REFRACTED AND REFLECTED LONGITUDINAL WAVES 43 



The intensity of possible interbed echoes above a shot point can also be 

 readily determined by means of formula (11). Comparison of their intensi- 

 ties with the intensity of single echoes from low-lying boundaries can help 

 in recognizing the nature of any particular wave recorded on the seismogram. 



THE FORM OF SINGLE AND MULTIPLE ECHOES IN MULTI-LAYERED MEDIA 



We can say the followng about the form of seismic traces for echoes 

 ■excited as a result of shots in multi-layered media up to the points of emer- 

 gence of the corresponding head waves: all reflected waves, whether 

 simple or multiple, have the same trace form when the influences are the 

 same. Their phases cannot be opposite. Displacements of points on the 

 ground surface repeat the form of the given pulse. As to the onset of echoes 

 when the angles at which they strike the reflecting boundaries are small, 

 all the facts that are well known from plane wave theory can be repeated. 

 If the wave is reflected from a layer which has a lower speed than the layer 

 from which the wave has arrived, and if the discontinuity in the transverse 

 velocities at the reflecting layer is greater than that in the longitudinal velo- 

 cities (that is y < A), then when the angle at which the wave strikes such 

 a reflecting boundary increases, the intensity of the echo from it can pass 

 through zero and the wave can change the sign of its onset. For waves reflected 

 from boundaries (with parameters from Table 2) this can occur when the 

 -angles at which the wave strikes the reflecting boundary have a sine greater 

 than 0.6 (not discussed in the present paper). If the reflected wave under 

 consideration nowhere undergoes reflection from such boundaries then, 

 as the angles of incidence increase to critical, it does not change its sign of 

 onset and has the same form as it has above the shot point. What we have 

 said does not refer to the vicinity in •which the corresponding head waves 

 ■emerge. 



REFERENCES 



1. A. M. Epinat "iTLVA, Experimental data on refracted waves in media with poor speed 



differentiation. Izv. Akad. Nauk SSSR, ser. geofiz. No. 2 (1955). 



2. G. I. Petrashf.n', Problems in Dynamic Theory of Seismic Wave Propagation. Coll. 



1. Gostoptekhizdat, 1957. 



3. G. I. Petrashen', Propagation of elastic waves in layered isotropic media divided by 



parallel planes. Sci. Rec. Zhdanov State Univ., Leningrad, No. 162, Pt. 26, 1953. 



4. D. B. Tal'-Virskii, Tectonics of the Tobolsk Zone from Seismic Prospecting and 



Deep-drilling Data. Thesis. VNII Geofiz. Foundation. 



