Abstract: Permanent electromagnetic (EM) monitoring of the regions around and between wells may employ a casing string positioned within a borehole through the subsurface formations of interest. At least two passivated electrodes are mounted on the casing string to sense electric fields in the formation. Though only capacitively coupled to the formation, the passivated electrodes nevertheless provide a potential difference to an electro-optical transducer, which in turn modifies a property of the light passing along an optical fiber attached to the casing string. An interface unit senses the modified property to derive a measure of the electric field between each pair of passivated electrodes. The passivated electrodes have a contact surface that is conductive but for one or more layers of non-reactive (and thus electrically insulating) materials. Illustrative materials include metal oxides, polymers and ceramics, but the layers are preferably kept very thin to maximize the coupling capacitance with the formation.