Abstract:
The present invention is directed to a nuclear magnetic resonance apparatus and method for generating an axisymmetric magnetic field having long, straight contour lines in the resonance region. A magnetically permeable member is used to shape the static magnetic field generated by an array of permanent magnets. The magnetically permeable member minimizes variations of the static magnetic field in the formation due to vertical motion of the apparatus while obtaining a nuclear magnetic resonance measurement. Further, the magnetically permeable member may minimize variations of the static magnetic field in the formation due to lateral motion of the apparatus while obtaining a nuclear magnetic resonance measurement.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to an apparatus and method for measuring nuclear magnetic resonance properties of an earth formation traversed by a borehole, and more particularly, to an apparatus and method for generating a substantially axisymmetric static magnetic field having long, straight contour lines in the resonance region. 
   It is well recognized that particles of an earth formation having non-zero nuclear spin magnetic moment, for example protons, have a tendency to align with a static magnetic field imposed on the formation. Such a magnetic field may be naturally generated, as is the case for the earth&#39;s magnetic field, B E . After an RF pulse applies a second oscillating magnetic field B 1 , transverse to B E , the protons will tend to precess about the B E  vector with a characteristic resonance or Larmor frequency ω L  which depends on the strength of the static magnetic field and the gyromagnetic ratio of the particle. Hydrogen nuclei (protons) precessing about a magnetic field B E  of 0.5 gauss, for example, have a characteristic frequency of approximately 2 kHz. If a population of hydrogen nuclei were made to precess in phase, the combined magnetic fields of the protons can generate a detectable oscillating voltage, known to those skilled in the art as a free induction decay or a spin echo, in a receiver coil. Hydrogen nuclei of water and hydrocarbons occurring in rock pores produce nuclear magnetic resonance (NMR) signals distinct from signals arising from other solids. 
   U.S. Pat. Nos. 4,717,878 issued to Taicher et al. and 5,055,787 issued to Kleinberg et al., describe NMR tools which employ permanent magnets to polarize hydrogen nuclei and generate a static magnetic field, B 0 , and RF antennas to excite and detect nuclear magnetic resonance to determine porosity, free fluid ratio, and permeability of a formation. The atomic nuclei align with the applied field, B 0 , with a time constant of T 1 . After a period of polarization, the angle between the nuclear magnetization and the applied field can be changed by applying an RF field, B 1 , perpendicular to the static field B 0 , at the Larmor frequency f L =γB 0 /2λ, where γ is the gyromagnetic ratio of the proton and B 0  designates the static magnetic field strength. After termination of the RF pulse, the protons begin to precess in the plane perpendicular to B 0 . A sequence of refocusing RF pulses generates a sequence of spin-echoes which produce a detectable NMR signal in the antenna. 
   U.S. Pat. No. 5,557,201 describes a pulsed nuclear magnetism tool for formation evaluation while drilling. The tool includes a drill bit, drill string, and a pulsed nuclear magnetic resonance device housed within a drill collar made of nonmagnetic alloy. The tool includes a channel, within the drill string and pulsed NMR device, through which drilling mud is pumped into the borehole. The pulsed NMR device comprises two tubular magnets, which are mounted with like poles facing each other, surrounding the channel, and an antenna coil mounted in an exterior surface of the drill string between the magnets. This tool is designed to resonate nuclei at a measurement region known to those skilled in the art as the saddle point. 
   Great Britain Pat. App. No. 2 310 500, published on Aug. 27, 1997, describes a measurement-while-drilling tool which includes a sensing apparatus for making nuclear magnetic resonance measurements of the earth formation. The NMR sensing apparatus is mounted in an annular recess formed into the exterior surface of the drill collar. In one embodiment, a flux closure is inserted into the recess. A magnet is disposed on the outer radial surface of the flux closure. The magnet is constructed from a plurality of radial segments which are magnetized radially outward from the longitudinal axis of the tool. The flux closure is required to provide suitable directional orientation of the magnetic field. 
   The tools developed in the prior art have disadvantages which limit their utility in nuclear magnetic resonance logging applications. Magnet designs of prior art tools do not simultaneously produce a highly axisymmetric static magnetic field with long straight contour lines in the resonance region of the formation under evaluation. These factors adversely affect the NMR measurement given the vertical motion of a wireline tool and the vertical and lateral motion of a logging-while-drilling tool. 
