1. An explosive charge or charge array in which the explosion front travels at approximately the velocity of seismic waves in the surrounding rock, so that energy traveling in a desired direction (usually vertically) adds constructively as opposed to that traveling in other directions. The charge must either be shaped to focus the energy in the desired direction or have a length that is a significant fraction of a wavelength to achieve appreciable directivity. Sometimes involves the use of broomstick charges, delay caps, impulse blasters, or sausage powder. In refraction shooting, a horizontal directional charge detonating at the refractor velocity is sometimes used to concentrate the energy traveling as a head wave. 2. Shaped charges as used for armor piercing or perforating casing.