Chap. 9] 



SEISMIC METHODS 



537 



velocity in the lower layer. The underlayer fronts are advanced farther 

 than the corresponding overburden fronts. Since there can be n3 dis- 

 continuities, the lower wave front has to be connected to the corresponding 



5 ■ sec. 



J — . — , — . ll—X 



SVKO 



M 



Inttrupt 



wftntff 



Fig. 9-69. Travel-time curve and corresponding wave fronts in three horizontal 

 layers (adapted from Thornburg). 



upper front. This is done by drawing lines through the upper points of 

 the underlayer wave front normal to the critical angle of the emerging ray 

 to intersection with the corresponding wave front in the upper medium. 

 As seen from Fig. 9-69, a straight wave front (contact front) results, 



