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For many years a single explosive charge was the most often used source of seismic energy. A single charge is an impulsive point source; that is, all of its energy is generated at one time in one location. For small charges, the amount of seismic energy produced per shot can be increased simply by increasing the charge size. However, as years of experimentation have shown, there are diminishing returns of seismic energy as large charges are made even larger. Single impulsive point sources cannot efficiently produce the amount of seismic energy needed to image deeper targets well. Three strategies evolved for overcoming the limitations of an impulsive point source: (1) distribute the source energy in space, (2) distribute the source energy in time, and (3) distribute the source energy in space and time.

A source can be distributed in space by dividing the single large charge into smaller point charges and firing them together in spatial patterns. The resulting source array produces more seismic energy than does its single-charge counterpart. As an additional benefit, the array pattern can be designed to reduce noise problems, just as can be done with a receiver array (see Chapter 2). Primacord is an example of a spatially distributed explosive source.

There are two ways a source can be distributed in time. In the discrete method, a single massive charge is replaced by many small charges that are fired sequentially from a single shotpoint. The resulting data records are stacked to simulate the shot of

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