An apparatus for determining the resistivity of formation surrounding a borehole comprising: an elongate conductive body; a pair of transmitter antennas comprising first and second antennas mounted on the body for inducing a current in the formation; and a pair of receiver antennas comprising first and second antennas mounted on the body for measuring an axial current running in the tool body at the location of the receivers; wherein the pair of transmitter antennas is located to one side of the pair of receiver antennas.