   SUMMARY OF THE INVENTION 
   The above disadvantages of the prior art are overcome by means of the subject invention for an apparatus and method for generating a substantially axisymmetric static magnetic field having long, straight contour lines in the resonance region. A wireline or logging-while-drilling apparatus within a borehole traversing an earth formation determines a formation characteristic by obtaining a nuclear magnetic resonance measurement. The apparatus produces a static magnetic field, B 0 , into the formation such that the contour lines generated by the static magnetic field are substantially straight in the axial direction at the depth of investigation where the nuclear magnetic resonance measurement is obtained. An oscillating field, B 1 , is produced in the same region of the formation as the static magnetic field to obtain the NMR measurement. The apparatus includes at least one magnetically permeable member for focusing the static magnetic field. The magnetically permeable member minimizes variations of the static magnetic field in the formation due to vertical motion of the apparatus while obtaining the nuclear magnetic resonance measurement. Further, the magnetically permeable member may minimize variations of the static magnetic field in the formation due to lateral motion of the apparatus while obtaining the nuclear magnetic resonance measurement. In addition, the magnetically permeable member can add significant, prepolarization by causing the B 0  field to have substantial magnitude well ahead of the actual region of investigation which can permit increased logging speed. 
   The static magnetic field is produced using either an axial, radial, or bobbin magnet design. For the axial design, the static magnetic field is produced by an upper magnet surrounding the carrying means and a lower magnet surrounding the carrying means and axially separated from the upper magnet by a distance such that the contour lines generated by the static magnetic field are substantially straight in the axial direction at the depth of investigation where the nuclear magnetic resonance measurement is obtained. The magnets are axially magnetized giving a radially polarized B 0  field in the region of investigation. At least one magnetically permeable member for shaping the static magnetic field is located between the lower magnet and the upper magnet. The static magnetic field has either a low gradient or a high gradient, depending on the separation of the magnets, at the depth of investigation where the nuclear magnetic resonance measurement is obtained. 
   For the radial design, the static magnetic field is produced by an annular cylindrical array of magnets surrounding the carrying means. The array of magnets comprises a plurality of segments, each segment is magnetized in a direction radially outward from and perpendicular to the longitudinal axis of the apparatus. The magnetically permeable member comprises a section of the carrying means, a chassis surrounding a section of the carrying means, or a combination of the chassis and the carrying means section. 
   For the bobbin design, the static magnetic field is produced by a plurality of geometrically and axisymmetric magnet rings surrounding the carrying means. The plurality of rings comprises an upper ring, a plurality of inner rings, and a lower ring. The radius of the upper and lower rings is greater then the radius of each inner ring. Each of the plurality of rings is axisymmetrically polarized and the direction of polarization for each ring differs progressively along the ring of magnets. The polarization direction of the upper ring is radially opposite to the polarization direction of the lower ring. The polarization of each inner ring changes progressively such that an angle between the polarization and a transverse radius vector varies linearly for each inner ring. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages of the present invention will become apparent from the following description of the accompanying drawings. It is to be understood that the drawings are to be used for the purpose of illustration only, and not as a definition of the invention. 
     In the drawings: 
       FIG. 1  illustrates a nuclear magnetic resonance logging-while-drilling tool; 
       FIG. 2  depicts the low gradient magnet design; 
       FIGS. 2   a - 2   d  illustrate the contour lines |  B   0 | corresponding to four low gradient magnet configurations; 
       FIGS. 3   a - 3   d  represent the contour lines of the gradient |∇  B   0 | corresponding to four low gradient magnet configurations; 
       FIG. 4  depicts the high gradient magnet design; 
       FIG. 4   a  represents the contour lines |  B   0 | corresponding to the high gradient magnet configuration; 
       FIG. 4   b  represents the contour lines of the gradient |∇  B   0 | corresponding to the high gradient magnet configuration; 
       FIG. 5  depicts the bobbin magnet design; 
       FIG. 5   a  represents the contour lines |  B   0 | corresponding to the bobbin magnet configuration with a non-magnetically permeable member; 
       FIG. 5   b  represents the contour lines |  B   0 | corresponding to the bobbin magnet configuration with a magnetically permeable member; 
       FIG. 6  depicts the radial magnet design; 
       FIG. 6   a  represents the contour lines |  B   0 | corresponding to the radial magnet configuration with a non-magnetically permeable member; and 
       FIG. 6   b  represents the contour lines |  B   0 | corresponding to the radial magnet configuration with a magnetically permeable member; 
       FIG. 7  depicts a combination magnet arrangement using three magnets; and, 
       FIG. 7   a  represents the contour lines |  B   0 | corresponding to a combination low gradient-low gradient magnet arrangement. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a nuclear magnetic resonance (NMR) logging-while-drilling tool  10  is illustrated. The tool  10  includes a drill bit  12 , drill string  14 , a magnet array  16 , RF antenna  18 , and electronic circuitry  20  housed within the drill collar  22 . A means for drilling a borehole  24  in the formation comprises drill bit  12  and drill collar  22 . The mud flow sleeve  28  defines a channel  30  for carrying the drilling fluid through the drill string  14 . A drive mechanism  26  rotates the drill bit  12  and drill string  14 . This drive mechanism is adequately described in U.S. Pat. No. 4,949,045 issued to Clark et al., the disclosure of which is incorporated by reference into this specification. However, it is also within contemplation of the subject invention to use a downhole mud motor placed in the drill string as the drive mechanism  26 . 
