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). Mini cores of 25.4 mm diameter and approximately 50 mm length are drilled from original cores. Anisotropy information is obtained by different core orientations. The complex resitivity is measured in an 4-electrode elctrolytic cell by use of a lock-in-amplifier. The applied frequency ranges from 0.5 Hz to 120 kHz. The samples are first evacuated and back-saturated with destilled water. In this case, mainly sigmas and sigmam contribute to the rock resistivity. A second cycle of evacuating and back-saturating with 0.1 molar NaCl solution leads to sigmav (Rauen 1994).