Method and apparatus for obtaining electrical images of a borehole wall through nonconductive mud

A resistivity logging apparatus has an array of electrodes (1-8) projecting from imaging pads (29). The electrodes penetrate nonconductive mud lining the borehole wall. Some of the electrodes (2-7) are moveable in and out of the pad while others of the electrodes (1,8) can be fixed. The electrodes, which are arranged in an array along a circumferential portion of the borehole wall, are able to make contact with the borehole wall. Sequencing electronics causes one electrode to be a source, another to be a measuring electrode, with the measurements of source electrode and measuring electrode moving along the array in order to log a circumferential portion of the borehole wall.