826 EXPLORATION GEOPHYSICS 



film handling ten channels. The films, each about thirty inches in length, 

 are wrapped side by side in film-wide grooves around a drum which is 

 driven at a uniform rate of speed by a governed motor. The speed of 

 the motor is such that the films are driven at the rate of five inches per 

 second. A commutator attached to this recording drum synchronizes the 

 firing of the shot with the beginning of the record. The speed of the drum, 

 although maintained by a governor, is checked by a stroboscopic device 

 operated by the timing fork. 



The lamps are heated by a bias battery, with their temperatures adjusted 

 to produce a mean track density. When the amplified impulses are super- 

 posed on this bias current, the track becomes alternately darker and 

 lighter than the mean value. Thus, a variable density record, similar to 

 that used in motion picture sound recording, is obtained. The recording 

 camera for this system is illustrated in Figure 509. 



Figure 510 shows four Sonograph films. The left two records 

 carry five tracks each and were made on a ten-track recorder. The right 

 two records contain ten tracks each. Although the prime purpose of 

 using this type of film record is to permit later analysis, a direct visual 

 examination often may be made by comparing the wave bands. Printing 

 all the films from a continuous profile side by side on a single sheet 

 facilitates the correlation. If desired, records of this type may be passed 

 through a photoelectric light intensity recorder, which yields traces similar 

 to those obtained from the conventional galvanometer systems. Usually, 

 however, the records are analyzed with the aid of a special device called 

 the "analyzer." 



It is the purpose of the analyzer to present the information contained 

 in the density-modulated traces of the film in a form that can be readily 

 interpreted. In the analyzer, the film carrying the recorded traces passes 

 a series of narrow illuminated slots, one slot for each trace. The beam 

 intensities are modulated by the density variations of the traces originally 

 made in accordance with the signals derived from the geophones. A single 

 photocell collects all light beams. The photocell output is applied to an 

 amplifier, passed through a filter, and controls an electromagnetically- 

 driven pen which traces the amplitude of the composite response on a 

 paper record. 



It is a basic feature of the Sonograph method to select and combine 

 signals derived from different traces on one record. As is well known, 

 all signals arriving from a given direction will arrive in a time sequence 

 which can be computed on the basis of geophysical data pertaining to the 

 particular region being explored. Time delays opposite to those occurring 

 in this sequence are introduced between the signals derived from the dif- 

 ferent traces, and these time-corrected signals are then combined. Signals 

 originating at one reflecting layer and received by a plurality of geophones 

 will be in phase, while signals arriving from a different direction will be out 

 of phase. It will be seen that the composite signal discriminates against 



