894 



EXPLORATION GEOPHYSICS 



ing horizon. The vaHdity of this explanation is favored by the fact that 

 multiple reflections are often obtained between the air-water interface 

 and the ocean bottom, in offshore seismic work. Here there is a sharp 

 change in velocity between the surface of the water and the air above it. 



An analysis of Figure 468- A will show that the placing of dynamite 

 below the low velocity or weathered zone serves to minimize the possibility 

 of these shallow multiple reflections, while at the same time much more 

 energy is transmitted to the higher velocity horizons. 



Fig. 563. — Multiple reflections between two reflecting beds. 



Figure 563 illustrates the general conditions where multiple reflections 

 may occur between two reflecting horizons substantially parallel, located 

 below the low velocity layer. In this figure the shot-point again is located 

 at the surface of the ground. Reflecting beds 1 and 2 are illustrated. The 

 image of bed 1 with respect to bed 2 is shown as bed V. When the angle 

 between the beds is not too great, 0' may be considered as the image of O 

 with respect to the image bed 1'; and 0" may be considered as the image 

 of with respect to the reflecting bed 2; while 0'" is the image of O" , 

 with respect to the reflecting bed 1. Using the real and the imaginary 

 origins, the paths of the rays may be drawn. A multiple reflection between 



