244 MISCELLANEOUS TRANSDUCERS 



types of strata, the confirmation of the various strata may be determined. 

 Oil pools are located in curved strata termed by geologists as anticlines. 

 The presence of anticlines may be determined from seismic measurements. 

 Magnetic, carbon, crystal, condenser and dynamic microphones have 

 been used for detectors. The large amplitude frequency components of 

 seismic waves are usually confined to the lower frequencies. Therefore, 

 the response of the microphone is confined to the range below 100 cycles. 

 For these applications a magnetic microphone having a system similar to 

 the reed type. Fig. 6.1, has been found to be very satisfactory. The arma- 

 ture is usually made massive and the stiffness small in order to obtain high 

 sensitivity in the low frequency range. The microphone is placed directly 

 upon the earth's surface. The microphone proper then vibrates with the 

 earth's surface. The massive armature opposes any change from its posi- 

 tion of rest. As a consequence, there is relative motion between the arma- 

 ture and the microphone proper which results in the production of a voltage 

 corresponding to the vibrations of the earth's surface. By suitable orien- 

 tation, the microphone can be made responsive to only vertical vibrations. 

 As a consequence, the wave transmitted directly through the earth is not 

 reproduced. 



