In this type log, provision is made for electrodes to be pressed firmly against the borehole wall. By doing this, current flowing from the electrodes to the wall of the borehole no longer has to traverse the mud. The path from the electrodes through the mud filter cake that sheaths permeable beds is also reduced to a mininium. The electrode spacing of contact logging devices is very small by comparison with the spacings used in conventional logging devices. Consequently, contact logging devices see very much more detail in the beds they pass through.