Abstract: A device for detecting changes of resistivity or dielectrical properties due to changes of fluid composition in the near-well area about a well in a geological formation, comprising an electrically conductive tubing string, an electrical energy source, a signal generator, at least one transmitting antenna for emitting electromagnetic waves along the tubing string, one or more receiver antennas for receiving electromagnetic waves reflected along the tubing string, devices for receiving signals induced in the receiver antennas, signal processing means for processing the received signals, and communication devices for transmitting signals representing the electrical signals and for receiving control signals.