Abstract: An apparatus for cleaning calcites and other extraneous matter from the perforated casing of a sub-terrainian water well comprises a plurality of elongate explosive members 14 for lowering into the well, detonators 16 for detonating each member 14 to produce an explosion which propagates longitudinally of the well, and a plurality of short tubular attenuators 19 circumscribing the explosive members 14 at intervals along the length thereof. Each tubular attenuator 19 comprises a steel tube 20 and an annular ring 21 extending around the tube at a point along the length thereof.