   The magnetic field generated by magnet array  16  is focused by at least one magnetically permeable member  36  positioned inside the drill collar. With this arrangement, member  36  can extend a considerable length in the axial direction without decreasing the mechanical strength of the drill collar  22 . Furthermore, if member  36  consists of a mechanically weak material, a separate, underlying mud flow sleeve  28  provides a degree of protection from the pressure, cuttings, and abrasion of drilling mud. Placement of member  36  outside the drill collar  22  would significantly weaken the mechanical integrity of the tool since that arrangement requires cutting a recessed area from the outside of the drill collar to accommodate member  36  thereby weakening collar  22  due to the section of drill collar between channel  30  and the recess having a decreased thickness in comparison to other sections of the drill collar. It is within contemplation of the subject invention that the magnetically permeable member  36  comprises a segment  38  of sleeve  28 . In this case, an additional layer of space is not required inside the drill collar for member  36  and the available space is sufficient to accommodate a magnet array having a larger volume. 
   Low Gradient Design 
   Referring to  FIG. 2 , in a preferred embodiment of the invention, hereinafter referred to as the low gradient design, magnet array  16  comprises an upper magnet  32  axially separated from a lower magnet  34 . The area between magnets  32 ,  34  is suitable for housing elements such as electronic components, an RF antenna, and other similar items. Both magnets  32 ,  34  surround sleeve  28 . A magnetically permeable member  36  is positioned inside the drill collar  22  between the magnets  32 ,  34 . Member  36  may consist of a single piece or a plurality of sections combined between the magnets. Member  36  is constructed of a suitable magnetically permeable material, such as ferrite, permeable steel or another alloy of iron and nickel, corrosion resistant permeable steel, or permeable steel having a structural role in the member design, such as 15-5 Ph stainless steel. The magnetically permeable member  36  focuses the magnetic field and either carries drilling fluid through the drill string or provides structural support to the drill collar. Further, member  36  improves the shape of the static magnetic field generated by magnets  32 ,  34  and minimizes variations of the static magnetic field due to vertical and lateral tool motion during the period of acquisition of the NMR signal. The segment  38  of sleeve  28  between magnets  32 ,  34  may comprise magnetically permeable member  36 . In this case, the segments  40 ,  42  of sleeve  28  under magnets  32 ,  34  shall consist of a non-magnetic member. Alternatively, a magnetically permeable chassis  44  surrounding segment  38  defines member  36 . In this case, segment  38  may consist of a magnetic or non-magnetic material. It is within contemplation of this invention to integrate chassis  44  and segment  38  to form member  36 . 
   The magnets  32 ,  34  are polarized in a direction parallel to the longitudinal axis of the tool  10  with like magnetic poles facing each other. For each magnet  32 ,  34 , the magnetic lines of induction travel outward from an end of the magnet  32 ,  34  into the formation to create a static field parallel to the axis of the tool  10  and travel inward to the other end of the magnet  32 ,  34 . In the region between upper magnet  32  and lower magnet  34 , the magnetic lines of induction travel from the center outward into the formation, creating a static field in the direction perpendicular to the axis of the tool  10 . The magnetic lines of induction then travel inward symmetrically above the upper magnet  32  and below the lower magnet  34  and converge in the longitudinal direction inside sleeve  28 . Because of the separation, the magnitude of the static magnetic field in the central region between the upper  32  and lower  34  magnet is relatively homogeneous. The amount of separation between the magnets  32 ,  34  is determined by selecting the requisite magnetic field strength and homogeneity characteristics. As the separation between the magnets  32 ,  34  decreases, the magnetic field becomes stronger and less homogeneous. Conversely, as the separation between the magnets  32 ,  34  increases, the magnetic field becomes weaker and more homogeneous. 
