794 



EXPLORATION GEOPHYSICS 



Fig. 480. — Moving coil conductor type of dynamic seismometer: 

 (A) magnet; (B) spring; (C) bobbin on which coil is wound; (D) 

 waterproof conductor. (Courtesy Western Geophysical Company.) 



Fig. 481. — Moving coil type of 

 geophone in waterproof case de- 

 signed to be pushed into the mud. 

 (Courtesy of Century Geophysical 

 Corporation.) 



type of seismometer would be proportional to 

 the velocity of motion of the inertia reactor. 

 Providing that the natural frequency of the 

 seismometer was not too far outside the range 

 of frequency of the impressed force, the out- 

 put would be approximately proportional to 

 the velocity of the ground motion. The elec- 

 trical output of a velocity type seismometer 

 lags behind the motion of the ground because, 

 when the amplitude is the greatest at the end 

 of a swing, the velocity passes through zero 

 and changes algebraic sign. The velocity is 

 probably greatest when the amplitude of the 

 motion is passing through the zero or rest 

 position, 



A form of moving conductor seismometer 

 is shown schematically by Figure 479. This 

 instrument comprises a circular conducting 

 coil, supported by two elastic springs, and a 

 magnetic system rigidly attached to the case. 

 The arrival of seismic waves produces a rela- 

 tive motion of the elastically supported con- 



