556 



SEISMIC METHODS 



[Chap. 9 



tion system between shot point and receiving point, and arrangements for 

 transmission of the instant of the shot and the vertical time break. Figs. 

 9-78 and 9-79 show the interior and exterior of a recording truck for such 

 equipment, with six amplifiers on each side, switch panel and camera be- 

 tween them, communication system on upper left, compander unit in 

 upper center, tuning fork arrangement at upper right. Developing cans 

 are in the rear of the recording compartment within easy reach of the 

 operator, whose position is in a swivel chair in front of the equipment. 

 Fig. 9-80 illustrates a portable six-channel apparatus. 



HeUand Research Corp. 



Fig. 9-80. Portable seismic reflection equipment. Upper row: two amplifier 

 boxes, each with three amplifiers; camera between them. Lower row (left to right): 

 communication unit, detectors, timing system. 



3. Travel-time relations. Simple travel-time relations are readily calcu- 

 lated on the assumption that the reflections originate on plane horizontal 

 or inclined surfaces. A further simplification may be introduced by as- 

 suming that the rays are straight. Experience indicates that in many 

 cases the curved path may be replaced by the straight path. Another 

 simplification results from a substitution of the straight for the complex 

 path. The latter has offsets due to refractions in reflecting beds higher 

 up in the section. 



(a) Horizontal layer. In Fig. 9-8 la, let SR = x, which is the dis- 



