614 SEISMIC METHODS [Chap. 9 



F. Photographic Recording; Time Marking 



In station seismographs, most recording is done mechanically by pens 

 writing white lines on blackened paper fastened to a drum. The drum 

 rotates and is shifted sideways by a thread cut on the drum shaft. Time 

 marks are recorded by means of the pens being lifted or shifted by an 

 electromagnet actuated from a contact chronometer. In light-weight 

 station seismometers, such as the Wood-Anderson seismograph, and in 

 galvanometric recording instruments, such as the Beniofif and Wenner 

 seismometers, records are taken photographically on sensitized paper. 

 Time marks are provided by interrupting the light beam with a shutter 

 actuated from an electromagnet or by deflecting the trace for a short in- 

 terval by a mirror or prism mounted on a relay connected to the contact 

 chronometer. Both mechanical and photographic recording is used in 

 vibrographs, depending on the sensitivity of the instrument. In some 

 types the record is made on celluloid strips by a pointed needle and is 

 inspected under a microscope. Disk recording has not been applied in 

 seismology. In high-speed recorders time marks are projected by me- 

 chanically started vibrating reeds with shutters, or by electrically sus- 

 tained indicators (galvanometers, reeds, oscillographs) driven from tuning 

 forks or V.T. oscillators. 



In virtually all prospecting seismographs, records are taken photo- 

 graphically on rapidly moving paper varying in width from 2 to 6 inches. 

 To obtain good quality of reproduction, attention must be paid in the field 

 to the proper concentration, temperature, and freshness of the developing 

 and fixing solutions.^" The speed at which the paper travels is about 3 

 to 10 centimeters per second in refraction recording and 30 to 40 centi- 

 meters per second in reflection recording. Provision is usually made for 

 simultaneous visual observation and photographic recording. This may 

 be done by spUtting the reflected light beam, by providing separate inci- 

 dent and reflected beams for the visual and photographic system, or by 

 viewing the Hght spots from the rear of the camera through paper. Even 

 paper speed increases the accuracy of record evaluation ; hence, the recorder 

 drive is usually equipped with a fairly elaborate governor. 



Time marking may be accomplished in various ways. In recorders used 

 with mechanical seismographs, a small pendulum provided with a shutter 

 arrangement is set in motion at the instant of firing. In reflection cameras, 

 a reed of higher frequency (50 cycles) electrically sustained from a tuning 

 fork or vacuum tube oscillator may be used. This has a shutter arrange- 

 ment to project dashes on the paper (if the reed is mounted close to the 

 paper), or to project time lines across its entire width (by reflecting light 



"F. A. Tompkins, Geophysics, 1(1), 107-114 (Jan., 1936). 



