A seismic-energy impulse produced when a heavy weight is allowed to fall and strike the ground. Weights vary from a few kilograms dropped from 2–3 m, for shallow seismic-reflection surveys, up to several tonnes dropped from 3–4 m, in large-scale, commercial, seismic-reflection surveys. In an accelerated weight drop, a mass (usually of no more than a few tens of kilograms) is accelerated towards the ground by compressed air, thus increasing the impact force and energy impulse. See also hammer; shot.