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 .