet and the housing and resiliently mounted with respect to the magnet.  
           [0020]    The transducer preferably has a circular cross-section with the pole piece at the centre and the housing, magnet, and coil in a concentric arrangement around the pole piece.  
           [0021]    The housing can be formed from a wall section closed at either end by end caps so as to define a cavity. The pole piece can be attached to the end caps and extend through the cavity. Any suitable material such ass steel or soft iron can be used for these parts. One or other end cap can be formed integrally with the housing. The pole piece can also be formed integrally with an end cap. The end caps can fit over the end of the housing or inside an open end of the housing.  
           [0022]    Any suitable magnetic material can be used although rare earth-cobalt (e.g. (Sm.Pr)Co 5 ) or neodimium-iron-boron(e.g. Nd 2 Fe 14 B) materials are preferred for magnetic properties and ferrite for its low cost. The magnet is polarised in the radial direction for optimum effect. While the magnet can be formed as a single piece, it is also possible for it to be formed from a number of discrete components which are either connected to the housing to encircle the coil and pole piece, or connected around the pole piece itself.  
           [0023]    The coil is preferably mounted on a bobbin. In one embodiment, the bobbin is connected to the magnet by means of springs although other resilient mounting arrangements can be used. The mounting preferably allows freedom of movement in the axial direction for the bobbin and hence the coils.  
           [0024]    Modification of the natural frequency of the transducer may be required. This can be achieved electronically, for example by use of an imaginary short, or operational amplifier and shunt resister arrangement.  
           [0025]    A second aspect of the invention provides a method of manufacturing a vibration transducer which comprises a housing having a central magnet structure disposed therein and a bobbin and coil structure disposed around the central magnet structure and resiliently mounted relative to the housing and central magnet structure, the method characterised in that the bobbin is formed from a substantially tubular body which is positioned on a mandrel and at least one coil is wound around its outer surface, the mandrel being removed from the bobbin when the coil is complete.  
           [0026]    The bobbin can be formed either from a complete tube, such as an extruded or welded tube, or a flat sheet formed into a tube shape without welding or the like. The mandrel can be inserted into the bobbin and expanded to support the bobbin while the coil is wound.  
           [0027]    A third aspect of the invention provides a further method of manufacturing a vibration transducer which comprises a housing having a central magnet structure disposed therein and a bobbin and coil structure disposed around the central magnet structure and resiliently mounted relative to the housing and central magnet structure, the method characterised in that the coil is formed separately and the bobbin is formed from a substantially tubular body which is positioned inside the coil and expanded to contact the coil when in position.  
           [0028]    Both methods of manufacturing allow the use of light and thing materials which improves the sensitivity of the resulting transducer. Also, an expensive machining step is avoided which allows dramatic cost reduction in the manufacturing of the transducer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    [0029]FIG. 1 shows a schematic view of a prior art geophone;  
         [0030]    [0030]FIG. 2 shows a second prior art geophone;  
         [0031]    [0031]FIG. 3 shows an electronic circuit for modifying the frequency response of a geophone;  
         [0032]    [0032]FIG. 4 shows a scematic view of a geophone according to an embodiment of the invention;  
         [0033]    [0033]FIG. 5 shows a view on AA of FIG. 4;  
         [0034]    [0034]FIGS. 6 a - 6   d  show alternative constructions for the housing and pole piece assembly;  
         [0035]    [0035]FIGS. 7 a - 7 C show alternative arrangements of coil and magnet mountings in the housing;  
         [0036]    [0036]FIG. 8 shows a detailed view of the spring mounting for the coil;  
         [0037]    [0037]FIG. 9 shows an alternative embodiment of a geophone according to the invention;  
         [0038]    [0038]FIG. 10 shows an alternative circuit for modifying frequency response;  
         [0039]    [0039]FIG. 11 shows one form of bobbin for use in a method of the invention;  
         [0040]    [0040]FIG. 12 shows another form of bobbin for use in a method of the invention;  
         [0041]    [0041]FIG. 13 shows a mandrel for use in a method of the invention;  
         [0042]    [0042]FIG. 14 shows a sea bed cable;  
