EWINO 



[chap. 1 



explosions is capable of penetrating to dc])ths of thousands of meters, and the 

 seisinic-reriection techni(iiie using explosions has been one of the major tools of 

 sub-surface exploration in the past few decades. 



The propagation of seismic energy can be considered adequately for most 

 purposes to behave according to ray theory. The shot point is the origin of ray 

 paths which extend in all directions, and each ray can be considered to be a 

 bundle of energy travelling that particular ])ath. Only the rays that leave the 

 origin and eventually arrive at a detector are of interest. In a multi-layered 

 medium, there are several possible paths, most of them involving reflection or 

 refraction at the interfaces between the layers. 



The simplest case to consider is that involving reflections from horizontal 

 plane interfaces. The lower jjart of Fig. 1 shows the ray paths of the direct 



Fig. 1. Ray diagram and time-distance graph foi- reflections in a tliree-layer model, 

 Ci<C2<C'3. 



O = detector 

 X = shot 

 Oi = Of 



waves and of the first-order reflected waves from two shots to a detector. The 

 shots and detector are in Layer 1 , which has a seismic velocity Ci and thickness 

 ^1. Underneath are two layers with velocities C2 and C3. The upper part of the 

 figure shows the travel time vs. distance graph for the arrivals. The direct 

 waves determine the straight line, D. The reflected waves determine the hyper- 

 boUc curves, Ri and Rn.^ The inverse slope of D, ADfAT, gives the seismic 

 velocity in Layer 1. The Ri curve is asymptotic to D. Its intercept on the 

 ordinate, 2Ai/C*i, gives the travel time for a reflected wave from the top of 



1 In this and succeeding discussions, Rj, Rh, Rm- . .designate reflected waves from 

 the 1st, 2nd, 3rd. . .interfaces below the surface. Arabic subscripts R2, Ra- ■ .designate 

 second, third . . . order reflections from the sea floor. 



