80 S. D. Shushakov 



Interbed echoes have different travel paths and are correspondingly 

 different in their kinematic properties, which are determined separately 

 in each case. 



When the velocity increases monotonically with depth, multiple reflections 

 show the following Idnematic properties (^). 



1. Depending on the change of type which the multiple wave undergoes 

 as it passes from one hodograph to another with increase of Iq, (when x = 0), 

 there may be increase or decrease of the effective velocity calculated from 

 hodographs which have been mistaken for the hodographs of single waves. 

 A decrease but not an increase in effective velocity mth growth of Iq can 

 be used for recognizing multiple waves. 



2. As the number of multiples increases, these effective velocities may either 

 increase or decrease, depending on whether the greater part of their journey 

 is through a high-velocity layer or a low-velocity one. 



3. As a criterion for recognizing multiple waves, it is possible to use the 

 scatter of the effective velocity values corresponding to identical values of t^ 

 and determined in different sectors of the operational area from the hodographs 

 of various multiple waves, or on condition that the various multiple waves 

 predominate in the interference vibrations. 



A multiple wave of which the first reflection has occurred above the 

 excitation point is distinguished by the fact that the travel time of this 

 wave increases with the increase of the shot depth, whereas the travel time 

 of an ordinary single wave decreases. 



DYNAMIC CHARACTERISTICS 



Shape oscillation — \t is not unusual to find the phase of an oscillation 

 changing through 180° when the number of multiplications of the wave 

 alters to an odd number (^^). The explanation is that in such cases, as the 

 wave passes from top to bottom through a lower boundary and from bottom 

 to top through an upper boundary, there is in the one case an increase and 

 in the other a decrease in wave resistance. Fig. 4 shows traces of multiple 

 reflections obtained by modelling. The phase of the vibration can be seen 

 to vary mth the number of multiplications. This effect could not be observed 

 if the wave resistances increased or decreased both when the wave was passing 

 through the lower boundary and when it was passing through the upper 

 one (^). In general the direction of arrival is reversed if the wave is reflected 

 an odd number of times from boundaries in which the wave resistance 

 diminishes as the wave passes through them. Given some information about 

 the structure of the sector under stiidy, we can get a more accurate idea 



