Abstract: An apparatus for determining radial distribution of resistivity of earth formations surrounding a wellbore. The apparatus includes a sonde mandrel having an insulating exterior surface and electrodes disposed on the insulating surface at spaced apart locations. The electrodes are connected to circuits for measuring a focused current resistivity within a predetermined longitudinal span. The electrodes are also connected to circuits for measuring electrical impedance between pairs of electrodes spaced apart at a plurality of different longitudinal spacings. The preferred embodiment includes a voltage measuring circuit interconnected between a pair of electrodes which is positioned between a closest spaced pair of electrodes connected to the impedance measuring circuits, so that a vertical resolution of the impedance measurements can be limited to approximately the axial spacing of the pair of electrodes connected to the voltage measuring circuit.