SEISMIC METHODS 



831 



Mirragraph. — One recording and analyzing system which is being 

 utilized in seismic work is the Mirragraph. f This system, which was 

 developed during the war for the study of vibration and flutter in aircraft, 

 comprises a portable recorder, a play-back unit or reproducer, and an 

 analyzer. The system has a flat frequency response over the range from 

 2 cps to 6,000 cps, and provides means for recording data in the field for 

 later play-back and analysis by electrical methods at headquarters. 



A 



L 



B C D E /F 



A^ J. ^1 riilifaMi III Ti ii wiiWifjM 



HOI '0:i ^•. 



Fig. 513. — Mirragraph recorder, front view with doors open. A, slater lamp; B, foot- 

 age counter; C, footage counter reset; D, magazine locking screw; E, light trap rollers; 

 F, feed spool; G, takeup spool; H, cam gear and switch assembly. (Courtesy of Western 

 Electric Company.) 



The recorder, illustrated in Figure 513, is a thirteen-channel device 

 which uses a light valve, similar to those employed in recording sound 

 for motion pictures, to convert the electrical signals from the individual 

 seismometers into photographic images on moving 35 mm, film (Figure 

 514). This light valve is a thirteen-ribbon Einthoven galvanometer of 

 special design, and the records made with it are similar to oscillograph 

 traces except that the trace width is much greater and the excursion from 

 the zero axis is held to less than the trace width. This is necessary in order 

 to achieve records which may be played back in the reproducer unit as 

 variable area type records. Figure 514 illustrates a typical Mirragraph 

 record. The bottom trace is a 1000-cycle timing track. The recorder has a 

 motor control device which holds the speed constant to one part in 25,000. 



The reproducer unit illustrated on the left of Figure 515 consists 

 essentially of a motor drive film mechanism, an exciter lamp which 

 illuminates all thirteen traces of the moving film record, a masking device 

 to mask off one-half of each trace (which converts the constant width 



t Trade mark, Western Electric Company. 



