magnets the resultant field is substantially radial in direction and reaches a maximum value at some axial distance away from the magnets and then decreases at increasing distances in the vicinity of the maximum is a toroidal region of substantially homogeneous radial magnetic field 140 see fig2 perpendicular to the pair of axially aligned magnets 133 and 135 the field in this toroidal region is radially symmetric still referring to fig2 the pulsed rf coil 136 creates the pulsed rf field 142 illustrated by dashed lines the distance of the toroid from the borehole is a function of the magnetization of the rfms their size and their separation calculations of the magnetic field produced by the rfms may be determined by suitable modeling methods such as a finite element analysis and would be known to those versed in the art examples of such modeling calculations are given in u s pat no 5 488 342 issued to hanley rock pores not shown in the earth formations are filled with fluid typically water or hydrocarbon the hydrogen nuclei in the fluid are aligned in the region of homogeneous magnetic field 140 generated by the magnets 133 and 135 as discussed above the hydrogen nuclei are then u201c flipped u201d away from the homogeneous magnetic field 140 by the pulsed rf field 142 produced by rf coil 136 at the termination of the pulsed rf field from rf coil 136 the hydrogen.