A in situ probe for the measurement of the geoelectrical properties of a geological formation. The probe serves as the interface between a standard acquisition system and the earth. The probe is customizable in its length and the type of electrodes used to make contact with the geologic formation. The probe is permanently installed for long term measurements, it contains closely spaced electrodes enabling a high vertical or depth resolution of measurements. The electrode type is customizable either stainless steel or non-polarizing.