MULTIPLE REFLECTED WAVES 97 



is not sufficiently unambiguous, and must be supplemented by the use 

 of dynamic indications. 



The first point to be noted is the variation or lack of variation in the 

 direction of the oscillations when the number of multiples changes to an 

 odd number. This depends on the combination of properties of the interfaces 

 from •which the multiples are reflected. 



The prevailing oscillation frequencies, caused by multiple waves reflected 

 from thin layers and travelling through poorly absorbent media, can under 

 certain conditions rise ■when there is a simultaneous increase in the number 

 of multiples and in the distance from the source. The prevailing frequencies 

 of single reflections usually fall, but in certain cases can also rise, with increase 

 in the depth of the reflecting boundary. Consequently, under certain condi- 

 tions, a frequency analysis of the waves recorded may be an aid to recognizing 

 the multiple waves. Sometimes it may help to compare the duration of the 

 vibrations, expressed by the nmnljer of their periods; for multiple reflections 

 from thin layers this increases with the number of multiples. 



The damping of multiple reflections with distance from the source is 

 weaker the more multiples there are; at some distances from the source 

 (of the order of the depth of the reflecting boundary) some multiple reflections 

 with a larger number of multiples can be more intensive than multiple 

 reflections with a smaller number of nndtiples. Consequently, at greater 

 distances from the source in comparison with the depth of the principal 

 reflecting boundary, multiple reflections may predominate over single ones 

 with respect to the amplitudes of the traces. Multiple reflections from thin 

 layers may be characterized by a peculiar shape of the curve representing 

 the relationship between a quantity connected with the amplitude of the 

 vibration on the one hand, and the distance from the source and the number 

 of multiples on the other. 



REFERENCES 



1. I. S. Berzon, Hodographs of multiple reflected, reflected-refracted and refracted- 



reflected waves. Izv. Akad. Nauk SSSR, ser. geogr. i geofiz. No. 6 (1942). 



2. I. S. Berzon, Effective velocities and depths determined from hodographs of multiple 



reflections. Izv. Akad. Nauk, SSSR, ser. geofiz. No. 8 (1956). 



3. I. I. GuRviCH, Reflections from thin strata in seismic prospecting. Applied geofiz. 



Pt. 9. Gostoptekhizdat (1952). 



4. A. M. Yepinat'yeva, Some seismic waves with long travel times. Izv. Akad. Nauk 



SSSR, ser. geofiz., No. 6 (1952). 



5. A; M. Yepinat'yeva, Some types of multiple seismic waves. Izv. Akad. Nauk 



SSSR, ser. geofiz. No. 8 (1956). 



6. N. V. ZvoLiNSKii, Multiple reflections of elastic waves in a layer. Tr. Geofiz. in-ta Akad. 



Nauk SSSR, No. 22 (149) (1954). 



Applied geophysics 7 