     FIGS. 2   a - 2   d  illustrate the contour lines of |  B   0 | corresponding to four configurations of upper  32  and lower  34  magnets. The configuration corresponding to  FIG. 2   a  comprises a non-magnetically permeable member separating an upper  32  and lower  34  magnet by 25 inches. The configuration corresponding to  FIG. 2   b  comprises a non-magnetically permeable member separating an upper  32  and lower  34  magnet by 18 inches. The configuration corresponding to  FIG. 2   c  comprises a non-magnetically permeable member separating an upper  32  and lower  34  magnet by eight inches. The low gradient design, corresponding to  FIG. 2   d , comprises a magnetically permeable member  36  separating an upper  32  and lower  34  magnet by 25 inches.  FIGS. 3   a - 3   d  represent the contour lines of the gradient |∇  B   0 | corresponding respectively to configurations illustrated in  FIGS. 2   a - 2   d.    
   In the low gradient design, a significant portion of the magnetic flux is shunted by the magnetically permeable member  36  into the center of the tool  10 . To illustrate, the magnitude of the B 0  field shown in  FIG. 2   d  at a distance of approximately seven inches radially from the longitudinal axis of tool  10  is twice as large as the B 0  field shown in  FIG. 2   a  which was generated by the same magnet configuration separated by a non-magnetically permeable member. Furthermore, the low gradient design produces a longer and more uniform extent of the static magnetic field in the axial direction. The NMR signal measured in this embodiment is substantially less sensitive to the vertical motion of the tool. Referring to  FIG. 3   d , with the low gradient design, a relatively small, approximately 3 Gauss/cm, gradient is measured at a distance of approximately seven inches radially from the longitudinal axis of tool. This low gradient results in a measured NMR signal which is substantially less sensitive to the lateral motion of the tool  10 . Moreover, with the low gradient design, the proton rich borehole region surrounding the tool  10  will resonate only at frequencies higher than those being applied to the volume of investigation, i.e., there is no borehole signal. This is a characteristic of all embodiments of this invention. Other NMR sensitive nuclei found in drilling mud, such as sodium-23, resonate at significantly higher static magnetic field strengths than hydrogen when excited at the same RF frequency. These higher field strengths are not produced in the borehole region surrounding the tool or near the antenna where such unwanted signals could be detected. This is a characteristic of the axial magnet designs of this invention, including the high gradient design. 
   High Gradient Design 
   As previously described, with the low gradient design, a significant portion of the magnetic flux is shunted by the magnetically permeable member  36  into the center of the tool  10 . Without the shunting of magnetically permeable member  36 , a high gradient design is achieved by separating the upper  32  and lower  34  magnet to obtain the same |  B   0 | illustrated in  FIG. 2   d . As shown in  FIG. 2   b , a magnetic field strength, 60 Gauss, at a distance of approximately seven inches radially from the longitudinal axis of tool  10 , is achieved by a non-magnetically permeable member separating the magnets  32 ,  34  by 18 inches. However, the shape of the volume of investigation in which the static magnetic field strength is in resonance with the RF frequency remains curved, and the field contour lines are relatively short in the axial direction. Furthermore, the receiver for detecting the NMR signal is sensitive to the borehole signal, as indicated by the two separate magnetic field regions shown in  FIG. 2   b.    
   For a high gradient design using a non-magnetically permeable member, the curved shape of the volume of investigation and the borehole signal are overcome by decreasing the separation between magnets  32  and  34 . As illustrated in  FIG. 2   c , if the magnet separation is decreased to approximately eight inches, the contour lines of the static magnetic field strength become straighter and the strength of |  B   0 | increases. However, the gradient |∇  B   0 | becomes larger, as illustrated in  FIG. 3   c , at a distance of approximately seven inches radially from the longitudinal axis of the tool. The contour lines of |∇  B   0 | are curved denoting variation of the gradient in the axial direction. 
