SEISMIC METHODS 699 



lengths, we have 



-^=— TT— cos<^ + — ^sin<^* (66) 



»J t^ CO ^ y 



Solving this expression for the desired dJ^x/S^ 



cos 



(67) 



or 



ATa, AT' , ^Ty ^ , ,,o. 



-— ^=-— sec<^ ^tanc/. (68) 



This corrected value is used in the conventional manner of computation 

 in which the components are at right angles to each other. 



This transformation is not limited to the use described above but can 

 readily be applied to other problems. For instance, it can be used to 

 calculate the move-out time for any direction if the two component move- 

 outs are known. 



The Recorded Reflection 



The problem of primary importance in the study of the reflection 

 seismogram is to recognize reflections on the records. The ability to 

 distinguish the reflected impulses from all other impulses is acquired to a 

 certain degree by practice and a knowledge of the various wave properties 

 and paths. 



From a physical point of view, probably the simplest analogy to a 

 seismic reflection is the understanding of an echo in the study of sound in 

 air. For example, one hears a sound from a source as the sound waves 

 pass his position, the sound wave continuing its path until it strikes a wall 

 and is turned back. As this "turned-back" sound wave passes the observer, 

 he hears the sound for the second time, as an echo. In the case of reflection 

 recording, the seismometer corresponds to the listener, picking up the 

 passing of the seismic waves through the movement of the ground from 

 their source and again when the reflected seismic wave returns to the ground 

 surface after being reflected from certain subterranean strata. A means 

 of observing the length of time required for the seismic wave to travel the 

 round trip from the origin to the reflecting strata and back to the seismom- 

 eter is provided by recording the efifects on the seismometer on a strip of 

 moving photographic material. 



This would be simple indeed except for the fact that the origin of the 

 seismic wave is not a single impulse, but consists of a complex train of 

 impulses which follow the initial one. To the resulting recorded events 

 appearing on the seismogram have been applied such terms as surface waves, 

 direct waves, refracted zvaves, diffracted zvaves, transverse waves, sustained 



* The subscripts x and y refer to the directions X and Y along which the particular 

 AT" and spread 5" are measured. 



