MULTIPLE REFLECTED WAVES 95 



The amplitudes of multiple waves recorded on the surface, when the 

 first reflection occurs above the shot point, can be less than, equal to or 

 greater than the amphtudes of the corresponding single waves (^). 



Multiple waves have been found superimposed on single ones. The peak 

 of the combined wave must occur when the shot depth is such that the 

 distance from the excitation point to the upper reflecting boundary is equal 

 to one quarter of the wave length. Furthermore the intensity and shape 

 of the oscillations in a multiple wave depends on the properties of the several 

 boundaries from which it is reflected. This partly explains its greater variability 

 along the observation Hne than that of a corresponding single wave. This 

 intensity and shape of vibration may be different at reciprocal points omng 

 to different conditions of reflection at these points from the upper boundary. 



REMARKS ON THE USE OF MULTIPLE REFLECTIONS 



A question which frequently arises in regions where records of multiple 

 reflections predominate in intensity over records of single waves, so that 

 the single waves cannot be distinguished, is whether the multiple reflections 

 can be used. 



In these regions the follomng problems must first be solved: 



(1) recognizing multiple reflections (establishing the boundaries from 

 which they are reflected, and determining the types of vibration — longi- 

 tudinal or transverse — with which they travel between these boundaries) ; 



(2) determining whether it is possible to follow these waves in the region 

 under investigation. 



To recognize multiple reflections one must use the kinematic and dynamic 

 characteristics of multiple reflections which have been indicated above, 

 and on the basis of appropriate observations show to which type of waves 

 they belong. The second problem is dealt with by trying to follow these 

 waves under the various conditions which exist in the region concerned. 



For regions where both initial problems can be settled satisfactorily, it 

 only remains to decide whether the corollary task of interpretation can be 

 dealt with. This question can be decided in a first approximation by construc- 

 ting a seismic cross -section with the condition that all the distinguishable 

 waves are regarded as single waves. Then, in the case of full-path reflections, 

 and with a known number of n, one must compute the angle of i ncide nce 

 of the lower reflecting boundary, which is equal to y^^\\ Z'^' 



' WOODS 



HOLE. 

 MASS. 



