Borehole imaging

Apparatus for imaging the wall of a borehole drilled through an underground formation, comprising: a light source (16); an optical detector device such as a CCD camera (18); a sensor head (24) including a window (26) for application against the wall of the borehole, the light source (16) being connected to the sensor head (24) so as to illuminate the region of the borehole wall when the sensor head is applied to the wall; and an optical fibre bundle (34) 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 fibre bundle (34) comprises a coherent bundle, each fibre of the coherent bundle providing one pixel of a two-dimensional, multi-pixel image of the borehole wall.