         [0043]    [0043]FIG. 15 shows a land cable; and  
         [0044]    [0044]FIG. 16 shows a borehole tool. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]    A geophone suitable for use in seismic surveying and embodying the present invention is shown in FIGS. 4 and 5. The geophone  100  comprises a hollow, tubular housing  102  formed from steel having its ends closed by steel end caps  104 ,  106  so as to form a cavity  108  inside the housing  102 . A cylindrical steel centre pole piece  110  extends between the end caps  104 ,  106  through the cavity  108 . In the embodiment shown in FIG. 4, one end cap  106  is integrally formed with the housing  102  and the other end cap  104  fits inside the upper part of the housing  102  to define the cavity. The pole piece  110  is formed separately from the end caps  104 ,  106 , but is connected to them when the geophone is assembled. Various constructions of housing  102 , end caps  104 ,  106  and pole piece  110  are shown in FIGS. 6 a - d . Where an end cap  104  is integral with the housing  102  the pole piece  110  can be formed integrally with the other end cap  106  (FIGS. 6 a  and  6   b ). The end caps  104 ,  106  can also be formed separately from the housing  102  and connected either over the ends of the housing  102  (FIG. 6 d ) or inside the open end of the housing  102  (FIG. 6 c ) in a similar way to the single end caps  106  shown in FIGS. 6 a  and  6   b . Again, the pole piece  110  can be integral with one or other end cap.  
         [0046]    A tubular magnet  112  is fixed to the inside of the housing  102  in the cavity  108 . The magnet  112  is formed from a number of discrete pieces  112 ′,  112 ″ (two are shown here although other numbers may be appropriate). Alternatively a single piece magnet could be used. Whichever construction is selected, the direction of polarisation of the magnet  112  should be in the radial direction of the geophone (indicated by NS in FIG. 4).  
         [0047]    The magnet is preferably made of neogium (Nd 2 Fe 14 B) but other materials such as rare earth cobalt magnetic materials can be used. Since materials such as these can have differing properties, especially with regard to temperature, the most suitable material may differ from application to application. Manufacturers of such materials provide indications of induction, demagnetising force, energy product and permanence coefficient for their products and these properties should be consulted when selecting a suitable material.  
         [0048]    A tubular bobbin  114  is positioned around the pole piece  110  and secured to the ends of the magnet  112  by means of springs  116 . The springs  116  allow freedom of movement of the bobbin  114  in the axial direction but locate it relatively securely in the radial direction. FIG. 8 shows a detailed view of the manner in which the spring  116  is connected to the bobbin  114  and magnet  112 . The spring  116  itself is a circular spring, examples of which are shown in U.S. Pat. No. 4,623,991. Other spring designs can be used where appropriate. In FIG. 8, the spring  116  is attached to the bobbin  114  and magnet  112  by means of plastic spring mounts  115 . These allow secure connection of the spring  116  but are durable and, in the case of the bobbin spring mount, protect the connection in the event that the end of the bobbin  114  contacts the end cap  104  in use. It is to be noted that in this arrangement, the spring  116  operates in a reversed configuration to the prior art arrangements. In the prior art, the centre of the spring is fixed and the outer part deflects. In this case, the outer part is fixed and the inner part deflects.  
         [0049]    A coil  118  is wound around the outer surface of the bobbin  114  and so is likewise moveable relative to the housing  102  and magnet  112 . Various arrangements of magnet and coil are shown in FIGS. 7 a - c . In FIG. 7 a , the magnet is mounted in a recess formed in the housing wall and the axial extent of the coil is less than that of the magnet (i.e. the magnet extends beyond the ends of the coil. Alternatively, the coil can extend beyond the ends of the magnet as shown in FIGS. 7 b  and  7   c . In FIG. 7 c , the magnet is mounted directly on the inner surface of the housing.  
         [0050]    Electric terminals  120 ,  122  are provided at either end of the magnet  112  an lead to the outside of the geophone via ports  124 ,  126  in the pole piece  110  and end cap  104 .  
         [0051]    The output of this geophone can be modified using an op-amp circuit such as that shown in FIG. 3. Alternatively, a shunt resistor RS and op-amp OP circuit such as that shown in FIG. 10 can be connected across the coil output “ 118 ”, or any other arrangement to modify the vibrational behaviour of the geophone to optimise its response at frequencies of interest.  