   Referring to  FIG. 4 , the high gradient design is improved by inserting a magnetically permeable member  36  between magnets  32 ,  34 .  FIG. 4   a  represents contour lines of |  B   0 | corresponding to a configuration where magnetically permeable member  36  separates the upper  32  and lower  34  magnets by eight inches. The contour lines of  FIG. 4   a  show less curvature in the axial direction than the contour lines of  FIG. 2   c . Also, as illustrated in  FIG. 4   b , the magnetically permeable member  36  produces a more constant gradient |∇  B   0 | in the axial direction. 
   Bobbin Design 
   Referring to  FIG. 5 , in a second embodiment of the invention, hereinafter referred to as the bobbin design, magnet array  16  comprises a geometrically and magnetically axisymmetric array of magnets  40  surrounding sleeve  28 . Preferably, sleeve  28  is constructed of a suitable magnetically permeable material, such as ferrite, permeable steel or another alloy of iron and nickel, corrosion resistant permeable steel, or permeable steel having a structural role in the member design, such as 15-5 Ph stainless steel. However, it is within contemplation of the subject invention to have a non-magnetically permeable sleeve. The magnet array  40  comprises a ring of magnets  43 ,  44 ,  45 ,  46 ,  47 , and  48 . The radius of the uppermost ring  47  and the lowermost ring  48  is greater than the plurality of rings  43 ,  44 ,  45 ,  46  defining a central array  42 . The area between rings  47  and  48  can accommodate a deep RF antenna mounted on the drill collar  22 . 
   With the bobbin design, each ring of the array  40  is axisymmetrically polarized but the directions of polarization differ progressively along the array  40 . The polarizations of the uppermost ring  47  and the lowermost ring  48  are oriented such that their respective lines of extension intersect in the NMR measurement zone of investigation in the formation. Consequently, the orientations of the magnetization of rings  47  and  48  are radially opposite to each other. By way of example,  FIG. 5  illustrates the orientation of ring  47  directed outward into the formation and the orientation of ring  48  directed inward. Progressing away from uppermost ring  47 , the polarization of each ring  43 ,  44 ,  45 ,  46  is tipped and changes progressively in a manner such that the angle between the polarization and the transverse radius vector varies linearly for each ring in the central array  42 . With the bobbin design, the path of magnetic lines of induction travels outward, away from the upper ring  47 , into the formation to create a static magnetic field parallel to the axis of the borehole at the center of the tool  10  and travels inward, towards the lower ring  48 . 
   Referring to  FIG. 5   b , the magnet configuration depicted in  FIG. 5 , used in conjunction with a magnetically permeable sleeve  28 , produces a longer and more uniform static field in the axial direction. The contour lines of |  B   0 | depicted in  FIG. 5   b  are straighter in the middle of the tool  10  than the contour lines of |  B   0 | illustrated in  FIG. 5   a . Also, the magnetically permeable sleeve of the subject invention permits the magnet array  34  to generate a stronger field at the same location in the formation in comparison to the magnet array  34  surrounding a non-magnetically permeable sleeve. The increased strength of the static field significantly improves the signal-to-noise ratio and enhances the depth of investigation. 
   Radial Design 
   Referring to  FIG. 6 , in a third embodiment of the invention, hereinafter referred to as the radial design, magnet array  16  comprises an annular cylindrical magnet array  50  surrounding a segment  38  of sleeve  28 . The magnet array  50  is comprised of a plurality of segments, each segment is magnetized radially, that is, outward from the longitudinal axis of the tool  10 . Such a magnet array is described in U.S. Pat. No. 4,717,876 to Masi et al., for example. An antenna  52  is mounted in an exterior recess  54  of the drill collar  22 . A non-conductive, magnetically permeable layer of material  56 , such as ferrite, fills recess  54 . The antenna  52  also surrounds sleeve  28 . The RF magnetic field, B 1 , generated by current flowing through antenna  52  has field directions substantially parallel to the longitudinal axis of the tool  10 . Alternatively, the RF magnetic field, B 1 , is generated by an array of antennas and B 1  extends azimuthally about the longitudinal axis of the tool  10 . 
