SEISMIC METHODS 



823 



distortion of wave shape and the introduction of spurious frequencies. 

 Certain modifications in design are necessary to compensate for the lower 

 frequency range of the seismic waves as compared to sound waves, but 

 these modifications may be made without special difficulty. The following 

 descriptions are illustrative of the instrumentation and general results to 

 be obtained by the playback system. 



Effects of Filtering. — Figure 506 illustrates a five-track variable-area 

 recording camera which is designed to translate electrical signals into 

 corresponding amplitude variations on a variable area film.f The camera 

 magazine capacity is 150 feet of film, 5 inches wide. This device is provided 

 with two film speeds, 8 or 16 inches per second. The galvanometers are 

 coil-type, electromagnetically damped to 0.6 of critical and with a natural 



Fig. 506. — A 5-track variable area recording 

 camera. (Courtesy of Seismograph Service Corp- 

 oration.) 



Fig. 507. — Variable area reproducer with 

 center panel open, showing transparent drum. 

 (Courtesy of Seismograph Service Corporation.) 



frequency of 225 cycles per second. The records may be reproduced by 

 means of a reproducer (Figure 507), consisting of an exciter lamp pro- 

 viding light which is passed by mirrors thru the five-track variable area 

 film, traveling on a rotating transparent drum, to five photocells. As the film 

 passes between the light source and the photocells, the variation in the 

 intensity of light causes a similar variation in the photocell current which 

 is ampHfied by means of a vacuum-tube amplifier. The output of the 

 amplifier drives the recording galvanometer. At A in Figure 508 are shown 

 three variable-area tracks on which are recorded the original outputs of 

 three seismometers. At B, C and D are conventional-type seismograms 

 recorded with dififerent amplifier filters, showing the output of six tracks. 

 The upper seismogram was recorded with filtering peaked at approximately 

 30 cycles per second; the second seismogram at approximately 50 cycles 



t J. E. Hawkins, "Method and Apparatus for Analysing Seismographic Records," U. S. Patent 

 2,463,534, March 8, 1949. 



