76 S. D. Shushakov 



aries are formed by the surfaces of the crystaUine basement'^*' or by car- 

 boiiate layers, lava flows and so forth' •'^' ■^^' ■'■^K 



Some investigators maintain that multiple reflections are found in those 

 regions where surface conditions favour their formation' -'^■'^' ■'■^^ (ground- 

 water level near the surface, this low -velocity zone, level ground surface). 

 This relationship is indeed observed in many areas, but it accounts only 

 for multiple weaves reflected from the Earth's surface. 

 . Various answers are given to the question whether the Earth's surface 

 or the base of the low-velocity zone acts as an upper reflecting boundary, 

 but most investigators think that the base of the low-velocity zone plays the 

 main part here ^^> ^^' ^^K Yepinat'yeva '^' explains this on the grounds that 

 a wave reflected from the surface loses a good deal of energy in the process 

 of being reflected by the bottom surface of the low-velocity zone of absorption 

 in this zone. The influence of the conditions of excitation and reception 

 on the intensity of the recorded multiple waves as yet has not been studied. 

 PoULTER, however, in a paper on the grouping of aerial explosions, notes 

 that after comparing a large number of seismograms obtained from single 

 well shots and from arrays of aerial explosions he is almost completely 

 convinced that no traces of multiple reflections occur on the film when 

 grouping is used. This can be explained in the case of multiple waves reflected 

 first above the excitation point, and also in the case of interfaces Avhich 

 dip, when the apparent velocities of multiple waves near the source may 

 be less than the apparent velocities of single waves, and the loAver these 

 velocities, the greater the number of multiple waves. Conditions which 

 are more favourable to the recording of single rather than multiple waves 

 can therefore be created not merely in grouping the shots but also in grouping 

 the detectors and with other forms of direction finding. This possibility 

 disappears as the angles of gradient of the reflecting boundaries diminishe. 



KINEMATIC CHARACTERISTICS 



Various types of multiple reflections can be excited in layered geological 

 media. These waves can be subdivided according to the layers in which 

 they are propagated and according to the layers in which they appear as 

 longitudinal or as transverse waves. 



The symbols P^ and 5^ are used for longitudinal and transverse waves 

 respectively, the subscripts indicating the layers in which these waves are 

 propagated (Fig. 1). 



When we are discussing a vuiiform wave we may confine ourselves to 

 indicating merely the particular boundaries which reflect multiple waves. 



