I7g B. McCOLLUM AND F. A. SNELL 
From this it follows that a perfectly uniform dip, in this sense, will not result 
in any velocity asymmetry. 
On the other hand, if the dip is non-uniform as illustrated in Fig. 5 and as 
is most commonly the case under actual conditions, especially close to an 
anticline, the wave paths will not be the same provided the surface points 
are shifted. Thus, in Fig. 5b, the shooting distance, L, is the same as in 5a 
Unstratified 
Formation 


Un stratified 
Formation 



Stratified 
Rock 
yy, 

Yi——_~ 
2 ae 

Fig. 5b. 
but the setup is shifted slightly so as to intercept different conditions of dip. 
Consequently, the wave paths are not the same for the two cases, and both the 
overall times and the station-to-station time intervals will be different. In 
other words, Af, will not, in general, have the same value as at At, and the 
apparent velocities will show an asymmetrical effect. 
CasE 3 
A third type of velocity asymmetry is often due to a relationship which 
frequently exists between the hardness and the age of rocks and shales. In 
shooting up-dip (except in the case of overturned folds), the wave is traveling 
from younger to older beds. Now in many cases it has been found that the 
hardness of rocks increases with their age and the velocity closely corresponds 
to the hardness. Consequently, in shooting from a given shot point to a de- 
tector setup located in an up-dip direction, the tendency is for the velocity 
to increase, and the converse is true in shooting down-dip. This tendericy, 
while by no means invariable, must nevertheless be taken into consideration 
as a causative factor. Whenever this effect exists it gives a true indication of 
the direction of the dip of the bedding planes. 
222 