         [0052]    An alternative form of geophone is shown in FIG. 9 in which the relative positions of the magnet and bobbin/coil are reversed. In this case, the magnet  112 ′″ is secured around the centre pole piece  110 ′ and the coil  118 ′ is wound on the the bobbin  114 ′ which is located around the magnet  112 ′″ by means of a spring mount  116 ′ as before. The various optoins in construction and configuration described above in relation to the embodiment of FIG. 4 apply here also, mutatis mutandis.  
         [0053]    A method according to the second aspect of the invention can be applied to the prior art designs of geophone shown in FIGS. 1 and 2. The mass of the bobbin carrying the coil(s)in any geophone design impacts upon its sensitivity and it is desirable to have the bobbin as light as possible. Previously, the bobbin was machined from metal making it relatively heavy, expensive and difficult to machine. For a complex design such as shown in FIG. 1, the mass might be 10 g, for a simpler design such as FIG. 2, the mass might be 4 g, both bobbins requiring machining to thicknesses of 0.1 mm in places. In the method of the present invention, the bobbin is formed from a simple tube of suitable thickness and material. For example a plastic tube  150  might be 0.15 mm thick and have a mass of about 2 g (FIG. 11). This form can be extruded or formed in any conventional manner. An alternative is to form a flat sheet into a tubular shape  160  with a slot  162  down one side (FIG. 12), in which case aluminium having a thickness of 0.1 mm might be used. One of the properties of the bobbin which affects the performance is its damping effect. Where the bobbin is a continuous metal tube, eddy currents can be set up which damp the motion of the bobbin. If the tube is incomplete (FIG. 12), eddy currents cannot be set up. The damping effect can be improved by welding the slot closed or incorporating complete metal rings or “c” rings to complete the short circuit into the bobbin.  
         [0054]    A problem with such approaches is that the bobbins are very flexible and unable to support the operation of winding the coil(s) onto their outer surface. There are two ways in which this can be accomplished. In the first, a mandrel is inserted into the bobbin to support it while coils are wound. After winding an adhesive compound is applied to the coil(s) and once this is set, the mandrel can be removed. One form of mandrel is shown in FIG. 13 and comprises a tubular body  170  with a slit  172  cut in one side. This allows the outer diameter of the mandrel to be reduced by compressing the mandrel to close the slit. The mandrel can then be inserted into the bobbin  150  and expanded to contact is inner surface. After winding of the coils, the mandrel can again be compressed for removal.  
         [0055]    In an alternative method, the coils are wound directly on the mandrel and the adhesive applied. Once the adhesive is set, the coils are self supporting and can be removed from the mandrel. The completed coils can then be positioned on a bobbin. For this approach a bobbin of the type shown in FIG. 12 can be used. The bobbin can be compressed to close the opening and reduce its outer diameter and allow the coils to pass over it. The proper diameter can be restored by releasing the compression and allowing the natural elasticity of the bobbin to restore its shape. Alternatively, and expanding mandrel can be used, for example of the type described above. It will be appreciated that the method is not restricted to one particular type of mandrel as long as its diameter can be changed as described.  
         [0056]    Geophones embodying the present invention find particular applications in seismic surveying equipment. FIG. 14 shows a sea bed cable  200  which includes a number of geophone packages  202  spaced at regular intervals and connected through the cable  200  to processing equipment  204 . FIG. 15 shows a land cable  200 ′ which has essentially the same configuration as the sea bed cable with geophones  202 ′ spaced apart and connected to processing equipment  204 ′.  
         [0057]    [0057]FIG. 16 shows a borehole tool comprising a tool body  220  which can be lowered into a borehole  222  on a wireline cable  224  connected to surface processing equipment  226 . The tool body  220  includes an operable arm  228  which can be caused to bear against the borehole wall  230 , and a sensor package  232  which is forced against the borehole wall  230  due to the action of the arm  228 . The sensor package  232  contains three orthogonally oriented geophones  234   x ,  234   y ,  234   z  (x,y,z directions) which can receive three component seismic signals and pass data back to the surface via the wireline cable  224 .