referring to fig1 , 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 . referring to fig2 , 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 . fig2 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 fig2 a comprises a non - magnetically permeable member separating an upper 32 and lower 34 magnet by 25 inches . the configuration corresponding to fig2 b comprises a non - magnetically permeable member separating an upper 32 and lower 34 magnet by 18 inches . the configuration corresponding to fig2 c comprises a non - magnetically permeable member separating an upper 32 and lower 34 magnet by eight inches . the low gradient design , corresponding to fig2 d , comprises a magnetically permeable member 36 separating an upper 32 and lower 34 magnet by 25 inches . fig3 a - 3 d represent the contour lines of the gradient |∇ b 0 | corresponding respectively to configurations illustrated in fig2 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 fig2 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 fig2 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 fig3 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 . 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 fig2 d . as shown in fig2 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 fig2 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 fig2 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 fig3 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 fig4 , the high gradient design is improved by inserting a magnetically permeable member 36 between magnets 32 , 34 . fig4 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 fig4 a show less curvature in the axial direction than the contour lines of fig2 c . also , as illustrated in fig4 b , the magnetically permeable member 36 produces a more constant gradient |∇ b 0 | in the axial direction . referring to fig5 , 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 , fig5 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 fig5 b , the magnet configuration depicted in fig5 , 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 fig5 b are straighter in the middle of the tool 10 than the contour lines of | b 0 | illustrated in fig5 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 . referring to fig6 , 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 fig6 , 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 . fig6 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 fig6 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 fig6 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 fig6 b are straighter in the middle of the tool 10 than the contour lines of | b 0 | illustrated in fig6 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 fig6 a , the magnetic field strength is 50 gauss where r = 6 ″ and z = 5 ″. in contrast , as illustrated in fig6 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 , fig7 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 . fig7 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 .