Abstract: Apparatus for investigating subsurface earth formations traversed by a borehole comprising a system of electrodes consisting of a central electrode A.sub.0 and four pairs of electrodes M.sub.1 -M'.sub.1, M.sub.2 -M'.sub.2, A.sub.1 -A'.sub.1, A.sub.2 -A'.sub.2 respectively short-circuited and aligned symmetrically on both sides of the electrode A.sub.0.This apparatus comprises arrangements which, for a frequency .function..sub.2, establish a potential gradient between electrodes A.sub.1 -A'.sub.1 and A.sub.2 -A'.sub.2. A source of alternating current of frequency .function..sub.1 is connected between the electrodes A.sub.1 -A'.sub.1 and A.sub.2 -A'.sub.2 and a source of alternating current of frequency .function..sub.2 between a surface electrode B and electrodes A.sub.2 -A'.sub.2. The potential difference between electrodes M.sub.1 -M.sub.2 and M'.sub.1 -M'.sub.2 is maintained substantially at zero by circulating a current between electrodes A.sub.1 -A'.sub.1 and electrode A.sub.0.