After years of development it has been determined that the energy from the detonation of a low velocity explosive in an elongated column approaches a unidirectional pattern. This column is placed in the subweathering material extending ¼ to 2 wavelengths in lengths depending on the frequency of energy desired.
A study of the theory of propagation showed that greater directivity can be obtained from the same length of charge by using a powder with a detonation rate less than the side wall velocity of the shot hole. As the detonation rate of the column is lowered relative to the side wall the directivity increases and the downward energy decreases. The optimum balance of these factors is a detonation rate about ⅘ of the side wall velocity.
Experiments on elongated charges indicate that a very important effect of directivity is the elimination of “ghosts” as well as the reduction of horizontal energy.
Interpretation may be greatly facilitated by a reduction in unwanted energy. Character is no longer dependent upon hole depth. Records have a more uniform appearance. Elongated charges produce many of the same effects as shallow pattern holes, but under many circumstances they are easier or less expensive to use.