an elongated body member adapted for coupling to a well logging or perforating instrument in a fixed rotational relation , and formed such that at least a portion of the mass is eccentrically disposed relative to the longitudinal axis of the body member . located within the eccentric mass of the body member is a recess for retaining one or more magnets . when the orienting decentralizer is introduced into a borehole , virtually any deviation of the borehole from true vertical will cause the longitudinal axis of the body member to tilt and the eccentric mass will cause the decentralizer to rotate about its longitudinal axis and seek equilibrium with the eccentric mass uniformly disposed beneath the tilted axis of the body member , thus rotating the attached logging or perforating instrument to a predictable orientation relative to the lower side of the borehole . in this orientation , within a cased borehole , the magnets will exert an attractive force upon the well casing to resist any departure from this predictable orientation .