or suspended on fins or spacers to space them away from the body of casing 104 if actual contact with the formation is desired the sensors 114 may be mounted on swellable packers 302 as indicated in fig3 b such packers 302 expand when exposed to downhole conditions pressing the sensors 114 into contact with the borehole wall fig3 c shows the use of bow spring centralizers 304 which also operate to press the sensors 114 into contact with the borehole walls to minimize insertion difficulties a restraining mechanism may hold the spring arms 304 against the casing 104 until the casing has been inserted in the borehole thereafter exposure to downhole conditions or a circulated fluid e g an acid degrades the restraining mechanism and enables the spring arms to extend the sensors against the borehole wall if made of conductive material the spring arms may further serve as current injection electrodes concentrating the measurable fields in the vicinity of the sensors to further concentrate the fields the spring arms outside the zone of interest may be insulated other extension mechanisms are known in the oilfield and may be suitable for placing the sensors 114 in contact with the borehole wall or into some other desired arrangements such as those illustrated in fig3 d and 3e in fig3 d the sensors are positioned near the radial midpoint of the annular region in fig3 e the sensors are placed in a spatial distribution having axial azimuthal and radial variation balloons hydraulic arms and projectiles are other.