Abstract: A device for monitoring the position of an oil/water contact (OWC, 22) between an oil-continuous fluid (2o) overlying a water-continuous fluid (2w) inside a casing pipe (7), comprising the following features: a transmitter (5) for a generating an electro-magnetic signal (ST), said transmitter (5) provided with electrical energy (GT) from a voltage signal generator (G); said transmitter (5) being arranged inside said oil-continuous fluid (2o) and being above said oil-water contact (22), and being inside said casing pipe (7); said electromagnetic wave signal (ST) for partly propagating downwards from said transmitter (5); said electromagnetic wave signal (ST) for being partly reflected from said oil-water contact (22), and partly reflected by the end of the casing, giving rise to an upward propagating, reflected electro-magnetic signal (SR); a sensor (6) for detecting said reflected electromagnetic signal (SR), said sensor (6) also arranged above said oil-water contact (22), providing a sensor signal (RR) to a

Abstract: A method for monitoring a high-resistivity reservoir rock formation (2) below one or more less resistive formations (3), comprising the following steps: Transmitting an electromagnetic signal (S) propagating from near a seafloor or land surface (1) by means of an electromagnetic transmitter (5) powered by a voltage signal generator (G). The electromagnetic signal (S) propagates from the seafloor (1) and is guided along a conductive string (7) to the high-resistive formation (2), and propagates as a guided-wave electromagnetic signal (S2) at a relatively higher speed (V2) inside the high-resistivity formation (2) than a propagation speed (V3) in the less resistive formations (3).

Abstract: The invention comprises a geophysical sensor apparatus for use under water in the sea, comprising a plurality of seismic sensors (1) for sensing seismic waves associated with underground formations, and a plurality of EM-sensors constituted preferably by electrodes (4) for sensing electromagnetic waves associated with said underground formations. In a preferred receiver cable configuration embodiment of the invention, the geophysical sensor apparatus comprises a seismic receiver cable with a linear array of a plurality of seismic sensors (1) and EM-sensors arranged inside a flexible outer skin (25), with said EM-sensors having electrodes on the outside of said outer skin. The cable is operated on the seafloor by a surface vessel, said vessel towing an electromagnetic transmitter antenna in addition to the seismic source.