MULTIPLE REFLECTED WAVES 85 



decreases, as the velocity in this layer increases and as the predominant 

 frequencies of the excited oscillations rise. The fact that the predominant 

 frequencies of multiple waves rise as the thickness of the reflecting layer 

 decreases and the velocity in it increases has been confirmed theoretically <^) 

 and from observations under natural conditions. For example, multiple 

 waves reflected from the gypsum-anhydrite layer in the outer zone of the 

 Cis-Carpathian depression, and waves reflected from a thin basalt layer (^*) 

 are of higher frequency than waves reflected from argillaceous sandstone 

 deposits in the same regions. 



The duration of the oscillations which is expressed by the number of 

 visible periods, noticeably increases as the number of multiples increases 

 (Fig. 4) and the velocities in the reflecting layer diminish. The reason is 

 that the phase shifts between the componental oscillations increase. 

 Oscillation intensity — Since the shape of a reflected wave changes shape 

 with distance from the source, the concept of "intensity" in such cases is 

 an arbitrary one, and we shall often take it as meaning a quantity which is 

 a function of the greatest amplitude of the oscillations, irrespectively of the 

 phase to which this amplitude belongs. 



In many regions, oscillations caused by multiple reflections are more 

 intensive than many other vibrations. Usually, the multiple waves stand 

 out in sharper relief the greater the wave resistance of the lower reflecting 

 layer and the shallower the depth of absorption. This is particularly noticeable 

 in the outer zone of the Cis-Carpathian depression, where there are sectors 

 close to one another in which multiple waves are found to be reflected from 

 a thin gypsum-anhydrite layer bedded at depths of the order of 500 m and 

 less in some parts and at depths of the order of 1000 m and more in others. 

 In the Ukraine, in the West Siberian plain, in Austria, California and certain 

 other regions <^' ^^» ^'^' ^^) where there is a near-surface crystalline basement 

 records of waves possessing distinctive amplitude and reflected singly and 

 repeatedly from the surface of this basement have been obtained. In neigh- 

 bouring sectors within these regions, where the surface of the crystalHne 

 basement is deep-seated, the waves reflected singly or repeatedly from it 

 are lost among the numerous other waves. The portion of wave energy which 

 is lost through reflection from intermediate boundaries increases as the 

 number of these boundaries increases with greater depth of the crystalline 

 basement surface. 



Hansen (^*) reports that on apparently good records obtained in the 

 River Salado basin, Argentina, all the distinguishable deep reflections are 

 multiple and so intensive that it is impossible to pick out single reflections 

 among them. Conversely, on seemingly poorer traces, multiple reflection? 



