An apparatus for investigating a geological formation surrounding a borehole including a logging tool moveable through the borehole, an electromagnetic probe 1 including a pad 2 mounted on the logging tool for engagement with the borehole by a wall-engaging face of the pad, at least one transmitting antenna TxA, TxB mounted in the wall-engaging face, and a plurality of spaced receiving antennas RxA, RxB mounted in the wall-engaging face spaced in relation to the transmitting antenna TxA, TxB. At least one of the antennas RxA, RxB, TxA, TxB is an open-ended antenna forming a substantially pure electric dipole normal to the pad face and wherein the first transmitting antenna of the at least one pair of transmitting antennas is positioned above the plurality of spaced receiving antennas and the second transmitting antenna of the at least one pair of transmitting antennas is positioned below the plurality of spaced receiving antennas.