618 



SEISMIC METHODS 



[Chap. 9 



are readily made ; (c) tests can be made in the recording truck to determine 

 whether coupling with timing or shot instance transmission circuits, and 

 the like, is present. 



The combined amplifier and galvanometer response is measured in terms 

 of galvanometer deflection for constant signal input. By varying the fre- 

 quency of the signal, the combined amplifier and galvanometer djoiamic 

 response is obtained. This test should be made for all possible filter 

 positions. 



5. Over-all response. The over-all response of an entire seismic channel 

 is measured by placing the detector on a shaking table, connecting it to 

 its amplifier and galvanometer, and measuring the galvanometer deflection 

 for a given ground amplitude at varying frequencies. Measurements are 

 made for the various filter positions and for various settings of gain con- 

 trols in amplifier and galvanometers to determine efifects of automatic 

 volume control devices, if these are incorporated. 



6. Various other tests of seismic equipment include tests of the timing 

 system by standard forks, parallax tests to determine time lag between 

 shot instant record and galvanometric record, and phase tests. When all 

 component parts of a recording channel have been balanced properly in 

 respect to their mechanical and electrical characteristics, phase differences 

 should not occur. Whether and to what extent they exist may be deter- 

 mined by a phase shot, that is, a reflection shot taken with all receivers 

 set together closely at the same location. Fig. 9-119 shows a phase shot 

 made with a 12-channel apparatus. 



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Fig. 9-119. Twelve-element alignment record. 



