Abstract: There is described a downhole electromagnetic induction communication system with a receiver arranged to receive from a transmitter a magnetic signal. The downhole electromagnetic induction communication system receiver comprising: a first inductive coil, a second inductive coil, and an amplifier; wherein the first inductive coil and the second inductive coil are configured to convert the magnetic signal into a first input electrical signal and a second input electrical signal. Additionally, the first inductive coil and the second inductive coil are differentially connected to inputs of the amplifier, such that a differential input electrical signal is found, the differential input electrical signal comprising reduced noise compared with the first input electrical signal and the second input electrical signal. The downhole electromagnetic induction communication system reduces noise and improves a signal-to-noise ratio of communications.

Abstract: A telemetry transmitter located in a portion of a drill string comprises an input for receiving and transmitting data obtained from at least one transducer arranged to sense a parameter of the drill string and/or a parameter of the surrounding medium. The transmitter comprises a magnetic dipole arranged to transmit an electromagnetic data signal, a current dipole arranged to transmit an electromagnetic data signal and control means for adaptively varying the power output from the magnetic and current dipoles, according to the orientation of the portion of the drill string containing the transmitter and according to the electrical resistivity of the medium surrounding the portion of the drill string containing the transmitter.