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Of fundamental importance in the discussion of seismic interpretation theory and technique is the concept of the emergence angle of a seismic wave.

As a seismic wave emerges to the surface of the earth, its direction of approach is not necessarily vertical; that is to say, the wave front need not be horizontal. A moving wave front, at any particular instant is the totality of points, at that instant, which are in the same cycle and phase of cycle in the propagation. This implies that the travel-times from the source, wherever it is, to each of the points of the wave front are the same.

For our purposes we may reduce the concept to a two-dimensional situation and look at a section of the wave fronts as they emerge to the surface, somewhat as indicated in Figure 1. The various positions of sections of a wave front, at times t1, t2, t3, t4, etc., as the wave travels toward the surface, are indicated by WW and the trace of the surface of the earth by SS.

Closely allied to these wave fronts is the concept of wave paths. These wave paths are curves, or straight lines, which represent the direction of travel of the waves. Thus, these paths are curves which intersect the wave fronts at right angles. A series of wave fronts with the related wave paths is shown in Figure 2.

In the discussion which follows, it is sometimes more convenient to speak of wave fronts and, at others, of wave paths.

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