An apparatus for investigating a geological formation GF surrounding a borehole WB, comprises a logging tool TL moveable through the borehole, an electromagnetic probe 1 comprising a pad 2 mounted on the logging tool, adapted 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.