SEISMIC METHODS 



825 



per second ; and the bottom seismogram at about 70 cycles per second. 

 This is an illustration of the effect of filtering on the recorded seismogram. 

 The turning points of the recorded traces become progressively less sharp 

 with decreasing peaked-frequencies of the filters. 



Sonograph Equipment for Phase Effects. — This equipmentf is 

 described in detail because of its novel features and different operating 



Fig. 509. — Recording oscillograph employing variable density light tracks, a, two film drums 

 for 35 mm. film; b, clamping slots for film; c, motor drive; d, motor control tubes; e, commutator 

 control for firing circuit to insure proper synchronization of firing and start of film record; /, 

 relays for recording lamp circuits; g, housing for recording lamps. (Courtesy of Rieber 

 Laboratories.) 



principles. One characteristic feature is the elimination of the recording 

 galvanometers. The amplified outputs of the seismometers are impressed 

 on small overheated incandescent lamps, having special filaments capable 

 of heating and cooling in approximate synchronism with the wave motions 

 being recorded. Twenty such lamps are used to record twenty channels, 

 additional lamps being employed to make records of the tuning fork motion 

 and the shot-break. Each lamp shines through a small well-defined slit, 

 approximately 0.010 by 0.070 inch, twelve such slits being placed side by 

 side to record on a strip of 35 mm. positive motion-picture film. 



In the twenty-trace recorder, two films are used for each shot, each 



t F. Rieber, "A New Reflection System with Controlled Directional Sensitivity," Geophysics, 

 Vol. 1, No. 1, Jan. 1936. 



