In general, four-electrode devices are used to measure the resistivity on original (unprepared) cores and on mini cores drilled from these original cores. The in-phase and the out-of-phase signal is measured and therefore the complex resistivity is determined. All measurements are performed under surface conditions (room temperature and atmospheric pressure). Four point like electrodes are situated at the core surface along a semicircle in a plane perpendicular to the core axis. The two current electrodes are opposite to each other. The electrodes are in contact to the sample by a porous plastic material soaked with 0.1 molar NaCl solution. Voltage and current is measured by a lock-in-amplifier at 120 Hz frequency. Due to high contact resistances, only the magnitude of complex resistivity is used. By computer controlled rotating of the core and moving of the electrode arrangement along the cores z axis, resistivity as a function of azimuth and length is measured.