Patent ID: 7151377

Claim:
An apparatus for monitoring an underground formation trough which a borehole passes from surface comprising: a conductive casing located in the borehole passing through the formation; a plurality of sensors for measuring an electrical property of the formation wherein the plurality of sensors is located in the borehole outside the casing, is connected to the surface and is electrically insulated from the casing; an electrical current source connected to the casing and to a reference electrode offset from the well and the casing; wherein said electrical current source is arranged to inject a current into the formation; whereby the casing has an electrically insulating coating over the outer surface thereof, at least in the region of the sensors, and an electrically conductive portion in electrical contact with the formation such that the current flows into the formation from the electrically conductive portion, and whereby the sensors measure the electrical property of the formation resulting from such current flow.