550 



SEISMIC METHODS 



[Chap. 9 



in proportion to the speed of their respective media, whereas the reflected 

 waves (because of their almost vertical incidence) arrive virtually at the 

 same time and have therefore a high ap-po.rent velocity. 



When multiple receivers are connected in series groups, there is an addi- 

 tional possibility of eliminating refractions and reinforcing reflections. In 

 the example illustrated in Fig. 9-76, eight seismometers, connected to- 

 gether, are set up at distances varying from 1000 to 1100 feet from the 

 shot point. When a reflection of 40-cycle frequency with an apparent 

 velocity of 100,000 feet per second strikes the group of receivers, the im- 

 pulses are virtually in phase and hence reinforced. Conversely, when a 



Fig. 9-76. Wave summation in series detectors (after McDermott). 



refraction, surface, or "ground roll" wave passes the receivers, the velocity 

 is so low (2000 ft. sec.~^ in the example) that the phase difference between 

 each receiver is 1/140 sec, and cancellation occurs except for the beginning 

 and end of the ground wave. The peaks shown in the figure are not serious, 

 as they are much smaller than the amplitude of the reinforced reflected 

 wave. 



A decrease in the amplitude of interfering impulses is often made pos- 

 sible by judicious placement of shots and selection of charges. By placing 

 them under the unconsolidated weathered layer, the amplitude of the 

 surface waves at the end of the seismogram (ground roll) is reduced, and 