   Still referring to  FIG. 6 , magnetically permeable member  36  is comprised of segment  38 . Similar to the low gradient design, a chassis surrounding segment  38  may define the permeable member  36 . For illustrative purposes only, the radial design described herein refers to a magnetically permeable member  36  consisting of segment  38  constructed of a suitable magnetically permeable material, such as ferrite, permeable steel or another alloy of iron and nickel, corrosion resistant permeable steel, or permeable steel having a structural role in the sleeve design, such as 15-5 Ph stainless steel. The use of a magnetically permeable material for the segment  38  improves the shape of the static magnetic field generated by magnet array  50  and minimizes variations of the static magnetic field due to vertical tool motion during the period of acquisition of the NMR signal. The direction of the static field is illustrated by vectors. The path of the magnetic lines of induction travels from the central section of the magnet array  50  outward into the formation creating a static magnetic field in the direction perpendicular to the borehole axis, travels inward symmetrically above and below the magnet array  50  through segment  38 , and then converges in the longitudinal direction inside sleeve  28 , returning to the central section of the magnet array  50 . The magnetically permeable material forces the return magnetic lines of induction to be more orthogonal to the axial direction when crossing the outer surface of segment  38 .  FIGS. 6   a  and  6   b  compare the field strength of the array of magnets  50  surrounding a non-magnetically permeable segment  38  versus the field strength of the array of magnets  50  surrounding a magnetically permeable segment  38 . 
   Referring to  FIG. 6   a , with a non-magnetically permeable segment  38 , the magnetic energy is primarily concentrated at the extremities of the cylindrical array of magnets  50 . This heterogeneity characteristic of B 0  extends into the surrounding formation. The portions of the static field near the ends of the array  50  are larger than the field located in the middle of the tool  10 . The shape of the formation volume in which the static magnetic field strength is in resonance with the RF frequency is curved, and the field contour lines are relatively short in the axial direction. 
   Referring to  FIG. 6   b , with a magnetically permeable sleeve  28 , a longer and more uniform static field is generated in the axial direction. The contour lines of |  B   0 | depicted in  FIG. 6   b  are straighter in the middle of the tool  10  than the contour lines of |  B   0 | illustrated in  FIG. 6   a . The magnetically permeable sleeve  28  serves a dual purpose of focusing the magnetic field and carrying the drilling fluid through the drill string. Also, the magnetically permeable sleeve of the subject invention permits the magnet array  50  to generate a stronger field at the same location in the formation in comparison to the magnet array  50  surrounding a non-magnetically permeable sleeve. For example, as illustrated in  FIG. 6   a , the magnetic field strength is 50 Gauss where r=6″ and z=5″. In contrast, as illustrated in  FIG. 6   b , with a magnetically permeable sleeve, the magnetic field strength increases to 200 Gauss where r=6″ and z=5″. The increased strength of the static field significantly improves the signal-to-noise ratio of the NMR measurement and enhances the depth of measurement investigation. 
   It is within contemplation of the subject invention to generate a static magnetic field by combining N+1 magnet arrays  16  to obtain at least N regions of investigation in the formation. The combinations contemplated by this invention include, but are not limited to, a low gradient-low gradient, high gradient-high gradient, high gradient-low gradient, or low gradient-high gradient combination of arrays  16 . By way of example,  FIG. 7  illustrates a first low gradient magnet array in combination with a second low gradient magnet array. In the region between upper magnet  60  and central magnet  62 , the magnetic lines of induction travel from the center outward into outward into formation creating a first static field in the direction perpendicular to the axis of the tool  10 . In the region between central magnet  62  and lower magnet  64 , the magnetic lines of induction travel from the center outward into outward into formation creating a second static field in the direction perpendicular to the axis of the tool  10 .  FIG. 7   a  illustrates the contour lines of |  B   0 | corresponding to a configuration where a first magnetically permeable member separates upper magnet  60  and central magnet  62  by approximately 25 inches and a second magnetically permeable member separates central magnet  62  and lower magnet  64  by approximately 25 inches. 
   The low gradient, high gradient, bobbin, and radial magnet designs of the present invention are also useful in a wireline logging tool application. Sleeve  28  would define a tubular member within the wireline tool which provides structural strength to the tool. Where sleeve  28  is the magnetically permeable member, the sleeve is designed to withstand substantial axial forces exerted on the tool during fishing operations. If sleeve  28  is the magnetically permeable member, the sleeve can be used for magnetic shielding of electronics, such as electromagnetic relays, that must be within the high magnetic field region produced by the nearby magnets. Moreover, member  36  can be used for the magnetic shielding. 
   The foregoing description of the preferred and alternate embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive nor to limit the invention to the precise form disclosed. Obviously, many modifications and variations will be apparent to those skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the accompanying claims and their equivalents.