The impression packer is designed to make an impression of the sides of a borehole wall on a wax-covered paper. This provides the information needed to determine the location of discontinuities in the strata at different depths. The impression packer consists of a split steel tube of the same diameter as the borehole itself. The two halves of this tube are pressed against the sides of the borehole when a pneumatic packer in the centre of the assembly is inflated. To the outer surface of the split steel tube is attached a thin layer of highly compressible rubber onto which the wax paper is taped.
On deflation, the packer reverts to its original diameter and the split steel tube closes by the action of a spring so that the assembly can be easily removed or repositioned inside the hole. These packers are manufactured to order and designs exist for borehole diameters of 101 mm and above. Waxed “parafilm” impression paper is supplied in rolls 500 mm wide by 15 m long which can be cut into strips of the appropriate size.