Patent ID: 7456632

Claim:
A method for monitoring a high-resistivity reservoir rock formation located below at least one less resistive formation, wherein said method comprises: transmitting an electromagnetic signal (S) propagating from near a sea-floor by means of an electromagnetic transmitter powered by a voltage or current signal generator; said electromagnetic transmitter comprises two electrodes of which one is connected to an upper end of an electrically conductive string in a well, said upper end being arranged near said seafloor; said electromagnetic signal (S) propagating from said sea-floor to said high resistive formation as a guided-wave electromagnetic signal (S 1 ) along said conductive string, and further propagating as a guided-wave electromagnetic signal (S 2 ) inside said high-resistivity formation; said electromagnetic signal (S 2 ) giving rise to an upward refracting electromagnetic signal (R 3 ) in said less resistive formation, said electromagnetic signal (R 3 ) rising from an interface between said high-resistivity formation and said lower-resistivity formation, and giving rise to a steeply rising refraction wave front (F 3 ); and detecting said refracted electromagnetic wave front (F 3 ) comprising refracted electromagnetic signals (R 3 ), along an array of sensor antennas located along said sea-floor, said array of sensor antennas having a direction away from said transmitter.