necessary not only to compute the depth to the interface but also to give the 

 lateral location of the reflection points. Even though in this split spread the 

 distances to the Nos. 1 and 11 seismometers are equal, a time difference is shown 

 on the record. This time difference is related to the dip of the interface — the 

 greater the dip, the greater the dip stepout time, and consequently the greater 

 the migration distance. 



If the depth of the reflector is large compared to the spread length, then 

 the assumption of an emergent plane wave is often used. Figure 26-7 represents 

 the relations for the dipping case to a somewhat more realistic scale. Under the 

 assumption of an emergent plane wave, the dip angle cf> is related to the dip 

 stepout, ATcf>, to the spread length, Ax, and to the average velocity, V, by the 

 expression sin = VAT<t>j Ax. 



Reflected travel paths may undergo substantial refraction. Figure 26-8 

 illustrates refracted-reflected ray paths. In this idealized figure, the ray-path 

 geometry clearly shows that distorted results would be anticipated from under- 



Figure 26-8. Ray-path diagram to illustrate refracted-reflected rays. Even though the re- 

 flection horizon is horizontal and the shotpoint-seismometer distances are equal, the 

 arrival times at seismometers 1 and 11 would not be the same; thus a false dip and 

 incorrect coincident point would probably be plotted. Although the figure is idealized, 

 nevertheless it indicates that anomalous results are not unexpected in faulted areas. 



571 



