The present invention relates to apparatus for a tool for investigating the wall of a borehole in a geological formation, the apparatus comprising: a non-conductive pad (3) having an inside face and an outside face for pressing against the wall; a set of measurement electrodes (6) mounted on the outside face of the pad and means for measuring the potential difference between two measurement electrodes; and both a source electrode (4) adapted to inject current into the geological formation and a return electrode (5), the set of measurement electrodes (6) being situated between the source electrode (4) and the return electrode (5) for the current; the apparatus being characterized in that the pad comprises an electrically conductive portion (8) covering the entire measurement electrode zone and electrically insulated from the measurement electrodes.
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