Chap. 9] 



SEISMIC METHODS 



555 



One method employs a hundred-cycle tuning fork which drives a syn- 

 chronous motor whose shaft carries a wheel with ten spokes, one spoke 

 being heavier to mark tenths of seconds. ^^^ This arrangement is used 

 for shadow photography with string galvanometers. For black on white 

 records the spoked wheel is replaced by a disk with the same number of 

 slots. In another arrangement, the prongs of a fifty-cycle tuning fork 

 are provided with slotted diaphragms to project the slot opening every 

 hundredth of a second directly upon the paper.^^ Lastly, vibrating reed 

 timers of fifty-cycle frequency are used, driven by vacuum tube oscillators 

 or tuning forks. 



(e) Shot-instant transmission is accomplished by wire or radio. The 

 shot-instant line also serves for communication with the shot point. In 



HeUand Research Corp. 



Fig. 9-79. Seismic recording truck with detector case and reels. 



most reflection equipment the shot instant is recorded on one of the 

 galvanometers as previously described. A second galvanometer may be 

 used to indicate the vertical (or up-hole) time recorded by a shot point 

 detector. 



In Fig. 9-77 a schematic and greatly simplified wiring diagram for a 

 twelve-channel seismic apparatus is given, including twelve regular reluc- 

 tance detectors, one shot-point detector, twelve amplifiers, a twelve-gal- 

 vanometer camera, synchronous timing arrangement, two-way communica- 



"»See records Nos. 1 and 2 of Fig. 9-91a. 



« See records Nos. 4 to 8 of Fig. 9-916 and c. 



" See record No. 3 of Fig. 9-91a with 1/200 sec. time lines from 100 cycle fork. 



