Apparatus for imaging the wall of a borehole drilled through an underground formation, comprising: a light source; an optical detector device such as a CCD camera; a sensor head including a window for application against the wall of the borehole, the light source being connected to the sensor head so as to illuminate the region of the borehole wall when the sensor head is applied to the wall; and an optical fiber bundle connecting the window to the optical detector device so as to pass optical signals from the wall to the optical detector device; wherein the optical fiber bundle comprises a coherent bundle, each fiber of the coherent bundle providing one pixel of a two-dimensional, multi-pixel image of the borehole wall.